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Sample records for proterozoic diamond-bearing kimberlites

  1. The potential for diamond-bearing kimberlite in northern Michigan and Wisconsin

    USGS Publications Warehouse

    Cannon, William F.; Mudrey, M.G.

    1981-01-01

    because of the extensive cover of glacial drift and the typical small size of kimberlite bodies. If all are magnetic, they might be found by detailed aeromagnetic surveys. However, the magnetism of the Lake Ellen kimberlite appears to be caused by secondary magnetite formed during serpentinization of olivine, so an unserpentinized kimberlite may not be strongly magnetic. We suggest that one or more diamond-bearing kimberlites may exist in northern Michigan or Wisconsin, but the discovery of such bodies is unlikely unless a very thorough search is undertaken.

  2. Diamond-bearing Rocks among Mantle Xenoliths in Kimberlites as Indicatory for the Chambers of Diamond-parental Carbonatite Magma

    NASA Astrophysics Data System (ADS)

    Litvin, Yuriy; Kuzyura, Anastasia

    2014-05-01

    Origin of diamond-bearing peridotite and eclogite rocks in kimberlites is cleared up using mantle-carbonatite model of diamond genesis (Litvin, 2007, 2009, 2013). Data of analytical mineralogy of primary inclusions in diamonds and results of physicochemical experiments on syngenetic diamond and inclusion phase relations are co-ordinated in this model (Litvin et al., 2012). It proved that diamond-parental media are presented by changeable carbon-saturated peridotite-carbonatite and eclogite-carbonatite melts. The melts are capable to form not diamonds only but their major and minor inclusions. The upper mantle is mainly composed of diamond-free peridotites which dominate over eclogites as 9 to 5 % (Mathias et al., 1970). Howewer diamond-bearing peridotites and eclogites occur rarely as demonstrated for S.Africa and Yakutia (Sobolev N., 1977). Nevertheless, origin of diamond-bearing rocks belongs to key problems of genetic mineralogy of diamond and mantle petrology due to dissimilar physicochemical and environmental conditions of formation of comparatively diamond-free rocks. Symptomatic that garnets included in diamond and these of diamond-bearing eclogite are compositionally similar (Sobolev V. et al., 1972). Garnets of diamond-bearing eclogites, inclusions in diamonds and intergrowths with them are marked by increased Na2O content (0.10-0.22%) because of Na-majorite component Na2MgSi5O12 (Bobrov & Litvin, 2011). Peridotitic garnets of diamond-bearing rocks, inclusions and intergrowths are indicated by high Cr2O3 and low CaO content over diamond-free ones. This compositional dissimilarity is compatible with formation of diamond-bearing rocks, inclusions and intergrowths in chambers of partially melted peridotite-eclogite-carbonatite-sulphide-carbon system of changeable composition. However, diamond-free rocks are products of upper-mantle magmatism based on carbonatite-free peridotite-eclogite-sulphide-carbon system. Chambers of diamond-parental carbonatite magma

  3. Découverte d'un diatrème de kimberlite diamantifère à Séguéla en Côte-d'IvoireDiscovery of a diamond-bearing kimberlite diatreme at Séguéla in Ivory Coast

    NASA Astrophysics Data System (ADS)

    Pouclet, André; Allialy, Marc; Daouda-Yao, Bertin; Esso, Botty

    2004-01-01

    The Séguéla area in Ivory Coast is known for its diamond-bearing field related to dykes of kimberlite and lamproite. These dykes, devoid of any deformation and metamorphism, crosscut the Birimian formations. Their N 170° orientation is controlled by the tectonized contact between the Archean and the Paleoproterozoic shields of the West-African craton. Discovery of a diatreme, with its pipe breccias and well-preserved maar sediments, below the present-day colluvia, attests for the probable recent geological age (Cretaceous?) of the kimberlitic activity. To cite this article: A. Pouclet et al., C. R. Geoscience 336 (2004).

  4. Archean diamond-bearing mantle in the expanding-earth model

    NASA Astrophysics Data System (ADS)

    Milanovskii, E. E.; Malkov, B. A.

    It is suggested that the Archean diamond-bearing mantle and its diamond-bearing parageneses experience in time a chain of retrograde transformations, making it possible to decide whether or not isobars were really submerged in the paleolithosphere from the Archean to the present age and to calculate the rate of this submergence. Kimberlite volcanism, transporting diamond-bearing matter from the upper mantle to the earth's surface, lends support to the global pulsational character of tectonic-magmatic processes. The regularities revealed in the analysis of diamond-bearing parageneses and the regularities in the features of the Precambrian diamond-bearing mantle agree well with a model of moderate pulsational expansion of the earth with a total increment of the earth's radius of the order of 10-15 percent from its initial value.

  5. Lower-crustal xenoliths from Jurassic kimberlite diatremes, upper Michigan (USA): Evidence for Proterozoic orogenesis and plume magmatism in the lower crust of the southern Superior Province

    USGS Publications Warehouse

    Zartman, Robert E.; Kempton, Pamela D.; Paces, James B.; Downes, Hilary; Williams, Ian S.; Dobosi, Gábor; Futa, Kiyoto

    2013-01-01

    Jurassic kimberlites in the southern Superior Province in northern Michigan contain a variety of possible lower-crustal xenoliths, including mafic garnet granulites, rare garnet-free granulites, amphibolites and eclogites. Whole-rock major-element data for the granulites suggest affinities with tholeiitic basalts. P–T estimates for granulites indicate peak temperatures of 690–730°C and pressures of 9–12 kbar, consistent with seismic estimates of crustal thickness in the region. The granulites can be divided into two groups based on trace-element characteristics. Group 1 granulites have trace-element signatures similar to average Archean lower crust; they are light rare earth element (LREE)-enriched, with high La/Nb ratios and positive Pb anomalies. Most plot to the left of the geochron on a 206Pb/€204Pb vs 207Pb/€204Pb diagram, and there was probably widespread incorporation of Proterozoic to Archean components into the magmatic protoliths of these rocks. Although the age of the Group 1 granulites is not well constrained, their protoliths appear to be have been emplaced during the Mesoproterozoic and to be older than those for Group 2 granulites. Group 2 granulites are also LREE-enriched, but have strong positive Nb and Ta anomalies and low La/Nb ratios, suggesting intraplate magmatic affinities. They have trace-element characteristics similar to those of some Mid-Continent Rift (Keweenawan) basalts. They yield a Sm–Nd whole-rock errorchron age of 1046 ± 140 Ma, similar to that of Mid-Continent Rift plume magmatism. These granulites have unusually radiogenic Pb isotope compositions that plot above the 207Pb/€204Pb vs 206Pb/€204Pb growth curve and to the right of the 4·55 Ga geochron, and closely resemble the Pb isotope array defined by Mid-Continent Rift basalts. These Pb isotope data indicate that ancient continental lower crust is not uniformly depleted in U (and Th) relative to Pb. One granulite xenolith, S69-5, contains quartz, and has a

  6. Orthopyroxene survival in deep carbonatite melts: implications for kimberlites

    NASA Astrophysics Data System (ADS)

    Stone, Rebecca S.; Luth, Robert W.

    2016-07-01

    Kimberlites are rare diamond-bearing volcanic rocks that originate as melts in the Earth's mantle. The original composition of kimberlitic melt is poorly constrained because of mantle and crustal contamination, exsolution of volatiles during ascent, and pervasive alteration during and after emplacement. One recent model (Russell et al. in Nature 481(7381):352-356, 2012. doi: 10.1038/nature10740) proposes that kimberlite melts are initially carbonatitic and evolve to kimberlite during ascent through continuous assimilation of orthopyroxene and exsolution of CO2. In high-temperature, high-pressure experiments designed to test this model, assimilation of orthopyroxene commences between 2.5 and 3.5 GPa by a reaction in which orthopyroxene reacts with the melt to form olivine, clinopyroxene, and CO2. No assimilation occurs at 3.5 GPa and above. We propose that the clinopyroxene produced in this reaction can react with the melt at lower pressure in a second reaction that produces olivine, calcite, and CO2, which would explain the absence of clinopyroxene phenocrysts in kimberlites. These experiments do not confirm that assimilation of orthopyroxene for the entirety of kimberlite ascent takes place, but rather two reactions at lower pressures (<3.5 GPa) cause assimilation of orthopyroxene and then clinopyroxene, evolving carbonatitic melts to kimberlite and causing CO2 exsolution that drives rapid ascent.

  7. Diamond resource potential of kimberlites from the Zimny Bereg field, Arkhangel'sk oblast

    NASA Astrophysics Data System (ADS)

    Kononova, V. A.; Golubeva, Yu. Yu.; Bogatikov, O. A.; Kargin, A. V.

    2007-12-01

    Kimberlites with different diamond grades from the Zolotitsa, Verkhotina, and Kepina occurrences of the Zimny Bereg field (Arkangel’sk oblast) have been compared in order to ascertain geochemical criteria of their diamond resource potential. A new collection of 21 core samples taken within a depth interval of 207 940 m from nine boreholes drilled in the central and western portions of the high-grade diamond-bearing Grib kimberlite pipe was subjected to comprehensive petrographic and geochemical examination, including Sr, Nd, and Pb isotopes and trace elements determined with ICP-MS. The compositional variations in kimberlites are controlled by the structural types of rocks. Porphyritic kimberlite (PK) distinctly differs from autolithic kimberlite breccia (AKB). Autoliths (Av) and PK are enriched in Th, U, Nb, Ta, La, Ce, Pr, P, Nd, Sm, Eu, Ti, LREE, and MREE, whereas HREE contents are rather uniform in all types of kimberlites. No lateral zoning was observed in pipes pertaining to the same structural type. The composition of kimberlites in the Zimny Bereg field and their diamond resource potential are variable. In the series of the Zolotitsa, Verkhotina, and Kepina occurrences, the Ti content increases, the La/Yb ratio grows from 18 44 to 70 130, and the diamond grade diminishes in the Kepina occurrence. The variations in kimberlite compositions are considered in terms of the degree of partial melting in the mantle, the role of volatiles, etc. As follows from the variation in the Ce/Y ratio, kimberlites from the Zolotitsa occurrence were formed at a lower degree of partial melting in comparison with the Kepina occurrence. Products of different degrees of partial melting are recognized within the Grib pipe; Av were likely formed at a somewhat higher degree of melting than AKB. An appreciable isotopic heterogeneity of the mantle is recorded in variable Nd and Sr isotopic compositions of kimberlites. The Kepina kimberlites were derived from a source slightly

  8. Model of kimberlite formation

    NASA Astrophysics Data System (ADS)

    Kostrovitsky, Sergey; Fiveyskaya, Lyudmila

    2013-04-01

    The critical goals in recognizing the nature of kimberlites are to find out: (1) the primary composition of melt of these rocks and (2) the principal processes of evolution of primary composition of kimberlites while ascending from mantle depth towards earth surface. Suppose, that the primary composition of kimberlite melt-fluid was in fact the composition of asthenosphere melt geochemically being close to alkaline-basalt (Hi-µ) saturated with high CO2. The genetic relation of kimberlites with basaltoids is indicated by a spatial and temporal affinity of their formation (Carlson et al, 2006; Lehmann et al, 2010; Tappe et al, 2012), similarity of the pattern of incompatible elements distribution, presence of megacryst minerals in alkaline basaltoids, Pyr-Alm garnet included, and finally, model calculation of parent melt composition for low-Cr megacryst minerals; it showed this composition to be typical for the alkaline basaltoid (Jones, 1980). At the asthenosphere level there was differentiation of basaltoid melt-fluid which was responsible for formation of its different parts with varying melt to fluid ratio and possibly varying content of alkalis (K2O). The outbreak of asthenosphere substance through lithosphere mantle proceeded by different scenarios: (a) With a noticeable dominance of fluid component kimberlites were formed by the capture and contamination of high-Mg, high-Cr rocks of lithosphere mantle that caused formation of high-Mg kimberlites. That corresponds to model of Russell (2012). (b) With a considerable proportion of melt phase depending on saturation in fluid there formed magnesium-ferriferous and ferriferous-titaniferous petrochemical types of kimberlites. There is no doubt that in formation of these kimberlite types the contamination of lithosphere material was the case, at the much lower level than in formation of high-Mg kimberlites. This model logically explains steady differences of petrochemistry of kimberlites making up clusters of

  9. Kimberlite ascent and eruption.

    PubMed

    Sparks, R S J; Brown, R J; Field, M; Gilbertson, M

    2007-12-13

    Wilson and Head model kimberlite ascent and eruption by considering the propagation of a volatile-rich dyke. Wilson and Head's model has features in common with Sparks et al., but it is inconsistent with geological observations and constraints on volatile solubility. Here we show that this may be due to erroneous physical assumptions.

  10. Kimberlite ascent and eruption.

    PubMed

    Sparks, R S J; Brown, R J; Field, M; Gilbertson, M

    2007-12-13

    Wilson and Head model kimberlite ascent and eruption by considering the propagation of a volatile-rich dyke. Wilson and Head's model has features in common with Sparks et al., but it is inconsistent with geological observations and constraints on volatile solubility. Here we show that this may be due to erroneous physical assumptions. PMID:18075522

  11. Did diamond-bearing orangeites originate from MARID-veined peridotites in the lithospheric mantle?

    PubMed

    Giuliani, Andrea; Phillips, David; Woodhead, Jon D; Kamenetsky, Vadim S; Fiorentini, Marco L; Maas, Roland; Soltys, Ashton; Armstrong, Richard A

    2015-04-17

    Kimberlites and orangeites (previously named Group-II kimberlites) are small-volume igneous rocks occurring in diatremes, sills and dykes. They are the main hosts for diamonds and are of scientific importance because they contain fragments of entrained mantle and crustal rocks, thus providing key information about the subcontinental lithosphere. Orangeites are ultrapotassic, H2O and CO2-rich rocks hosting minerals such as phlogopite, olivine, calcite and apatite. The major, trace element and isotopic compositions of orangeites resemble those of intensely metasomatized mantle of the type represented by MARID (mica-amphibole-rutile-ilmenite-diopside) xenoliths. Here we report new data for two MARID xenoliths from the Bultfontein kimberlite (Kimberley, South Africa) and we show that MARID-veined mantle has mineralogical (carbonate-apatite) and geochemical (Sr-Nd-Hf-O isotopes) characteristics compatible with orangeite melt generation from a MARID-rich source. This interpretation is supported by U-Pb zircon ages in MARID xenoliths from the Kimberley kimberlites, which confirm MARID rock formation before orangeite magmatism in the area.

  12. Did diamond-bearing orangeites originate from MARID-veined peridotites in the lithospheric mantle?

    PubMed

    Giuliani, Andrea; Phillips, David; Woodhead, Jon D; Kamenetsky, Vadim S; Fiorentini, Marco L; Maas, Roland; Soltys, Ashton; Armstrong, Richard A

    2015-01-01

    Kimberlites and orangeites (previously named Group-II kimberlites) are small-volume igneous rocks occurring in diatremes, sills and dykes. They are the main hosts for diamonds and are of scientific importance because they contain fragments of entrained mantle and crustal rocks, thus providing key information about the subcontinental lithosphere. Orangeites are ultrapotassic, H2O and CO2-rich rocks hosting minerals such as phlogopite, olivine, calcite and apatite. The major, trace element and isotopic compositions of orangeites resemble those of intensely metasomatized mantle of the type represented by MARID (mica-amphibole-rutile-ilmenite-diopside) xenoliths. Here we report new data for two MARID xenoliths from the Bultfontein kimberlite (Kimberley, South Africa) and we show that MARID-veined mantle has mineralogical (carbonate-apatite) and geochemical (Sr-Nd-Hf-O isotopes) characteristics compatible with orangeite melt generation from a MARID-rich source. This interpretation is supported by U-Pb zircon ages in MARID xenoliths from the Kimberley kimberlites, which confirm MARID rock formation before orangeite magmatism in the area. PMID:25882074

  13. Did diamond-bearing orangeites originate from MARID-veined peridotites in the lithospheric mantle?

    NASA Astrophysics Data System (ADS)

    Giuliani, Andrea; Phillips, David; Woodhead, Jon D.; Kamenetsky, Vadim S.; Fiorentini, Marco L.; Maas, Roland; Soltys, Ashton; Armstrong, Richard A.

    2015-04-01

    Kimberlites and orangeites (previously named Group-II kimberlites) are small-volume igneous rocks occurring in diatremes, sills and dykes. They are the main hosts for diamonds and are of scientific importance because they contain fragments of entrained mantle and crustal rocks, thus providing key information about the subcontinental lithosphere. Orangeites are ultrapotassic, H2O and CO2-rich rocks hosting minerals such as phlogopite, olivine, calcite and apatite. The major, trace element and isotopic compositions of orangeites resemble those of intensely metasomatized mantle of the type represented by MARID (mica-amphibole-rutile-ilmenite-diopside) xenoliths. Here we report new data for two MARID xenoliths from the Bultfontein kimberlite (Kimberley, South Africa) and we show that MARID-veined mantle has mineralogical (carbonate-apatite) and geochemical (Sr-Nd-Hf-O isotopes) characteristics compatible with orangeite melt generation from a MARID-rich source. This interpretation is supported by U-Pb zircon ages in MARID xenoliths from the Kimberley kimberlites, which confirm MARID rock formation before orangeite magmatism in the area.

  14. Kimberlitic olivines derived from the Cr-poor and Cr-rich megacryst suites

    NASA Astrophysics Data System (ADS)

    Moore, Andy; Costin, Gelu

    2016-08-01

    Reversed-zoned olivines (Fe-richer cores compared to rims), appear to be ubiquitous in kimberlites with a wide distribution. These olivines generally comprise a subordinate population relative to the dominant normally zoned olivines. However, they are notably more abundant in the megacryst-rich mid-Cretaceous Monastery and early Proterozoic Colossus kimberlites, located on the Kaapvaal and Zimbabwe cratons, respectively. The reverse-zoned olivines at these two localities define compositional fields that are closely similar to those for two olivine megacryst populations of the Cr-poor association which have been documented in the Monastery kimberlite. This points to a genetic link between megacrysts and the reversed zoned olivines. The ubiquitous, occurrence of the Fe-rich (relative to the field for rims) olivines in kimberlites with a wide geographic distribution in turn argues for an intimate link between megacrysts and the host kimberlite. Some large olivines have inclusions of rounded Cr-rich clinopyroxenes, garnets and/or spinel, characterized by fine-scale, erratic internal compositional zoning. Olivines with such chemically heterogeneous Cr-rich inclusions are not derived from disaggregated mantle peridotites, but are rather linked to the Cr-rich megacryst suite. Consequently, they cannot be used as evidence that cores of a majority of kimberlitic olivines are derived from disaggregated mantle peridotites.

  15. Genesis of Diamond-bearing and Diamond-free Podiform Chromitites in the Luobusa Ophiolite, Tibet

    NASA Astrophysics Data System (ADS)

    Yang, J.; Xiong, F.; Xu, X.; Robinson, P. T.; Dilek, Y.; Griffin, W. L.

    2014-12-01

    Micro-diamonds, moissanite and many highly reduced minerals, such as native Fe, Cr, Ni, Si, Al, and metallic alloys, have been reported previously from podiform chromitites and peridotites of the Luobusa ophiolite in the eastern segment of the Yarlung-Zangbo suture of southern Tibet.. Similar mineral associations have now been confirmed in mantle peridotites or chromitites of 11 other ophiolites in 5 orogenic belts, in Tibet, Myanmar, North China and the Polar Urals. However, detailed studies of the Luobusa ophiolite show that not all chromitites contain these UHP and highly reduced minerals. Diamond-bearing chromitites are chiefly massive bodies composed of over 95 modal% magnesiochromite with Cr#s [100Cr/(Cr+Al)] of 77-83 and Mg#s [100Mg/(Mg+Fe)] of 71-82. Most of these bodies have sharp contacts with the host harzburgites and are only rarely enclosed in dunite envelopes. Many magnesiochromite grains in the massive chromitites contain inclusions of forsterite and pyroxene. Forsterite inclusions have Fo numbers of 97-99 and NiO contents of 1.11-1.29 wt%. Mg#s of clinopyroxene inclusions are 96-98 and those of orthopyroxene are 96-97. X-ray studies show that the olivine inclusions have very small unit cells and short cation-oxygen bond distances, suggesting crystallization at high pressure. In contrast, diamond-free chromitites typically occur as layers within thick dunite sequences or as irregular patches surrounded by dunite envelopes. They consist of variable proportions of magnesiochromite (Cr# = 76-78; Mg# = 58-61) and olivine, and have banded, nodular and disseminated textures. The dunite envelopes consist chiefly of granular olivine with a few relatively large, amoeboidal grains of magnesiochromite, and typically grade into the host peridotites with increasing pyroxene. Unlike those in the massive ores, magnesiochromite grains in nodular and disseminated chromitites lack pyroxene inclusions, and their olivine inclusions have relatively low Fo (94-96) and Ni

  16. Kimberlite Ascent: Insights from Olivine

    NASA Astrophysics Data System (ADS)

    Brett, C.; Russell, K.

    2009-05-01

    Olivine is ubiquitous in both extrusive and intrusive kimberlite deposits worldwide. Within kimberlite, it is mainly present as xenocrysts derived from the disaggregation of mantle-derived peridotitic xenoliths. Many textural and chemical features within the mantle-derived olivine xenocrysts result from post entrainment processes. On that basis, these features record physical and chemical changes attending kimberlite ascent and can be used to elucidate the transport and eruption of kimberlite magma. Our textural study of kimberlitic olivine is based on intrusive and pyroclastic kimberlite from the Diavik kimberlite cluster and from the Igwisi Hills kimberlitic lava flows. Based on these observations and the physical and chemical properties of olivine we derive a relative sequence of textural events. Textural features include: sealed cracks, healed cracks, phases trapping in cracks, rounded grains, overgrowths and phase trapping in overgrowths. These features record processes that operate in kimberlite during ascent, and from these features we create a summary model for kimberlite ascent: -- Olivine is incorporated into kimberlitic melts in peridotitic mantle xenoliths continuously during ascent. Xenolith incorporation is focused at the crack tip where the stress regime is highest. -- Shortly after the incorporation of these xenocrysts the tensile strength of the xenoliths is reached at a maximum of 2 km from its source. Disaggregation of mantle xenoliths (producing xenocrysts) is facilitated by expansion of the minerals within the xenoliths causing intra-crystal slip (i.e. along grain boundaries). -- Continued decompression causes olivine to also break in tension approximately 20 km from source. The void space produced by the failure of the crystals (inter-crystal cracks) is filled with melt and crystals consisting of primary carbonate (high-Sr), chromite and spinel crystals. The carbonate later crystallizes to produce sealed fractures. -- Mechanical rounding of

  17. Emplacement of crystal-rich magmas: insights from the Snap Lake kimberlite intrusion (NW Territories, Canada)

    NASA Astrophysics Data System (ADS)

    Gernon, Thomas; Sparks, Steve; Field, Matthew; Ogilvie-Harris, Rachael

    2010-05-01

    The Cambrian Snap Lake kimberlite intrusion (Northwest Territories, Canada) is a complex segmented diamond-bearing ore-body. Detailed geological investigations suggest that the intrusion is a multi-phase body with at least four different magmatic lithofacies. In particular, olivine-rich (phlogopite-poor) and olivine-poor (phlogopite-rich) varieties of hypabyssal kimberlite have been identified. Key observations are that olivine-rich lithofacies (ORK) has a strong tendency to be located where the intrusion is thickest and that there is a good correlation between intrusion thickness, olivine crystal size and crystal content. Accordingly the olivine-poor lithofacies (OPK) tends to be most abundant where the intrusion is thinnest. Complimentary studies demonstrate that the lithofacies are geochemically distinct, and are characterised by different diamond abundances and size distributions. Our data and observations suggest that the ORK and OPK represent different magma phases that have experienced different processes during transport and emplacement. Heterogeneities in the kimberlite lithofacies are attributed to variations in intrusion thickness and structural complexities. Auto-xenoliths of ORK within the OPK suggest that the magmas are closely related in time and have clearly exploited the same fracture system during intrusion, resulting in various degrees of intermingling. The geometry and distribution of lithofacies points to magmatic cointrusion, and flow differentiation driven by fundamental rheological differences between the ORK and OPK phases. The presence of such low viscosity, crystal-poor magmas may explain how extremely crystal-rich kimberlite magmas (> 60 vol.%) are able to reach the surface and erupt in kimberlite diatremes. We envisage that the low viscosity OPK magma acted as a lubricant for the highly viscous ORK magma; such rheological segregation is a common feature in other magmatic systems. The Snap Lake intrusion provides important insights into

  18. Lake Ellen kimberlite, Michigan, U.S.A.

    USGS Publications Warehouse

    McGee, E.S.; Hearn, B.C.

    1983-01-01

    The recently discovered Lake Ellen kimberlite, in northern Michigan, indicates that bedrock sources of diamonds found in glacial deposits in the Great Lakes area could lie within the northern U.S. Magnetic surveys show a main kimberlite 200 m in diameter and an adjacent body 25 x 90 m(?). The kimberlite cuts Proterozoic volcanic rocks that overlie Archean basement, but is post-Ordovician in age based on abundant Ordovician(?) dolomite inclusions. Xenocrysts and megacrysts are ilmenite (abundant, 12.5-19% MgO), pyropealmandine and Cr-pyrope (up to 9.3% Cr2O3), Cr-diopside (up to 4.5% Cr2O3), olivine (Fo 91), enstatite and phlogopite. The kimberlite contains fragments of crustal schist and granulite, as well as disaggregated crystals and rare xenoliths of eclogites, garnet pyroxenites and garnet peridotites from a heterogeneous upper mantle. Eclogites, up to 3 cm size, show granoblastic equant or tabular textures and consist of jadeitic cpx (up to 8.4% Na20, 15.3% Al2O3), pyrope-almandine, ? rutile ? kyanite ? sanidine ? sulfide. Garnet pyroxenite contains pyrope--(0.44% Cr2O3) + cpx (0.85% Na2O, 0.53% Cr2O3) + Mg-Al spinel. Mineral compositions of rare composite xenocrysts of garnet + cpx are distinctively peridotitic, pyroxenitic or eclogitic. Calculated temperatures of equilibration are 920-1060 ?C for the eclogites and 820-910?C for the garnet pyroxenite using the Ellis-Green method. Five peridotite garnet-clinopyroxene composite xenocrysts have calculated temperatures of 980-1120?C using the Lindsley-Dixon 20 kb solvus. Spinel pyroxenite and clinopyroxene-orthopyroxene composites have lower calculated temperatures of 735?C and 820-900?C, respectively. Kyanite-bearing eclogites must have formed at pressures greater than 18-20 kb. Using the present shield geotherm with a heat flow value of 44mW/m 2 for the time of kimberlite emplacement, the eclogite temperatures imply pressures of 35-48 kb (105-140 km) and the garnet pyroxenite temperatures indicate pressures of

  19. Origin of the Luobusa diamond-bearing peridotites from the sub-arc mantle

    NASA Astrophysics Data System (ADS)

    Liu, Chuanzhou; Zhang, Chang; Wu, Fuyuang; Chung, Sunlin

    2016-04-01

    Ophiolites are the remnants of ancient oceanic lithosphere that were emplaced onto continental margins. Ophiolites along the E-W trending Yarlung-Tsangpo Suture (YTS), which separates the Indian plate from the Eurasian plate, have been regarded as relics of the Neo-Tethys Ocean. The Luobusa ophiolite outcrops at the eastern YTS and mainly consists of harzburgites and dunites that have been intruded by gabbroic/diabase dykes at ca 130 Ma (Zhang et al., 2015). Basaltic lavas are rarely outcropped, and volumetrically minor (< 1% by volume) chromitites are enveloped as lens and layers within dunites (Zhou et al., 1996). The Luobusa peridotites have been interpreted as mantle residues experienced melt extraction at the mid-ocean ridge and subsequently reacted with boninitic magmas in subduction zone, which gave rise to podifiorm chromitites (Zhou et al., 1996). However, such a shallow depth origin fails to explain the occurrence of diamond and other ultra-high pressure (UHP) minerals in both peridotites and chromitites (Yang et al., 2007, 2014). A mantle plume origin has been proposed for the Luobusa ophiolite to explain the UHP minerals. However, this model is not reconciled with the occurrence of low-pressure crustal minerals in both chromitites and peridotites (Robinson et al., 2015). Here we report whole-rock Re-Os isotope data, which suggest that most Luobusa peridotites have subjected to ancient melting events older than 1.9 billion years. High contents of heavy rare earth elements in clinopyroxenes support the occurrence of ancient melting in garnet stability field. Hf-Nd isotopes of clinopyroxenes, which yield young model ages as 110 Ma, with one showing the lowest ɛNd(T) value of -3, do not preserve the signatures of ancient melting but record metasomatism by subduction-related agents. Consequently, we argue that protoliths of the Luobusa peridotites originated from ancient domains in the transition zone and, together with diamond-bearing chromitites, were

  20. Ultrabasic-basic evolution of upper mantle magmas: petrogenetic links between diamond-bearing peridotites and eclogites (on evidence of physico-chemical experiments)

    NASA Astrophysics Data System (ADS)

    Litvin, Y.

    2012-04-01

    1. Upper mantle primordial and differentiated rocks. Present notion of primordial "pyrolitic" (Ringwood, 1962) and differentiated rocks is based on peridotite-pyroxenite and eclogite-grospydite xenoliths in kimberlites. Peridotites are dominant (~95%) respectively to eclogites (~5%) but Roberts-Victor mine is more eclogitic (80%) than peridotitic (20%). Bimineral Cpx-Grt eclogites present ~63% of eclogites, that was explained by "eclogitic thermal barrier" stable over 27 GPa (O'Hara, 1968). This led to subduction version of eclogite formation contrary to mechanism of mantle peridotite differentiation that was expanding to relationship between diamond-bearing varieties. Nevertheless, Qtz/Coes-Opx and Ky/Crd eclogites exist. This stimulates experimental searching for physico-chemical mechanism of formation of all eclogite verieties from primordial peridotite during ultrabasic-basic magmatic differentiation. 2. Physico-chemical reasons for "eclogitic thermal barrier". Liquidus of primordial multicomponent peridotite (Litvin, 1991) is determined by univariant curves Ol+Opx+Cpx+L, Ol+Opx+Grt+L, Opx+Cpx+Grt+L linking together to form invariant peritectics Ol+Opx+Cpx+Grt+L (primary melt is komatiitic). Univariant curve Ol+Cpx+Grt+L emerges from the peritectics. Liquidus of peridotite-eclogite system includes "eclogitic" peritectics Coes+Opx+Cpx+Grt+L tied by emerging univariant curve Coes+ Cpx+Grt+L with another "eclogitic" peritectics Coes+Ky+Cpx+Grt+L. "Eclogitic thermal barrier" is located on Opx-Cpx-Grt plane (separating peridotitic and eclogitic compositions) as temperature maximum of univariant curve Opx+Cpx+Grt+L being connecting link between peridotitic Ol+Opx+Cpx+Grt+L and eclogitic Coes+Opx+Cpx+Grt+L peritectics. "Eclogitic thermal barrier" is insuperable obstacles for ultrabasic-basic magmatic differentiation for both equilibrium and fractional crystallization mechanisms. 3. Fractional crystallization of ultrabasic-basic magmas and continuous change-over from

  1. Kimberlite emplacement record in diamond morphology

    NASA Astrophysics Data System (ADS)

    Fedortchouk, Y.; Chinn, I.

    2015-12-01

    Diamond resorption morphology reflects conditions and events in the host kimberlite magma and in diamond sources in subcratonic mantle. Recent experimental studies on diamond dissolution enable us now to use surface features of diamonds to examine magmatic fluid in kimberlites. This study uses optical and scanning electron microscopy examination of ~750 macro-diamonds from two kimberlites in Orapa cluster, Botswana. Kimberlite A is a simple body filled with coherent kimberlite facies (CK); kimberlite B is a complex body with two facies of coherent kimberlite and a massive volcaniclastic kimberlite facies (MVK). Distinction between kimberlite-induced and mantle-derived resorption was based on: the type of the most abundant resorption style, morphology of crystals with attached kimberlite fragments, and the study of pseudohemimorphic diamonds. Kimberlite-induced resorption is the focus of this work. The three facies in the pipe B show three contrasting diamond resorption types. Resorption in MVK facies leads to glossy rounded surfaces with fine striation and hillocks, and is identical to the resorption style in CK facies of pipe A. This type of resorption is typical for volcaniclastic facies and indicates emplacement in the presence of abundant COH fluid with high H2O:CO2 ratio (>50mol% of H2O). We propose that pipe A is a root zone supplying material to a larger kimberlite body filled with VK. The two CK in pipe B have very different resorption style. One forms similar glossy surfaces but with regular small cavities of rounded outline, while the other seems more corrosive and develops extremely rough features and deep cavities. Comparison to the experimental data suggests that the former had almost pure H2O fluid at low pressure (where solubility of SiO2 is low). The later CK facies was emplaced in the absence or very low abundance of a free fluid, and possibly in melt closer to carbonatitic composition.

  2. Introduction: The Proterozoic

    NASA Astrophysics Data System (ADS)

    Jenkins, Gregory S.; McKay, Christopher P.; McMenamin, Mark A. S.

    The Proterozoic (2.5 Ga-545 Ma) is perhaps the most intriguing period in Earth's history. In a typical high school physical science textbook it may be presented as a rather boring period that today's student is happy to pass over in lieu of the Mesozoic and the extinction of Tyrannosaurus rex by a large asteroid. In reality this was a period full of excitement as it opens (in the PalaeoProterozoic) with low-latitude glaciation in concert with a rise in atmospheric oxygen. The Proterozoic ends with a glacial period and a possible rise in atmospheric oxygen levels. Other highlights of the Proterozoic include: three or more severe glacial events, a long period (1 billion years) of apparent warmth without evidence of glacial deposits, significant fluctuations in δC13, two or more periods where supercontinents were assembled, cap carbonates, banded iron formations, the rise of eukaryotes and the first complex life. The juxtaposition of extreme climate conditions and major evolutionary change among complex organisms during the Proterozoic is particularly puzzling, and begs the following question: What are the factors controlling the appearance of complex life?

  3. The Igwisi Hills extrusive 'kimberlites'

    NASA Technical Reports Server (NTRS)

    Reid, A. M.; Donaldson, C. H.; Dawson, J. B.; Brown, R. W.; Ridley, W. I.

    1975-01-01

    The petrography and mineral chemistry of volcanic rocks from the Igwisi Hills in Tanzania are discussed. There is considerable evidence to suggest that the Igwisi rocks are extrusive kimberlites: a two-component nature with high P-T minerals in a low P-T matrix; the presence of chrome pyrope, Al enstatite, chrome diopside, chromite and olivine; a highly oxidized, volatile-rich matrix with serpentine, calcite, magnetite, perovskite; high Sr, Zr, and Nb contents; occurrence in a narrow isolated vent within a stable shield area. The Igwisi rocks differ from kimberlite in the lack of magnesian ilmenite, the scarcity of matrix phlogopite, and the overall low alkali content. They apparently contain material from phlogopite-bearing garnet peridotites with a primary mineral assemblage indicative of equilibrium at upper mantle temperatures and pressures. This primary assemblage was brought rapidly to the surface in a gas-charged, carbonate-rich fluid. Rapid upward transport, extrusion, and rapid cooling have tended to prevent reaction between inclusions and the carbonate-rich matrix that might otherwise have yielded a more typical kimberlite.

  4. Cracks preserve kimberlite melt composition

    NASA Astrophysics Data System (ADS)

    Brett, R. C.; Vigouroux-Caillibot, N.; Donovan, J. J.; Russell, K.

    2009-12-01

    The chemical composition of kimberlite melts has previously been estimated by measuring aphanitic intrusive rocks (deposit composition) or by partial melting experiments on carbonated lherzolites (source composition). Pervasively altered, degassed and contaminated material preclude the determination of the primitive melt composition. Here we present data on melt compositions trapped in unaltered olivine cracks that have been healed and overgrown prior to shallow level emplacement. During the ascent of kimberlite magma the prograding crack tip samples mantle peridotite xenoliths. Xenoliths rapidly disaggregate over the first few kilometers of transport producing a population of olivine xenocrysts that are released to the fluid-rich melt. Rapid ascent of the kimberlite magma causes depressurization and creates internal elastic stresses in the olivine crystals that can only be alleviated by volumetric expansion or brittle failure. On the time scales operative during kimberlite ascent volume expansion is negligible and brittle failure occurs. Small wetting angles between the fluid-rich melt and olivine allow infiltration of the melt into the crack. These very thin cracks (<5 µm) heal rapidly and preserve primary kimberlitic material en route to the surface. We use the electron microprobe with a focused beam (interaction volume less than 2 µm) to analyze the small volumes of material found in the healed cracks of the olivine. We analyzed for 18 elements including oxygen, which we obtained by utilizing a non-linear time dependent intensity acquisition and empirically determined mass absorption coefficients. By accurately knowing the amount of oxygen in a sample, we assign oxygen molecules to all other analyzed elements (e.g. MgO, Al2O3) and the remaining oxygen is assigned to hydrogen and carbon. The analysis total is used as a constraint on the proportion of each species. Mg/Ca ratios of the cracks vary from 0.6-5 indicating a compositional continuum between alkali

  5. Blueball, a new kimberlite from Arkansas

    SciTech Connect

    Salpas, P.A.; Taylor, L.A.; Shervais, J.W.

    1985-01-01

    Kimberlites provide direct observation of the upper-mantle/lower-crust by their constituent minerals and inclusions. Blueball is a previously unreported kimberlite from Scott County, Arkansas. It is unrelated to the Murfreesboro kimberlite and is composed of phenocrysts of phlogopite and olivine (serpentinized), along with spinels, in a ground mass of calcite and phlogopite, with minor perovskite and apatite. Phlogopites are compositionally homogeneous, often with euhedral spinel inclusions, and occur as embayed and corroded laths (2 mm). Si+Al are insufficient to fill tetrahedral sites. Reverse pleochroism suggests that Fe/sup 3 +/ or Ti may occupy the remaining tetrahedral sites, similar to other kimberlitic phlogopites. Besides inclusions, spinel also occurs as discrete, anhedral grains with skeletal and atoll habits. Compositions of the two occurrences are the same. These have Mg-Al-chr cores and Mg-Al-mt rims (2-5 um). Based on mineral compositions, as well as whole-rock REE data, Blueball is a true kimberlite. Blueball minerals are compositionally similar to those from kimberlites occurring in other stable cratons (e.g., South Africa); they are dissimilar from those in kimberlites at plate margins (e.g., the Appalachians) indicating heterogeneity in the mantle underlying different tectonic regimes.

  6. The Kokchetav Massif, Kazakhstan: "Type locality" of diamond-bearing UHP metamorphic rocks

    NASA Astrophysics Data System (ADS)

    Schertl, H.; Sobolev, N. V.

    2012-12-01

    Since the first discovery of in situ metamorphic diamond in the Kokchetav Massif in 1990, numerous scientists from different disciplines paid special attention to petrological and geochemical features, geodynamics and the mechanism of deep subduction and exhumation of UHP rocks. This discovery was a trigger for an intense search of further possible diamondiferous UHP terranes worldwide, successfully documented by numerous new findings. Thus far, more than 350 papers have been published on various aspects of the Kokchetav UHP rocks. A compilation of the most important and far-reaching implications are outlined here. PT-estimates yield peak metamorphic conditions of at least 43 kbar at temperatures of about 950-1000°C. Zircon separates show inherited Proterozoic cores and mantle with the peak of UHP metamorphism at about 537-530 Ma. Several Ar-Ar-ages on micas scatter around 529-528 and 521-517 Ma and reflect different stages of the exhumation history. Migmatization occurred during exhumation at about 526-520 Ma. Microdiamonds which reach a grain size of 300 micrometers, contain highly potassic fluid inclusions as well as solid inclusions like carbonates, different silicates and metal sulfides, which favours the idea of a diamond formation from a C-O-H bearing fluid. Nitrogen isotope data and negative δ13C values of Kokchetav diamonds indicate a metasedimentary origin. δ18O studies on garnet and clinopyroxene of a layered calcsilicate are ruling out a primitive mantle protolith; similar studies on eclogite indicate their basaltic protolith to have experienced water-rock interaction prior to UHP metamorphism. A number of unique mineralogical findings have been made. K-feldspar exsolutions in clinopyroxene demonstrate that potassium can be incorporated into the clinopyroxene structure under upper mantle pressures. Other significant observations are coesite exsolutions in titanite, quartz-rods in clinopyroxene, the discovery of K-tourmaline as well as new minerals

  7. Constraints on kimberlite ascent mechanisms revealed by phlogopite compositions in kimberlites and mantle xenoliths

    NASA Astrophysics Data System (ADS)

    Giuliani, Andrea; Phillips, David; Kamenetsky, Vadim S.; Goemann, Karsten

    2016-01-01

    Kimberlite magmas are of economic and scientific importance because they represent the major host to diamonds and are probably the deepest magmas from continental regions. In addition, kimberlite magmas transport abundant mantle and crustal xenoliths, thus providing fundamental information on the composition of the sub-continental lithosphere. Despite their importance, the composition and ascent mechanism(s) of kimberlite melts remain poorly constrained. Phlogopite is one of the few minerals that preserves a history of fluid migration and magmatism in the mantle and crust and is therefore an invaluable petrogenetic indicator of kimberlite magma evolution. Here we present major and trace element compositional data for phlogopite from the Bultfontein kimberlite (Kimberley, South Africa; i.e. the kimberlite type-locality) and from entrained mantle xenoliths. Phlogopite macrocrysts (~ > 0.3-0.5 mm) and microcrysts (between ~ 0.1 and 0.3 mm) in the Bultfontein kimberlite display concentric compositional zoning patterns. The cores of these phlogopite grains exhibit compositions typical of phlogopite contained in peridotite mantle xenoliths. However, the rims of some grains show compositions analogous to kimberlite groundmass phlogopite (i.e. high Ti, Al and Ba; low Cr), whereas other rims and intermediate zones (between cores and rims) exhibit unusually elevated Cr and lower Al and Ba concentrations. The latter compositions are indistinguishable from matrix phlogopite in polymict breccia xenoliths (considered to represent failed kimberlite intrusions) and from Ti-rich overgrowth rims on phlogopite in other mantle xenoliths. Consequently, it is likely that these phlogopite grains crystallized from kimberlite melts and that the high Ti-Cr zones originated from earlier kimberlite melts at mantle depths. We postulate that successive pulses of ascending kimberlite magma progressively metasomatised the conduit along which later kimberlite pulses ascended, producing

  8. Kimberlite, lamproite, ultramafic lamprophyre, and carbonatite relationships on the Dharwar Craton, India; an example from the Khaderpet pipe, a diamondiferous ultramafic with associated carbonatite intrusion

    NASA Astrophysics Data System (ADS)

    Smith, C. B.; Haggerty, S. E.; Chatterjee, B.; Beard, A.; Townend, R.

    2013-12-01

    typical of ultramafic lamprophyres such as alnoite or aillikite. The significant carbonate content and the presence of accessory Ti-andradite would lead to the KCR being classified as an ultramafic lamprophyre under the IUGS classification. The Khaderpet carbonatite component shows extreme enrichment in REE approaching that of world-average carbonatite. Given the chemical and petrological characteristics of a carbonatite, along with the associated and widespread aureole of metasomatism, this phase is considered to be a late-stage fractionation product of the Khaderpet diamond-bearing ultramafic magma.

  9. Salts in southern Yakutian kimberlites and the problem of primary alkali kimberlite melts

    NASA Astrophysics Data System (ADS)

    Kopylova, M. G.; Kostrovitsky, S. I.; Egorov, K. N.

    2013-04-01

    Alkali carbonates, sodalite, gypsum, anhydrite, halite and sylvite are present in the groundmass and matrix of many kimberlites in the southern part of the Yakutian kimberlite province. The kimberlites were emplaced through 2 km-thick evaporite-bearing carbonate sediments saturated with brines. In the global context, southern Yakutian kimberlites are unprecedented in the amount of the crustal carbonate and evaporite material included in the pipes, as evidenced by the bulk major element chemistry and isotopic compositions of Sr, C, O, Cl and S. We present geological and hydrogeological data on country rocks and kimberlites of the Udachnaya, Mir and International'naya pipes. The secondary, crustal origin of Na, K, Cl and S-rich minerals is supported by the following: 1. A regional correlation between the geology and hydrogeology of the local country rocks and the kimberlite mineralogy, in particular the difference between southern and northern Yakutian kimberlites; 2. A restriction of halite or gypsum mineralization in the Mir and International'naya pipes to depths where pipes intersect country rock strata with similar mineralogy; 3. The localization of the highest abundances of Nasbnd Ksbnd Clsbnd S-bearing minerals in the Udachnaya East kimberlite at a depth interval that correlates across three magmatic phases of kimberlites and coincides with the roof of the halite-bearing country rock and an aquifer carrying anomalously Na-rich brines; 4. The presence of evaporite xenoliths and veins of halite, gypsum and carbonate cutting through the kimberlite and xenoliths; 5. A secondary origin of halite and alkali carbonates as observed in their textural relationships to serpentine and other groundmass minerals; 6. The geochemical and isotopic evidence for crustal contamination. Addition of crustal salts to kimberlite melt began prior to the volcanic fragmentation as a result of preferential melting and assimilation of evaporite xenoliths and may have continued in

  10. OH and H2O of garnets in diamond-bearing and diamond-free garnet-clinopyroxene rocks from the Kumdy-kol area, Kokchetav Massif

    NASA Astrophysics Data System (ADS)

    Sakamaki, K.; Ogasawara, Y.

    2013-12-01

    Garnet-clinopyroxene (Grt-Cpx) rocks consisting mainly of Grt + Na-poor Cpx + calcite with various proportions, occur in the Kumdy-kol area. Diamond-bearing Grt-Cpx rock was first reported by Sobolev and Shatsky (1990) and has been well-known as one of the Kokchetav diamond-bearing rocks. UHP evidence, e.g., coesite exsolution from supersilicic titanite, was discovered also in the diamond-free Grt-Cpx rock (Inoue and Ogasawara, 2003). Presence/lack of diamond in Grt-Cpx rocks is one of important features to understand the stability of diamond in the Kokchetav UHP calcsilicate and carbonate rocks. We focused on OH and H2O in garnets in two types of Grt-Cpx rock to clarify fluid conditions during UHP metamorphism. One of the samples, the diamond-bearing Grt-Cpx rock (sample no. 25018; provided by Prof. H.-P. Schertl) is composed of two monomineralic layers, Grt-layer and Cpx-layer, with minor amounts of rutile and calcite. Coarse-grained diamond (up to 0.15 mm across) is included in garnet. Another Grt-Cpx rock, diamond-free one (sample no. XX16) shows a glanoblastic texture, and consists of Grt (30 %) + Cpx (30 %) + calcite (30 %) × titanite (5 %) with exsolved coesite-needles and plates. The precursor supersilicic compositions of titanite indicate six-coordinated Si at UHP conditions (Ogasawara et al, 2002; Sakamaki and Ogasawara, IGR in press). To understand the fluid environments during the formation of these two calcsilicate rocks, we chose garnets and conducted micro FT-IR spectroscopy. IR spectra of garnets in the diamond-bearing Grt-Cpx rock showed OH bands at 3430 and 3570 cm-1, sometimes with a week band at 3675 cm-1. The absorption band at 3570 cm-1 is the strongest and is sharp, whereas the band at 3430 cm-1 is broad. IR spectra of garnets in diamond-free one show strong OH bands at 3400 and 3555 cm-1, sometimes with week bands at 3590 and 3640 cm-1. The OH band at 3555 cm-1 is the strongest and is sharp, whereas the band at 3400 cm-1 is broad. IR

  11. Kimberlite ascent by assimilation-fuelled buoyancy.

    PubMed

    Russell, James K; Porritt, Lucy A; Lavallée, Yan; Dingwell, Donald B

    2012-01-18

    Kimberlite magmas have the deepest origin of all terrestrial magmas and are exclusively associated with cratons. During ascent, they travel through about 150 kilometres of cratonic mantle lithosphere and entrain seemingly prohibitive loads (more than 25 per cent by volume) of mantle-derived xenoliths and xenocrysts (including diamond). Kimberlite magmas also reputedly have higher ascent rates than other xenolith-bearing magmas. Exsolution of dissolved volatiles (carbon dioxide and water) is thought to be essential to provide sufficient buoyancy for the rapid ascent of these dense, crystal-rich magmas. The cause and nature of such exsolution, however, remains elusive and is rarely specified. Here we use a series of high-temperature experiments to demonstrate a mechanism for the spontaneous, efficient and continuous production of this volatile phase. This mechanism requires parental melts of kimberlite to originate as carbonatite-like melts. In transit through the mantle lithosphere, these silica-undersaturated melts assimilate mantle minerals, especially orthopyroxene, driving the melt to more silicic compositions, and causing a marked drop in carbon dioxide solubility. The solubility drop manifests itself immediately in a continuous and vigorous exsolution of a fluid phase, thereby reducing magma density, increasing buoyancy, and driving the rapid and accelerating ascent of the increasingly kimberlitic magma. Our model provides an explanation for continuous ascent of magmas laden with high volumes of dense mantle cargo, an explanation for the chemical diversity of kimberlite, and a connection between kimberlites and cratons.

  12. Kimberlite ascent by assimilation-fuelled buoyancy.

    PubMed

    Russell, James K; Porritt, Lucy A; Lavallée, Yan; Dingwell, Donald B

    2012-01-19

    Kimberlite magmas have the deepest origin of all terrestrial magmas and are exclusively associated with cratons. During ascent, they travel through about 150 kilometres of cratonic mantle lithosphere and entrain seemingly prohibitive loads (more than 25 per cent by volume) of mantle-derived xenoliths and xenocrysts (including diamond). Kimberlite magmas also reputedly have higher ascent rates than other xenolith-bearing magmas. Exsolution of dissolved volatiles (carbon dioxide and water) is thought to be essential to provide sufficient buoyancy for the rapid ascent of these dense, crystal-rich magmas. The cause and nature of such exsolution, however, remains elusive and is rarely specified. Here we use a series of high-temperature experiments to demonstrate a mechanism for the spontaneous, efficient and continuous production of this volatile phase. This mechanism requires parental melts of kimberlite to originate as carbonatite-like melts. In transit through the mantle lithosphere, these silica-undersaturated melts assimilate mantle minerals, especially orthopyroxene, driving the melt to more silicic compositions, and causing a marked drop in carbon dioxide solubility. The solubility drop manifests itself immediately in a continuous and vigorous exsolution of a fluid phase, thereby reducing magma density, increasing buoyancy, and driving the rapid and accelerating ascent of the increasingly kimberlitic magma. Our model provides an explanation for continuous ascent of magmas laden with high volumes of dense mantle cargo, an explanation for the chemical diversity of kimberlite, and a connection between kimberlites and cratons. PMID:22258614

  13. The timing of kimberlite magmatism in North America: implications for global kimberlite genesis and diamond exploration

    NASA Astrophysics Data System (ADS)

    Heaman, L. M.; Kjarsgaard, B. A.; Creaser, R. A.

    2003-12-01

    Based on a compilation of more than 100 kimberlite age determinations, four broad kimberlite emplacement patterns can be recognized in North America: (1) a northeast Eocambrian/Cambrian Labrador Sea province (Labrador, Québec), (2) an eastern Jurassic province (Ontario, Québec, New York, Pennsylvania), (3) a Cretaceous central corridor (Nunavut, Saskatchewan, central USA), and (4) a western mixed (Cambrian-Eocene) Type 3 kimberlite province (Alberta, Nunavut, Northwest Territories, Colorado/Wyoming). Ten new U-Pb perovskite/mantle zircon and Rb-Sr phlogopite age determinations are reported here for kimberlites from the Slave and Wyoming cratons of western North America. Within the Type 3 Slave craton, at least four kimberlite age domains exist: I-a southwestern Siluro-Ordovician domain (˜450 Ma), II-a SE Cambrian domain (˜540 Ma), III-a central Tertiary/Cretaceous domain (48-74 Ma) and IV-a northern mixed domain consisting of Jurassic and Permian kimberlite fields. New U-Pb perovskite results for the 614.5±2.1 Ma Chicken Park and 408.4±2.6 Ma Iron Mountain kimberlites in the State Line field in Colorado and Wyoming confirm the existence of at least two periods of pre-Mesozoic kimberlite magmatism in the Wyoming craton. A compilation of robust kimberlite emplacement ages from North America, southern Africa and Russia indicates that a high proportion of known kimberlites are Cenozoic/Mesozoic. We conclude that a majority of these kimberlites were generated during enhanced mantle plume activity associated with the rifting and eventual breakup of the supercontinent Gondwanaland. Within this prolific period of kimberlite activity, there is a good correlation between North America and Yakutia for three distinct short-duration (˜10 my) periods of kimberlite magmatism at 48-60, 95-105 and 150-160 Ma. In contrast, Cenozoic/Mesozoic kimberlite magmatism in southern Africa is dominated by a continuum of activity between 70-95 and 105-120 Ma with additional less

  14. Carbonate dissolution and transport in H2O fluids during subduction revealed by diamond-bearing rocks from the Alps

    NASA Astrophysics Data System (ADS)

    Frezzotti, M.; Selverstone, J.; Sharp, Z. D.; Compagnoni, R.

    2011-12-01

    Here we discuss the fate of subducted carbonates and its implications for recycling of crustal carbon. Thermodynamic models predict little decarbonation along most subduction geotherms, and the mechanisms by which carbon is transferred from the subducting slab to the overlying mantle remain poorly constrained. Diamond-bearing fluid inclusions in garnet in oceanic metasedimentary rocks from Lago di Cignana (western Alps) represent the first occurrence of diamond from a low-temperature subduction complex of clearly oceanic origin (T ≤600°C; P ≥3.5 GPa). The presence of diamonds in and associated with fluid inclusions provides clear evidence of carbon transport by fluids at depths that are directly relevant to slab-mantle fluid transfer during subduction. At room temperature, the fluid inclusions contain aqueous fluid, a vapor bubble, and multiple solid daughter crystals. Daughter crystals identified by Raman spectroscopy and microprobe analysis include ubiquitous Mg-calcite/calcite and rutile, and less common diamond, quartz, paragonite, dawsonite, rhodochrosite, dypingite, and pentahydrite. Molecular CO2 is absent or in trace amounts. The aqueous liquid phase contains ≥0.2 wt%, HCO3-, CO32-, and SO42- ions. In Raman spectra, broad peaks at 773 and 1017 cm-1 point to the presence of both Si(OH)4(aq) and deprotonated monomers (e.g., SiO(OH)3-(aq), and SiO2(OH)22-(aq)), indicative of alkaline solutions. The absence of CO2 in the vapor, and the presence of carbonate daughter minerals, CO32-(aq), and HCO3-(aq) also show that the trapped fluids are alkaline at ambient conditions. High activities of aqueous carbon species reveal that carbonate dissolution is an important mechanism for mobilizing slab carbon at sub-arc depths (100-200 km) during oceanic subduction. Our results imply that the magnitude of carbon release and transport from the slab at sub-arc depths is greater than experimentally predicted on the basis of decarbonation reactions alone.

  15. Kimberlite emplacement temperatures from conodont geothermometry

    NASA Astrophysics Data System (ADS)

    Pell, Jennifer; Russell, James K.; Zhang, Shunxin

    2015-02-01

    Kimberlites are mantle-derived ultramafic rocks preserved in volcanic and sub-volcanic edifices and are the main primary source of diamonds. The temperatures of formation, transport, eruption and deposition remain poorly constrained despite their importance for understanding the petrological and thermodynamic properties of kimberlite magmas and styles of volcanic eruption. Here, we present measured values of Colour Alteration Indices (CAI) for conodonts recovered from 76 Paleozoic carbonate xenoliths found within 11 pipes from the Chidliak kimberlite field on Baffin Island, Nunavut, Canada. The dataset comprises the largest range of CAI values (1.5 to 8) and the highest CAI values reported to date for kimberlite-hosted xenoliths. Thermal models for cooling of the Chidliak kimberlite pipes and synchronous heating of conodont-bearing xenoliths indicate time windows of 10-20 000 h and, for these short time windows, the measured CAI values indicate heating of the xenoliths to temperatures of 225 to >925 °C. We equate these temperatures with the minimum temperatures of the conduit-filling kimberlite deposit (i.e. emplacement temperature, TE). The majority of the xenoliths record CAI values of between 5 and 6.5 suggesting heating of xenoliths to temperatures of 460 °C-735 °C. The highest CAI values are consistent with being heated to 700 °C-925 °C and establish the minimum conditions for welding or formation of clastogenic kimberlite deposits. Lastly, we use TE variations within and between individual pipes, in conjunction with the geology of the conduit-filling deposits, to constrain the styles of explosive volcanic eruption.

  16. Paleoproterozoic Kimozero kimberlite (Karelian Craton): Geological setting and geochemical typing

    NASA Astrophysics Data System (ADS)

    Kargin, A. V.; Babarina, I. I.; Bogatikov, O. A.; Yutkina, E. V.; Kondrashov, I. A.

    2015-11-01

    Geological and structural mapping of Paleoproterozoic Kimozero kimberlite with account for lithological facies and geochemical specialization provides evidence for the multiphase structure of the kimberlite pipe, which underwent fragmentation as a result of shear-faulting deformations. Two geochemical types of kimberlite (magnesium and carbonate) are distinguished.

  17. The Kimberlites and related rocks of the Kuruman Kimberlite Province, Kaapvaal Craton, South Africa

    NASA Astrophysics Data System (ADS)

    Donnelly, Cara L.; Griffin, William L.; O'Reilly, Suzanne Y.; Pearson, Norman J.; Shee, Simon R.

    2011-03-01

    The Kuruman Kimberlite Province is comprised of 16 small pipes and dikes and contains some of the oldest known kimberlites (>1.6 Ga). In this study, 12 intrusions are subdivided into three groups with distinct petrology, age, and geochemical and isotopic compositions: (1) kimberlites with groundmass perovskites defining a Pb-Pb isochron age of 1787 ± 69 Ma, (2) orangeite with a U-Pb perovskite age of 124 ± 16 Ma, and (3) ultramafic lamprophyres (aillikite and mela-aillikite) with a zircon U-Pb age of 1642 ± 46 Ma. The magma type varies across the Province, with kimberlites in the east, lamprophyres in the west and orangeite and ultramafic lamprophyres to the south. Differences in the age and petrogenesis of the X007 orangeite and Clarksdale and Aalwynkop aillikites suggest that these intrusions are probably unrelated to the Kuruman Province. Kimberlite and orangeite whole-rock major and trace element compositions are similar to other South African localities. Compositionally, the aillikites typically lie off kimberlite and orangeite trends. Groundmass mineral chemistry of the kimberlites has some features more typical of orangeites. Kimberlite whole-rock Sr and Nd isotopes show zoning across the Province. When the kimberlites erupted at ~1.8 Ga, they sampled a core volume (ca 50 km across) of relatively depleted SCLM that was partially surrounded by a rim of more metasomatized mantle. This zonation may have been related to the development of the adjacent Kheis Belt (oldest rocks ~2.0 Ga), as weaker zones surrounding the more resistant core section of SCLM were more extensively metasomatized.

  18. Distinct Thermal and Metasomatic Characteristics of Mantle Lithosphere Beneath Two Proterozoic Terranes Bordering the Kaapvaal Craton of Southern Africa

    NASA Astrophysics Data System (ADS)

    Janney, P. E.; Shiimi, E. T.

    2015-12-01

    There is a first order contrast in compositional and thermal properties between cold and infertile Archean cratonic mantle and younger, warmer and more fertile Proterozoic lithosphere, but it has also become apparent that coherent thermal and compositional differences exist between adjacent Proterozoic terranes, even in regions that have been stable for over 1 Ga. We report new thermobarometry and in-situ trace element data for garnet peridotite xenoliths from several late Cretaceous (100-70 Ma) kimberlite localities in the western Namaqua-Natal Belt (NNB) and Rehoboth Province (RP), which bound the Archean Kaapvaal craton to the west and south, respectively. The localities include some for which no data have been reported previously. Re-depletion model ages from Os isotopes indicate that the lithosphere beneath the NNB and RP is mainly Early Proterozoic (Pearson et al., Chem. Geol., 2004; Janney et al. J. Petrol., 2010) and there is no evidence from xenolith modal proportions for significant differences in average fertility between lithospheric terranes. Equilibration pressures for garnet peridotites from both terranes fall in a similar range (2 to 5 GPa). However, peridotites from the RP typically have P and T values that fall on or very close to the Kaapvaal cratonic geotherm (apart from a group of peridotites from the Gibeon kimberlites with pressures > 4 GPa that follow an adiabatic gradient; e.g. Franz et al., J. Geol., 1996) whereas peridotites from the western NNB have temperatures roughly 100°C warmer than the cratonic geotherm over the whole depth range. Peridotites from the 140 Ma Melton Wold kimberlite, also in the western NNB, lack these warmer temperatures and suggest that warming was contemporaneous with Late Cretaceous kimberlite magmatism. Metasomatic enrichment in incompatible elements (consistent with interaction with kimberlitic melts) is more pronounced in NNB as compared to RP peridotites. The association of higher temperatures with a greater

  19. Lithosphere Structure in Southern Africa: Mantle Density, Dynamic Topography, Moho Sharpness, and Kimberlite Magmatism

    NASA Astrophysics Data System (ADS)

    Artemieva, I. M.; Vinnik, L. P.

    2015-12-01

    In southern Africa, both the Archean and Proterozoic blocks have the topography 500-700 m higher than in any other craton worldwide, except for the Tanzanian craton. An unusually high topography may be caused by a low density of the cratonic lithospheric mantle and/or by the dynamic support of the mantle with origin below the depth of isostatic compensation (assumed to be at the lithosphere base). We use free-board constraints to examine the relative contributions of the both factors to surface topography in the cratons of southern Africa and present regional model of density structure of the lithospheric mantle. The results indicate that 0.5-1.0 km of topography requires the dynamic contribution from the sublithospheric mantle because it cannot be explained by the lithosphere structure within the petrologically permitted range of mantle densities. The calculated lithospheric mantle density values are in an overall agreement with xenolith-based data and show an overall trend in mantle density increase from Archean to younger lithospheric terranes. Notable exceptions are the Limpopo belt and the Bushveld Intrusion Complex, which have an increased mantle density, probably as a result of melt-metasomatism. The Western Cape Fold Belt has a moderately depleted mantle with density within the range expected for Phanerozoic mantle, while mantle densities beneath the Eastern Cape Fold Belt require the presence of a significant amount of eclogite in the mantle. Mantle density structure correlates with distribution of kimberlites and with seismic velocity contrast across the Moho: kimberlite-rich regions have sharp Moho and low-density (3.32-3.33 g/cc) mantle, while kimberlite-poor regions have transient Moho and denser mantle (3.34-3.35 g/cc). We explain this pattern by melt-metasomatism which affects both mantle depletion and the Moho sharpness. We also find that regions with high mantle density host non-diamondiferous kimberlites, while diamondiferous kimberlites are

  20. Frequency of Proterozoic geomagnetic superchrons

    NASA Astrophysics Data System (ADS)

    Driscoll, Peter E.; Evans, David A. D.

    2016-03-01

    Long-term geodynamo evolution is expected to respond to inner core growth and changing patterns of mantle convection. Three geomagnetic superchrons, during which Earth's magnetic field maintained a near-constant polarity state through tens of Myr, are known from the bio/magnetostratigraphic record of Phanerozoic time, perhaps timed according to supercontinental episodicity. Some geodynamo simulations incorporating a much smaller inner core, as would have characterized Proterozoic time, produce field reversals at a much lower rate. Here we compile polarity ratios of site means within a quality-filtered global Proterozoic paleomagnetic database, according to recent plate kinematic models. Various smoothing parameters, optimized to successfully identify the known Phanerozoic superchrons, indicate 3-10 possible Proterozoic superchrons during the 1300 Myr interval studied. Proterozoic geodynamo evolution thus appears to indicate a relatively narrow range of reversal behavior through the last two billion years, implying either remarkable stability of core dynamics over this time or insensitivity of reversal rate to core evolution.

  1. Criteria for interpreting kimberlite as coherent: insights from the Muskox and Jericho kimberlites (Nunavut, Canada)

    NASA Astrophysics Data System (ADS)

    Hayman, P. C.; Cas, R. A. F.

    2011-10-01

    The Jurassic Muskox and Jericho kimberlites (Northern Slave Province, Nunavut, Canada) contain a variety of facies exhibiting different geometries, contact relationships, internal organisation, country rock abundance and olivine shapes, although many have similar matrix/groundmass mineralogies and textures. Five facies are examined that either have characteristics consistent with coherent rocks in general (i.e. intrusive and extrusive non-fragmental rocks) or are mineralogically and texturally similar to kimberlite described as coherent (or apparent coherent). Three facies are interpreted as coherent on the basis of: (1) geological setting, (2) apparent-porphyritic texture, (3) sharp contacts with fragmental kimberlite, (4) relative abundance of elongate and unbroken olivine crystals and (5) paucity of country rock xenoliths, while the remaining two facies are interpreted as fragmental on the basis of: (1) the gradational contacts with demonstrably fragmental kimberlite, (2) relative abundance and range of sizes of country rock lithic clasts and (3) numerous broken olivine crystals. Comparisons are made with coherent and apparent-coherent kimberlite from the literature. Our three coherent facies are similar to literature reported coherent kimberlite dykes hosted in country rock (CKd) in terms of internal organisation, low abundance of country rock xenoliths, and apparent-porphyritic texture. Conversely, our two fragmental facies share attributes with previously described pipe-filling coherent and apparent-coherent kimberlite (CKpf) in terms of geometry, internal organisation and abundance of country rock xenoliths. We conclude that CKd and most CKpf, although similar in matrix/groundmass mineralogy and texture, can be distinguished on the basis of internal organisation, country rock lithic clast abundance, texture (e.g. apparent-porphyritic texture) and possibly olivine crystal shapes and suggest that fragmental kimberlite is more common than reported.

  2. Phlogopite and Quartz Lamellae in Diamond-bearing Diopside from Marbles of the Kokchetav Massif Kazakhstan: Exsolution or Replacement Reaction

    SciTech Connect

    L Dobrzhinetskaya; R Wirth; D Rhede; Z Liu; H Green

    2011-12-31

    Exsolution lamellae of pyroxene in garnet (grt), coesite in titanite and omphacite from UHPM terranes are widely accepted as products of decompression. However, interpretation of oriented lamellae of phyllosilicates, framework silicates and oxides as a product of decompression of pyroxene is very often under debate. Results are presented here of FIB-TEM, FEG-EMP and synchrotron-assisted infrared (IR) spectroscopy studies of phlogopite (Phlog) and phlogopite + quartz (Qtz) lamellae in diamond-bearing clinopyroxene (Cpx) from ultra-high pressure (UHP) marble. These techniques allowed collection of three-dimensional information from the grain boundaries of both the single (phlogopite), two-phase lamellae (phlogopite + quartz), and fluid inclusions inside of diamond included in K-rich Cpx and understanding their relationships and mechanisms of formation. The Cpx grains contain in their cores lamellae-I, which are represented by topotactically oriented extremely thin lamellae of phlogopite (that generally are two units cell wide but locally can be seen to be somewhat broader) and microdiamond. The core composition is: (Ca{sub 0.94}K{sub 0.04}Na{sub 0.02})(Al{sub 0.06}Fe{sub 0.08}Mg{sub 0.88})(Si{sub 1.98}Al{sub 0.02})O{sub 6.00}. Fluid inclusions rich in K and Si are recognized in the core of the Cpx, having no visible connections to the lamellae-I. Lamellar-II inclusions consist of micron-size single laths of phlogopite and lens-like quartz or slightly elongated phlogopite + quartz intergrowths; all are situated in the rim zone of the Cpx. The composition of the rim is (Ca{sub 0.95}Fe{sub 0.03}Na{sub 0.02})(Al{sub 0.05}Fe{sub 0.05}Mg{sub 0.90})Si{sub 2}O{sub 6}, and the rim contains more Ca, Mg than the core, with no K there. Such chemical tests support our microstructural observations and conclusion that the phlogopite lamellae-I are exsolved from the K-rich Cpx-precursor during decompression. It is assumed that Cpx-precursor was also enriched in H{sub 2}O, because

  3. Lithospheric mantle structure and the diamond potential of kimberlites in southern D.R. Congo

    NASA Astrophysics Data System (ADS)

    Batumike, J. M.; Griffin, W. L.; O'Reilly, S. Y.

    2009-11-01

    Mantle-derived peridotitic garnet xenocrysts from kimberlites in the Mbuji Mayi and Kundelungu areas and from heavy-mineral concentrates collected in the Luebo area, D.R. Congo, have been analysed for major- and trace-element compositions in order to understand the structure and composition of the subcontinental lithospheric mantle (SCLM) and the diamond potential of the kimberlites. The lithosphere beneath the Kundelungu Plateau is ca 175 km thick and has been affected by pronounced melt metasomatism. Garnets from the Kundelungu Plateau indicate an initially cool geotherm (~ 35 mW/m 2), which was disturbed by asthenospheric melts that penetrated the SCLM shortly before kimberlite intrusion ca 32 Ma ago. Harzburgitic garnets are very rare, but some lherzolitic garnets display compositions similar to garnets included in diamond. Garnets from the Mbuji Mayi region indicate a cool geotherm (35 mW/m 2); the SCLM is ~ 210 km thick and was affected by melt-related and phlogopite-related metasomatisms. Harzburgitic garnets form about 33% of the analysed population. The garnets from the Luebo region indicate a cool lithospheric geotherm (35 mW/m 2) typical of cratonic areas. The SCLM from which the garnets were derived was relatively thick (205 km), affected by melt-related and phlogopite-related metasomatisms and characterised by the presence of a ~ 80-km thick harzburgite-rich layer. In terms of peridotitic diamond potential, Mbuji Mayi and Luebo are more prospective than Kundelungu. The initially cool conductive geotherm, the presence of some garnets with compositions similar to garnets included in diamond and the presence of sporadic diamond in the Kundelungu Plateau suggest that diamond initially was present in the lithosphere and the observed paucity of diamond may be due to the melt-related metasomatism that affected the lithosphere in the region. We suggest that the lithospheric mantle beneath Kundelungu is a strongly modified Archean cratonic lithosphere that has

  4. Metamorphic Diamond Formation under H2O-Fluid Conditions in Diamond-bearing Garnet-Clinopyroxene Rock from the Kokchetav Massif

    NASA Astrophysics Data System (ADS)

    Sakamaki, K.; Ogasawara, Y.; Schertl, H. P.

    2015-12-01

    H2O-fluid inclusions and carbonate inclusions were identified in metamorphic diamond in garnet-clinopyroxene rock from the Kokchetav massif by micro-FTIR spectroscopy. Metamorphic diamond was first reported in the Kumdy-Kol area of the Kokchetav Massif (Sobolev & Shatsky 1990). Kokchetav metamorphic diamonds occur in dolomite marbles, gneisses, and garnet-clinopyroxene rock, and show various features of morphology and occurrence. Among these diamond-bearing rocks, dolomite marble contains the highest concentrations of microdiamond (10-20 μm across, 2700 carat/t; Yoshioka et al. 2001). The largest "microdiamonds" (> 100 μm across) occur in garnet-clinopyroxene (Schertl et al. 2004). Recently, the same rock type but diamond-free one was studied; this diamond-free garnet-clinopyroxene rock contains exsolved coesite-bearing titanite suggesting precursor supersilicic compositions at UHP conditions (Sakamaki & Ogasawara 2014). Garnet and clinopyroxene in both diamond-bearing and diamond-free garnet-clinopyroxene rocks contain significant amounts of water (3000 ppm wt. H2O at average) as structural OH and submicron-sized H2O-fluid inclusions. The host garnet and clinopyroxene of diamond were grown under H2O-rich environments (AGU Fall Meeting 2014 abstract, #V13B-4775). Cubic diamond grains (approximately 100 μm across) chemically separated from diamond-bearing garnet-clinopyroxene rock was used in this study. Micro-FTIR analyses were conducted using a KBr pellet as an IR transparent window in N2 gas atmosphere. The IR spectra shows CO32- bands at 1455 cm-1 (weak), broad H2O bands at 3428 cm-1 (strong), and sharp OH bands at 3555 cm-1 (strong) were identified. These bands are assigned to H2O-fluid inclusions, aragonite, and a hydrous silicate mineral (probably phengite), respectively. These bands are similar to those in De Corte et al. (1998). Strong IR absorption bands by C-N bonds at 1282 cm-1 (A center, very strong), 1180 cm-1 (B center, very weak), and 1133 cm-1 (C

  5. Distinct kimberlite pipe classes with contrasting eruption processes

    NASA Astrophysics Data System (ADS)

    Skinner, E. M. W.; Marsh, J. S.

    2004-09-01

    Field and Scott Smith [Field, M., Scott Smith, B.H., 1999. Contrasting geology and near-surface emplacement of kimberlite pipes in southern Africa and Canada. Proc. 7th Int. Kimb. Conf. (Eds. Gurney et al.) 1, 214-237.] propose that kimberlite pipes can be grouped into three types or classes. Classical or Class 1 pipes are the only class with characteristic low temperature, diatreme-facies kimberlite in addition to hypabyssal- and crater-facies kimberlite. Class 2 and 3 pipes are characterized only by hypabyssal-and crater-facies kimberlite. In an increasing number of Class 1 pipes a new kimberlite facies, transitional-facies kimberlite, is being found. In most cases this facies forms a zone several metres wide at the interface between the hypabyssal- and diatreme-facies. The transitional-facies exhibits textural and mineralogical features, which are continuously gradational between the hypabyssal and the diatreme types. The textural gradations are from a coherent magmatic texture to one where the rock becomes increasingly magmaclastic and this is accompanied by concomitant mineralogical gradations involving the decline and eventual elimination of primary calcite at the expense of microlitic diopside. Both transitional- and diatreme-facies kimberlites are considered to have formed in situ from intruding hypabyssal kimberlite magma as a consequence of exsolution of initially CO 2-rich volatiles from the volatile-rich kimberlite magma. The transitional-facies is initiated by volatile exsolution at depths of about 3 km below the original surface. With subsequent cracking through to the surface and resultant rapid decompression, the further catastrophic exsolution of volatiles and their expansion leads to the formation of the diatreme facies. Thus diatreme-facies kimberlite and Class 1 pipes are emplaced by essentially magmatic processes rather than by phreatomagmatism. Distinctly different petrographic features characterize crater-facies kimberlite in each of the

  6. Fission track dating of kimberlitic zircons

    USGS Publications Warehouse

    Haggerty, S.E.; Raber, E.; Naeser, C.W.

    1983-01-01

    The only reliable method for dating kimberlites at present is the lengthy and specialized hydrothermal procedure that extracts 206Pb and 238U from low-uranium zircons. This paper describes a second successful method by fission track dating of large single-crystal zircons, 1.0-1.5 cm in dimension. The use of large crystals overcomes the limitations imposed in conventional fission track analysis which utilizes crushed fragments. Low track densities, optical track dispersion, and the random orientation of polished surfaces in the etch and irradiation cycle are effectively overcome. Fission track ages of zircons from five African kimberlites are reported, from the Kimberley Pool (90.3 ?? 6.5 m.y.), Orapa (87.4 ?? 5.7 and 92.4 ?? 6.1 m.y.), Nzega (51.1 ?? 3.8 m.y.), Koffiefontein (90.0 ?? 8.2 m.y.), and Val do Queve (133.4 ?? 11.5 m.y.). In addition we report the first radiometric ages (707.9 ?? 59.6 and 705.5 ?? 61.0 m.y.) of crustal zircons from kimberlites in northwest Liberia. The fission track ages agree well with earlier age estimates. Most of the zircons examined in this study are zoned with respect to uranium but linear correlations are established (by regression analysis) between zones of variable uranium content, and within zones of constant uranium content (by analysis of variance). Concordance between the fission track method and the U/Pb technique is established and we concluded that track fading from thermal annealing has not taken place. Kimberlitic zircons dated in this study, therefore, record the time of eruption. ?? 1983.

  7. Using of clinopyroxene thermobarometry for the eclogites and omphacite diamond inclusions of Yakutian and worldwide kimberlites .

    NASA Astrophysics Data System (ADS)

    Ashchepkov, Igor; Spetsius, Zdislav; Downes, Hilary; Logvinova, Alla; Ravi, Subramanian; Ntaflos, Theodoros

    2014-05-01

    Modified clinopyroxene thermobarometry (Ashchepkov et al., 2010) in combination with (Krogh, 1988) or (Nimis, Taylor, 2000) thermometers checked using 570 published runs in eclogite system clarified position of eclogites in Siberian and Worldwide SCLM (Ashchepkov et al, 2010; 2012; 2013). In Siberia Fe- eclogites related to Fe- basalts or TTG cumulates sediments and are found in the middle pyroxenite layer formed in Early Archean when eclogites can't be subducted and were remelted in near 100 -130 km (3.5-4GPa) (Udachnaya, Mir, Prianabarie) . In Middle and late Archean they locate ~5 GPa forming several deeper levels (Udachnaya). Hi- Mg arc cumulates (Horodyskyj ea, 2007) are related to the different depth and relate to Low-T geotherms starting from 7.5 to 4 GPa. Diamond omphacite inclusions from melt metasomatized eclogites or protokimberlite cumulates often trace HT geotherm. In Siberia eclogites positions in SCLM differ. In Magan terrain abundant eclogites of varying (Mg') correspond to different types. Majority (4-5 GPa, MaloBotupbinsky and Khramai) form several trends decreasing P- Fe corresponding to melt differentiation and reaction with kimberlites referring to high -T conditions. The 3.0-3.5 GPa lens traced by both high and low-Fe eclogites. Cold low Fe type are probably referring to subduction type eclogites (LT) but HT -to protokimberlite crystallization . In West Daldyn (Alakit) terrain eclogites locate in middle SCLM part. In Daldyn West they are distributed in all section. In Nakyn field (Markha terrane) Fe-rich eclogites dominate in lower SCLM like in Upper Muna fields. In northern Siberian craton part in Hapchan (Kuoyka) and Birekte terrain most eclogites belong to middle part. Those from Upper part may corresponds to TTG cumulates. Abundant eclogite diamond inclusions suggest that they should be also in the low SCLM. Proterozoic kimberlites commonly carry hot eclogites from middle part like in Wajrakarur field (KL-4) in India where Ca- rich

  8. On the geodynamic setting of kimberlite genesis

    NASA Technical Reports Server (NTRS)

    England, P.; Houseman, G.

    1984-01-01

    The emplacement of kimberlites in the North American and African continents since the early Palaeozoic appears to have occurred during periods of relatively slow motion of these continents. The distribution of kimberlites in time may reflect the global pattern of convection, which forces individual plates to move faster or slower at different times. Two-dimensional numerical experiments on a convecting layer with a moving upper boundary show two different regimes: in the first, when the upper boundary velocity is high, heat is transferred by the large-scale circulation and in the second, when the upper boundary velocity is lower, heat is predominantly transferred by thermal plumes rising from the lower boundary layer. For a reasonable mantle solidus, this second regime can give rise to partial melting beneath the moving plate, far from the plate boundaries. The transition between these modes takes place over a small range of plate velocities; for a Rayleigh number of 1,000,000 it occurs around 20 mm/yr. It is suggested that the generation of kimberlite magmas may result from thermal plumes incident on the base of a slowly moving plate.

  9. The ascent of kimberlite: Insights from olivine

    NASA Astrophysics Data System (ADS)

    Brett, R. C.; Russell, J. K.; Andrews, G. D. M.; Jones, T. J.

    2015-08-01

    Olivine xenocrysts are ubiquitous in kimberlite deposits worldwide and derive from the disaggregation of mantle-derived peridotitic xenoliths. Here, we provide descriptions of textural features in xenocrystic olivine from kimberlite deposits at the Diavik Diamond Mine, Canada and at Igwisi Hills volcano, Tanzania. We establish a relative sequence of textural events recorded by olivine during magma ascent through the cratonic mantle lithosphere, including: xenolith disaggregation, decompression fracturing expressed as mineral- and fluid-inclusion-rich sealed and healed cracks, grain size and shape modification by chemical dissolution and abrasion, late-stage crystallization of overgrowths on olivine xenocrysts, and lastly, mechanical milling and rounding of the olivine cargo prior to emplacement. Ascent through the lithosphere operates as a "kimberlite factory" wherein progressive upward dyke propagation of the initial carbonatitic melt fractures the overlying mantle to entrain and disaggregate mantle xenoliths. Preferential assimilation of orthopyroxene (Opx) xenocrysts by the silica-undersaturated carbonatitic melt leads to deep-seated exsolution of CO2-rich fluid generating buoyancy and supporting rapid ascent. Concomitant dissolution of olivine produces irregular-shaped relict grains preserved as cores to most kimberlitic olivine. Multiple generations of decompression cracks in olivine provide evidence for a progression in ambient fluid compositions (e.g., from carbonatitic to silicic) during ascent. Numerical modelling predicts tensile failure of xenoliths (disaggregation) and olivine (cracks) over ascent distances of 2-7 km and 15-25 km, respectively, at velocities of 0.1 to >4 m s-1. Efficient assimilation of Opx during ascent results in a silica-enriched, olivine-saturated kimberlitic melt (i.e. SiO2 >20 wt.%) that crystallizes overgrowths on partially digested and abraded olivine xenocrysts. Olivine saturation is constrained to occur at pressures <1 GPa; an

  10. New window on Proterozoic life

    PubMed

    Knoll, A H; Butterfield, N J

    1989-02-16

    This article briefly discusses the the new fossil assemblage found in the 600-650-million-year-old shales of the Pertatataka formation in central Australia. It includes about two dozen taxa of spinose, process-bearing or otherwise ornamented acritarchs, many of them extremely large relative to comparably ornamented fossils in younger rocks. These acritarchs reveal a glimpse of early evolution and Proterozoic life.

  11. Oxygen isotope ratios in eclogites from kimberlites.

    PubMed

    Garlick, G D; Macgregor, I D; Vogel, D E

    1971-06-01

    The oxygen isotope compositions (delta(18)O) of eclogitic xenoliths from the Roberts Victor kimberlite range from 2 to 8 per mil relative to SMOW (standard mean ocean water). This surprising variation appears to be due to fractional crystallization: the eclogites rich in oxygen-18 represent early crystal accumulates; the eclogites poor in oxygen-18 represent residual liquids. Crystal-melt partitioning probably exceeded 3 per mil and is interpreted to be pressure-dependent. Anomalous enrichment of oxygen-18 in cumulate eclogites relative to ultramafic xenoliths suggests that crystal-melt partitioning increased after melt-formation but prior to crystallization.

  12. Spectroscopy of Moses Rock Kimberlite Diatreme

    NASA Technical Reports Server (NTRS)

    Pieters, C. M.; Mustard, J. F.

    1985-01-01

    Three types of remote sensing data (Airborne Imaging Spectroscopy (AIS), NS001, Zeiss IR-photographs) were obtained for the Moses Rock kimberlite dike in southern Utah. The goal is to identify and characterize the mantle derived mafic component in such volcanic features. The Zeiss and NS001 images provide information on the regional setting and allow units of the dike to be distinguished from surrounding material. A potential unmapped satellite dike was identified. The AIS data provide characterizing information of the surface composition of the dike. Serpentized olivine-bearing soils are (tentatively) identified from the AIS spectra for a few areas within the dike.

  13. Oxygen isotope ratios in eclogites from kimberlites.

    PubMed

    Garlick, G D; Macgregor, I D; Vogel, D E

    1971-06-01

    The oxygen isotope compositions (delta(18)O) of eclogitic xenoliths from the Roberts Victor kimberlite range from 2 to 8 per mil relative to SMOW (standard mean ocean water). This surprising variation appears to be due to fractional crystallization: the eclogites rich in oxygen-18 represent early crystal accumulates; the eclogites poor in oxygen-18 represent residual liquids. Crystal-melt partitioning probably exceeded 3 per mil and is interpreted to be pressure-dependent. Anomalous enrichment of oxygen-18 in cumulate eclogites relative to ultramafic xenoliths suggests that crystal-melt partitioning increased after melt-formation but prior to crystallization. PMID:17798552

  14. Latest Proterozoic stratigraphy and earth history

    NASA Technical Reports Server (NTRS)

    Knoll, Andrew H.; Walter, Malcolm R.

    1992-01-01

    Novel biostratigraphic and chemostratigraphic data furnish an improved framework for stratigraphic correlation of the Proterozoic Eon as well as tools for a chronostratigraphic division of the late Proterozoic. It is argued that, in conjunction with geochronometric data, protistan microfossils and isotope geochemistry can furnish a means for an eventual integration of the latest Proterozoic Eon. Attention is given to the emerging methodologies of fossil protists and prokaryotes and of isotopic chemostratigraphy.

  15. Grade-tonnage and other models for diamond kimberlite pipes

    USGS Publications Warehouse

    Bliss, J.D.

    1992-01-01

    Grade-tonnage and other quantitative models help give reasonable answers to questions about diamond kimberlite pipes. Diamond kimberlite pipes are those diamondiferous kimberlite pipes that either have been worked or are expected to be worked for diamonds. These models are not applicable to kimberlite dikes and sills or to lamproite pipes. Diamond kimberlite pipes contain a median 26 million metric tons (mt); the median diamond grade is 0.25 carat/metric ton (ct/mt). Deposit-specific models suggest that the median of the average diamond size is 0.07 ct and the median percentage of diamonds that are industrial quality is 67 percent. The percentage of diamonds that are industrial quality can be predicted from deposit grade using a regression model (log[industrial diamonds (percent)]=1.9+0.2 log[grade (ct/mt)]). The largest diamond in a diamond kimberlite pipe can be predicted from deposit tonnage using a regression model (log[largest diamond (ct)]=-1.5+0.54 log[size (mt]). The median outcrop area of diamond pipes is 12 hectares (ha). Because the pipes have similar forms, the tonnage of the deposits can be predicted by the outcrop area (log[size (mt)]=6.5+1.0 log[outcrop area (ha)]). Once a kimberlite pipe is identified, the probability is approximately .005 that it can be worked for diamonds. If a newly discovered pipe is a member of a cluster that contains a known diamond kimberlite pipe, the probability that the new discovery can be mined for diamonds is 56 times that for a newly discovered kimberlite pipe in a cluster without a diamond kimberlite pipe. About 30 percent of pipes with worked residual caps at the surface will be worked at depth. Based on the number of discovered deposits and the area of stable craton rocks thought to be well explored in South Africa, about 10-5 diamond kimberlite pipes are present per square kilometer. If this density is applicable to the South American Precambrian Shield, more than 70 undiscovered kimberlite pipes are predicted to

  16. Redox state of earth's upper mantle from kimberlitic ilmenites

    NASA Technical Reports Server (NTRS)

    Haggerty, S. E.; Tompkins, L. A.

    1983-01-01

    Temperatures and oxygen fugacities are reported on discrete ilmenite nodules in kimberlites from West Africa which demonstrate that the source region in the upper mantle is moderately oxidized, consistent with other nodule suites in kimberlites from southern Africa and the United States. A model is presented for a variety of tectonic settings, proposing that the upper mantle is profiled in redox potential, oxidized in the fertile asthenosphere but reduced in the depleted lithosphere.

  17. Eukaryotic organisms in Proterozoic oceans.

    PubMed

    Knoll, A H; Javaux, E J; Hewitt, D; Cohen, P

    2006-06-29

    The geological record of protists begins well before the Ediacaran and Cambrian diversification of animals, but the antiquity of that history, its reliability as a chronicle of evolution and the causal inferences that can be drawn from it remain subjects of debate. Well-preserved protists are known from a relatively small number of Proterozoic formations, but taphonomic considerations suggest that they capture at least broad aspects of early eukaryotic evolution. A modest diversity of problematic, possibly stem group protists occurs in ca 1800-1300 Myr old rocks. 1300-720 Myr fossils document the divergence of major eukaryotic clades, but only with the Ediacaran-Cambrian radiation of animals did diversity increase within most clades with fossilizable members. While taxonomic placement of many Proterozoic eukaryotes may be arguable, the presence of characters used for that placement is not. Focus on character evolution permits inferences about the innovations in cell biology and development that underpin the taxonomic and morphological diversification of eukaryotic organisms.

  18. Eukaryotic organisms in Proterozoic oceans

    PubMed Central

    Knoll, A.H; Javaux, E.J; Hewitt, D; Cohen, P

    2006-01-01

    The geological record of protists begins well before the Ediacaran and Cambrian diversification of animals, but the antiquity of that history, its reliability as a chronicle of evolution and the causal inferences that can be drawn from it remain subjects of debate. Well-preserved protists are known from a relatively small number of Proterozoic formations, but taphonomic considerations suggest that they capture at least broad aspects of early eukaryotic evolution. A modest diversity of problematic, possibly stem group protists occurs in ca 1800–1300 Myr old rocks. 1300–720 Myr fossils document the divergence of major eukaryotic clades, but only with the Ediacaran–Cambrian radiation of animals did diversity increase within most clades with fossilizable members. While taxonomic placement of many Proterozoic eukaryotes may be arguable, the presence of characters used for that placement is not. Focus on character evolution permits inferences about the innovations in cell biology and development that underpin the taxonomic and morphological diversification of eukaryotic organisms. PMID:16754612

  19. PIXE micro-mapping of minor elements in Hypatia, a diamond bearing carbonaceous stone from the Libyan Desert Glass area, Egypt: Inheritance from a cold molecular cloud?

    NASA Astrophysics Data System (ADS)

    Andreoli, M. A. G.; Przybylowicz, W. J.; Kramers, J.; Belyanin, G.; Westraadt, J.; Bamford, M.; Mesjasz-Przybylowicz, J.; Venter, A.

    2015-11-01

    Matter originating from space, particularly if it represents rare meteorite samples, is ideally suited to be studied by Particle Induced X-ray Emission (PIXE) as this analytical technique covers a broad range of trace elements and is per se non-destructive. We describe and interpret a set of micro-PIXE elemental maps obtained on two minute (weighing about 25 and 150 mg), highly polished fragments taken from Hypatia, a controversial, diamond-bearing carbonaceous pebble from the SW Egyptian desert. PIXE data show that Hypatia is chemically heterogeneous, with significant amounts of primordial S, Cl, P and at least 10 elements with Z > 21 (Ti, V, Cr, Mn, Fe, Ni, Os, Ir) locally attaining concentrations above 500 ppm. Si, Al, Ca, K, O also occur, but are predominantly confined to cracks and likely represent contamination from the desert environment. Unusual in the stone is poor correlation between elements within the chalcophile (S vs. Cu, Zn) and siderophile (i.e.: Fe vs. Ni, Ir, Os) groups, whereas other siderophiles (Mn, Mo and the Platinum group elements (PGEs)) mimic the distribution of lithophile elements such as Cr and V. Worthy of mention is also the presence of a globular domain (Ø ∼ 120 μm) that is C and metals-depleted, yet Cl (P)-enriched (>3 wt.% and 0.15 wt.% respectively). While the host of the Cl remains undetermined, this chemical unit is enclosed within a broader domain that is similarly C-poor, yet Cr-Ir rich (up to 1.2 and 0.3 wt.% respectively). Our data suggest that the pebble consists of shock-compacted, primitive carbonaceous material enriched in cold, pre-solar dust.

  20. Petrogenesis of the Late Cretaceous northern Alberta kimberlite province

    NASA Astrophysics Data System (ADS)

    Eccles, D. Roy; Heaman, Larry M.; Luth, Robert W.; Creaser, Robert A.

    2004-09-01

    At present, 48 Late Cretaceous (ca. 70-88 Ma) kimberlitic pipes have been discovered in three separate areas of the northern Alberta: the Mountain Lake cluster, the Buffalo Head Hills field and the Birch Mountains field. The regions can be distinguished from one another by their non-archetypal kimberlite signature (Mountain Lake) or, in the case of kimberlite fields, primitive (Buffalo Head Hills) to evolved (Birch Mountains) magmatic signatures. The dominant process of magmatic differentiation is crystal fractionation and accumulation of olivine, which acts as the main criteria to distinguish between primitive and evolved Group I-type kimberlite fields in the northern Alberta. This is important from the viewpoint of diamond exploration because the majority (about 80%) of the more primitive Buffalo Head Hills kimberlites are diamondiferous, whereas the more evolved Birch Mountains pipes are barren of diamonds for the most part. Petrographically, the Buffalo Head Hills samples are distinct from the Birch Mountains samples in that they contain less carbonate, have a smaller modal abundance of late-stage minerals such as phlogopite and ilmenite, and have a higher amount of fresh, coarse macrocrystal (>0.5 mm) olivine. Consequently, samples from the Buffalo Head Hills have the highest values of MgO, Cr and Ni, and have chemistries similar to those of primitive hypabyssal kimberlite in the Northwest Territories. Based on whole-rock isotopic data, the Buffalo Head Hills K6 kimberlite has 87Sr/ 86Sr and ɛNd values similar to those of South African Group I kimberlites, whereas the Birch Mountains Legend and Phoenix kimberlites have similar ɛNd values (between 0 and +1.9), but distinctly higher 87Sr/ 86Sr values (0.7051-0.7063). The lack of whole-rock geochemical overlap between kimberlite and the freshest, least contaminated Mountain Lake South pipe rocks reflects significant mineralogical differences and Mountain Lake is similar geochemically to olivine alkali basalt

  1. Hydrothermal alteration of kimberlite by convective flows of external water

    NASA Astrophysics Data System (ADS)

    Afanasyev, A. A.; Melnik, O.; Porritt, L.; Schumacher, J. C.; Sparks, R. S. J.

    2014-07-01

    Kimberlite volcanism involves the emplacement of olivine-rich volcaniclastic deposits into volcanic vents or pipes. Kimberlite deposits are typically pervasively serpentinised as a result of the reaction of olivine and water within a temperature range of 130-400 °C or less. We present a model for the influx of ground water into hot kimberlite deposits coupled with progressive cooling and serpentisation. Large-pressure gradients cause influx and heating of water within the pipe with horizontal convergent flow in the host rock and along pipe margins, and upward flow within the pipe centre. Complete serpentisation is predicted for wide ranges of permeability of the host rocks and kimberlite deposits. For typical pipe dimensions, cooling times are centuries to a few millennia. Excess volume of serpentine results in filling of pore spaces, eventually inhibiting fluid flow. Fresh olivine is preserved in lithofacies with initial low porosity, and at the base of the pipe where deeper-level host rocks have low permeability, and the pipe is narrower leading to faster cooling. These predictions are consistent with fresh olivine and serpentine distribution in the Diavik A418 kimberlite pipe, (NWT, Canada) and with features of kimberlites of the Yakutian province in Russia affected by influx of ground water brines. Fast reactions and increases in the volume of solid products compared to the reactants result in self-sealing and low water-rock ratios (estimated at <0.2). Such low water-rock ratios result in only small changes in stable isotope compositions; for example, δO18 is predicted only to change slightly from mantle values. The model supports alteration of kimberlites predominantly by interactions with external non-magmatic fluids.

  2. Geochemical Analysis and Classification of the Gates-Adah Kimberlite Dike

    NASA Astrophysics Data System (ADS)

    Jurkowski, C.; Harris, D.; Patton, N. R.

    2014-12-01

    The Gates-Adah Kimberlite dike is a NW-SE striking vertical ultramafic igneous intrusion located in Adah, southwestern Pennsylvania. A previous compositional study of the kimberlite considered classifying the kimberlite, either Group I or Group II, to be problematic. Major and trace element (including REE) analysis using X-ray Fluoresence and ICP-MS was performed on a sample collected from the dike by the Washington State University GeoAnalytical lab in order to better classify the kimberlite as Group I or II. Comparison of major elements to South African kimberlite suggests that the Gates-Adah kimberlite most closely resembles a Group I kimberlite. Comparable major element concentrations between South African Group I kimberlite and Gates-Adah kimberlite include TiO2, Al2O3, FeO, MgO, and CaO. Assessment of calculated clay mineral and tectosilicate content relative to unaltered phlogopite and olivine was performed using a contamination equation in order to understand the extent of emplacement conditions and weathering of the kimberlite using weight percent of normalized major elements (contamination index C.I.). Uncontaminated Group I kimberlite has a C.I. near 1.0 and some apparently fresh and contamination-free micaceous Group II kimberlite has a C.I. up to 1.5 . The Gates-Adah kimberlite has a C.I. level of 1.14 suggesting greater similarity to a Group I kimberlite. Similarly an Ilmenite index (Ilm.I.) was calculated using the weight percents of normalized major elements to further classify the Gates-Adah kimberlite. Group I kimberlite and Group II kimberlite should not exceed 0.52 and 0.47, respectively and the Gates-Adah kimberlite has an Ilm.I. of 0.42. Two thin sections were produced from the Gates-Adah kimberlite dike. Minerals present in Group I kimberlite include: Olivine, phlogopite, serpentine, ilmenite, and diopside. Phenocrysts of anhedral serpentinized olivine were found along with ilmenite, diopside, enstatie, and phlogopite in thin section. Large

  3. Origin of carbonate xenoliths in Siddanpalli kimberlites, Southern India

    NASA Astrophysics Data System (ADS)

    Anand, M.; Osborne, I.; Gilmour, M. A.; Chalapath Rao, N. V.

    2009-04-01

    A number of carbonate xenoliths have been recently discovered from the three kimberlites (designated as SK1, SK2, SK3) of the Siddanpalli kimberlite cluster (SKC; Sridhar et al., 2004) of Southern India. These kimberlites intrude the Precambrian granite-greenstone terrain of Gadwal schist belt of Eastern Dharwar Craton (e.g., Dongre et al., 2008). Rb-Sr phlogopite/whole-rock dating of one of these bodies, SK1, has yielded an age of 1093 ± Ma (Kumar et al. 2007). Previously, Dongre et al. (2008) reported an occurrence of a limestone xenolith from the SK2 kimberlite. Based on petrographic, geochemical, and C and O isotope data these authors suggested a sedimentary origin for the limestone xenolith. However, now we have documented a larger variation in the nature of carbonate material occurring as xenoliths in these kimberlites, ranging from micro-crystalline segregations to well-formed carbonate crystals. Thus, it is likely that a number of sources and/or processes have been involved in their formation. We are in the process of measuring carbon and oxygen isotopic compositions of individual carbonate xenoliths in order to further constrain their origin.

  4. An Unusual Example of Coherent Kimberlite From the Muskox Kimberlite (Nunavut, Canada): a Re-evaluation of the Criteria for Recognising Coherent Kimberlite

    NASA Astrophysics Data System (ADS)

    Hayman, P.; Cas, R.

    2009-05-01

    Although there is no published treatise on the criteria for recognising coherent kimberlite (CK), a review of the kimberlite literature will reveal that there is some consensus on its characteristics. For example, in most publications in the last kimberlite conference (8IKC volume 1) in which CK is discussed, the groundmass is described as 'uniform and well-crystallised' and spinel crystals are almost always present in abundance. Macroscopic features and contact relationships, however, are less commonly reported and do not seem to be important for the distinction of CK from fragmental rocks. Here we report contact relationships, facies distributions, textural features and mineralogy for one coherent and two fragmental facies from the Muskox kimberlite (Nunavut, Canada) to initiate debate on the criteria for distinguishing coherent from fragmental kimberlite, which we also refer to as volcaniclastic kimberlite (VK). Particular focus is placed on the coherent facies, which has unusual groundmass characteristics that have not been previously described in the kimberlite literature. Emplaced within granodiorite country rocks, the Muskox kimberlite is a steeply-tapering, single-vent, kimberlite body, measuring 200 x 220 m, that is infilled with VK and minor amounts of CK. VK is divided into two main facies, a light-coloured, country rock-rich facies (VK1) and a dark-coloured, olivine-rich facies (VK2). VK2 is mostly enclosed by VK1, forming an off-centre nested architecture. The contact between the facies is roughly vertical and gradational over distances up to 10 meters, however, in most drillholes that intersect the contact, the two VK facies alternate several times, thus forming an apparent inter-digitating relationship. Both facies are massive, poorly sorted and range from matrix- to clast-supported and contain olivine crystals, country rock lithic clasts of limestone and granodiorite and rare juvenile pellets. The matrix of VK1 consists mainly of serpentine, calcite

  5. Coalingite from kimberlite breccia of the Manchary pipe, Central Yakutia

    NASA Astrophysics Data System (ADS)

    Zayakina, N. V.; Oleinikov, O. B.; Vasileva, T. I.; Oparin, N. A.

    2015-12-01

    Coalingite, Mg10Fe2(CO3)(OH)24 · 2H2O, rare Mg-Fe hydrous carbonate, has been found in the course of the mineralogical study of a disintegrated kimberlite breccia from the Manchary pipe of the Khompu-May field located in the Tamma Basin, Central Yakutia, 100 km south of Yakutsk. Coalingite occurs as small reddish brown platelets, up to 0.2 mm in size. It is associated with lizardite, chrysotile and brucite, which are typical kimberlitic assemblage. Coalingite is a supergene mineral, but in this case, it is produced by the interaction of brucite-bearing kimberlite and underground water circulating through a vertical or oblique fault zone.

  6. Garnets from the Camafuca-Camazambo kimberlite (Angola).

    PubMed

    Correia, Eugénio A; Laiginhas, Fernando A T P

    2006-06-01

    This work presents a geochemical study of a set of garnets, selected by their colors, from the Camafuca-Camazambo kimberlite, located on northeast Angola. Mantle-derived garnets were classified according to the scheme proposed by Grütter et al. (2004) and belong to the G1, G4, G9 and G10 groups. Both sub-calcic (G10) and Ca-saturated (G9) garnets, typical, respectively, of harzburgites and lherzolites, were identified. The solubility limit of knorringite molecule in G10D garnets suggests they have crystallized at a minimum pressure of about 40 to 45 kbar (4-4.5 GPa). The occurrence of diamond stability field garnets (G10D) is a clear indicator of the potential of this kimberlite for diamond. The chemistry of the garnets suggests that the source for the kimberlite was a lherzolite that has suffered a partial melting that formed basaltic magma, leaving a harzburgite as a residue.

  7. An integrated model of kimberlite ascent and eruption.

    PubMed

    Wilson, Lionel; Head Iii, James W

    2007-05-01

    Diatremes are carrot-shaped bodies forming the upper parts of very deep magmatic intrusions of kimberlite rock. These unusual, enigmatic and complex features are famous as the source of diamonds. Here we present a new model of kimberlite ascent and eruption, emphasizing the extremely unsteady nature of this process to resolve many of the seemingly contradictory characteristics of kimberlites and diatremes. Dyke initiation in a deep CO2-rich source region in the mantle leads to rapid propagation of the dyke tip, below which CO2 fluid collects, with a zone of magmatic foam beneath. When the tip breaks the surface of the ground, gas release causes a depressurization wave to travel into the magma. This wave implodes the dyke walls, fragments the magma, and creates a 'ringing' fluidization wave. Together, these processes form the diatreme. Catastrophic magma chilling seals the dyke. No precursor to the eruption is felt at the surface and the processes are complete in about an hour.

  8. Samarium-neodymium systematics in kimberlites and in the minerals of garnet lherzolite inclusions.

    PubMed

    Basu, A R; Tatsumoto, M

    1979-07-27

    The initial ratios of neodymium-143 to neodymium-144 in kimberlites ranging in age between 90 x 10(6) to 1300 x 10(6) years from South Africa, India, and the United States are different from the corresponding ratios in the minerals of peridotite inclusions in the kimberlites but are identical to the ratios in the basaltic achondrite Juvinas at the times of emplacement of the respective kimberlite pipes. This correlation between the kimberlites and Juvinas, which represents the bulk chondritic earth in rare-earth elements, strongly indicates that the kimberlite's source in the mantle is chondritic in rare-earth elements and relatively primeval in composition. PMID:17790851

  9. Samarium-neodymium systematics in kimberlites and in the minerals of garnet lherzolite inclusions.

    PubMed

    Basu, A R; Tatsumoto, M

    1979-07-27

    The initial ratios of neodymium-143 to neodymium-144 in kimberlites ranging in age between 90 x 10(6) to 1300 x 10(6) years from South Africa, India, and the United States are different from the corresponding ratios in the minerals of peridotite inclusions in the kimberlites but are identical to the ratios in the basaltic achondrite Juvinas at the times of emplacement of the respective kimberlite pipes. This correlation between the kimberlites and Juvinas, which represents the bulk chondritic earth in rare-earth elements, strongly indicates that the kimberlite's source in the mantle is chondritic in rare-earth elements and relatively primeval in composition.

  10. Samarium-neodymium systematics in kimberlites and in the minerals of garnet lherzolite inclusions

    USGS Publications Warehouse

    Basu, A.R.; Tatsumoto, M.

    1979-01-01

    The initial ratios of neodymium-143 to neodymium-144 in kimberlites ranging in age between 90 ?? 106 to 1300 ?? 106 years from South Africa, India, and the United States are different from the corresponding ratios in the minerals of peridotite inclusions in the kimberlites but are identical to the ratios in the basaltic achondrite Juvinas at the times of emplacement of the respective kimberlite pipes. This correlation between the kimberlites and Juvinas, which represents the bulk chondritic earth in rare-earth elements, strongly indicates that the kimberlite's source in the mantle is chondritic in rare-earth elements and relatively primeval in composition. Copyright ?? 1979 AAAS.

  11. CO2 degassing in ascending magmas: from MORBs to kimberlites.

    NASA Astrophysics Data System (ADS)

    Guillot, Bertrand; Folliet, Nicolas; Sator, Nicolas

    2013-04-01

    Kimberlites and MORB samples exhibit very different CO2 contents (generally much less than 1wt% CO2 for MORBs and up to 15 wt% or more for kimberlites). For MORBs a majority of the CO2 content is found in the vesicles whereas for kimberlites it is dissolved in the groundmass. These differences in CO2 abundance are assigned to a large variation of the CO2 solubility with melt composition. However, the composition of MORBs is well established while that of kimberlite magmas is badly constrained due to alteration. Recent studies (Canil and Bellis, 2008; Sparks et al., 2009; Brooker et al., 2011) have suggested that primary kimberlite magmas originally had lower SiO2 contents than the commonly reconstructed compositions, and that the latter ones could be transitional between silicate (×25 wt% SiO2) and carbonate (˜5wt% SiO2) melts. Indeed, CO2 solubility data suggest that a melt composed of 25-35 wt% SiO2 (as estimated in reconstructed compositions) should be almost fully degassed in CO2 when the magma enters the root zone of kimberlites (~1-2 kbar) whereas the observed CO2 abundance mostly exceed 10 wt% CO2. This has prompted us to investigate by molecular dynamics (MD) simulations the degassing trajectory of CO2-rich silicate melts of various composition ascending adiabatically. In using a force field recently developed by us to describe CO2-bearing silicate melts (Guillot and Sator, 2011), we have simulated three magma compositions in the CO2-CMAS system: a basaltic (with ~49 wt% SiO2 on a volatile free basis), a kimberlitic (~36 wt% SiO2) and a transitional (~13 wt% SiO2) composition. In considering a CO2-rich source region located at 250 km depth in the astenosphere (Tp ~1450° C), the three CO2-saturated magmas are then decompressed adiabatically in the course of the MD simulation. The adiabatic expansion of the melts induces at once a cooling effect and a CO2-degassing which are consistent with observations. In particular, our simulations show that only the

  12. Chasing the Late Jurassic APW Monster Shift in Ontario Kimberlites

    NASA Astrophysics Data System (ADS)

    Kent, D. V.; Muttoni, G.; Gee, J. S.; Kjarsgaard, B. A.

    2012-12-01

    A 30° gap was recognized in a composite APW path when global poles from predominantly igneous rocks were assembled in North American coordinates using plate reconstructions (Kent & Irving 2010 JGR). The 'monster shift' occurred between a 160-190 Ma cluster of mean poles at 75-80°N 90-110°E to a 140-145 Ma grouping centered at 60-65°N ~200°E. There are hardly any intermediate igneous poles whereas the rather divergent directions from the Late Jurassic Morrison Formation published by Steiner & Helsley (1975 GSA Bulletin) are subject to adjustments for Colorado Plateau rotation and sedimentary inclination error, neither of which are precisely known for this redbed unit sampled in Colorado. On the other hand, similar large rapid swings have been recognized in the Late Jurassic APW path for Adria (Channell et al. 2010 Paleo3), suggesting a global phenomena. In an effort to fill the data gap between ~145 and 160 Ma, we sampled accessible outcrops/subcrops of kimberlites in the Timiskaming area of Ontario, Canada, that are associated with high precision U-Pb perovskite ages (Heamon & Kjarsgaard 2000 EPSL). We report initial results from two of the intrusions: the 153.6±2.4 Ma Peddie kimberlite from outcrop and the Triple B kimberlite that was accessible by trenching and is assumed to be the same age as the nearby 153.7±1.8 Ma Seed kimberlite as delineated by aeromagnetic surveys and borings. Systematic progressive thermal demagnetization indicated in each unit a dominant characteristic component with unblocking temperatures to 575° that presumably reflect a magnetite carrier that will be checked by further rock magnetic experiments. Samples from the Peddie kimberlite had stable downward (normal polarity) magnetizations whose mean direction gives a paleopole at 73°N 184°E. In contrast, samples from the Triple B kimberlite have upward (reverse polarity) magnetizations with a well-grouped direction whose (north) paleopole is 78°N 197°E, proximal to the Peddie

  13. How unique is the Udachnaya-East kimberlite? Comparison with kimberlites from the Slave Craton (Canada) and SW Greenland

    NASA Astrophysics Data System (ADS)

    Kamenetsky, Vadim S.; Kamenetsky, Maya B.; Weiss, Yakov; Navon, Oded; Nielsen, Troels F. D.; Mernagh, Terrence P.

    2009-11-01

    The origin of alkali carbonates and chlorides in the groundmass of unaltered Udachnaya-East kimberlites in Siberia is still controversial. Contrary to existing dogma that the Udachnaya-East kimberlite was either contaminated by the crustal sediments or platform brines, magmatic origin of the groundmass assemblage has been proposed on the basis of melt immiscibility textures, melt inclusion studies, and strontium and neon isotope compositions. We further tested the idea of alkali- and chlorine enrichment of the kimberlite parental melt by studying olivine-hosted melt inclusions and secondary serpentine in kimberlites from the Slave Craton, Canada (Gahcho Kué, Jericho, Aaron and Leslie pipes) and southern West Greenland (Majuagaa dyke). Host olivine phenocrysts closely resemble groundmass olivine from the Udachnaya-East kimberlite in morphology, compositions (high-Fo, low-Ca), complex zoning with cores of varying shapes and compositions and rims of constant Fo. Melt inclusions in olivine consist of several translucent and opaque daughter phases and vapour bubble(s). The daughter crystals studied in unexposed inclusions by laser Raman spectroscopy and in carefully exposed inclusions by WDS-EDS are represented by Na-K chlorides, calcite, dolomite, magnesite, Ca-Na, Ca-Na-K and Ca-Mg-Ba carbonates, bradleyite Na 3 Mg(CO 3)(PO 4), K-bearing nahpoite Na 2(HPO 4), apatite, phlogopite and tetraferriphlogopite, unidentified sulphates, Fe sulphides, djerfisherite, pyrochlore (Na,Ca) 2Nb 2O 6(OH,F), monticellite, Cr-spinel and Fe-Ti oxides. High abundances of Na, K (e.g., (Na + K)/Ca = 0.15-0.85) and incompatible trace elements in the melt inclusions are confirmed by LA-ICPMS analysis of individual inclusions. Heating experiments show that melting of daughter minerals starts and completes at low temperatures (~ 100 °C and 600 °C, respectively), further reinforcing the similarity with the Udachnaya-East kimberlite. Serpentine minerals replacing olivine in some of the studied

  14. Metaconglomerate preserves evidence for kimberlite, diamondiferous root and medium grade terrane of a pre-2.7 Ga Southern Superior protocraton

    NASA Astrophysics Data System (ADS)

    Kopylova, M. G.; Afanasiev, V. P.; Bruce, L. F.; Thurston, P. C.; Ryder, J.

    2011-12-01

    detrital material. The clasts could have originated as close as the northern Wawa-Abitibi Terrane or as distant as the Opatica terrane. The pre-2.7 Ga diamonondiferous cratonic root below the Southern Superior was removed in the Neoarchean-Proterozoic. The existence of Archean kimberlites and deep diamondiferous roots below smaller pre-2.7 Ga protocratons emphasizes the similarity of Neoarchean and Phanerozoic mantle processes.

  15. Constraining kimberlite geology through integration of geophysical, geological and geochemical methods: A case study of the Mothae kimberlite, northern Lesotho

    NASA Astrophysics Data System (ADS)

    Galloway, M.; Nowicki, T.; van Coller, B.; Mukodzani, B.; Siemens, K.; Hetman, C.; Webb, K.; Gurney, J.

    2009-11-01

    The Cretaceous Mothae kimberlite is located in northern Lesotho on the southeast margin of the Kaapvaal craton. Historical work suggests that Mothae has a low average diamond grade of ~ 3 cpht and the economic viability therefore depends on the presence of large, high quality (and thus value) diamonds as does that of the nearby Letseng Diamond Mine. Defining such a diamond population requires a very large and representative bulk sample. The near surface geology of the Mothae kimberlite was investigated using ground geophysical surveys, pit mapping, petrography, measurements of the mantle components and whole rock compositions. Integration of data from these different approaches clearly defines the outline of the kimberlite at the surface and permits definition, with varying confidence levels, of at least six geologically distinct domains within the body. The domains are defined primarily on the basis of variations in the relative abundances of certain mantle-derived minerals extracted from exploratory pit samples, supported to varying extents by geophysically-defined zones, variations in kimberlite type (established petrographically) and variations in whole rock composition. The domains are interpreted to reflect the presence of multiple phases of volcaniclastic kimberlite each with a potentially different diamond content. The map of the near surface geology constructed on the basis of the work described in this paper provides a valuable framework for planning of further drilling and sampling work aimed at constraining the diamond resource at Mothae. This study illustrates the value of an integrated, multidisciplinary approach to understanding the geology of a complex kimberlite body during the early stages of evaluation.

  16. Discovery of kimberlite in a magnetically noisy environment: a case study of the Syferfontein and Goedgevonden kimberlites (Invited)

    NASA Astrophysics Data System (ADS)

    Webb, S. J.; Van Buren, R.

    2013-12-01

    Airborne geophysical methods play an important role in the exploration for kimberlites. As regions become more intensively explored, smaller kimberlites, which can be extremely difficult to find, are being targeted. These smaller kimberlites, as evidenced by the M-1 Maarsfontein pipe in the Klipspringer cluster in South Africa, can be highly profitable. The Goedgevonden and Syferfontein pipes are small kimberlites (~0.2 ha) ~25 km NNE of Klerksdorp in South Africa. The Goedgevonden pipe has been known since the 1930s and is diamondiferous, but not commercially viable due to small stone size and low quality of stones. In the early 1990s, Gold Fields used this pipe as a typical kimberlite to collect example geophysical data. The nearby (~1 km to the east) Syferfontein pipe is not diamondiferous but was discovered in 1994 as part of a speculative airborne EM survey conducted by Gold Fields and Geodass (now CGG) as part of their case study investigations. Both kimberlites have had extensive ground geophysical survey data collected and have prominent magnetic, gravity and EM responses that aided in the delineation of the pipes. These pipes represent a realistic and challenging case study target due to their small size and the magnetically noisy environment into which they have been emplaced. The discovery of the Syferfontein pipe in 1994 stimulated further testing of airborne methods, especially as the surface was undisturbed. These pipes are located in a region that hosts highly variably magnetized Hospital Hill shales, dolerite dykes and Ventersdorp lavas, a 2-3 m thick resistive ferricrete cap and significant cultural features such as an electric railroad and high tension power line. Although the kimberlites both show prominent magnetic anomalies on ground surveys, the airborne data are significantly noisy and the pipes do not show up as well determined targets. However, the clay-rich weathered zone of the pipes provides an ideal target for the EM method, and both

  17. UHP-UHT peak conditions and near-adiabatic exhumation path of diamond-bearing garnet-clinopyroxene rocks from the Eger Crystalline Complex, North Bohemian Massif

    NASA Astrophysics Data System (ADS)

    Haifler, Jakub; Kotková, Jana

    2016-04-01

    Intermediate garnet-clinopyroxene rocks from the Eger Crystalline Complex, North Bohemian Massif, contain microdiamonds enclosed in garnet and zircon. The variable mineral assemblage of these rocks allows for an evaluation of the P-T evolution using numerous univariant equilibria and thermodynamic modelling, in addition to the ternary feldspar solvus, Ti-in-garnet, Zr-in-rutile and Ti-in-zircon thermometry. Zircon mantle domains with diamond inclusions contain 111-189 ppm Ti, reflecting temperatures of 1037-1117 °C. The peak pressure consistent with diamond stability corresponds to c. 4.5-5.0 GPa. Ti-in-garnet thermometry using the Ti content of diamond-bearing garnet core yielded temperatures of 993-1039 °C at c. 5.0 GPa. An omphacite inclusion in garnet (reflecting c. 2.3-2.4 GPa at c. 1050 °C) and metastably preserved kyanite represent relics of eclogite-facies conditions. The dominant high-pressure granulite-facies mineral assemblage of low-Ca garnet, diopsidic clinopyroxene, antiperthitic feldspar and quartz equilibrated at 1.8-2.1 GPa and c. 1050 °C, based on the XGrs isopleth of the garnet mantle, garnet-feldspar-kyanite-quartz univariant equilibria and ternary feldspar solvus. Our thermodynamic modelling shows that a steep decrease of XGrs from a maximum core value of 0.32 to 0.17 at the rim as well as a rimward XMg increase (from 0.42 to 0.50) are consistent with significant decompression without heating. The latter is related to omphacite and kyanite breakdown reactions producing garnet and plagioclase. The Ti content in the rim zone of zircon (13-42 ppm), exsolved plagioclase and K-feldspar associated with matrix diopside and garnet rim, and late biotite reflect temperatures of c. 830-900 °C at c. 1.4 GPa. A similar temperature is recorded by matrix rutile grains, containing 2028-4390 ppm Zr and representing a relatively homogeneous population in contrast to rutile enclosed in garnet with variable Zr content. Our results show that the garnet

  18. The discovery of kimberlites in Antarctica extends the vast Gondwanan Cretaceous province.

    PubMed

    Yaxley, Gregory M; Kamenetsky, Vadim S; Nichols, Geoffrey T; Maas, Roland; Belousova, Elena; Rosenthal, Anja; Norman, Marc

    2013-01-01

    Kimberlites are a volumetrically minor component of the Earth's volcanic record, but are very important as the major commercial source of diamonds and as the deepest samples of the Earth's mantle. They were predominantly emplaced from ≈2,100 Ma to ≈10 ka ago, into ancient, stable regions of continental crust (cratons), but are also known from continental rifts and mobile belts. Kimberlites have been reported from almost all major cratons on all continents except for Antarctica. Here we report the first bona fide Antarctic kimberlite occurrence, from the northern Prince Charles Mountains, emplaced during the reactivation of the Lambert Graben associated with rifting of India from Australia-Antarctica. The samples are texturally, mineralogically and geochemically typical of Group I kimberlites from more classical localities. Their ≈120 Ma ages overlap with those of many kimberlites from other world-wide localities, extending a vast Cretaceous, Gondwanan kimberlite province, for the first time, into Antarctica.

  19. The discovery of kimberlites in Antarctica extends the vast Gondwanan Cretaceous province.

    PubMed

    Yaxley, Gregory M; Kamenetsky, Vadim S; Nichols, Geoffrey T; Maas, Roland; Belousova, Elena; Rosenthal, Anja; Norman, Marc

    2013-01-01

    Kimberlites are a volumetrically minor component of the Earth's volcanic record, but are very important as the major commercial source of diamonds and as the deepest samples of the Earth's mantle. They were predominantly emplaced from ≈2,100 Ma to ≈10 ka ago, into ancient, stable regions of continental crust (cratons), but are also known from continental rifts and mobile belts. Kimberlites have been reported from almost all major cratons on all continents except for Antarctica. Here we report the first bona fide Antarctic kimberlite occurrence, from the northern Prince Charles Mountains, emplaced during the reactivation of the Lambert Graben associated with rifting of India from Australia-Antarctica. The samples are texturally, mineralogically and geochemically typical of Group I kimberlites from more classical localities. Their ≈120 Ma ages overlap with those of many kimberlites from other world-wide localities, extending a vast Cretaceous, Gondwanan kimberlite province, for the first time, into Antarctica. PMID:24346162

  20. Proterozoic geology and ore deposits of Arizona

    USGS Publications Warehouse

    Karlstrom, Karl E.

    1991-01-01

    Proterozoic rocks in Arizona have been the focus of interest for geologists since the late 1800's. Early investigations, led by the U.S. Geological Survey, focused on the extensive ore deposits hosted by Proterozoic rocks. By the 1960's, these studies, combined with theses from academic institutions and the efforts of the Arizona Geological Survey, had produced a rich data base of geologic maps, primarily of the central part of the Transition Zone. The chronological significance of these maps became much better known with the application of U-Pb geochronology by L.Y. Silver and his students starting in the 1960's. The 1970's and early 1980's were marked by numerous contributions from Masters and Ph.D students at a variety of academic institutions, and continued work by the U.S. Geological Survey. Interest in ore deposits persisted and there was an increasing interest in interpretation of the tectonic history of Proterozoic rocks in terms of plate tectonic models, as summarized in papers by Phillip Anderson, Ed DeWitt, Clay Conway, Paul Lindberg, and J.L Anderson in the 1989 Arizona Geological Society Digest 17: "Geologic Evolution of Arizona". The present volume: "Proterozoic Geology and Ore deposits of Arizona" builds upon A.G.S. Digest 17, and presents the results of geologic investigations from the latter part of the 1980's. A number of the papers are condensed versions of MS theses done by students at Northern Arizona University. These papers are based upon 1:10,000 mapping and structural analysis of several areas in Arizona. The geologic maps from each of these studies are available separately as part of the Arizona Geological Survey Contributed Map Series. These detailed maps, plus the continuing mapping efforts of the U.S.G.S. and students at other academic institutions, form an ever improving data base for continuing attempts to understand the Proterozoic geology and ore deposits of Arizona

  1. Petrological characteristics of the Masontown, Pennsylvania kimberlite dike

    SciTech Connect

    Prellwitz, H.S.; Bikerman, M. . Dept. of Geology and Planetary Science)

    1993-03-01

    The Masontown, PA, kimberlite dike intrudes flat-lying Pennsylvanian and early Permian sedimentary rocks, via a pre-existing vertical fault zone, contact relationship indicate a low temperature of intrusion. The kimberlite consists of a phenocryst mineral assemblage which includes olivine, phlogopite, Ti rich oxides, and very fine grained carbonate, that is believed to be of primary origin. Most of the olivine has been altered to serpentine, and post emplacement fractures are filled with secondary carbonate. Most of the mineral grains have reaction rims, which record high pressure/temperature melt conditions that later changed into a lower pressure/temperature environment. Vertical alignment of the mineral grains suggest an upward flow direction. Lithospheric mantle xenoliths of garnet lherzolite and crustal xenoliths of biotite gneiss show probable compositions of deep-seated rocks. These rocks are normally inaccessible because they are converted by a thick Paleozoic sedimentary blanket in this area.

  2. The origin of pelletal lapilli in explosive kimberlite eruptions.

    PubMed

    Gernon, T M; Brown, R J; Tait, M A; Hincks, T K

    2012-05-15

    Kimberlites are volatile-rich magmas from mantle depths of ≥ 150  km and are the primary source of diamonds. Kimberlite volcanism involves the formation of diverging pipes or diatremes, which are the locus of high-intensity explosive eruptions. A conspicuous and previously enigmatic feature of diatreme fills are 'pelletal lapilli'--well-rounded clasts consisting of an inner 'seed' particle with a complex rim, thought to represent quenched juvenile melt. Here we show that these coincide with a transition from magmatic to pyroclastic behaviour, thus offering fundamental insights into eruption dynamics and constraints on vent conditions. We propose that pelletal lapilli are formed when fluid melts intrude into earlier volcaniclastic infill close to the diatreme root zone. Intensive degassing produces a gas jet in which locally scavenged particles are simultaneously fluidised and coated by a spray of low-viscosity melt. A similar origin may apply to pelletal lapilli in other alkaline volcanic rocks, including carbonatites, kamafugites and melilitites.

  3. Isotope fractionation related to kimberlite magmatism and diamond formation

    SciTech Connect

    Galimov, E.M. )

    1991-06-01

    This paper deals with a model of carbon isotope fractionation presumed to accompany the movement of mantle fluids. In the first part of the article, the experimental data and the relationships revealed are generalized and discussed; the remainder of the paper describes the model. The isotope compositions of different forms of carbon related to kimberlite magmatism vary widely. In diamonds, {delta}{sup 13}C values range from {minus}34.5 to +2.8{per thousand}. Carbonate-bearing autholiths in kimberlites occur enriched in {sup 13}C up to +35{per thousand}. Organic matter, including that occurring in fluid inclusions of magmatic minerals of kimberlites, is depleted in {sup 13}C down to {minus}30{per thousand}. It is concluded that the {delta}{sup 13}C-distribution for diamonds is specific for a particular occurrence. Principal differences in isotopic distribution patterns for diamonds of ultrabasic and basic paragenesis exist. Isotopically light diamonds are related only to the latter. The intention of the model is to explain the observed variations of carbon isotope composition of diamond and other carbonaceous substances related to kimberlite magmatism. The model is based on the interaction of reduced sub-asthenospehric fluid with a relatively oxidized lithosphere. It is suggested that diamonds of ultrabasic paragenesis are produced during interaction of the fluid with sheared garnet lbherzolite which is considered to be primitive mantle rock. During contact with the more oxidized mantle, reduced carbon (CH{sub 4}) may partially be converted to CO{sub 2}. Isotope exchange in CO{sub 2}-CH{sub 4} system, conbined with Rayleigh distillation, may provide a significant isotope fractionation. Diamonds of the basic (eclogitic) paragenesis are considered to be realted to this fractionated carbon. Also, occurrence of carbonate material highly enriched in {sup 13}C is explained by the model.

  4. Linear stability analysis for hydrothermal alteration of kimberlitic rocks

    NASA Astrophysics Data System (ADS)

    Afanasyev, Andrey; Belyaeva, Ekaterina

    2016-06-01

    The influx of groundwater into hot kimberlite deposits results in the reaction of water with olivine-rich rocks. The products of the reaction are serpentine and release of latent heat. The rise of temperature due to the heat release increases the rate of the reaction. Under certain conditions, this self-speeding up of the reaction can result in instabilities associated with a significantly higher final serpentinization in slightly warmer regions of the kimberlite deposit. We conduct linear stability analysis of serpentinization in an isolated volume of porous kimberlitic rocks saturated with water and an inert gas. There is a counteracting interplay between the heat release tending to destabilize the uniform distribution of parameters and the heat conduction tending to stabilize it by smoothing out temperature perturbations. We determine the critical spatial scale separating the parameters where one phenomenon dominates over another. The perturbations of longer-than-critical length grow, whereas the perturbations of shorter-than-critical length fade. The analytical results of the linear stability analysis are supported by direct numerical simulations using a full nonlinear model.

  5. Garnets from the Camafuca-Camazambo kimberlite (Angola).

    PubMed

    Correia, Eugénio A; Laiginhas, Fernando A T P

    2006-06-01

    This work presents a geochemical study of a set of garnets, selected by their colors, from the Camafuca-Camazambo kimberlite, located on northeast Angola. Mantle-derived garnets were classified according to the scheme proposed by Grütter et al. (2004) and belong to the G1, G4, G9 and G10 groups. Both sub-calcic (G10) and Ca-saturated (G9) garnets, typical, respectively, of harzburgites and lherzolites, were identified. The solubility limit of knorringite molecule in G10D garnets suggests they have crystallized at a minimum pressure of about 40 to 45 kbar (4-4.5 GPa). The occurrence of diamond stability field garnets (G10D) is a clear indicator of the potential of this kimberlite for diamond. The chemistry of the garnets suggests that the source for the kimberlite was a lherzolite that has suffered a partial melting that formed basaltic magma, leaving a harzburgite as a residue. PMID:16710568

  6. An integrated model of kimberlite ascent and eruption.

    PubMed

    Wilson, Lionel; Head Iii, James W

    2007-05-01

    Diatremes are carrot-shaped bodies forming the upper parts of very deep magmatic intrusions of kimberlite rock. These unusual, enigmatic and complex features are famous as the source of diamonds. Here we present a new model of kimberlite ascent and eruption, emphasizing the extremely unsteady nature of this process to resolve many of the seemingly contradictory characteristics of kimberlites and diatremes. Dyke initiation in a deep CO2-rich source region in the mantle leads to rapid propagation of the dyke tip, below which CO2 fluid collects, with a zone of magmatic foam beneath. When the tip breaks the surface of the ground, gas release causes a depressurization wave to travel into the magma. This wave implodes the dyke walls, fragments the magma, and creates a 'ringing' fluidization wave. Together, these processes form the diatreme. Catastrophic magma chilling seals the dyke. No precursor to the eruption is felt at the surface and the processes are complete in about an hour. PMID:17476260

  7. The Fazenda Largo off-craton kimberlites of Piauí State, Brazil

    NASA Astrophysics Data System (ADS)

    Kaminsky, Felix V.; Sablukov, Sergei M.; Sablukova, Ludmila I.; Zakharchenko, Olga D.

    2009-10-01

    In the late 1990s, the Fazenda Largo kimberlite cluster was discovered in the Piauí State of Brazil. As with earlier known kimberlites in this area - Redondão, Santa Filomena-Bom Jesus (Gilbues) and Picos - this cluster is located within the Palaeozoic Parnaiba Sedimentary Basin that separates the São Francisco and the Amazonian Precambrian cratons. Locations of kimberlites are controlled by the 'Transbrasiliano Lineament'. The Fazenda Largo kimberlites are intensely weathered, almost completely altered rocks with a fine-grained clastic structure, and contain variable amounts of terrigene admixture (quartz sand). These rocks represent near-surface volcano-sedimentary deposits of the crater parts of kimberlite pipes. By petrographic, mineralogical and chemical features, the Fazenda Largo kimberlites are similar to average kimberlite. The composition of the deep-seated material in the Fazenda Largo kimberlites is quite diverse: among mantle microxenoliths are amphibolitised pyrope peridotites, garnetised spinel peridotites, ilmenite peridotites, chromian spinel + chromian diopside + pyrope intergrowths, and large xenoliths of pyrope dunite. High-pressure minerals are predominantly of the ultramafic suite, Cr-association minerals (purplish-red and violet pyrope, chromian spinel, chromian diopside, Cr-pargasite and orthopyroxene). The Ti-association minerals of the ultramafic suite (picroilmenite and orange pyrope), as well as rare grains of orange pyrope-almandine of the eclogite association, are subordinate. Kimberlites from all four pipes contain rare grains of G10 pyrope of the diamond association, but chromian spinel of the diamond association was not encountered. By their tectonic position, by geochemical characteristics, and by the composition of kimberlite indicator minerals, the Fazenda Largo kimberlites, like the others of such type, are unlikely to be economic.

  8. In-situ assimilation of mantle minerals by kimberlitic magmas - Direct evidence from a garnet wehrlite xenolith entrained in the Bultfontein kimberlite (Kimberley, South Africa)

    NASA Astrophysics Data System (ADS)

    Soltys, Ashton; Giuliani, Andrea; Phillips, David; Kamenetsky, Vadim S.; Maas, Roland; Woodhead, Jon; Rodemann, Thomas

    2016-07-01

    The lack of consensus on the possible range of initial kimberlite melt compositions and their evolution as they ascend through and interact with mantle and crustal wall rocks, hampers a complete understanding of kimberlite petrogenesis. Attempts to resolve these issues are complicated by the fact that kimberlite rocks are mixtures of magmatic, xenocrystic and antecrystic components and, hence, are not directly representative of their parental melt composition. Furthermore, there is a lack of direct evidence of the assimilation processes that may characterise kimberlitic melts during ascent, which makes understanding their melt evolution difficult. In this contribution we provide novel constraints on the interaction between precursor kimberlite melts and lithospheric mantle wall rocks. We present detailed textural and geochemical data for a carbonate-rich vein assemblage that traverses a garnet wehrlite xenolith [equilibrated at ~ 1060 °C and 43 kbar (~ 140-145 km)] from the Bultfontein kimberlite (Kimberley, South Africa). This vein assemblage is dominated by Ca-Mg carbonates, with subordinate oxide minerals, olivine, sulphides, and apatite. Vein phases have highly variable compositions indicating formation under disequilibrium conditions. Primary inclusions in the vein minerals and secondary inclusion trails in host wehrlite minerals contain abundant alkali-bearing phases (e.g., Na-K bearing carbonates, Mg-freudenbergite, Na-bearing apatite and phlogopite). The Sr-isotope composition of vein carbonates overlaps those of groundmass calcite from the Bultfontein kimberlite, as well as perovskite from the other kimberlites in the Kimberley area. Clinopyroxene and garnet in the host wehrlite are resorbed and have Si-rich reaction mantles where in contact with the carbonate-rich veins. Within some veins, the carbonates occur as droplet-like, globular segregations, separated from a similarly shaped Si-rich phase by a thin meniscus of Mg-magnetite. These textures are

  9. Eumetazoan fossils in terminal proterozoic phosphorites?

    PubMed

    Xiao, S; Yuan, X; Knoll, A H

    2000-12-01

    Phosphatic sedimentary rocks preserve a record of early animal life different from and complementary to that provided by Ediacaran fossils in terminal Proterozoic sandstones and shales. Phosphorites of the Doushantuo Formation, South China, contain eggs, egg cases, and stereoblastulae that document animals of unspecified phylogenetic position; small fossils containing putative spicules may specifically record the presence of sponges. Microfossils recently interpreted as the preserved gastrulae of cnidarian and bilaterian metazoans can alternatively be interpreted as conventional algal cysts and/or egg cases modified by diagenetic processes known to have had a pervasive influence on Doushantuo phosphorites. Regardless of this interpretation, evidence for Doushantuo eumetazoans is provided by millimeter-scale tubes that display tabulation and apical budding characteristic of some Cnidaria, especially the extinct tabulates. Like some Ediacaran remains, these small, benthic, colonial fossils may represent stem-group eumetazoans or stem-group cnidarians that lived in the late Proterozoic ocean. PMID:11095754

  10. Eumetazoan fossils in terminal proterozoic phosphorites?

    PubMed

    Xiao, S; Yuan, X; Knoll, A H

    2000-12-01

    Phosphatic sedimentary rocks preserve a record of early animal life different from and complementary to that provided by Ediacaran fossils in terminal Proterozoic sandstones and shales. Phosphorites of the Doushantuo Formation, South China, contain eggs, egg cases, and stereoblastulae that document animals of unspecified phylogenetic position; small fossils containing putative spicules may specifically record the presence of sponges. Microfossils recently interpreted as the preserved gastrulae of cnidarian and bilaterian metazoans can alternatively be interpreted as conventional algal cysts and/or egg cases modified by diagenetic processes known to have had a pervasive influence on Doushantuo phosphorites. Regardless of this interpretation, evidence for Doushantuo eumetazoans is provided by millimeter-scale tubes that display tabulation and apical budding characteristic of some Cnidaria, especially the extinct tabulates. Like some Ediacaran remains, these small, benthic, colonial fossils may represent stem-group eumetazoans or stem-group cnidarians that lived in the late Proterozoic ocean.

  11. Regulation of atmospheric oxygen during the Proterozoic

    NASA Astrophysics Data System (ADS)

    Laakso, Thomas A.; Schrag, Daniel P.

    2014-02-01

    Many studies suggest that oxygen has remained near modern levels throughout the Phanerozoic, but was much less abundant from the “Great Oxygenation Event” around 2.4 Ga until the late Neoproterozoic around 600 Ma (Kump, 2008). Using a simple model, we show that the maintenance of atmospheric pO2 at ∼1% of present atmospheric levels (PAL) is inconsistent with modern biogeochemical cycling of carbon, sulfur and iron unless new feedbacks are included. Low oxygen conditions are stable in our model if the flux of phosphorus to the oceans was greatly reduced during the Proterozoic. We propose a mechanism to reduce this flux through the scavenging of phosphate ions with an “iron trap” driven by greater surface mobility of ferrous iron in a low pO2 world. Incorporating this feedback leads to two stable equilibria for atmospheric oxygen, the first quantitative hypothesis to explain both Proterozoic and Phanerozoic O2 concentrations.

  12. A Laughing Gas Greenhouse for the Proterozoic?

    NASA Astrophysics Data System (ADS)

    Roberson, A. L.; Roadt, J.; Halevy, I.; Kasting, J. F.

    2010-12-01

    An anoxic, sulfidic ‘Canfield ocean’ during the Proterozoic (0.75-2.3 Ga) would have had limited trace metal abundances because of the low solubility of metal sulfides. The limitation on copper, specifically, would have had a significant impact on marine denitrification. Copper is needed for the enzyme that controls the final step of denitrification, from N2O to N2. Today, only about 5-6 percent of denitrification results in release of N2O. If all denitrification stopped at N2O during the Proterozoic, the N2O flux could have been 15-20 times higher than today. Other parts of the nitrogen cycle should have been able to operate at rates comparable to today, as catalysts for these reactions should have existed. The high N2O flux should have created higher atmospheric N2O concentrations; although this effect may have been offset by faster rates of N2O photolysis if O2 concentrations were lower than today. N2O concentrations of 0.3 to 30 ppmv, along with methane levels of 30-100 ppm, could have kept the surface warm during the Proterozoic without necessitating high CO2 levels. The high methane concentrations were a consequence of lack of dissolved O2 and sulfate in the deep ocean, which should have led to a high CH4 flux from marine sediments. A second oxygenation event at the end of the Proterozoic would have resulted in a shift to a more modern ocean and, consequently, more modern concentrations of atmospheric N2O and CH4.

  13. Proterozoic ocean redox and biogeochemical stasis

    PubMed Central

    Reinhard, Christopher T.; Planavsky, Noah J.; Robbins, Leslie J.; Partin, Camille A.; Gill, Benjamin C.; Lalonde, Stefan V.; Bekker, Andrey; Konhauser, Kurt O.; Lyons, Timothy W.

    2013-01-01

    The partial pressure of oxygen in Earth’s atmosphere has increased dramatically through time, and this increase is thought to have occurred in two rapid steps at both ends of the Proterozoic Eon (∼2.5–0.543 Ga). However, the trajectory and mechanisms of Earth’s oxygenation are still poorly constrained, and little is known regarding attendant changes in ocean ventilation and seafloor redox. We have a particularly poor understanding of ocean chemistry during the mid-Proterozoic (∼1.8–0.8 Ga). Given the coupling between redox-sensitive trace element cycles and planktonic productivity, various models for mid-Proterozoic ocean chemistry imply different effects on the biogeochemical cycling of major and trace nutrients, with potential ecological constraints on emerging eukaryotic life. Here, we exploit the differing redox behavior of molybdenum and chromium to provide constraints on seafloor redox evolution by coupling a large database of sedimentary metal enrichments to a mass balance model that includes spatially variant metal burial rates. We find that the metal enrichment record implies a Proterozoic deep ocean characterized by pervasive anoxia relative to the Phanerozoic (at least ∼30–40% of modern seafloor area) but a relatively small extent of euxinic (anoxic and sulfidic) seafloor (less than ∼1–10% of modern seafloor area). Our model suggests that the oceanic Mo reservoir is extremely sensitive to perturbations in the extent of sulfidic seafloor and that the record of Mo and chromium enrichments through time is consistent with the possibility of a Mo–N colimited marine biosphere during many periods of Earth’s history. PMID:23515332

  14. Proterozoic ocean redox and biogeochemical stasis.

    PubMed

    Reinhard, Christopher T; Planavsky, Noah J; Robbins, Leslie J; Partin, Camille A; Gill, Benjamin C; Lalonde, Stefan V; Bekker, Andrey; Konhauser, Kurt O; Lyons, Timothy W

    2013-04-01

    The partial pressure of oxygen in Earth's atmosphere has increased dramatically through time, and this increase is thought to have occurred in two rapid steps at both ends of the Proterozoic Eon (∼2.5-0.543 Ga). However, the trajectory and mechanisms of Earth's oxygenation are still poorly constrained, and little is known regarding attendant changes in ocean ventilation and seafloor redox. We have a particularly poor understanding of ocean chemistry during the mid-Proterozoic (∼1.8-0.8 Ga). Given the coupling between redox-sensitive trace element cycles and planktonic productivity, various models for mid-Proterozoic ocean chemistry imply different effects on the biogeochemical cycling of major and trace nutrients, with potential ecological constraints on emerging eukaryotic life. Here, we exploit the differing redox behavior of molybdenum and chromium to provide constraints on seafloor redox evolution by coupling a large database of sedimentary metal enrichments to a mass balance model that includes spatially variant metal burial rates. We find that the metal enrichment record implies a Proterozoic deep ocean characterized by pervasive anoxia relative to the Phanerozoic (at least ∼30-40% of modern seafloor area) but a relatively small extent of euxinic (anoxic and sulfidic) seafloor (less than ∼1-10% of modern seafloor area). Our model suggests that the oceanic Mo reservoir is extremely sensitive to perturbations in the extent of sulfidic seafloor and that the record of Mo and chromium enrichments through time is consistent with the possibility of a Mo-N colimited marine biosphere during many periods of Earth's history.

  15. (U-Th)/He dating of kimberlites-A case study from north-eastern Kansas

    USGS Publications Warehouse

    Blackburn, T.J.; Stockli, D.F.; Carlson, R.W.; Berendsen, P.

    2008-01-01

    Dating kimberlite intrusive rocks by radiogenic isotope geochronology often is a difficult task, complicated by both the lack of dateable minerals within kimberlite as well as significant sample alteration that can degrade samples and alter parent-daughter ratios. This study presents a new geochronologic tool for timing the emplacement of kimberlites using the (U-Th)/He system to date the cooling of common kimberlite phenocrystic and xenocrystic minerals. To demonstrate the use of this technique, new apatite, titanite, zircon, magnetite and garnet (U-Th)/He ages constrain the timing of emplacement for the Stockdale, Tuttle, Baldwin Creek, Bala, and Leonardville kimberlite pipes, located in Riley County, Kansas. Zircon from the Tuttle pipe and titanite from the Stockdale pipe yield (U-Th)/He ages of 108.6 ?? 9.6??Ma and 106.4 ?? 3.1??Ma, respectively. These data are consistent with new Tuttle kimberlite Rb-Sr analyses of phlogopite megacrysts that give a five point isochron age of 106.6 ?? 1.0??Ma. Similarly, an apatite (U-Th)/He age of 85.3 ?? 2.3??Ma from the Baldwin Creek kimberlite is in agreement with a Rb-Sr phlogopite age of 88.4 ?? 2.7??Ma. These dates demonstrate that (U-Th)/He thermochronometry provides reliable timing constraints on the cooling of common kimberlite xenocrystic phases, thereby timing kimberlite emplacement. In addition to the use of more commonly used apatite and zircon (U-Th)/He thermochronometers, we produced reliable emplacement ages of 103.0 ?? 7.5??Ma for the Bala kimberlite using (U-Th)/He dating of phenocrystic magnetite and an age of 98.8 ?? 8.9??Ma for the Tuttle kimberlite using (U-Th)/He dating of megacrystic garnet. In contrast, kimberlitic apatite (U-Th)/He ages from the Stockdale, Bala, Tuttle, and Leonardville kimberlites yield ages ranging from 67.3 ?? 4.4??Ma to 64.3 ?? 5.6??Ma, suggesting a local, possibly hydrothermal reheating event resulting in resetting of the apatite (U-Th)/He clock in latest Cretaceous to earliest

  16. Multi-stage kimberlite evolution tracked in zoned olivine from the Benfontein sill, South Africa

    NASA Astrophysics Data System (ADS)

    Howarth, Geoffrey H.; Taylor, Lawrence A.

    2016-10-01

    Olivine is the dominant mineral present in kimberlite magmas; however, due to the volatile-rich nature of most kimberlites, they rarely survive late-stage serpentinisation. Here we present major and trace element data for a rare example of ultra-fresh olivine in a macrocrystic calcite kimberlite from the Benfontein kimberlite sill complex. Olivines are characterised by xenocrystic cores surrounded by multiple growth zones representing melt crystallisation and late-stage equilibration. Two distinct core populations are distinguished: Type 1) low Fo (88-89), Ni-rich, Ca- and Na-rich cores, interpreted here to be the result of carbonate-silicate metasomatism potentially as part of the earliest stages of kimberlite magmatism, and Type 2) high Fo (91-93), Ni-rich, low-Ca cores derived from a typical garnet peridotite mantle source. In both cases, the cores have transitional margins (Fo89-90) representing equilibration with a proto-kimberlite melt. Trace element concentrations, in particular Cr, of these transition zones suggest formation of the proto-kimberlite melt through assimilation of orthopyroxene from the surrounding garnet peridotite lithology. Trace element trends in the surrounding melt-zone olivine (Fo87-90) suggest evolution of the kimberlite through progressive olivine crystallisation. The final stages of olivine growth are represented by Fe-rich (Fo85) and P-rich olivine indicating kimberlite evolution to mafic compositions. Fine (< 60 μm), Mg-rich olivine rims (Fo94-98) represent equilibration with the final stages of kimberlite evolution back to Fe-poor carbonatitic melts. We present a step-by-step model for kimberlite magma genesis and evolution from mantle to crust tracked by the chemistry of olivines in the Benfontein kimberlite. These steps include early stages of metasomatism and mantle assimilation followed by direct crystallisation of the kimberlite melt and late-stage equilibration with the evolved carbonatitic residual liquids. The Ca contents

  17. Use of high-resolution ground-penetrating radar in kimberlite delineation

    USGS Publications Warehouse

    Kruger, J.M.; Martinez, A.; Berendsen, P.

    1997-01-01

    High-resolution ground-penetrating radar (GPR) was used to image the near-surface extent of two exposed Late Cretaceous kimberlites intruded into lower Permian limestone and dolomite host rocks in northeast Kansas. Six parallel GPR profiles identify the margin of the Randolph 1 kimberlite by the up-bending and termination of limestone reflectors. Five radially-intersecting GPR profiles identify the elliptical margin of the Randolph 2 kimberlite by the termination of dolomite reflectors near or below the kimberlite's mushroom-shaped cap. These results suggest GPR may augment magnetic methods for the delineation of kimberlites or other forceful intrusions in a layered host rock where thick, conductive soil or shale is not present at the surface.

  18. Exceptional preservation of fossils in an Upper Proterozoic shale

    NASA Technical Reports Server (NTRS)

    Butterfield, N. J.; Knoll, A. H.; Swett, K.

    1988-01-01

    An exceptionally well-preserved and distinctive assemblage of Late Proterozoic fossils from subtidal marine shales is reported. In addition to the spheromorphic acritarchs and cyanobacteria sheaths routinely preserved in Proterozoic rocks, this assemblage includes multicellular algae, a diverse assortment of morphologically complex protistan vesicles, and probable heterotrophic bacteria. Thus, it provides one of the clearest and most taxonomically varied views of Proterozoic life yet reported.

  19. Results of 40Ar/39Ar dating of phlogopites from kelyphitic rims around garnet grains (Udachnaya-Vostochnaya kimberlite pipe)

    NASA Astrophysics Data System (ADS)

    Yudin, D. S.; Tomilenko, A. A.; Alifirova, T. A.; Travin, A. V.; Murzintsev, N. G.; Pokhilenko, N. P.

    2016-07-01

    40Ar/39Ar dating of phlogopite from kelyphitic rims around garnet grains from the Udachnaya-Vostochnaya kimberlite pipe in the Sakha (Yakutia) Republic (Russia) revealed that when this mineral has contact with a kimberlite melt its age corresponds (within error limits) to that of the formation of the kimberlite pipe, thus indicating that the method may be used for dating kimberlites and related rocks. In mantle xenoliths, kelyphitic phlogopites rimming garnet grains partially lose radiogenic Ar, which results in a complex age spectrum. Rejuvenation of the K/Ar system in them is determined by the thermal impact of the kimberlite melt on captured rocks.

  20. Subcalcic diopsides from kimberlites: Chemistry, exsolution microstructures, and thermal history

    USGS Publications Warehouse

    McCallister, R.H.; Nord, G.L.

    1981-01-01

    Twenty-six subcalcic diopside megacrysts (Ca/(Ca+ Mg)) = 0.280-0.349, containing approximately 10 mol% jadeite, from 15 kimberlite bodies in South Africa, Botswana, Tanzania, and Lesotho, have been characterized by electron microprobe analysis, X-ray-precession photography, and transmission electron microscopy. Significant exsolution of pigeonite was observed only in those samples for which Ca/(Ca+Mg)???0.320. The exsolution microstructure consists of coherent (001) lamellae with wavelengths ranging from 20 to 31 nm and compositional differences between the hosts and lamellae ranging from 10 to 30 mol% wollastonite. These observations suggest that the exsolution reaction mechanism was spinodal decomposition and that the megacrysts have been quenched at various stages of completion of the decomposition process. Annealing experiments in evacuated SiO2 glass tubes at 1,150?? C for 128 hours failed to homogenize microstructure, whereas, at 5 kbar and 1,150?? C for only 7.25 hours, the two lattices were homogenized. This "pressure effect" suggests that spinodal decomposition in the kimberlitic subcalcic diopside megacrysts can only occur at depths less than ???15 km; the cause of the effect may be the jadeite component in the pyroxene. "Apparent quench" temperatures for the exsolution process in the megacrysts range from 1,250?? C to 990?? C, suggesting that decomposition must have commenced at temperatures of more than ???1,000?? C. These P-T limits lead to the conclusion that, in those kimberlites where spinodal decomposition has occurred in subcalcic diopside megacrysts, such decomposition occurred at shallow levels (<15 km) and, at the present erosion level, temperatures must have been greater than 1,000?? C. ?? 1981 Springer-Verlag.

  1. New data for Eclogites and mantle xenocrysts and megacrysts from kimberlites of Dharwar craton , southern India.

    NASA Astrophysics Data System (ADS)

    Ashchepkov, Igor; Ravi, Subramanian; Shankar Nayak, Shiva; Kaminsky, , Felix; Naflos, Theodoros; Vladykin, Nikolai

    2013-04-01

    Eclogitic mantle xenoliths from Proterozoic (1100) Kalyandurg kimberlite field (KL-4 pipe) Dharwar craton, India as well as the xenocrysts from pipes Wajrakarur kimberlite field pipes and others were analyzed by EPMA and LAM ICP MS methods. The eclogites (often with kyanite) (Patel et al., 2006) are composed mainly from garnet and Cpx, intergrain material is mainly represented by the carbonates and Ca- silicates. Garnets reveal Hi- CaO content to (10-12%) and Cpx are omphacites very low in FeO (1-3%) high Al2O3 (8-14% ) and Na2O (2-6 %) differing from the studied samples (Patel et al., 2009). The typical grosspidites (with kyanites coesite, K-Cpx and sanidine) have irregular compositions of minerals and fingerprint structures probably related to the crystallization from fluid. The compositions of the Cpx from Wajrakarur and other pipes reveal Hi - Cr2O3 (5%) content often higher then FeO and Na2O (4%). Garnets are in Lherzolite field in CaO - Cr2O3 (to 12%) diagram. Ilmenites with TiO2 variations (58-42%) show two trends of Cr2O3 enrichments accompanied by the general NiO and V2O5 decrease. Trace element s for eclogitic Cpx reveal high La/Ybn ratios, Eu peaks and flattened HREE. Garnet REE are not equilibrated and highly inclined. The TRE spiderdiagrams show depletion in HFSE (Ta>Nb), the most depleted show Y through for most depleted varieties. Garnets reveal U peak but low Sr CPx peaks in both Ba and Sr. The REE patterns Cpx xenocrysts from Wajrakarur are very similar in shape with varying incompatible part. They are showing high La/Ybn by the order of 2 and small humps in Ce-Pr. Spidergrams show small depletion in Zr- Hf and U and all incompatible elements and through in Pb . The REE of ilmenite xenocrysts show two models: high La/Ybn by 2 orders or nearly flattened patterns. Chromites show depletion in La-Pr elements. Ilmenite's TRE spiderrams show peaks in Nb-Ta and Pb and Zr- Hf . PTXFO2 diagrams for SCLM beneath the Wajrakarur and nearby fields show rather

  2. Temporal, geomagnetic and related attributes of kimberlite magmatism at Ekati, Northwest Territories, Canada

    NASA Astrophysics Data System (ADS)

    Lockhart, Grant; Grütter, Herman; Carlson, Jon

    2004-09-01

    This paper outlines the development of a multi-disciplinary strategy to focus exploration for economic kimberlites on the Ekati property. High-resolution aeromagnetic data provide an over-arching spatial and magnetostratigraphic framework for exploration and kimberlite discovery at Ekati, and hence also for this investigation. The temporal, geomagnetic, spatial and related attributes of kimberlites with variable diamond content have been constrained by judiciously augmenting the information gathered during routine exploration with detailed, laboratory-based or field-based investigations. The natural remanent magnetisation of 36 Ekati kimberlites has been correlated with their age as determined by isotopic dating techniques, and placed in the context of a well-constrained geomagnetic polarity timescale. Kimberlite magmatism occurred over the period 75 to 45 Ma, in at least five temporally discrete intrusive episodes. Based on current evidence, the older kimberlites (75 to 59 Ma) have low diamond contents and are distributed throughout the property. Younger kimberlites (56 to 45 Ma) have moderate to high diamond contents and occur in three distinct intrusive corridors with NNE to NE orientations. Economic kimberlite pipes erupted at 55.4±0.4 Ma along the A154-Lynx intrusive corridor, which is 7 km wide and oriented at 015°, and at 53.2±0.3 Ma along the Panda intrusive corridor, which is 1 km wide and oriented at 038°. The intrusion ages straddle a paleopole reversal at Chron C24n, consistent with the observation that the older economic kimberlites present as aeromagnetic "low" anomalies while the younger economic pipes are characterised as aeromagnetic "highs". The aeromagnetic responses for these kimberlites are generally muted because they contain volcaniclastic rock types with low magnetic susceptibility. Kimberlites throughout the Ekati property carry a primary natural magnetic remanence (NRM) vector in Ti-bearing groundmass magnetite, and it dominates over

  3. Zircon megacrysts from kimberlite: oxygen isotope variability among mantle melts

    NASA Astrophysics Data System (ADS)

    Valley, John W.; Kinny, Peter D.; Schulze, Daniel J.; Spicuzza, Michael J.

    The oxygen isotope ratios of Phanerozoic zircons from kimberlite pipes in the Kaapvaal Craton of southern Africa and the Siberian Platform vary from 4.7 to 5.9‰ VSMOW. High precision, accurate analyses by laser reveal subtle pipe-to-pipe differences not previously suspected. These zircons have distinctive chemical and physical characteristics identifying them as mantle-derived megacrysts similar to zircons found associated with diamond, coesite, MARID xenoliths, Cr-diopside, K-richterite, or Mg-rich ilmenite. Several lines of evidence indicate that these 18O values are unaltered by kimberlite magmas during eruption and represent compositions preserved since crystallization in the mantle, including: U/Pb age, large crystal size, and the slow rate of oxygen exchange in non-metamict zircon. The average 18O of mantle zircons is 5.3‰, 0.1 higher and in equilibrium with values for olivine in peridotite xenoliths and oceanic basalts. Zircon megacrysts from within 250 km of Kimberley, South Africa have average 18O=5.32+/-0.17 (n=28). Small, but significant, differences among other kimberlite pipes or groups of pipes may indicate isotopically distinct reservoirs in the sub-continental lithosphere or asthenosphere, some of which are anomalous with respect to normal mantle values of 5.3+/-0.3. Precambrian zircons (2.1-2.7 Ga) from Jwaneng, Botswana have the lowest values yet measured in a mantle zircon, 18O=3.4 to 4.7‰. These zircon megacrysts originally crystallized in mafic or ultramafic rocks either through melting and metasomatism associated with kimberlite magmatism or during metamorphism. The low 18O zircons are best explained by subduction of late Archean ocean crust that exchanged with heated seawater prior to underplating as eclogite and to associated metasomatism of the mantle wedge. Smaller differences among other pipes and districts may result from variable temperatures of equilibration, mafic versus ultramafic hosts, or variable underplating. The narrow

  4. Stratigraphy of the intra-crater volcaniclastic deposits of the Victor Northwest kimberlite, northern Ontario, Canada

    NASA Astrophysics Data System (ADS)

    van Straaten, Bram I.; Kopylova, M. G.; Russell, J. K.; Webb, K. J.; Smith, B. H. Scott

    2009-11-01

    The Victor Northwest (VNW) kimberlite is one of several steep-sided pipes in the Victor kimberlite complex. In this paper detailed logging of ~ 4.2 km of drill core and petrographic studies of hundreds of samples and thin sections are used to reconstruct the intra-crater stratigraphy of the VNW kimberlite and to constrain the emplacement history of the pipe. The results show that the VNW pipe comprises numerous contrasting small-volume volcanic facies, including dark and competent kimberlite, sedimentary country rock breccias, pyroclastic kimberlite and resedimented volcaniclastic kimberlite. We interpret that the VNW pipe was formed by two separate eruption cycles. During the first eruption cycle, the main VNW crater is excavated and partly filled. The second eruption cycle excavates a smaller nested crater within the existing lithified pipe fill. Both eruption cycles can be subdivided into three comparable stages. The first stage in both cycles comprises a highly explosive eruption involving crater excavation and deposition of pyroclastic kimberlite. The eruption products contain abundant broken olivines, small angular country rock fragments and kimberlite ash. All these features indicate high degrees of fragmentation resulting from high eruption intensities. The high proportion of country rock fragments in these deposits suggests continuous pipe wall erosion. The second stage in both cycles is represented by dark and competent deposits, which have low country rock fragment abundances and mostly intact olivines. Based on contact relationships and textures within these units it is suggested that these rock types are formed by lower energy eruptions, during which no major pipe wall erosion took place. The eruption came to an end during the third and last stage. In both cycles, the uppermost deposits record resedimentation of kimberlite by water. Major pipe wall collapse results in the formation of voluminous sedimentary country rock breccia deposits that cap all

  5. Terminal Proterozoic reorganization of biogeochemical cycles

    NASA Technical Reports Server (NTRS)

    Logan, G. A.; Hayes, J. M.; Hieshima, G. B.; Summons, R. E.

    1995-01-01

    The Proterozoic aeon (2,500-540 million years ago) saw episodic increases in atmospheric oxygen content, the evolution of multicellular life and, at its close, an enormous radiation of animal diversity. These profound biological and environmental changes must have been linked, but the underlying mechanisms have been obscure. Here we show that hydrocarbons extracted from Proterozoic sediments in several locations worldwide are derived mainly from bacteria or other heterotrophs rather than from photosynthetic organisms. Biodegradation of algal products in sedimenting matter was therefore unusually complete, indicating that organic material was extensively reworked as it sank slowly through the water column. We propose that a significant proportion of this reworking will have been mediated by sulphate-reducing bacteria, forming sulphide. The production of sulphide and consumption of oxygen near the ocean surface will have inhibited transport of O2 to the deep ocean. We find that preservation of algal-lipid skeletons improves at the beginning of the Cambrian, reflecting the increase in transport by rapidly sinking faecal pellets. We suggest that this rapid removal of organic matter will have increased oxygenation of surface waters, leading to a descent of the O2-sulphide interface to the sea floor and to marked changes in the marine environment, ultimately contributing to the Cambrian radiation.

  6. The origin of pelletal lapilli in explosive kimberlite eruptions.

    PubMed

    Gernon, T M; Brown, R J; Tait, M A; Hincks, T K

    2012-01-01

    Kimberlites are volatile-rich magmas from mantle depths of ≥ 150  km and are the primary source of diamonds. Kimberlite volcanism involves the formation of diverging pipes or diatremes, which are the locus of high-intensity explosive eruptions. A conspicuous and previously enigmatic feature of diatreme fills are 'pelletal lapilli'--well-rounded clasts consisting of an inner 'seed' particle with a complex rim, thought to represent quenched juvenile melt. Here we show that these coincide with a transition from magmatic to pyroclastic behaviour, thus offering fundamental insights into eruption dynamics and constraints on vent conditions. We propose that pelletal lapilli are formed when fluid melts intrude into earlier volcaniclastic infill close to the diatreme root zone. Intensive degassing produces a gas jet in which locally scavenged particles are simultaneously fluidised and coated by a spray of low-viscosity melt. A similar origin may apply to pelletal lapilli in other alkaline volcanic rocks, including carbonatites, kamafugites and melilitites. PMID:22588294

  7. Potential for diamond in kimberlites from Michigan and Montana as indicated by garnet xenocryst compositions

    USGS Publications Warehouse

    McGee, E.S.

    1988-01-01

    The Williams kimberlite in north-central Montana and the Lake Ellen kimberlite in northern Michigan contain diagnostic xenoliths and xenocrysts which indicate that diamonds may be present. To date, however, no diamonds have been reported from either locality. In this study, particular compositions of garnet xenocrysts which are associated with diamond elsewhere were sought as an indication of the potential for diamond in the Williams and Lake Ellen kimberlites. For this study, garnets were carefully selected for purple color in order to increase the chance of finding the subcalcic chrome-rich compositions that are associated with the presence of diamond. -Author

  8. Kimberlite Wall Rock Fragmentation: Venetia K08 Pipe Development

    NASA Astrophysics Data System (ADS)

    Barnett, W.; Kurszlaukis, S.; Tait, M.; Dirks, P.

    2009-05-01

    Volcanic systems impose powerful disrupting forces on the country rock into which they intrude. The nature of the induced brittle deformation or fragmentation can be characteristic of the volcanic processes ongoing within the volcanic system, but are most typically partially removed or obscured by repeated, overprinting volcanic activity in mature pipes. Incompletely evolved pipes may therefore provide important evidence for the types and stages of wall rock fragmentation, and mechanical processes responsible for the fragmentation. Evidence for preserved stages of fragmentation is presented from a detailed study of the K08 pipe within the Cambrian Venetia kimberlite cluster, South Africa. This paper investigates the growth history of the K08 pipe and the mechanics of pipe development based on observations in the pit, drill core and thin sections, from geochemical analyses, particle size distribution analyses, and 3D modeling. Present open pit exposures of the K08 pipe comprise greater than 90% mega-breccia of country rock clasts (gneiss and schist) with <10% intruding, coherent kimberlite. Drill core shows that below about 225 m the CRB includes increasing quantities of kimberlite. The breccia clasts are angular, clast-supported with void or carbonate cement between the clasts. Average clast sizes define sub-horizontal layers tens of metres thick across the pipe. Structural and textural observations indicate the presence of zones of re-fragmentation or zones of brittle shearing. Breccia textural studies and fractal statistics on particle size distributions (PSD) is used to quantify sheared and non- sheared breccia zones. The calculated energy required to form the non-sheared breccia PSD implies an explosive early stage of fragmentation that pre-conditions the rock mass. The pre-conditioning would have been caused by explosions that are either phreatic or phreatomagmatic in nature. The explosions are likely to have been centered on a dyke, or pulses of preceding

  9. Mineralogical zoning of the diamondiferous areas: Application experience of paragenetic analysis of garnets from kimberlites

    NASA Astrophysics Data System (ADS)

    Samdanov, D. A.; Afanasiev, V. P.; Tychkov, N. S.; Pokhilenko, N. P.

    2016-03-01

    Paragenetic analysis of pyropes from alluvial deposits of the Muna—Markha interfluve (Sakha-Yakutia Republic) made it possible to distinguish relatively uniform areas that are promising for the discovery of kimberlite bodies.

  10. Spatial patterns in the distribution of kimberlites: relationship to tectonic processes and lithosphere structure

    NASA Astrophysics Data System (ADS)

    Chemia, Zurab; Artemieva, Irina; Thybo, Hans

    2015-04-01

    Since the discovery of diamonds in kimberlite-type rocks more than a century ago, a number of theories regarding the processes involved in kimberlite emplacement have been put forward to explain the unique properties of kimberlite magmatism. Geological data suggests that pre-existing lithosphere weakness zones may control the spatial patterns of kimberlites, but this hypothesis has never been tested by geophysical methods. As the first step in our analysis of tectonic and lithosphere control of kimberlite-type magmatism, we perform a detailed global analysis of the spatial patterns of kimberlites, and present the first results. The analysis is based on the assumption that the kimberlite emplacement is a two-stage process, and the two stages are controlled by the crustal and lithospheric mantle rheologies, respectively. Stage 1 includes the first-order, lithosphere-scale process that initiate the rise of kimberlite melts through the lithospheric mantle, which forms the major pipe. Stage 2 (second-order process) begins when the major pipe splits into daughter sub-pipes (tree-like pattern) at crustal depths. We apply cluster analysis to the spatial distribution of all known kimberlite fields with the goal of establishing characteristic scales for the stage 1 and stage 2 processes. To reveal similarities between the kimberlite data we use the density-based clustering technique, such as density-based spatial clustering of applications with noise (DBSCAN), which is efficient for large data sets, requires one input parameter, and can deal with clusters of any shape. The results indicate that characteristic scales for the stage 2 are almost globally uniform and thus are almost independent of the structure and the mantle lithosphere. In contrast, the characteristic scales for stage 1 (lithosphere-scale process) that initiate the rise of kimberlite melts through the lithospheric mantle forms the major pipes with characteristic distance ranging from 100 to 300 km and are

  11. Zircon U-Th-Pb-He double dating of the Merlin kimberlite field, Northern Territory, Australia

    NASA Astrophysics Data System (ADS)

    McInnes, Brent I. A.; Evans, Noreen J.; McDonald, Brad J.; Kinny, Peter D.; Jakimowicz, Janusz

    2009-11-01

    This paper discusses the development of a combined U/Pb and (U-Th)/He geochronology method for single zircon grains and the potential application of this technique in diamond exploration. Zircon entrained within kimberlite deposits should have distinctive U-Th-Pb-He signatures compared to those found in the host terrane. To investigate the application of zircon double dating to kimberlite diamond exploration, we analysed zircon from the Sacramore kimberlite pipe located in the Merlin field in the Northern Territory of Australia. We also conducted a double dating study of detrital zircon from a regional sample of the Bukalara Sandstone, the principal host rock for the Merlin field. Repeated analysis of Fish Canyon zircon verified that the ion microprobe sputtering process used for U/Pb dating does not disturb the He content of the sample. The zircon U/Pb age for the Sacramore pipe ( n = 14) ranged from 1541-2433 Ma, consistent with the Mesoproterozoic formation of the North Australian Craton and indicating that the zircon obtained from the kimberlite is of xenocrystic origin. (U-Th)/He thermochronometry of these kimberlite zircon xenocrysts ( n = 33) yielded a mean weighted average age of 368 ± 4 Ma (2 σ), concordant with a previously determined phlogopite Rb-Sr age of 367 ± 4 Ma for the Merlin field. The U/Pb age (1472-2939 Ma; n = 41) of detrital zircon from the Bukalara Sandstone is statistically indistinguishable from that of the kimberlite zircon xenocrysts, while the detrital (U-Th)/He ages range from 459 to 1279 Ma. The double dating study indicates that the majority of the zircon grains in the Sacramore kimberlite pipe had their helium clocks reset to the time of kimberlite emplacement. Thermal resetting of the zircon xenocrysts may have been caused by either one or a combination of these processes: (i) the grains originated from lower- to mid-crustal depths (> 6 km) where ambient geothermal temperatures allowed the helium to diffuse out of the zircon as

  12. The internal geology and emplacement history of the Renard 2 kimberlite, Superior Province, Quebec, Canada

    NASA Astrophysics Data System (ADS)

    Fitzgerald, C. E.; Hetman, C. M.; Lepine, I.; Skelton, D. S.; McCandless, T. E.

    2009-11-01

    The Renard 2 kimberlite is located in the Otish Mountains region of Quebec, Canada and is one of the largest pipes in the Renard cluster. The cluster consists of nine kimberlite bodies and was discovered in 2001 by Ashton Mining of Canada Inc. and its joint venture partner SOQUEM Inc. Renard 2 was emplaced into Archean meta-greywacke derived migmatite, gneiss and granite of the Opinaca Subprovince of the eastern Superior Province at approximately 640.5 ± 2.8 Ma. An undetermined amount of erosion has occurred since emplacement with the present surface expression of the pipe estimated to be 0.75 ha. This kimberlite is interpreted as a steep-sided diatreme with minor irregularities in the external shape. The dominant infill is a massive volcaniclastic kimberlite (MVK) that is classified as tuffisitic kimberlite breccia (TKB) and is characterized by a high proportion of granitoid country rock xenoliths. A second dominant infill is a texturally complex, less diluted coherent kimberlite (CK) characterized locally by a transitional textures between CK and TKB. Surrounding the diatreme is a significant zone of variable width comprised of extensively brecciated country rock (+/-kimberlite) and referred to as marginal breccia. In addition to the two main rock types infilling the pipe, a number of hypabyssal kimberlite (HK) dykes and irregular shaped intrusions occur throughout the body, along the pipe contacts, within the marginal breccia and in the surrounding country rock. Geological features displayed by Renard 2 are similar to those described from Class 1 kimberlites of the Kimberley area of South Africa, the Gahcho Kué cluster of Canada and the Pimenta Bueno kimberlite field of Brazil. The economic evaluation of Renard 2 is in progress and to date has included extensive diamond and reverse circulation drilling as well as the collection of an underground bulk sample. Results from material sampled from Renard 2, including a 2449 tonne bulk sample, suggest Renard 2 has

  13. Collapse of the Late Proterozoic ecosystem

    NASA Technical Reports Server (NTRS)

    Schopf, J. W.

    1991-01-01

    Evidence accumulated over the past two decades is now sufficient to permit an initial quantitative assessment of the patterns of biotic diversity and extinction that occurred during Proterozoic time. Because of limitations in both the quality and quantity of data currently available, however, generalizations thus derived must be regarded as tentative. Nevertheless, read literally, available palaeontological data appear to indicate that the global ecosystem experienced a gradual but massive collapse between 1 000 Ma and the beginning of the Phanerozoic, a supposition consistent with other lines of geological and geochemical evidence. A possible forcing agent for such a collapse appears to have been a decrease in ambient levels of carbon dioxide and a resultant decrease in average global temperature, photosynthetic efficiency, and primary productivity.

  14. Arsenic stress after the Proterozoic glaciations

    PubMed Central

    Chi Fru, Ernest; Arvestål, Emma; Callac, Nolwenn; El Albani, Abderrazak; Kilias, Stephanos; Argyraki, Ariadne; Jakobsson, Martin

    2015-01-01

    Protection against arsenic damage in organisms positioned deep in the tree of life points to early evolutionary sensitization. Here, marine sedimentary records reveal a Proterozoic arsenic concentration patterned to glacial-interglacial ages. The low glacial and high interglacial sedimentary arsenic concentrations, suggest deteriorating habitable marine conditions may have coincided with atmospheric oxygen decline after ~2.1 billion years ago. A similar intensification of near continental margin sedimentary arsenic levels after the Cryogenian glaciations is also associated with amplified continental weathering. However, interpreted atmospheric oxygen increase at this time, suggests that the marine biosphere had widely adapted to the reorganization of global marine elemental cycles by glaciations. Such a glacially induced biogeochemical bridge would have produced physiologically robust communities that enabled increased oxygenation of the ocean-atmosphere system and the radiation of the complex Ediacaran-Cambrian life. PMID:26635187

  15. Arsenic stress after the Proterozoic glaciations.

    PubMed

    Fru, Ernest Chi; Arvestål, Emma; Callac, Nolwenn; El Albani, Abderrazak; Kilias, Stephanos; Argyraki, Ariadne; Jakobsson, Martin

    2015-12-04

    Protection against arsenic damage in organisms positioned deep in the tree of life points to early evolutionary sensitization. Here, marine sedimentary records reveal a Proterozoic arsenic concentration patterned to glacial-interglacial ages. The low glacial and high interglacial sedimentary arsenic concentrations, suggest deteriorating habitable marine conditions may have coincided with atmospheric oxygen decline after ~2.1 billion years ago. A similar intensification of near continental margin sedimentary arsenic levels after the Cryogenian glaciations is also associated with amplified continental weathering. However, interpreted atmospheric oxygen increase at this time, suggests that the marine biosphere had widely adapted to the reorganization of global marine elemental cycles by glaciations. Such a glacially induced biogeochemical bridge would have produced physiologically robust communities that enabled increased oxygenation of the ocean-atmosphere system and the radiation of the complex Ediacaran-Cambrian life.

  16. Arsenic stress after the Proterozoic glaciations

    NASA Astrophysics Data System (ADS)

    Chi Fru, Ernest; Arvestål, Emma; Callac, Nolwenn; El Albani, Abderrazak; Kilias, Stephanos; Argyraki, Ariadne; Jakobsson, Martin

    2015-12-01

    Protection against arsenic damage in organisms positioned deep in the tree of life points to early evolutionary sensitization. Here, marine sedimentary records reveal a Proterozoic arsenic concentration patterned to glacial-interglacial ages. The low glacial and high interglacial sedimentary arsenic concentrations, suggest deteriorating habitable marine conditions may have coincided with atmospheric oxygen decline after ~2.1 billion years ago. A similar intensification of near continental margin sedimentary arsenic levels after the Cryogenian glaciations is also associated with amplified continental weathering. However, interpreted atmospheric oxygen increase at this time, suggests that the marine biosphere had widely adapted to the reorganization of global marine elemental cycles by glaciations. Such a glacially induced biogeochemical bridge would have produced physiologically robust communities that enabled increased oxygenation of the ocean-atmosphere system and the radiation of the complex Ediacaran-Cambrian life.

  17. Diamonds in an upper mantle peridotite nodule from kimberlite in southern wyoming.

    PubMed

    McCallum, M E; Eggler, D H

    1976-04-16

    Diamonds in a serpentinized garnet peridotite nodule from a diatreme in southern Wyoming are the first known occurrence in an upper mantle peridotite xenolith from a kimberlite intrusion in North America as well as the second authenticated occurrence of diamonds from kimberlite pipes in North America. The nodule is believed to have come from a section of depleted (partially melted) lherzolite at a depth of 130 to 180 kilometers. PMID:17831161

  18. The crater-facies kimberlite system of Tokapal, Bastar District, Chhattisgarh, India

    NASA Astrophysics Data System (ADS)

    Mainkar, Datta; Lehmann, Bernd; Haggerty, Stephen E.

    2004-09-01

    Discovery of diamondiferous kimberlites in the Mainpur Kimberlite Field, Raipur District, Chhattisgarh in central India, encouraged investigation of similar bodies in other parts of the Bastar craton. The earlier known Tokapal ultramafic intrusive body, located beyond the 19-km milestone in Tokapal village along the Jagdalpur-Geedam road, was reinterpreted as crater-facies kimberlite. Its stratigraphic position in the Meso-Neoproterozoic intracratonic sedimentary Indravati basin makes it one of the oldest preserved crater-facies kimberlite systems. Ground and limited subsurface data (dug-, tube-wells and exploratory boreholes) have outlined an extensive surface area (>550 ha) of the kimberlite. The morphological and surface color features of this body on enhanced satellite images suggest that there is a central feeder surrounded by a collar and wide pyroclastic apron. Exploration drilling indicates that the central zone probably corresponds to a vent overlain by resedimented volcaniclastic (epiclastic) rocks that are surrounded by a 2-km-wide spread of pyroclastic rocks (lapilli tuff, tuff/ash beds and volcaniclastic breccia). Drill-holes also reveal that kimberlitic lapilli tuffs and tuffs are sandwiched between the Kanger and Jagdalpur Formations and also form sills within the sedimentary sequence of the Indravati basin. The lapilli tuffs are commonly well stratified and display slumping. Base surges and lava flows occur in the southern part of the Tokapal system. The geochemistry and petrology of the rock correspond to average Group I kimberlite with a moderate degree of contamination. However, the exposed rock is intensely weathered and altered with strong leaching of mobile elements (Ba, Rb, Sr). Layers of vesicular fine-grained glassy material represent kimberlitic lava flows. Tuffs containing juvenile lapilli with pseudomorphed olivine macrocrysts are set in a talc-serpentine-carbonate matrix with locally abundant spinel and sphene. Garnet has not been

  19. Abundance and distribution of mineral components associated with Moses Rock (kimberlite) diatreme

    NASA Technical Reports Server (NTRS)

    Mustard, J. F.; Pieters, C. M.

    1986-01-01

    The surface mineralogy in and around Moses Rock diatreme, a kimberlite-bearing dike in SW Utah, was examined using internally calibrated Airborne Imaging Spectrometer (AIS) data. Distinct near-infrared absorption characteristics of clays, gypsum, and serpentine (a key marker for kinberlite concentration) allowed the surface units containing these components to be identified spatially and the relative abundance of each component measured. Within the dike itself, channels and dispersed components of kimberlite and blocks of country rocks were accurately determined.

  20. Diamonds in an upper mantle peridotite nodule from kimberlite in southern wyoming

    USGS Publications Warehouse

    McCallum, M.E.; Eggler, D.H.

    1976-01-01

    Diamonds in a serpentinized garnet peridotite nodule from a diatreme in southern Wyoming are the first known occurrence in an upper mantle peridotite xenolith from a kimberlite intrusion in North America as well as the second authenticated occurrence of diamonds from kimberlite pipes in North America. The nodule is believed to have come from a section of depleted (partially melted) Iherzolite at a depth of 130 to 180 kilometers.

  1. Diamonds in an upper mantle peridotite nodule from kimberlite in southern wyoming.

    PubMed

    McCallum, M E; Eggler, D H

    1976-04-16

    Diamonds in a serpentinized garnet peridotite nodule from a diatreme in southern Wyoming are the first known occurrence in an upper mantle peridotite xenolith from a kimberlite intrusion in North America as well as the second authenticated occurrence of diamonds from kimberlite pipes in North America. The nodule is believed to have come from a section of depleted (partially melted) lherzolite at a depth of 130 to 180 kilometers.

  2. The geology and emplacement history of the Pigeon kimberlite, EKATI Diamond Mine, Northwest Territories, Canada

    NASA Astrophysics Data System (ADS)

    Crawford, Barbara; Hetman, Casey; Nowicki, Tom; Baumgartner, Mike; Harrison, Sara

    2009-11-01

    The Pigeon kimberlite is located approximately 6 km to the northwest of the Koala cluster of the EKATI Diamond Mine, and is presently one of ten kimberlite occurrences in the EKATI resource development plan. It was emplaced along a regional lithological contact between syn-Yellowknife Supergroup granitoid rocks and Yellowknife Supergroup metasedimentary rocks that were covered by a now eroded veneer of poorly consolidated muddy sediments. Detailed age dating has not been undertaken, however the emplacement age is inferred from sedimentary xenoliths present within the pipe to range between 45-75 Ma. Pigeon is a small kimberlite body, estimated to be approximately 3.5 ha at surface, consisting of a steep-sided pipe that can be separated into four main geological domains that are characterized by contrasting textures, different diamond characteristics and unique mineral abundance and compositional signatures. The uppermost portion of the body consists of mud-rich resedimented volcaniclastic kimberlite that was formed by the deposition of extra crater deposits by debris flow type processes into an open diatreme. Texturally complex kimberlite is present within the lower portion of the kimberlite and includes rocks that display a range of features consistent with coherent (magmatic) and less common volcaniclastic (fragmental) rocks. This texturally complex zone is interpreted to represent a clastogenic deposit formed by a low energy eruption within an open diatreme.

  3. The first allanite-bearing eclogite xenolith in kimberlite

    NASA Astrophysics Data System (ADS)

    Trojman-Nichols, S.; Heaman, L.

    2015-12-01

    Here we report the first allanite-bearing mantle eclogite xenolith, entrained in the 173 Ma Jericho kimberlite pipe, located in the Slave craton, northwestern Canada. This eclogite is unique among the other Jericho eclogites by an extreme LREE enrichment in all phases, and garnet alteration rims that are more calcic than the garnet cores. Allanite is an abundant accessory phase, present as dull orange, subhedral crystals. Other minerals in the paragenesis are garnet, clinopyroxene, apatite and sulfides; two compositionally and texturally distinct generations of phlogopite constitute a secondary paragenesis where allanite is no longer stable. Allanite in this sample is La-, Ce- and Th- rich, with concentrations at the weight % level, while Y is only present at the relatively low concentration of ~100 ppm. Electron backscatter imaging reveals complex zonation within the allanite crystals that is off-centre, non-symmetric, and patchy. It is often asserted that eclogite xenoliths represent subducted oceanic lithosphere, despite significant differences in the composition and mineralogy between mantle-derived eclogite xenoliths and eclogite massif material. Both types of eclogite occurrences can contain quartz/coesite; massif eclogites often have small, sparse allanite inclusions, but allanite has never been reported in eclogite xenoliths in kimberlite. Allanite in massif eclogite is thought to form during subduction by the break-down of lawsonite and the incorporation of LREE into zoisite. Lawsonite breaks down into grossular and H20 at high pressures, which may explain the anomalous high-Ca rims measured in some garnets in this sample. This allanite-bearing eclogite may provide an unprecedented window for exploring a crucial stage of eclogite metamorphism and fluid mobilization in subduction zones. In addition, the U-Pb systematics currently under investigation may constrain the age of eclogitization.

  4. Kimberlitic sources of super-deep diamonds in the Juina area, Mato Grosso State, Brazil

    NASA Astrophysics Data System (ADS)

    Kaminsky, Felix V.; Sablukov, Sergei M.; Belousova, Elena A.; Andreazza, Paulo; Tremblay, Mousseau; Griffin, William L.

    2010-01-01

    The Juina diamond field, in the 1970-80s, was producing up to 5-6 million carats per year from rich placer deposits, but no economic primary deposits had been found in the area. In 2006-2007, Diagem Inc. discovered a group of diamondiferous kimberlitic pipes within the Chapadão Plateau (Chapadão, or Pandrea cluster), at the head of a drainage system which has produced most of the alluvial diamonds mined in the Juina area. Diamonds from placer deposits and newly discovered kimberlites are identical; they have super-deep origins from the upper-mantle and transition zone. Field observations and petrographic studies have identified crater-facies kimberlitic material at seven separate localities. Kimberlitic material is represented by tuffs, tuffisites and various epiclastic sediments containing chrome spinel, picroilmenite, manganoan ilmenite, zircon and diamond. The diamond grade varies from 0.2-1.8 ct/m 3. Chrome spinel has 30-61 wt.% Cr 2O 3. Picroilmenite contains 6-14 wt.% MgO and 0.2-4 wt.% Cr 2O 3. Manganoan ilmenite has less than 3 wt.% MgO and 0.38-1.41 wt.% MnO. The 176Hf/ 177Hf ratio in kimberlitic zircons is 0.028288-0.28295 with ɛHf = 5.9-8.3, and lies on the average kimberlite trend between depleted mantle and CHUR. The previously known barren and weakly diamondiferous kimberlites in the Juina area have ages of 79-80 Ma. In contrast, zircons from the newly discovered Chapadão kimberlites have a mean 206Pb/ 238U age of 93.6 ± 0.4 Ma, corresponding to a time of magmatic activity related to the opening of the southern part of the Atlantic Ocean. The most likely mechanism of the origin of kimberlitic magma is super-deep subduction process that initiated partial melting of zones in lower mantle with subsequent ascent of proto-kimberlitic magma.

  5. Argon isotopic studies of minerals in kimberlites, mantle xenoliths and diamonds, from selected southern African localities

    SciTech Connect

    Phillips, D.

    1989-01-01

    The occurrence, composition, behavior and origin of excess argon components, in mantle phases hosted by southern African kimberlites, is evaluated using furnace step-heating and laser-probe analytical techniques. Laser-probe {sup 40}Ar/{sup 39}Ar analyses of phlogopite from the swartruggens kimberlite dyke (145 Ma), and Premier diatreme ({approximately}1200 Ma) lherzolite xenoliths, yielded apparent ages decreasing from high ages at grain centres to values approaching the age of kimberlite intrusion, along grain margins. The old apparent ages are attributed excess radiogenic argon, with high {sup 40}Ar/{sup 39}Ar ratios (> 15,000), incorporated prior to kimberlite intrusion under conditions of locally high argon partial pressure. The preservation of the excess argon components is dependent on the timing of melt devolatilization, temperature, cooling rate and the characteristic radius for argon diffusion. Swartruggens phlogopite grains also display chlorine zonations, measured by a neutron activation technique and the laser probe. Fluorine contents, determined by electron microprobe were uniform. Halogen analyses of Premier xenolith phlogopite revealed minor variations. {sup 40}Ar/{sup 39}Ar laser-probe analyses of eclogitic suite clinopyroxene inclusions in diamonds from the Premier kimberlite yielded an age of 1198 {plus minus} 6 Ma, indistinguishable from the inferred time of intrusion of the host kimberlite ({approximately}1200 Ma). This implies diamond formation synchronous with, or no more than {approximately}20 Ma before kimberlite generation. The associated initial {sup 40}Ar/{sup 39}Ar ratio of 334 {plus minus} 50 is similar to the present day atmospheric composition. It is suggested that late-stage equilibration with {sup 36}Ar-rich fluids, derived either from primordial mantle, or from subducted atmospheric argon, is the most likely explanation for this low {sup 40}Ar/{sup 39}Ar value.

  6. Ultrafresh salty kimberlite of the Udachnaya-East pipe (Yakutia, Russia): A petrological oddity or fortuitous discovery?

    NASA Astrophysics Data System (ADS)

    Kamenetsky, Vadim S.; Kamenetsky, Maya B.; Golovin, Alexander V.; Sharygin, Victor V.; Maas, Roland

    2012-11-01

    An ultrabasic/ultramafic composition of kimberlite magmas is difficult to reconcile with existing models of the kimberlite mantle source and melting conditions, inferred magma temperatures and rheological properties, and the style of magma ascent and emplacement. The inconsistencies in current thinking indicate serious flaws in understanding kimberlite magma compositions. Much of the uncertainty over true kimberlite compositions may stem from almost ubiquitous hydration and leaching of kimberlite rocks. This study presents petrographic and geochemical data for kimberlite samples largely unaffected by postmagmatic modification, from the Devonian Udachnaya-East pipe in Siberia. These samples are unusually enriched in chlorine and sodium, yet they are essentially anhydrous. These features are consistent with the phase composition of the groundmass which is dominated by minerals such as Na-Ca carbonates, Na-K chlorides and sulphates which appear to be - in our samples - co-magmatic with common silicates and oxides, but are unknown in other kimberlites, or rarely found within magmatic assemblages. We suggest that a kimberlite parent melt of essentially non-silicate composition, with high concentrations of alkalis, CO2 and Cl may be a viable alternative to the currently favoured water-rich, high-Mg model primary melt. Entrainment of mantle silicates into such a melt en route to the surface, followed by gravitational accumulation of mantle olivine and liquidus oxides (perovskite, Cr-spinel) at the bottom of vertically extensive magma bodies after emplacement, would explain the observed properties of kimberlite magma/rock, notably enrichment in olivine and trace elements in the hypabyssal kimberlite facies. A carbonate melt composition would retain attributes of the standard model such as trace element enrichment via low degrees of partial melting, it would explain low temperatures of crystallisation and the exceptional rheological properties that enable kimberlite primary

  7. The environmental distribution of late proterozoic organisms

    NASA Technical Reports Server (NTRS)

    Knoll, A. H.

    1991-01-01

    Along present day coast lines, the environmental distributions of prokaryotic and protistan populations are often sharply delimited. Realized habitat ranges are generally narrower than those circumscribed by physiological tolerances, suggesting the importance of organism-organism interactions in the determination of population distributions. Microfossil populations preserved in silicified carbonates, dolomites, and shales of the 700-800 Ma old Akademikerbreen Group, Svalbard, and elsewhere indicate that the environmental distributions were defined equally clearly during the Proterozoic Eon. The Draken Conglomerate Formation is a tidal flat/lagoonal complex in which we have distinguished five principle biofacies containing a total of 42 taxa. Supratidal to subtidal gradients include the increasing abundance and diversity of both mat dweller microbenthos and allochthonous (principally planktonic) organisms, as well as a taphonomically important pattern of decreasing sheath thickness among mat builder microorganisms. The seaward barriers of Akademikerbreen lagoons were oolitic shoals, and these contain about a dozen endolithic and epilithic species not observed elsewhere in the group. Subtidal environments below fair weather wave base are represented by mudstones of the Svanbergfjellet Formation. These contain abundant and diverse cyanobacteria-like fossils generally similar to but specifically different from those found in tidal flat sediments, as well as diverse unicellular protists (some of impressive morphological complexity) and at least half a dozen cellularly preserved metaphyte populations. In all, more than 80 species are distributed among Akademikerbreen lithologies. Fossil assemblages from Svalbard and elsewhere illustrate the potential for a much finer paleoecological, biostratigraphic, and, hence, evolutionary resolution of the early fossil record.

  8. Crustal Signatures in Mantle Peridotites From Yakutian Kimberlites

    NASA Astrophysics Data System (ADS)

    Taylor, L. A.; Spetsius, Z.; Wiesli, R.; Anand, M.; Valley, J.

    2002-12-01

    Peridotites and eclogites are considered as the original hosts for diamonds in the mantle. However, it is now generally agreed that these "mantle" eclogites from kimberlites had their origin in the subduction of oceanic crust beneath the major cratons of the world. One of the first indications for such crustal protoliths was from studies of oxygen and carbon isotopes (e.g., Peter Deines and colleagues, Ian McGregor, as well as our group). Indeed, subsequent studies of such rocks have revealed several additional crustal signatures. A possible scenario involves the subduction of an ophiolite sequence, whereby the basaltic and lower mafic components were metamorphosed, devolatilized/partially melted, and otherwise transformed into eclogites. Being within the diamond-stability field, they later experience metasomatic diamond formation. Surprisingly, the closely associated diamondiferous peridotites are considered to be of original mantle origin. We pose the query: What became of the ultramafic portion at the bottom of the crustal sequence? Could this be the origin of at least some of the mantle peridotites? The restricted δ13C values for P-type (peridotitic) diamonds is commonly used as evidence for the mantle origin of peridotites. However, a compilation of δ13C data, published by Peter Deines and our group, for P-type diamonds, mainly from numerous south African pipes, also shows a significant number of values that are well below the mantle field (to -20 \\permil). Fresh, clean garnets were carefully selected from over a hundred peridotites collected from several Yakutian kimberlites. These were subjected to oxygen-isotope analyses by laser-fluorination at the University of Wisconsin. The majority of the δ18O values plot within the accepted mantle value of 5.5+/-0.4 \\permil (Mattey et al., 1994). However, a significant number (~20%) lies outside this window, both above and below. These values are interpreted to represent the effects of both high- and low

  9. Temperature and pressure dependences of kimberlite melts viscosity (experimental-theoretical study)

    NASA Astrophysics Data System (ADS)

    Persikov, Eduard; Bykhtiyarov, Pavel; Cokol, Alexsander

    2016-04-01

    Experimental data on temperature and pressure dependences of viscosity of model kimberlite melts (silicate 82 + carbonate 18, wt. %, 100NBO/T = 313) have been obtained for the first time at 100 MPa of CO2 pressure and at the lithostatic pressures up to 7.5 GPa in the temperature range 1350 oC - 1950 oC using radiation high gas pressure apparatus and press free split-sphere multi - anvil apparatus (BARS). Experimental data obtained on temperature and pressure dependences of viscosity of model kimberlite melts at moderate and high pressures is compared with predicted data on these dependences of viscosity of basaltic melts (100NBO/T = 58) in the same T, P - range. Dependences of the viscosity of model kimberlite and basaltic melts on temperature are consistent to the exponential Arrenian equation in the T, P - range of experimental study. The correct values of activation energies of viscous flow of kimberlite melts have been obtained for the first time. The activation energies of viscous flow of model kimberlite melts exponentially increase with increasing pressure and are equal: E = 130 ± 1.3 kJ/mole at moderate pressure (P = 100 MPa) and E = 160 ± 1.6 kJ/mole at high pressure (P = 5.5 GPa). It has been established too that the viscosity of model kimberlite melts exponentially increases on about half order of magnitude with increasing pressures from 100 MPa to 7.5 GPa at the isothermal condition (1800 oC). It has been established that viscosity of model kimberlite melts at the moderate pressure (100 MPa) is lover on about one order of magnitude to compare with the viscosity of basaltic melts, but at high pressure range (5.5 - 7.5 GPa), on the contrary, is higher on about half order of magnitude at the same values of the temperatures. Here we use both a new experimental data on viscosity of kimberlite melts and our structural chemical model for calculation and prediction the viscosity of magmatic melts [1] to determine the fundamental features of viscosity of

  10. Identification of kimberlite bodies in Brazil from a 3D audio-magnetotelluric survey

    NASA Astrophysics Data System (ADS)

    De Lugao, P. P.; Eric, C. D. O.; Loureiro, F. O.; Arantes, P. R.; Pastana, A. F.

    2015-12-01

    We report on a succesfull identification of kimberlite bodies in Brazil through the use of the electromagnetic technique audio-magnetotelluric (AMT). Macnae (1979) writes that "In one large survey in South Africa, electromagnetic (EM) techniques have proven to be remarkably effective in detecting the presence of weathered clays or epiclastic kimberlite contained within the pipes." Full tensor AMT data were acquired at 65 points (stations) in a 3D configuration with frequencies ranging from 10kHz to 1Hz. The survey was located in the NW portion of the Mato Grosso state, Brazil, in na area of thick jungle coverage. During the AMT survey, few outcrops were seen because of the dense forest cover. Usually, the occurrences found were of sand deposits, indicating the occurence of Fazenda Casa Branca and Utiariti Formations and gravel from Salto das Nuvens Formation, widely used in paving trails n this region. In the area of the survey, three main targets were confirmed/identified: Kimberlite Area 1 - a classic kimberlite in the region, with the crater facies with different clasts and distinct size. We noted the occurrence of a red-brown soil and an unusual vegetation in this area. The resistivity model provided confirmed the presence of Kimberlite Area 1 and was used to identify other two areas. Area of Interest 1 - area with atypical vegetation along a trail. There is an excavation that displays soil of white color with several blocks present, there are small quartz crystal agglomerates in these blocks. The resistivity model cleary shows a conductive body here, indicative of the presence of a kimberlite. Area of Interest 2 - the presence of a kimberlite was confirmed, not exactly where the targeted Area 2 was, but the southwest of it. Close to this area, there was a very fine rock and a few blocks of pure silica, probably indicating a kimberlitic intrusion. In summary, the 3D resistivity model in depth obtained from inversion of the AMT data confirmed and identified

  11. The evolution of calcite-bearing kimberlites by melt-rock reaction: evidence from polymineralic inclusions within clinopyroxene and garnet megacrysts from Lac de Gras kimberlites, Canada

    NASA Astrophysics Data System (ADS)

    Bussweiler, Y.; Stone, R. S.; Pearson, D. G.; Luth, R. W.; Stachel, T.; Kjarsgaard, B. A.; Menzies, A.

    2016-07-01

    Megacrystic (>1 cm) clinopyroxene (Cr-diopside) and garnet (Cr-pyrope) xenocrysts within kimberlites from Lac de Gras (Northwest Territories, Canada) contain fully crystallized melt inclusions. These `polymineralic inclusions' have previously been interpreted to form by necking down of melts at mantle depths. We present a detailed petrographical and geochemical investigation of polymineralic inclusions and their host crystals to better understand how they form and what they reveal about the evolution of kimberlite melt. Genetically, the megacrysts are mantle xenocrysts with peridotitic chemical signatures indicating an origin within the lithospheric mantle (for the Cr-diopsides studied here ~4.6 GPa, 1015 °C). Textural evidence for disequilibrium between the host crystals and their polymineralic inclusions (spongy rims in Cr-diopside, kelyphite in Cr-pyrope) is consistent with measured Sr isotopic disequilibrium. The preservation of disequilibrium establishes a temporal link to kimberlite eruption. In Cr-diopsides, polymineralic inclusions contain phlogopite, olivine, chromite, serpentine, and calcite. Abundant fluid inclusion trails surround the inclusions. In Cr-pyropes, the inclusions additionally contain Al-spinel, clinopyroxene, and dolomite. The major and trace element compositions of the inclusion phases are generally consistent with the early stages of kimberlite differentiation trends. Extensive chemical exchange between the host phases and the inclusions is indicated by enrichment of the inclusions in major components of the host crystals, such as Cr2O3 and Al2O3. This chemical evidence, along with phase equilibria constraints, supports the proposal that the inclusions within Cr-diopside record the decarbonation reaction: dolomitic melt + diopside → forsterite + calcite + CO2, yielding the observed inclusion mineralogy and producing associated (CO2-rich) fluid inclusions. Our study of polymineralic inclusions in megacrysts provides clear mineralogical

  12. Pristine Early Eocene wood buried deeply in kimberlite from northern Canada.

    PubMed

    Wolfe, Alexander P; Csank, Adam Z; Reyes, Alberto V; McKellar, Ryan C; Tappert, Ralf; Muehlenbachs, Karlis

    2012-01-01

    We report exceptional preservation of fossil wood buried deeply in a kimberlite pipe that intruded northwestern Canada's Slave Province 53.3±0.6 million years ago (Ma), revealed during excavation of diamond source rock. The wood originated from forest surrounding the eruption zone and collapsed into the diatreme before resettling in volcaniclastic kimberlite to depths >300 m, where it was mummified in a sterile environment. Anatomy of the unpermineralized wood permits conclusive identification to the genus Metasequoia (Cupressaceae). The wood yields genuine cellulose and occluded amber, both of which have been characterized spectroscopically and isotopically. From cellulose δ(18)O and δ(2)H measurements, we infer that Early Eocene paleoclimates in the western Canadian subarctic were 12-17°C warmer and four times wetter than present. Canadian kimberlites offer Lagerstätte-quality preservation of wood from a region with limited alternate sources of paleobotanical information.

  13. Pristine Early Eocene Wood Buried Deeply in Kimberlite from Northern Canada

    PubMed Central

    Wolfe, Alexander P.; Csank, Adam Z.; Reyes, Alberto V.; McKellar, Ryan C.; Tappert, Ralf; Muehlenbachs, Karlis

    2012-01-01

    We report exceptional preservation of fossil wood buried deeply in a kimberlite pipe that intruded northwestern Canada’s Slave Province 53.3±0.6 million years ago (Ma), revealed during excavation of diamond source rock. The wood originated from forest surrounding the eruption zone and collapsed into the diatreme before resettling in volcaniclastic kimberlite to depths >300 m, where it was mummified in a sterile environment. Anatomy of the unpermineralized wood permits conclusive identification to the genus Metasequoia (Cupressaceae). The wood yields genuine cellulose and occluded amber, both of which have been characterized spectroscopically and isotopically. From cellulose δ18O and δ2H measurements, we infer that Early Eocene paleoclimates in the western Canadian subarctic were 12–17°C warmer and four times wetter than present. Canadian kimberlites offer Lagerstätte-quality preservation of wood from a region with limited alternate sources of paleobotanical information. PMID:23029080

  14. Pristine Early Eocene wood buried deeply in kimberlite from northern Canada.

    PubMed

    Wolfe, Alexander P; Csank, Adam Z; Reyes, Alberto V; McKellar, Ryan C; Tappert, Ralf; Muehlenbachs, Karlis

    2012-01-01

    We report exceptional preservation of fossil wood buried deeply in a kimberlite pipe that intruded northwestern Canada's Slave Province 53.3±0.6 million years ago (Ma), revealed during excavation of diamond source rock. The wood originated from forest surrounding the eruption zone and collapsed into the diatreme before resettling in volcaniclastic kimberlite to depths >300 m, where it was mummified in a sterile environment. Anatomy of the unpermineralized wood permits conclusive identification to the genus Metasequoia (Cupressaceae). The wood yields genuine cellulose and occluded amber, both of which have been characterized spectroscopically and isotopically. From cellulose δ(18)O and δ(2)H measurements, we infer that Early Eocene paleoclimates in the western Canadian subarctic were 12-17°C warmer and four times wetter than present. Canadian kimberlites offer Lagerstätte-quality preservation of wood from a region with limited alternate sources of paleobotanical information. PMID:23029080

  15. Carbonate-silicate liquid immiscibility in the mantle propels kimberlite magma ascent

    NASA Astrophysics Data System (ADS)

    Kamenetsky, Vadim S.; Yaxley, Gregory M.

    2015-06-01

    Kimberlite is a rare volcanic rock renowned as the major host of diamonds and originated at the base of the subcontinental lithospheric mantle. Although kimberlite magmas are dense in crystals and deeply-derived rock fragments, they ascend to the surface extremely rapidly, enabling diamonds to survive. The unique physical properties of kimberlite magmas depend on the specific compositions of their parental melts that, in absence of historical eruptions and due to pervasive alteration of kimberlite rocks, remain highly debatable. We explain exceptionally rapid ascent of kimberlite magma from mantle depths by combining empirical data on the essentially carbonatite composition of the kimberlite primary melts and experimental evidence on interaction of the carbonate liquids with mantle minerals. Our experimental study shows that orthopyroxene is completely dissolved in a Na2CO3 melt at 2.0-5.0 GPa and 1000-1200 °C. The dissolution of orthopyroxene results in homogeneous silicate-carbonate melt at 5.0 GPa and 1200 °C, and is followed by unmixing of carbonate and carbonated silicate melts and formation of stable magmatic emulsion at lower pressures and temperatures. The dispersed silicate melt has a significant capacity for storing a carbonate component in the deep mantle (13 wt% CO2 at 2.0 GPa). We envisage that this component reaches saturation and is gradually released as CO2 bubbles, as the silicate melt globules are transported upwards through the lithosphere by the carbonatite magma. The globules of unmixed, CO2-rich silicate melt are continuously produced upon further reaction between the natrocarbonatite melt and mantle peridotite. On decompression the dispersed silicate melt phase ensures a continuous supply of CO2 bubbles that decrease density and increase buoyancy and promote rapid ascent of the magmatic emulsion.

  16. Composition of primary kimberlite magma: constraints from melting and diamond dissolution experiments

    NASA Astrophysics Data System (ADS)

    Sokol, A. G.; Khokhryakov, A. F.; Palyanov, Yu. N.

    2015-09-01

    Experiments are applied to constrain the composition of primary kimberlitic magmas which were in equilibrium with lithospheric peridotite and could resorb the entrained diamond to form typical dissolution features. The experiments are run on samples of a model carbonatite and a melt of the Udachnaya kimberlite at 6.3 GPa and 1400 °C, and at unbuffered or Re-ReO2-buffered oxygen fugacity (1-2 log units above Ni-NiO). Near-liquidus dry Fe3+-free carbonatitic melt (derived from carbonated harzburgite) is saturated with the Ol-Grt-Opx-Mgs assemblage and is almost inert to diamond. Carbonatitic melts that bear 4.6-6.8 wt% Fe2O3 or 1.5 wt% H2O are in equilibrium only with Mgs ± Ol near the liquidus. Dissolution of diamond by these melts produces surface textures uncommon (corrosion sculptures) or common (negative-oriented trigons, shield-shaped laminae and elongate hillocks) to kimberlitic diamonds. The near-liquidus melt of the Udachnaya kimberlite (Yakutia) with 10-12 wt% H2O is saturated with the Ol-Grt-Cpx assemblage and may result from melting of carbonated garnet-bearing wehrlite. Hydrous kimberlitic melt likewise resorbs diamonds forming typical negative-oriented trigons, shield-shaped laminae and elongate hillocks on their surfaces. Therefore, the melts that could originate in the thermal conditions of subcratonic lithosphere, entrain diamond and dissolve it to produce dissolution features on crystal surfaces, were compositionally close to kimberlite (16-19 wt% SiO2) and rich in H2O. Dry Fe3+-bearing carbonatites with fO2 controlled by the ferric/ferrous equilibrium slightly above the Ni-NiO buffer cannot be diamond carriers.

  17. Volcanology of the Aries micaceous kimberlite, central Kimberley Basin, Western Australia

    NASA Astrophysics Data System (ADS)

    Downes, Peter J.; Ferguson, Dale; Griffin, Brendan J.

    2007-01-01

    The Neoproterozoic (815.4 ± 4.3 Ma) Aries kimberlite intrudes the King Leopold Sandstone and the Carson Volcanics in the central Kimberley Basin, northern Western Australia. Aries is comprised of a N-NNE-trending series of three diatremes and associated hypabyssal kimberlite dykes and plugs. The diatremes are volumetrically dominated by massive, clast-supported, accidental lithic-rich kimberlite breccias that were intruded by hypabyssal macrocrystic phlogopite kimberlite dykes and plugs with variably uniform- to globular segregationary-textured groundmasses. Lower-diatreme facies, accidental lithic-rich breccias probably formed through fall-back of debris into the vent with a major contribution from the collapse of the vent walls. These massive breccias are overlain by a sequence of bedded volcaniclastic breccias in the upper part of the north lobe diatreme. Abundant, poorly vesicular to nonvesicular, juvenile kimberlite ash and lapilli, with morphologies that are indicative of phreatomagmatic fragmentation processes, occur in a reversely graded volcaniclastic kimberlite breccia unit at the base of this sequence. This unit and overlying bedded accidental lithic-rich breccias are interpreted to be sediment gravity-flow deposits (including possible debris flows) derived from the collapse of the crater walls and/or tephra ring deposits that surrounded the crater. Diatreme-forming eruptions may have been initiated by magma-water interactions along fracture and joint-controlled aquifers within the King Leopold Sandstone. The current level of exposure of the diatremes probably extends from the lower-diatreme facies up into the base of a bedded upper-diatreme sequence.

  18. New data on kimberlite and lamproite magmatism in diamondiferous areas in the Western Urals

    NASA Astrophysics Data System (ADS)

    Goloburdina, Marina

    2014-05-01

    Rare potassic alkaline-ultrabasic rocks were first studied in the western slope of the Middle Urals (Perm Region) in the multiphase Blagodatsky Massif. They are represented by olivine-sanidine lamproite and kimberlite. Based on materials of bore-hole sections of up to ~ 500 m deep and trenches (~ 3.5 m x 2 km), it was identified that thin alkaline-ultrabasic rock bodies consist of pipe-like, vein and dyke intrusions. Alkaline-ultrabasic rocks are associated in the massive with PZ2 essexite-dolerite, trachydolerite and V2 trachybasalt. Rock contacts between one another and with terrigenous rocks (V2) and sandstone (D1) are tectonic or intrusive. Kimberlite is of specific mineral composition. It is characterized by the presence not only of three generations of olivine, but also altered melilite, sanidine, leucite that suggests that these rocks are transitional between kimberlite and lamproite. According to the classification of R.H. Mitchell (1995), similar rocks are an extreme member of Group II kimberlites. Such kimberlites are known in the Kroonstad area, South Africa (Besterskraal North, Voorspoed Mine) (G.H. Howarth, E. Michael et al., 2011). Chemical composition of the rocks varies widely due to superimposed transformations expressed in chloritization, silicification, carbonatization, micatization, hematitization, leucoxenization, albitization. Distribution of rare elements and rare earth elements in alkaline-ultrabasic rocks are similar to those in kimberlites of the Timan and the Arkhangelsk diamondiferous province. Accessory minerals are rare pyrope of lherzolite paragenesis, diopside, chrome-spinelide, picroilmenite and large zircons similar to those of kimberlite. Single diamonds of dodecahedroid shape have been found in bulk samples. They are typical of alluvial diamond occurrences and deposits of the Urals. Isotopic dating of zircons (SRIMP-II) yields the age of the alkaline-ultrabasic rocks corresponding to the Middle Paleozoic (D2-C1) and shows that

  19. Rapid kimberlite ascent and the significance of Ar-Ar ages in xenolith phlogopites

    PubMed

    Kelley; Wartho

    2000-07-28

    Kimberlite eruptions bring exotic rock fragments and minerals, including diamonds, from deep within the mantle up to the surface. Such fragments are rapidly absorbed into the kimberlite magma so their appearance at the surface implies rapid transport from depth. High spatial resolution Ar-Ar age data on phlogopite grains in xenoliths from Malaita in the Solomon Islands, southwest Pacific, and Elovy Island in the Kola Peninsula, Russia, indicate transport times of hours to days depending upon the magma temperature. In addition, the data show that the phlogopite grains preserve Ar-Ar ages recorded at high temperature in the mantle, 700 degrees C above the conventional closure temperature.

  20. Volatiles in Kimberlitic Magmas: Forced Multiple Saturation with a Mantle Source

    NASA Astrophysics Data System (ADS)

    Stamm, N.; Schmidt, M. W.

    2015-12-01

    The geochemistry and mineralogy of the mantle source for primary kimberlite melts is still very much debated, the primary melt is argued to be either of carbonatitic or kimberlitic nature and proposed melting mechanisms range from low-degree partial melting of a carbonated peridotite to high-degree melting of strongly metasomatized veins. Experimental multiple saturation of a proposed close-to-primary kimberlitic composition from Jericho (Kopylova et al. 2007, GCA) at 7 GPa shows that saturation of a lherzolitic mineral assemblage occurs at 1300-1350 °C resulting in a carbonatitic melt with less than 8 wt% SiO2 and >35 wt% CO2. At higher temperatures, where the Jericho melt stays kimberlitic, it is only saturated in opx and garnet. We hence forced the close-to-primary Jericho kimberlite into multiple saturation with a lherzolitic assemblage (7 GPa, 1400-1650 °C) by adding a volatile-free peridotite with the aim to saturate the system in olivine, opx, cpx and garnet. This mineral assemblage is obtained over a wide temperature range (1400-1600 °C) for a starting Jericho composition with 20-22.5 wt% CO2, H2O was kept at 0.46 wt% corresponding to the K:H ratio of phlogopite. The transition from a carbonatitic melt with ~10 wt% SiO2 and >35 wt% CO2 to a kimberlitic melt with ~27 wt% SiO2 and <25 wt% CO2 occurs from 1450 to 1600 °C. Compared to the Jericho composition, these melts have higher Na2O and lower XMg. At lower CO2 contents (10 wt%) opx was absent, while at higher CO2 (30 wt%) olivine and cpx were not stable. Kimberlitic melts in equilibrium with a lherzolite are obtained for temperatures of >1500 °C, requiring a few hundred degrees more than estimated for the base of the cratonic lithosphere (1200-1400 °C at a heat flux of 40-45 mW/m2). If lower temperature carbonatites intrude into the base of the lithosphere it is questionable how these should develop into kimberlites within the lithosphere.

  1. Rapid kimberlite ascent and the significance of Ar-Ar ages in xenolith phlogopites

    PubMed

    Kelley; Wartho

    2000-07-28

    Kimberlite eruptions bring exotic rock fragments and minerals, including diamonds, from deep within the mantle up to the surface. Such fragments are rapidly absorbed into the kimberlite magma so their appearance at the surface implies rapid transport from depth. High spatial resolution Ar-Ar age data on phlogopite grains in xenoliths from Malaita in the Solomon Islands, southwest Pacific, and Elovy Island in the Kola Peninsula, Russia, indicate transport times of hours to days depending upon the magma temperature. In addition, the data show that the phlogopite grains preserve Ar-Ar ages recorded at high temperature in the mantle, 700 degrees C above the conventional closure temperature. PMID:10915621

  2. Metasomatic enrichment of Proterozoic mantle south of the Kaapvaal Craton, South Africa: origin of sinusoidal REE patterns in clinopyroxene and garnet

    NASA Astrophysics Data System (ADS)

    le Roex, Anton; Class, Cornelia

    2016-02-01

    indicates regional metasomatism by melts of various compositions. The strong HREEN depletion is interpreted to reflect the effect of initial melt depletion in the early Proterozoic, with melting extending into the spinel stability field requiring an oceanic realm, and again later in the Mesoproterozoic (Namaqua Orogeny). The superimposed incompatible element enrichment indicates subsequent multiple enrichment events by rising alkaline melts similar in composition to kimberlite or ultramafic alkaline lamprophyre, possibly related to Mesozoic plume upwelling beneath the region, that reintroduced clinopyroxene into the depleted Proterozoic harzburgite protolith.

  3. Modeling the Consequences of Proterozoic Oxygenation

    NASA Astrophysics Data System (ADS)

    Bachan, A.; Kump, L. R.

    2013-12-01

    state iron removal is dominated by pyrite precipitation allowing for elevated phosphate regeneration. Furthermore, in the model, the weathering input of cations and phosphate accompanied by sulfate moderates the reduction in pCO2 that otherwise results from elevated organic carbon burial. Thus, a positive feedback arises where O2 production from organic carbon burial triggers additional sulfide oxidation which leads to further organic carbon burial. This process continues until the sulfide reservoir is depleted, after which δ13C values return to near zero and the model settles into a new steady state with a higher pO2. In total, our modeling supports the idea that the late Proterozoic carbon isotope anomaly can be understood as the manifestation of oxidizing power, triggered externally, and amplified internally, propagating through the earth system.

  4. Composition of primary fluid and melt inclusions in regenerated olivines from hypabyssal kimberlites of the Malokuonapskaya pipe (Yakutia)

    NASA Astrophysics Data System (ADS)

    Tomilenko, A. A.; Kuzmin, D. V.; Bulbak, T. A.; Timina, T. Yu.; Sobolev, N. V.

    2015-11-01

    The primary fluid and melt inclusions in regenerated zonal crystals of olivine from kimberlites of the Malokuonapskaya pipe were first examined by means of microthermometry, optic and scanning electron microscopy, and Raman spectroscopy. The high-pressure genesis of homogenous central parts of the olivines was revealed, probably under intense metasomatism at early hypogene stages with subsequent regeneration in the kimberlitic melt. The olivine crystals were regenerated from silicate-carbonate melts at about 1100°C. The composition of the kimberlitic melt was changed by way of an increase in the calcium content.

  5. Diamond resorption features as a new method for examining conditions of kimberlite emplacement

    NASA Astrophysics Data System (ADS)

    Fedortchouk, Yana

    2015-10-01

    The study develops a new approach utilizing parameters of trigonal etch pits on diamond crystals to infer the conditions of diamond residence in kimberlite magma. Diamond crystals from dissolution experiments conducted at 1 GPa and 1150-1350 °C in the presence of H2O-rich or CO2-rich fluid were studied with atomic force microscopy (AFM). The AFM data of resorbed diamond surfaces show that much deeper surface relief was produced in CO2 fluid. It also clearly distinguishes the profiles of the trigonal etch pits forming regular flat-bottomed trigons in H2O fluid, and round- or pointed-bottomed trigons in CO2 fluid. The relationship between the diameter and the depth of the trigonal pits is found to be another important indicator of the fluid composition. Dissolution in H2O fluid develops trigons with constant diameter and variable depth where the diameter increases with temperature. Trigons developed in CO2 fluid have a large range of diameters showing a strong positive correlation with the depth. The developed criteria applied to the natural diamond crystals from three Ekati Mine kimberlites indicate significant variation in CO2-H2O ratio and temperature of their magmatic fluid. This conclusion based on diamond resorption agrees with the mineralogy of microphenocrysts and groundmass of the studied kimberlites offering new method to study crystallization conditions of kimberlite magma.

  6. Upper proterozoic geology and hydrocarbon prospects, Metropolitan Moscow District

    SciTech Connect

    Kuz`menko, Yu.T.; Kuklinskii, A.Ya.; Pimenov, Yu.G.

    1994-09-01

    New data on the geological makeup of the Teplostansk Graben of the Moscow Aulacogen deals with lithological-geochemical rock characteristics in deep drillholes in the Moscow area and about the enclosed bituminoids. Hydrocarbon prospects of the Upper Proterozoic beds in the graben have been evaluated.

  7. Macrocrystal phlogopite Rb-Sr dates for the Ekati property kimberlites, Slave Province, Canada: evidence for multiple intrusive episodes in the Paleocene and Eocene

    NASA Astrophysics Data System (ADS)

    Creaser, Robert A.; Grütter, Herman; Carlson, Jon; Crawford, Barbara

    2004-09-01

    New Rb-Sr age determinations using macrocrystal phlogopite are presented for 27 kimberlites from the Ekati property of the Lac de Gras region, Slave Province, Canada. These new data show that kimberlite magmatism at Ekati ranges in age from at least Late Paleocene (˜61 Ma) to Middle Eocene time (˜45 Ma). Older, perovskite-bearing kimberlites from Ekati extend this age range to Late Cretaceous time (˜74 Ma). Within this age range, emplacement episodes at ˜48, 51-53, 55-56 and 59-61 Ma can be recognized. Middle Eocene kimberlite magmatism of the previously dated Mark kimberlite (˜47.5 Ma) is shown to include four other pipes from the east-central Ekati property. A single kimberlite (Aaron) may be younger than the 47.5 Ma Mark kimberlite. The economically important Panda kimberlite is precisely dated in this study to be 53.3±0.6 Ma using the phlogopite isochron method, and up to six additional kimberlites from the central Ekati property have Early Eocene ages indistinguishable from that of Panda, including the Koala and Koala North occurrences. Late Paleocene 55-56 Ma kimberlite magmatism, represented by the Diavik kimberlite pipes adjacent to the southeastern Ekati property, is shown to extend onto the southeastern Ekati property and includes three, and possibly four, kimberlites. A precise eight-point phlogopite isochron for the Cobra South kimberlite yields an emplacement age of 59.7±0.4 Ma; eight other kimberlites from across the Ekati property have similar Late Paleocene Rb-Sr model ages. The addition of 27 new emplacement ages for kimberlites from the Ekati property confirms that kimberlite magmatism from the central Slave Province is geologically young, despite ages ranging back to Cambrian time from elsewhere in the Slave Province. With the available geochronologic database, Lac de Gras kimberlites with the highest diamond potential are currently restricted to the 51-53 and 55-56 Ma periods of kimberlite magmatism.

  8. Diamond Morphology: Link to Metasomatic Events in the Mantle or Record of Evolution of Kimberlitic Fluid?

    NASA Astrophysics Data System (ADS)

    Fedortchouk, Y.

    2009-05-01

    Morphology and surface features on diamonds show tremendous variation even within a single kimberlite body reflecting a complex history of growth and dissolution. But does the diamond surface record the conditions in the several mantle sources sampled by the rising kimberlite magma, or evolution of the fluid system in the kimberlite magma itself? To address this question I revised morphological classification of diamonds from several kimberlite pipes from EKATI Mine property, N.W.T., Canada. The novelty of the approach, compared to the existing classifications, is in utilizing a random but large dataset of diamond dissolution experiments accumulated by several researchers including myself. These experiments have shown that similar forms (e.g. trigon etch pits) can be produced in a variety of conditions and environments, whereas their shape and size would depend on the reactant. Similarly, different types of resorption features always form together and can be used for deriving the composition of oxidizing fluid. The proposed classification method is focused on relating various types of diamond surfaces to the composition and conditions of oxidizing media. The study uses parcels of micro-and macro-diamonds (total of 125 carats) from Misery, Grizzly, Leslie and Koala kimberlites, EKATI Mine property, Northwest Territories, Canada. Only octahedron and hexoctahedron diamonds were selected (total ~600 stones). Diamond surfaces were studied using an optical and Field- Emission Scanning Electron Microscope to define resorption elements - simple surface features. These elements were identified for each of the three categories: 1) present on octahedral faces (well-preserved diamonds), 2) present on hexoctahedral faces (rounded resorbed diamonds), and 3) frosting (micro-features). Consistent associations of several elements define Resorption Types of diamonds, which form during a single oxidizing event. We further relate these types to the composition of the C-H-O + chlorides

  9. Mineralogy of Juvenile Lapilli in Fort a la Corne Pyroclastic Kimberlites

    NASA Astrophysics Data System (ADS)

    Mitchell, R. H.; Kjarsgaard, B. A.; McBride, J.

    2009-05-01

    Pyroclastic-dominated kimberlites of the Fort a la Corne area (Saskatchewan, Canada) are characterized by the presence of juvenile ash and lapilli tuffs together with crystal tuffs composed of discrete crystals of olivine. Juvenile lapilli are amoeboid-to-curviplanar in shape and composed of olivine set in a fine-grained groundmass. Welding of juvenile lapilli is extremely rare. Clasts can be set in a matrix of calcite and/or serpentine. Commonly, olivine crystals protrude from the clast margins. Many, but not all, clasts contain vesicles filled with carbonates and/or serpentine. Carbonates in the vesicles include Sr-bearing calcite, dolomite and Ba-Mg carbonate. Individual kimberlite units in some instances contain several juxtaposed texturally- and mineralogically-different varieties of juvenile lapilli. In others, clasts are of similar petrographic character that differ in only in their spinel mineralogy. In many clasts early-forming microphenocrystal prisms of calcite are present. The groundmass of the ash and lapilli consists of perovskite, serpentine pseudomorphs after monticellite, diverse discrete euhedral, resorbed and atoll spinels, apatite, serpentine, and laths of quench dolomite. Groundmass mica appears to be absent. Spinel assemblages differ between different vents. At Candle Lake the overall trend of spinel compositions follows that typical of hypabyssal kimberlites with individual clasts within a given unit exhibiting segments of this trend. Compositions belong to a spinel- magnesiochromite-chromite-qandilite-magnetite solid solution series. At Smeaton (FALC 169 body), spinel compositions follow the same overall trend but are all relatively more evolved, and typically Ti-rich and Cr-poor. The majority of compositions plot on the rear face of the reduced spinel prism and belong to the spinel- qandilite-ulvospinel-magnetite series. It is concluded that spinels in Fort a la Corne kimberlites follow the "normal" evolutionary trend of spinel compositions

  10. Role of fluid in the mechanism of formation of volcaniclastic and coherent kimberlite facies: a diamond perspective

    NASA Astrophysics Data System (ADS)

    Fedortchouk, Yana; Chinn, Ingrid

    2016-04-01

    Dissolution features on diamonds recovered from kimberlites vary depending on the dissolution conditions and can be used as a reliable proxy for volatiles and their role in kimberlite emplacement. Volatiles determine the mechanism of magma emplacement; variation in volatile content and CO2/CO2+H2O ratio may affect the geology of kimberlite bodies and formation of coherent vs. volcaniclastic kimberlite facies. Here we examine the evolution of a kimberlite system during ascent using the resorption morphology of its diamond population. We use 655 macro-diamonds from a complex kimberlite pipe in the Orapa kimberlite field (Botswana) to examine the role of volatiles in the formation of the three facies comprising this pipe: two coherent kimberlite facies (CKA and CKB) and one massive volcaniclastic facies (MVK). The diamonds come from three drillholes through each of the studied kimberlite facies. Separate diamond samples derived from 2 - 13 m intervals were combined into 40 m depth intervals for statistical purposes. Four independent morphological methods allowed us to reliably discriminate products of resorption in kimberlite magma from resorption in the mantle, and use the former in our study. We found that the proportion of diamonds with kimberlitic resorption is the lowest in CKA - 22%, medium in MVK - 50%, and highest in CKB - 73%, and it increases with depth in each of the drillholes. Each kimberlite facies shows its own style of kimberlite-induced resorption on rounded tetrahexahedron (THH) diamonds: glossy surfaces in MVK, rough corroded surfaces in CKB, and combination of glossy surfaces with chains of circular pits in CKA, where these pits represent the initial stages of development of corrosive features observed on CKB diamonds. Based on the results of our previous experimental studies we propose that resorption of MVK diamonds is a product of interaction with COH fluid, resorption of CKB diamonds is a product of interaction with a volatile

  11. The Source of Proterozoic Anorthosites: Bringing It All Back Home

    NASA Astrophysics Data System (ADS)

    Scoates, J. S.

    2004-05-01

    Proterozoic anorthosites are coarse-grained cumulate igneous rocks dominated by plagioclase of intermediate composition (An70-35) that occur in spatial and temporal association with both intrusions of troctolite and Fe-enriched rocks (ferrodiorite, monzonite) and with predominantly crustally-derived granitic batholiths. Given the relatively limited range of plagioclase compositions within individual intrusions, differences in plagioclase anorthite content between intrusions likely reflects primarily differences in pressures of segregation of plagioclase-rich magma bodies (An content of plag decreases with increasing pressures of crystallization). More importantly, Proterozoic anorthosite plutonic suites formed over an extended interval of time (1.2 byr) during the Middle Proterozoic from 2.1-0.9 Ga and thus are recording fundamental relationships between plate tectonics, mantle temperatures, and crust-mantle interactions over 1/4 of Earth history. Experimental work on opx-normative gabbroic/dioritic rocks from Harp Lake and Rogaland appears to show that some proposed anorthosite parental liquids lie across the trace of the plag+2-px cotectic from 1-1.3 GPa and that they straddle the thermal divide on the plag+px liquidus surface, thus apparently requiring a mafic source region (i.e. lower continental crust). It is unlikely that small amounts of dry partial melting of lower crustal granulite will produce melt compositions that are strongly plag-saturated nor will it yield the large quantities of melt (and corresponding cumulates) required by mass balance constraints. In addition, noritic-gabbronoritic lower crust is opx-normative and cannot be responsible for producing the olivine-bearing anorthosites or troctolites typical of the largest Proterozoic anorthosites. A compilation of high-Al,Fe basaltic magmas from Proterozoic anorthosite plutonic suites worldwide shows them to have compositions that are significantly less silica-rich than the opx-normative rocks that

  12. [Study on the FTIR spectra of OH in olivines from mengyin kimberlite].

    PubMed

    Ai, Qun; Yang, Zhi-jun; Zeng, Xiang-qing; Zheng, Yun-long; Hu, Piao-ye

    2013-09-01

    The results of FTIR spectra study of OH in olivines from Mengyin kimberlite show that there are more than 60 OH absorption peaks in the range of 3800-3000 cm(-1). We identified four major spectral features in the OH absorption bands of kimberlitic olivines. One is with nuOH in the range of 3800-3700 cm(-1), which is caused by the vapour of the room circumstance, and can not be regarded as intrinsic or non-intrinsic nuOH of the olivines. Another one is with nuOH in the range of 3710-3620 cm(-1), which belongs to three "water"-bearing minerals including serpentine, talc and Mg-bearing amphiboles, which is the non-intrinsic nuOH of the olivines. There is the possibility that H in hydrous minerals mainly entered into olivines during post-emplacement processes of the kimberlite magma. The third one is with nuOH in the range of 3620-3425 cm(-1), which originated from H occupying the Si-defect in the olivine structure, forming humite-like defects, and/or the defects that H occupies (Mg,Fe)-depletion, which is certainly attributed to the intrinsic nuOH of the olivines. In this case, H possibly entered into olivines following its immersion in the high temperature and rich fluid kimberlite magma in the mantle circumstance. The last one is with nuOH in the range of 3425-3000 cm(-1). In this area, nuOH is assigned to fluid inclusions of the olivines, and is the non-intrinsic nuOH of olivines. Fluid inclusions can enter into the olivines either during post-emplacement processes of the kimberlite magma or during the periods that olivines were formed in the mantle.

  13. Mantle-derived argon components in phlogopite from southern African kimberlites

    SciTech Connect

    Phillips, D.; Onstott, T.C.

    1985-01-01

    Application of the /sup 40/Ar//sup 39/Ar dating technique to kimberlite geochronology often yields discordant spectra with ages that are much older than the inferred time of emplacement of the kimberlite body. In the past, these anomalously high ages have been attributed to the presence of excess radiogenic /sup 49/Ar incorporated into the mineral phases either pre- or syn- emplacement of the kimberlite. Detailed /sup 40/Ar//sup 39/Ar stepheating analyses on phlogopite xenocrysts from Southern African kimberlites revealed the presence of excess argon. Discrimination between different reservoirs of argon, contained in the phlogopite, was best achieved using plots of /sup 36/Ar//sup 40/Ar versus /sup 39/Ar//sup 40/Ar. High /sup 36/Ar//sup 40/Ar ratios for the low temperature steps are interpreted as resulting from atmospheric contamination (/sup 40/Ar//sup 36/Ar = 295.5). At the highest temperature steps (1100/sup 0/-1200/sup 0/C), the /sup 36/Ar//sup 40/Ar ratios increase dramatically. Least squares fits of the high temperature components yield /sup 40/Ar//sup 36/Ar initial ratios ranging from 340 to 366. These values correspond closely to those obtained by Allegre et al. (1983) for Hawaiian basalts. This suggest that either the xenocrystic phlogopites within the kimberlites or the vapor phase within which they crystallized were derived from an undegassed mantle source. The release of this mantle reservoir at high temperatures requires a high retentivity site for the argon within the phlogopite structure. As phlogopite appears to retain its structural water to high temperatures, the argon may be trapped within the hydroxyl sites of the mica.

  14. [Study on the FTIR spectra of OH in olivines from mengyin kimberlite].

    PubMed

    Ai, Qun; Yang, Zhi-jun; Zeng, Xiang-qing; Zheng, Yun-long; Hu, Piao-ye

    2013-09-01

    The results of FTIR spectra study of OH in olivines from Mengyin kimberlite show that there are more than 60 OH absorption peaks in the range of 3800-3000 cm(-1). We identified four major spectral features in the OH absorption bands of kimberlitic olivines. One is with nuOH in the range of 3800-3700 cm(-1), which is caused by the vapour of the room circumstance, and can not be regarded as intrinsic or non-intrinsic nuOH of the olivines. Another one is with nuOH in the range of 3710-3620 cm(-1), which belongs to three "water"-bearing minerals including serpentine, talc and Mg-bearing amphiboles, which is the non-intrinsic nuOH of the olivines. There is the possibility that H in hydrous minerals mainly entered into olivines during post-emplacement processes of the kimberlite magma. The third one is with nuOH in the range of 3620-3425 cm(-1), which originated from H occupying the Si-defect in the olivine structure, forming humite-like defects, and/or the defects that H occupies (Mg,Fe)-depletion, which is certainly attributed to the intrinsic nuOH of the olivines. In this case, H possibly entered into olivines following its immersion in the high temperature and rich fluid kimberlite magma in the mantle circumstance. The last one is with nuOH in the range of 3425-3000 cm(-1). In this area, nuOH is assigned to fluid inclusions of the olivines, and is the non-intrinsic nuOH of olivines. Fluid inclusions can enter into the olivines either during post-emplacement processes of the kimberlite magma or during the periods that olivines were formed in the mantle. PMID:24369634

  15. Geochemistry of Eclogite Xenoliths from Kimberlite Pipe Udachnaya

    NASA Astrophysics Data System (ADS)

    Agashev, Aleksey; Pokhilenko, Ludmila; Pokhilenko, Nikolai

    2016-04-01

    A suite of 17 unique big (1 to 20 kg) and fresh ecligite xenoliths from Udachnaya kimberlite pipe have been studied for their whole-rock and minerals major and trace elements composition.Whole rock major elements composition of the Udachnaya eclogite xenoliths suite have a great variability in their MgO contents (9-19Wt%). Based on major elements composition Udachnaya eclogites can be subdivided in two subsets, high magnesian (Mg# 68.8-81.9) and low magnesian (Mg# 56.8-59). High variations also shown by Al2O3 and Na2O concentrations and high Mg# samples tend to contain less of those oxides then low Mg# samples with some exceptions. Two eclogitic groups are clearly different in style of inter-elements correlations. FeO and CaO contents are positively correlate with MgO in low Mg# group of eclogites but negatively in high Mg# group. The same relations present between Al2O3 contents of eclogite group with their Mg#. Compared to present day MORB composition eclogite samples have similar contents of most of elements with some depletion in TiO2 and P2O5 and enrichment in MgO and K2O. The variability of these elements concentrations can be related to melt extraction while elevated K2O can indicate late metasomatic enrichment. In terms of trace elements composition Udachnaya eclogites are enriched over PM but comparable to that of MORB composition, except significant enrichment in LILE elements (Rb, Ba, K, Sr). The records of both subduction related processes and mantle metasomatism could be find in geochemical features of these rocks. Most of the eclogites show positive Eu anomaly which is direct evidence of plagioclase accumulation in eglogites protolith. Variation of La/Yb ratio (1-11), in majority of samples are the range 2-4 indicates different degrees of samples metasomatic enrichment in LREE. Udachnaya eclogites have range of Sm/Nd ratio from 0.25 to 0.5 (MORB is 0.32) which positive covariates with Nd content. This trend could not be a result of melt extraction nor

  16. A Raman microprobe study of melt inclusions in kimberlites from Siberia, Canada, SW Greenland and South Africa.

    PubMed

    Mernagh, Terrence P; Kamenetsky, Vadim S; Kamenetsky, Maya B

    2011-10-01

    Raman spectroscopy has been used for the identification of both common and uncommon minerals in melt inclusions in Group-I kimberlites from Siberia, Canada, SW Greenland and South Africa. The melt inclusions all contained high abundances of alkali-Ca carbonates, with varying proportions of cations, and Na-Ca-Ba sulphates. In accordance with its dry mineralogy, no hydrated carbonates or sulphates were detected in melt inclusions from the Udachnaya-East kimberlite. In contrast, the melt inclusions in kimberlites from Canada, South Africa and SW Greenland were found to contain bassanite, pirssonite, and hydromagnesite suggesting that greater amounts of water were present in their residual magmas. This suggests that enrichment in alkali carbonates and sulphates is widespread across a range of Group-I kimberlites and implies that they commonly have an alkali-, and sulphur-rich residual liquid.

  17. A Raman microprobe study of melt inclusions in kimberlites from Siberia, Canada, SW Greenland and South Africa.

    PubMed

    Mernagh, Terrence P; Kamenetsky, Vadim S; Kamenetsky, Maya B

    2011-10-01

    Raman spectroscopy has been used for the identification of both common and uncommon minerals in melt inclusions in Group-I kimberlites from Siberia, Canada, SW Greenland and South Africa. The melt inclusions all contained high abundances of alkali-Ca carbonates, with varying proportions of cations, and Na-Ca-Ba sulphates. In accordance with its dry mineralogy, no hydrated carbonates or sulphates were detected in melt inclusions from the Udachnaya-East kimberlite. In contrast, the melt inclusions in kimberlites from Canada, South Africa and SW Greenland were found to contain bassanite, pirssonite, and hydromagnesite suggesting that greater amounts of water were present in their residual magmas. This suggests that enrichment in alkali carbonates and sulphates is widespread across a range of Group-I kimberlites and implies that they commonly have an alkali-, and sulphur-rich residual liquid. PMID:21334252

  18. Erosion patterns and mantle sources of topographic change across the southern African Plateau derived from the shallow and deep records of kimberlites

    NASA Astrophysics Data System (ADS)

    Stanley, Jessica R.; Flowers, Rebecca M.; Bell, David R.

    2015-09-01

    Flow in the sublithospheric mantle is increasingly invoked as a mechanism to explain both modern and past surface topography, but the importance of this phenomenon and its influence at different localities are debated. Southern Africa is an elevated continental shield proposed to represent dynamically supported topography. However, this region is also characterized by a complex lithospheric architecture variably affected by Cretaceous heating, thinning, and metasomatic alteration. We used apatite (U-Th)/He thermochronometry on 15 Cretaceous kimberlites from an ˜600 km long transect across the Kaapvaal Craton, combined with information from xenoliths in these pipes, to determine the plateau interior erosion history. The goal was to determine the relationships with lithospheric modification patterns and thereby better isolate the sublithospheric contribution to elevation. The results document a wave of erosion from west to east across the craton from ˜120 to <60 Ma, initially focused along paleorivers and then retreating as a scarp across the landscape. This spatially variable erosion event was associated with limited modification of the Archean cratonic lithospheric mantle as recorded by mantle xenoliths and xenocrysts, implying that dynamic buoyancy sources may be required to explain the elevations. In contrast, off-craton to the southwest, a more pronounced regional erosion phase at ˜110-90 Ma was coincident with significant modification of the Proterozoic lithospheric mantle. This relationship suggests that lithospheric processes were more important contributors to erosion and topographic change off-craton than on-craton. Together, these results suggest that lithospheric architecture can have an important control on the surface expression of mantle dynamics.

  19. Unique Mineralogy of Triassic Diamondiferous Hypabyssal Kimberlite Postdated Siberian Flood Basalt (sfb) Province

    NASA Astrophysics Data System (ADS)

    Sobolev, N. V.; Sobolev, A. V.; Tomilenko, A. A.; Schertl, H. P.; Neuser, R. D.; Timina, T. Y.; Karmanov, N. S.; Batanova, V. G.; Logvinova, A. M.; Kuzmin, D.

    2014-12-01

    The Siberian flood basalt (SFB) province is the largest terrestrial province with the estimated volume of igneous rocks up to 5 million cubic km. The majority of SFB erupted over less than one million years at 251 Ma. The main epochs of kimberlites emplacement took place in Devonian (344-364 Ma) producing principal diamond mines including Udachnaya mine and in Triassic (about 240 Ma) with only one, Malokuonapskaya kimberlite pipe with near-commercial diamond grade. This indicates the availability of complete lithospheric cross section. It contains flood basalt and peridotite xenoliths. We report here preliminary data on mineralogy of this hypabyssal kimberlite containing fresh olivine. Homogeneous cores of zoned olivine with Fo 78.5-93 are different in compositional range from those of Udachnaya olivines (Fo 85-94). Outer rims composition are also different (Fo 85-86 and 89-90 respectively). Concentration of Ni, Mn, Co, Ca, Cr, Al, Ti, P, Na and Zn were measured by EPMA using an innovative method which has been developed based on earlier publication (Sobolev et al., Science, 2007, 316: 412-417). It made possible to obtain external precision down to 10 ppm (2SD) and detection limit down to 2 ppm. High resolution compositional maps of olivine zoning for all mentioned elements are produced. "Hot cathode" CL microscope was applied for study of luminescent minerals including calcite, apatite and baryte. Twenty percent of representative olivine samples are characterized by low Fo 78.5-85 and NiO from 600 to 2300 ppm. Clear zoning in concentration of some trace elements, P in particular, is detected in the cores of studied olivines. Ba-phlogopite containing BaO from less than 1 up to 14.5 wt.% is another specific feature of Malokuonapskaya kimberlite, which is different from any kimberlites and especially from Udachnaya with highest BaO - 4.85 wt.% of its phlogopite. Chromediopsides contain 1.3 - 5.2 wt.% FeO, 0.6 - 2.0 wt.% Cr2O3 and 0.45 - 2.0 wt.% Na2O. Pyropes

  20. Reply on: "Comment on: The ascent of kimberlite: Insights from olivine" authored by Brett R.C. et al. [Earth Planet. Sci. Lett. 424 (2015) 119-131

    NASA Astrophysics Data System (ADS)

    Brett, R. Curtis; Russell, J. K.; Andrews, G. D. M.; Jones, T. J.

    2016-04-01

    The Kamenetsky (2016) comment on the Kimberlite Factory model proposed by Brett et al. (2015) asserts, "A cornerstone of this model is a specific, carbonatitic composition of proto-kimberlite melts ascending through the sub-cratonic lithospheric mantle" and "… the major thrust of the study hinges on the premise that the parental kimberlite melt is carbonatitic". This is a clear misstatement of our central thesis, which is to utilize the attributes of olivine xenocrysts to constrain the physical ascent of kimberlite. Brett et al.'s study does not hinge on the premise that parental kimberlite melt is carbonatitic. Rather, our interpretation that kimberlite melt originates as near carbonatitic hinges on our novel observation that early "carbonate sealed cracks provide evidence of melt being drawn into decompression cracks and precipitating" (p. 129). Our connection between this observation and our interpretation is tied explicitly to earlier published works "in this regard, the carbonate-filled sealed cracks strongly support to the hypothesis that all kimberlite magmas originate as carbonatitic-melts (e.g.,Russell et al., 2012, 2013;Kamenetsky et al., 2013; Pilbeam et al., 2013; Kamenetsky and Yaxley, 2015; Bussweiler et al., 2015)" (p. 129). To state that our interpretation is based on a pre-existing bias towards a model of a carbonatitic origin of kimberlite magmas is incorrect. Rather, our new observational data independently demonstrates that the presence of carbonate-sealed cracks formed during kimberlite ascent.

  1. Bridging Two Worlds: From the Archean to the Proterozoic

    NASA Technical Reports Server (NTRS)

    Schopf, J. William

    2000-01-01

    As now known, the Archean and Proterozoic appear to have been different worlds: the geology (tectonic style, basinal distribution, dominant rock types), atmospheric composition (O2, CO21, CH4), and surface environment (day-length, solar luminosity, ambient temperature) all appear to have changed over time. And virtually all paleobiologic indicators can be interpreted as suggesting there were significant biotic differences as well: (1) Stromatolites older than 2.5 Ga are rare relative to those of the Proterozoic; their biotic components are largely unknown; and the biogenicity of those older than approx. 3.2 Ga has been questioned. (2) Bona fide microfossils older than approx. 2.4 Ga are rare, poorly preserved, and of uncertain biological relations. Gaps of hundreds of millions of years in the known record make it impossible to show that Archean microorganisms are definitely part of the 2.4 Ga-to-present evolutionary continuum. and (3) In rocks older than approx. 2.2 Ga, the sulfur isotopic record is subject to controversy; phylogenetically distinctive bio-markers are unknown; and nearly a score of geologic units contain organic carbon anomalously light isotopically (relative to that of the Proterozoic and Phanerozoic) that may reflect the presence of Archaeans ("Archaebacteria of earlier classifications) but may not (since cellularly preserved Archean-age Archaeans have never been identified).

  2. Lower Paleozoic and Proterozoic rocks of Southern Brooks Range, Alaska

    SciTech Connect

    Dillion, J.T.

    1985-04-01

    Lower Paleozoic or Proterozoic basement rocks occur in windows and thrust plates in several areas of the Brooks Range. Uranium-lead radiometric analyses of highly metamorphosed rocks from the Baird Mountains and Ernie Lake area have yielded Proterozoic ages. Structural, stratigraphic, petrologic, and isotopic evidence exists for Proterozoic(.) rocks in the schist belt; around the Chandalar, Arrigetch,and Igikpak plutons; and in the Cosmos Hills window. Fossiliferous, Lower Paleozoic, low-grade metasedimentary rocks occur in the Romanzof Mountains, Doonerak window, and Baird Mountains, and may also surround the Chandalar plutons. Locally, the Lower Paleozoic rocks are unconformably overlain by Devonian to Mississippian metasediments and may stratigraphically overlie older, higher grade metamorphic rocks. Similarities in the stratigraphic settings and lithologies and in fossil ages and affinities allow correlation of the Lower Paleozoic rocks in the southern Brooks Range. Correlation of Lower Paleozoic rocks exposed beneath the Endicott allochthon at the Doonerak fenster with coeval rocks in an overlying thrust plate to the south at Snowsden Mountain is especially significant. A west-trending thrust fault, which is rooted in Lower Paleozoic basement, along the north side of Snowsden Mountain is postulated to account for these relationships. Apparently, the Endicott allochthon roots beneath the Snowsden Mountain thrust fault. Evidence form conodont samples currently being studied by A. Harris may bear on the extent of the Lower Paleozoic rocks in the upper plate of the Snowsden Mountain thrust and in the Chandalar area.

  3. Why lower diatremes in kimberlitic and non-kimberlitic systems are non-stratified, homogenized, and contain steep internal contacts: episodic bursts and debris jets

    NASA Astrophysics Data System (ADS)

    Ross, P.; White, J. D.; Kurszlaukis, S.; Lorenz, V.; Zimanowski, B.; Buettner, R.; McClintock, M.

    2009-05-01

    In both kimberlitic and non-kimberlitic systems, the volcaniclastic fill of the lower diatreme zone is often described as "homogenized" or "well mixed". Although the components come from different sources, the deposits display "a crude degree of textural and lithological consistency" (Clement and Reid, 1989, "Kimberlites and related rocks", p. 632-646). Bedding is typically absent from the lower diatreme but in some pipes, columnar bodies of volcaniclastic material occur. These bodies have steep contacts with, and a different grain size, componentry, etc. than, the enclosing host. Sometimes the difference can be subtle and the contacts gradational, making recognition difficult. Good examples are documented from Arizona and Antarctica in basaltic systems and such columnar bodies are also known in kimberlites, where they are sometimes called "feeder conduits". Both the homogenized aspect of many diatremes, and the generation of steep internal contacts, have been attributed to whole-pipe fluidization by some recent workers. This process is unlikely to occur in large pipes because it would take a huge amount of gas being emitted at a sufficient rate to fluidize the whole pipe. Other recent models call for Plinian-scale eruptions. However it is clear that small episodic bursts, not sustained Plinian plumes, must explain the genesis of the hundreds of relatively thin beds in maar tephra rims (maar- diatreme volcanoes do not generate large ignimbrites or thick widespread pyroclastic fall layers). Here we examine what these episodic bursts may do to the underground part of the maar-diatreme volcano. An explosion at deep levels in the pipe will generate enough gas to mobilize newly fragmented magma and existing debris upward into a "debris jet", typically much narrower than the width of the diatreme. Debris jets propagate within the existing diatreme fill and may or may not reach the surface. Experimental studies can be used to illustrate the processes at work. With time

  4. Composition of garnet and clinopyroxene in peridotite xenoliths from the Grib kimberlite pipe, Arkhangelsk diamond province, Russia: Evidence for mantle metasomatism associated with kimberlite melts

    NASA Astrophysics Data System (ADS)

    Kargin, A. V.; Sazonova, L. V.; Nosova, A. A.; Tretyachenko, V. V.

    2016-10-01

    Here we present major and trace element data for garnet and clinopyroxene from mantle-derived peridotite xenoliths of the Grib kimberlite, the Arkhangelsk diamond province, Russia, and provide new insights into the metasomatic processes that occur within the subcontinental lithospheric mantle (SCLM) during the kimberlite generation and ascent. The mantle xenoliths examined in this study are both coarse and sheared garnet peridotites and consist of olivine, orthopyroxene, clinopyroxene, garnet with minor ilmenite, magnetite, and Cr-spinel. Based on garnet and clinopyroxene composition, two groups of peridotite are recognized. One group contains high-Ti, light rare earth elements (LREE) enriched garnets and low-Mg# clinopyroxenes with low (La/Sm)n (C1 chondrite-normalized) values. This mineral assemblage was in equilibrium with a high-temperature carbonate-silicate metasomatic agent, presumably, a protokimberlite melt. Pressure-temperature (P-T) estimates (T = 1220 °C and P = 70 kbar) suggest that this metasomatic event occurred at the base of the SCLM. Another group contains low-Ti garnet with normal to sinusoidal rare earth elements (REE) distribution patterns and high-Mg# clinopyroxenes with wide range of (La/Sm)n values. The geochemical equilibrium between garnet and clinopyroxene coupled with their REE composition indicates that peridotite mantle experienced metasomatic transformation by injection of a low-Ti (after crystallizations of the ilmenite megacrysts) kimberlite melt that subsequently percolated through a refractory mantle column. Peridotites of this group show a wide range of P-T estimates (T = 730-1070 °C and P = 22-44 kbar). It is suggested that evolution of a kimberlite magma from REE-enriched carbonate-bearing to carbonate-rich ultramafic silicate compositions with lower REE occurs during the ascent and interaction with a surrounding lithospheric mantle, and this process leads to metasomatic modification of the SCLM with formation of both high

  5. Application of Fe-Ti oxide dissolution experiments to the petrogenesis of the Ekati Diamond Mine kimberlites, Northwest Territories, Canada

    NASA Astrophysics Data System (ADS)

    Kressall, R.; Fedortchouk, Y.; McCammon, C. A.

    2015-12-01

    Composition of kimberlites is ambiguous due to assimilation and fractional crystallization. We propose that the evolution history of minerals can be used to decipher the magmatic history of kimberlites. We use Fe-Ti oxides (chromite and ilmenite) from six kimberlites from the Ekati Diamond Mine and dissolution experiments to elucidate the petrogenesis of kimberlites. Experiments at 0.1 MPa and variable ƒO2s in a diopside-anorthite melt show that the dissolution rate of ilmenite is highly sensitive to ƒO2. No significant difference was observed in chromite. Zoning in chromite is related to the Fe-content and oxidation state of the melt. Experiments at 1 GPa explore the development of chromite surface resorption features in the system Ca-Mg-Si-H-C-O. Five kimberlites contain a low abundance of ilmenite, owing to a relatively high ƒO2, though ilmenite constituted 65% of oxide macocrysts in one kimberlite. Chromite compositions evolve from Mg-chromite to magnesio-ulvöspinel-magnetite (MUM) in all but one kimberlite where chromite evolves to a pleonaste composition perhaps as a result of rapid emplacement. The high abundance of MUM spinel and low abundance of ilmenite in the matrix could be related to the change in the stable Ti-phase with increasing ƒO2. Core compositions of macrocrysts vary for different mantle sources but rims converge to a composition slightly more oxidized and Mg-rich than chromite from depleted peridotite. Ilmenite commonly has rims composed of perovskite, titanite and MUM. We suggest a model where the kimberlite melt composition is controlled by the co-dissolution and co-precipitation of silicates (predominantly orthopyroxene and olivine) to explain chromite evolution in kimberlites. Resorption-related surface features on chromite macrocrysts show trigon protrusions-depressions on {111} faces and step-like features along the crystal edges resembling products of experiments in H2O fluid. We propose predominantly H2O magmatic fluid in Ekati

  6. Ar-40/Ar-39 laser-probe dating of diamond inclusions from the Premier kimberlite

    NASA Technical Reports Server (NTRS)

    Phillips, D.; Onstott, T. C.; Harris, J. W.

    1989-01-01

    The results of Ar-40/Ar-39 laser-probe analyses of individual eclogitic clinopyroxene inclusions from Premier diamonds are reported which yield a mean age of 1198 + or - 14 Myr. This age agrees well with Sm-Nd and Ar-40/Ar-39 analyses on similar Premier inclusions and is indistinguishable from the inferred time of emplacement of the host kimberlite, which implies that diamond formation was essentially synchronous with kimberlite generation. The extrapolated nonradiogenic Ar-40/Ar-36 ratio of 334 + or - 102 is similar to the present-day atmospheric composition. This value is inconsistent with Sr and Nd isotopic signatures from Premier eclogite inclusions, which suggest a depleted mantle source. Preentrapment equilibration of the inclusions with an Ar-36-rich fluid is the most probable explanation for the low nonradiogenic composition.

  7. The petrogenesis of oceanic kimberlites and included mantle megacrysts: The Malaitan alnoite

    NASA Technical Reports Server (NTRS)

    Neal, Clive R.

    1988-01-01

    The study of unambiguous suboceanic mantle was facilitated by the occurrence of anomalous kimberlite-type intrusives on Malaita in the Solomon Islands. The pseudo-kimberlites were termed alnoites, and are basically mica lamprophyres with melilite in the ground mass. Alnoitic magmas were explosively intruded into the Ontong Java Plateau (OJP) 34 Ma ago. The OJP is a vastly overthickened portion of the Pacific plate which now abuts the Indo-Australian plate. Malaita is considered to be the obducted leading edge of the OJP. Initial diapiric upwelling beneath the OJP produced the proto-alnoite magma. After impingement on the rigid lithosphere, megacrysts fractionation occurred, with augites precipitating first, representing the parental magma. Sea water-altered oceanic crust, which underplated the OJP, was assimilated by the proto-alnoite magma during megacrysts fractionation (AFC).

  8. Inclusions of crichtonite group minerals in pyropes from the Internatsionalnaya kimberlite pipe, Yakutia

    NASA Astrophysics Data System (ADS)

    Rezvukhin, D. I.; Malkovets, V. G.; Sharygin, I. S.; Kuzmin, D. V.; Gibsher, A. A.; Litasov, K. D.; Pokhilenko, N. P.; Sobolev, N. V.

    2016-02-01

    The results of study of crichtonite group minerals in pyropes from the Internatsionalnaya kimberlite pipe are reported. Most of the studied samples are characterized by high concentrations of Sr, Ca, Na, and LREEs in comparison with minerals of the LIMA series from kimberlites of South Africa, whereas the average concentrations of Ba and K are significantly lower. Crichtonite group minerals in pyropes are characterized by predomination of Na over K in most samples and by a high concentration of Al2O3 (up to 4.5 wt %). Findings of inclusions of crichtonite group minerals with high concentrations of incompatible elements provide evidence for the metasomatic origin of host chromium-rich pyropes.

  9. Super-deep diamonds from kimberlites in the Juina area, Mato Grosso State, Brazil

    NASA Astrophysics Data System (ADS)

    Kaminsky, Felix V.; Khachatryan, Galina K.; Andreazza, Paulo; Araujo, Debora; Griffin, William L.

    2009-11-01

    One thousand three hundred sixty-five diamonds from seven newly discovered kimberlitic pipes in the Juina area were comprehensively studied. These diamonds, like the ones from previously studied Juina placer deposits, are very homogeneous in their morphology and optical properties. Two diamond populations exist in the Pandrea pipes: the major population with a highly aggregated nitrogen impurity (% N B = 75-100%), and a secondary population with a moderately aggregated nitrogen impurity (% N B = 20-65%), while only one major population is present in the diamonds from placers. The diamonds from the pipes have a permanent, relatively high hydrogen impurity concentration. Among the mineral inclusions in diamonds from the Juina pipes, ferropericlase is predominant; chrome spinel, picroilmenite, Mn-ilmenite, MgCaSi-'perovskite' phase, rutile, sulphide, native iron, and iron-oxides were also identified. Most of the inclusions belong to the lower-mantle paragenesis; some (rutile and sulphide) are of eclogitic paragenesis. Mineral inclusions in diamonds from kimberlitic pipes are different in composition from the same minerals in placer diamonds. Both kimberlitic and placer diamonds belong to the same carbon isotopic population, but have differences in the δ13C distribution and were probably formed from different local carbon sources. These data indicate that diamonds from both groups, kimberlites and placer deposits, in the Juina area, belong to the same genetic population with most of the stones originating within the super-deep conditions. However, there are differences between these two groups, which indicate that besides the known Pandrea pipes which may have partly supplied diamonds to the placer deposits, there may be other, still unknown primary sources of diamonds in the Juina area.

  10. Carbon matter in kimberlite-like rocks of the Charteskii Complex (Subpolar Urals)

    NASA Astrophysics Data System (ADS)

    Isaenko, S. I.; Shumilova, T. G.; Shevchuk, S. S.

    2015-10-01

    Results of the study of carbon material (CM) discovered in kimberlite-like rocks of the Charteskii Complex (Subpolar Urals) are considered. It is shown that CM is represented by partially oxidized graphite and optically transparent amorphous CM (presumably diamond-like carbon). The data obtained are important for estimation of the diamond potential of this object, as well as for understanding of the new mechanism of the formation of diamond-like carbon and diamond.

  11. Comment on: "The ascent of kimberlite: Insights from olivine" by Brett R.C. et al. [Earth Planet. Sci. Lett. 424 (2015) 119-131

    NASA Astrophysics Data System (ADS)

    Kamenetsky, Vadim S.

    2016-04-01

    Brett et al. (2015) proposed a kimberlite factory model that "…involves carbonatitic proto-kimberlite melts preferentially assimilating Opx xenocrysts as they transit the cratonic mantle lithosphere to evolve into silicic-hydrous melts that reach olivine saturation during ascent" (p. 130). A cornerstone of this model is a specific, carbonatitic composition of proto-kimberlite melts ascending through the subcratonic lithospheric mantle "…whereby parental carbonatitic magmas are progressively converted to kimberlite (e.g., Russell et al., 2012, 2013; Bussweiler et al., 2015)" (p. 120). The model by Brett et al. (2015) is based on observations of "the carbonate-sealed cracks" in olivine that "…strongly support to the hypothesis that all kimberlite magmas originate as carbonatitic-melts (e.g., Russell et al., 2012, 2013; Kamenetsky et al., 2013; Pilbeam et al., 2013; Kamenetsky and Yaxley, 2015; Bussweiler et al., 2015)." (p. 129). While the major thrust of the study by Brett et al. (2015) hinges on the premise that the parental kimberlite melt is carbonatitic, the overwhelming majority in the kimberlite community still prefers a carbonated ultramafic/ultrabasic composition for parental kimberlite melts. Thus the suggestion that kimberlites have an initial carbonatite composition is not less than "a paradigm shift" in the kimberlite petrology. It appears that a carbonatite origin for kimberlites has been proposed in many studies that significantly pre-date the publications starting from 2012 that they cite, but which unfortunately are overlooked by Brett et al. (2015). It is, therefore, worth acknowledging the research which has previously advanced this unorthodox idea.

  12. Syn- and post-eruptive volcanic processes in the Yubileinaya kimberlite pipe, Yakutia, Russia, and implications for the emplacement of South African-style kimberlite pipes

    NASA Astrophysics Data System (ADS)

    Kurszlaukis, S.; Mahotkin, I.; Rotman, A. Y.; Kolesnikov, G. V.; Makovchuk, I. V.

    2009-11-01

    The Yubileinaya kimberlite pipe, with a surface area of 59 ha, is one of the largest pipes in the Yakutian kimberlite province. The Devonian pipe was emplaced under structural control into Lower Paleozoic karstic limestone. The pipe complex consists of several smaller precursor pipes which are cut by the large, round Main pipe. While the precursor pipes show many features typical for root zones, Main pipe is younger, cuts into the precursor pipes and exposes well-bedded volcaniclastic sediments. The maximum estimated erosion since emplacement is 250 m. Open pit mapping of a 180 m thick kimberlite sequence documents the waning phases of the volcanic activity in the kimberlite pipe and the onset of its crater infill by resedimentation. Three volcanic lithofacies types can be differentiated. The deepest and oldest facies type is a massive volcaniclastic rock ("AKB") only accessible in drill core. It is equivalent to Tuffisitic Kimberlite in South African pipes and thought to be related to the main volcanic phase which was characterized by violent explosions. The overlying lithofacies type comprises primary and resedimented volcaniclastic sediments as well as rock avalanche deposits sourced from the exposed maar crater collar. It represents the onset of sedimentation onto the crater floor during the waning phase of volcanic eruptions, where primary pyroclastic deposition was contemporaneous with resedimentation from the tephra wall and the widening maar crater. Ongoing volcanic activity is also testified by the presence of a vertical feeder conduit marking the area of the last volcanic eruption clouds piercing through the diatreme. This feeder conduit is overlain by the third and youngest lithofacies type which consists mainly of resedimented volcaniclastic material and lake beds. During the sedimentation of this facies, primary volcanic activity was only minor and finally absent and resedimentation processes dominated the crater infill. The Yubileinaya pipe complex

  13. Garnet peridotites from Williams kimberlites, north-central Montana, U.S.A

    USGS Publications Warehouse

    Hearn, B.C.; McGee, E.S.

    1983-01-01

    Two Williams kimberlites, 250x350m and 37x390m, in the eastern part of a swarm of 30 middle Eocene alnoitic diatremes in north-central Montana, USA, contain xenoliths of garnet-bearing lherzolites, harzburgites and dunites, in addition to spinel peridotites and upper and lower crustal amphibolites and granulites. Colluvial purple, red, and pink garnets are dominantly Mg- and Cr-rich, indicating their derivation From peridotites or megacrysts, and have CaO and Cr2O3 contents that fall in the lherzolite trend. Temperatures were calculated by the Lindsley-Dixon 20 kb method for lherzolites and by the O'Neill-Wood method for harzburgites and dunites, and pressures were calculated by the MacGregor method, or were assumed to be 50 kb for dunites. Most peridotites equilibrated at 1220-1350?C and 50-60 kb, well above a 44mW/m2 shield geotherm and on or at higher P than the graphite-diamond boundary. Four lherzolites are low T-P (830-990?C, 23-42 kb) and are close to the shield geotherm. All four low T-P lherzolites have coarse textures whereas the high T-P cluster has both coarse and porphyroclastic textures, indicating a range of conditions of deformation and recrystallization in a restricted high T-P range. The tiny size (0.01-0.2 mm) of granulated and euhedral olivines in several xenoliths shows that deformation was occurring just prior to incorporation in kimberlite and that ascent was rapid enough (40-70 km/hr) to retard Further coarsening of fine-grained olivine. For other high T-P peridotites, cessation of deformation and beginning of recrystallization before or during inclusion in kimberlite is suggested by larger (up to 3mm) euhedral olivines in a matrix of fine granulated olivine or by optical continuity of large and nearby small olivines. Two low T-P lherzolites contain distinctive, phlogopite-rimmed, 5-8mm clots of moderate-Cr garnet + Cr-spinel + Cr-diopside + enstatite that are inferred to have formed by reaction of an initial high-Cr garnet brought into the

  14. Geology, geochronology, geochemistry, and Pb-isotopic compositions of Proterozoic rocks, Poachie region, west-central Arizona; a study of the east boundary of Proterozoic Mojave crustal province

    USGS Publications Warehouse

    Bryant, Bruce; Wooden, J.L.; Nealey, L. David

    2001-01-01

    The Poachie region at the south edge of the Colorado Plateau transition zone in western Arizona is underlain by early and middle Proterozoic plutonic rocks locally separated by screens of amphibolite-facies early Proterozoic metamorphic rocks and is in the eastern part of the Mojave crustal province, as shown by 35 Pb-isotope analyses of rocks and feldspars. Previously published mapping, 10 U-Pb zircon ages, 55 rock analyses, and other studies in the larger region of the transition zone lead to interpretation of the history and origin of the Proterozoic crust.

  15. Modified ion exchange separation for tungsten isotopic measurements from kimberlite samples using multi-collector inductively coupled plasma mass spectrometry.

    PubMed

    Sahoo, Yu Vin; Nakai, Shun'ichi; Ali, Arshad

    2006-03-01

    Tungsten isotope composition of a sample of deep-seated rock can record the influence of core-mantle interaction of the parent magma. Samples of kimberlite, which is known as a carrier of diamond, from the deep mantle might exhibit effects of core-mantle interaction. Although tungsten isotope anomaly was reported for kimberlites from South Africa, a subsequent investigation did not verify the anomaly. The magnesium-rich and calcium-rich chemical composition of kimberlite might engender difficulty during chemical separation of tungsten for isotope analyses. This paper presents a simple, one-step anion exchange technique for precise and accurate determination of tungsten isotopes in kimberlites using multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS). Large quantities of Ca and Mg in kimberlite samples were precipitated and removed with aqueous H(2)SO(4). Highly pure fractions of tungsten for isotopic measurements were obtained following an anion exchange chromatographic procedure involving mixed acids. That procedure enabled efficient removal of high field strength elements (HFSE), such as Hf, Zr and Ti, which are small ions that carry strong charges and develop intense electrostatic fields. The tungsten yields were 85%-95%. Advantages of this system include less time and less use of reagents. Precise and accurate isotopic measurements are possible using fractions of tungsten that are obtained using this method. The accuracy and precision of these measurements were confirmed using various silicate standard rock samples, JB-2, JB-3 and AGV-1.

  16. Modified ion exchange separation for tungsten isotopic measurements from kimberlite samples using multi-collector inductively coupled plasma mass spectrometry.

    PubMed

    Sahoo, Yu Vin; Nakai, Shun'ichi; Ali, Arshad

    2006-03-01

    Tungsten isotope composition of a sample of deep-seated rock can record the influence of core-mantle interaction of the parent magma. Samples of kimberlite, which is known as a carrier of diamond, from the deep mantle might exhibit effects of core-mantle interaction. Although tungsten isotope anomaly was reported for kimberlites from South Africa, a subsequent investigation did not verify the anomaly. The magnesium-rich and calcium-rich chemical composition of kimberlite might engender difficulty during chemical separation of tungsten for isotope analyses. This paper presents a simple, one-step anion exchange technique for precise and accurate determination of tungsten isotopes in kimberlites using multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS). Large quantities of Ca and Mg in kimberlite samples were precipitated and removed with aqueous H(2)SO(4). Highly pure fractions of tungsten for isotopic measurements were obtained following an anion exchange chromatographic procedure involving mixed acids. That procedure enabled efficient removal of high field strength elements (HFSE), such as Hf, Zr and Ti, which are small ions that carry strong charges and develop intense electrostatic fields. The tungsten yields were 85%-95%. Advantages of this system include less time and less use of reagents. Precise and accurate isotopic measurements are possible using fractions of tungsten that are obtained using this method. The accuracy and precision of these measurements were confirmed using various silicate standard rock samples, JB-2, JB-3 and AGV-1. PMID:16496054

  17. Tectonics and metallogenesis of Proterozoic rocks of the Reading Prong

    USGS Publications Warehouse

    Gundersen, L.C.S.

    2004-01-01

    Detailed geologic mapping, petrography, and major and trace-element analyses of Proterozoic rocks from the Greenwood Lake Quadrangle, New York are compared with chemical analyses and stratigraphic information compiled for the entire Reading Prong. A persistent regional stratigraphy is evident in the mapped area whose geochemistry indicates protoliths consistent with a back-arc marginal basin sequence. The proposed marginal basin may have been floored by an older sialic basement and overlain by a basin-fill sequence consisting of a basal tholeiitic basalt, basic to intermediate volcanic or volcaniclastic rocks and carbonate sediments, a bimodal calc-alkaline volcanic sequence, and finally volcaniclastic, marine, and continental sediments. The presence of high-chlorine biotite and scapolite may indicate circulation of brine fluids or the presence of evaporite layers in the sequence. Abundant, stratabound magnetite deposits with a geologic setting very unlike that of cratonic, Proterozoic banded-iron formations are found throughout the proposed basin sequence. Associated with many of the magnetite deposits is unusual uranium and rare-earth element mineralization. It is proposed here that these deposits formed in an exhalative, volcanogenic, depositional environment within an extensional back-arc marginal basin. Such a tectonic setting is consistent with interpretations of protoliths in other portions of the Reading Prong, the Central Metasedimentary Belt of the Canadian Grenville Province, and recent interpretation of the origin of the Franklin lead-zinc deposits, suggesting a more cohesive evolving arc/back-arc tectonic model for the entire Proterozoic margin of the north-eastern portion of the North American craton. Published by Elsevier Ltd.

  18. Tracing the stepwise oxygenation of the Proterozoic ocean.

    PubMed

    Scott, C; Lyons, T W; Bekker, A; Shen, Y; Poulton, S W; Chu, X; Anbar, A D

    2008-03-27

    Biogeochemical signatures preserved in ancient sedimentary rocks provide clues to the nature and timing of the oxygenation of the Earth's atmosphere. Geochemical data suggest that oxygenation proceeded in two broad steps near the beginning and end of the Proterozoic eon (2,500 to 542 million years ago). The oxidation state of the Proterozoic ocean between these two steps and the timing of deep-ocean oxygenation have important implications for the evolutionary course of life on Earth but remain poorly known. Here we present a new perspective on ocean oxygenation based on the authigenic accumulation of the redox-sensitive transition element molybdenum in sulphidic black shales. Accumulation of authigenic molybdenum from sea water is already seen in shales by 2,650 Myr ago; however, the small magnitudes of these enrichments reflect weak or transient sources of dissolved molybdenum before about 2,200 Myr ago, consistent with minimal oxidative weathering of the continents. Enrichments indicative of persistent and vigorous oxidative weathering appear in shales deposited at roughly 2,150 Myr ago, more than 200 million years after the initial rise in atmospheric oxygen. Subsequent expansion of sulphidic conditions after about 1,800 Myr ago (refs 8, 9) maintained a mid-Proterozoic molybdenum reservoir below 20 per cent of the modern inventory, which in turn may have acted as a nutrient feedback limiting the spatiotemporal distribution of euxinic (sulphidic) bottom waters and perhaps the evolutionary and ecological expansion of eukaryotic organisms. By 551 Myr ago, molybdenum contents reflect a greatly expanded oceanic reservoir due to oxygenation of the deep ocean and corresponding decrease in sulphidic conditions in the sediments and water column.

  19. Early Proterozoic ultrahigh pressure metamorphism: evidence from microdiamonds.

    PubMed

    Cartigny, Pierre; Chinn, Ingrid; Viljoen, K S; Robinson, Derek

    2004-05-01

    Microdiamonds from the Akluilâk minette dykes (Nunavut, Canada) are similar to diamonds formed in subducted metamorphic rocks. High concentrations of unaggregated nitrogen and positive delta(15)N suggest that the microdiamonds formed within rocks subducted to ultrahigh pressures before being sampled by the minette magma 1.8 billion years ago. This ultrahigh pressure metamorphism in North America, probably related to the Trans-Hudson orogen (about 2 billion years ago), extends the occurrence of ultrahigh pressure metamorphism from 0.6 billion years to before 1.8 billion years ago and suggests that Phanerozoic-type subductions were active by the Early Proterozoic.

  20. A NEW CHROOCOCCACEAN ALGA FROM THE PROTEROZOIC OF QUEENSLAND

    PubMed Central

    Licari, Gerald R.; Cloud, Preston E.; Smith, W. D.

    1969-01-01

    Nannofossils§ here described are from the middle Proterozoic Paradise Creek Formation, along Paradise Creek in northwestern Queensland, Australia. These fossils, in chert blebs associated with branched stromatolites, comprise cubic colonies analogous to living Eucapsis, a member of the blue-green algal family Chroococcaceae. The age of the enclosing rocks, bracketed by the ages of older and younger granitic events, is about 1.6 × 109 years. We record, therefore, a new chronological and biological datum in the currently accumulating sequence of pre-Paleozoic microbiotas. Images PMID:16591730

  1. Continental growth through time by underplating of subducted oceanic crust: Evidence from kimberlites in South Africa and SW Pacific

    NASA Technical Reports Server (NTRS)

    Taylor, Lawrence A.; Neal, Clive R.

    1988-01-01

    In the dynamic model of plate tectonics, it is evident that crustal components are returned to the mantle by subduction. Chemical signatures of these subducted components were identified in ocean island volcanics and in island arc volcanics. Indeed, an origin involving a subducted protolith was postulated for certain types of xenoliths in kimberlite, including diamonds. Recent studies of eclogite xenoliths in kimberlite from southern Africa and megacrysts form the Malaitan alnoite, Solomon islands, indicate that lithospheric underplating by subducted oceanic crust has occurred in these two contrasting areas. The results of new eclogite studies from the Bellsbank kimberlite, South Africa, and isotopic data from the Malaitan alnoite megacryst suite. This forms the basis for discerning the role of lithospheric underplating in the growth of cratons and in the evolution of mantle-derived magma.

  2. Molar tooth carbonates and benthic methane fluxes in Proterozoic oceans.

    PubMed

    Shen, Bing; Dong, Lin; Xiao, Shuhai; Lang, Xianguo; Huang, Kangjun; Peng, Yongbo; Zhou, Chuanming; Ke, Shan; Liu, Pengju

    2016-01-01

    Molar tooth structures are ptygmatically folded and microspar-filled structures common in early- and mid-Proterozoic (∼2,500-750 million years ago, Ma) subtidal successions, but extremely rare in rocks <750 Ma. Here, on the basis of Mg and S isotopes, we show that molar tooth structures may have formed within sediments where microbial sulphate reduction and methanogenesis converged. The convergence was driven by the abundant production of methyl sulphides (dimethyl sulphide and methanethiol) in euxinic or H2S-rich seawaters that were widespread in Proterozoic continental margins. In this convergence zone, methyl sulphides served as a non-competitive substrate supporting methane generation and methanethiol inhibited anaerobic oxidation of methane, resulting in the buildup of CH4, formation of degassing cracks in sediments and an increase in the benthic methane flux from sediments. Precipitation of crack-filling microspar was driven by methanogenesis-related alkalinity accumulation. Deep ocean ventilation and oxygenation around 750 Ma brought molar tooth structures to an end. PMID:26739600

  3. Microbenthic distribution of Proterozoic tidal flats: environmental and taphonomic considerations

    NASA Technical Reports Server (NTRS)

    Kah, L. C.; Knoll, A. H.

    1996-01-01

    Silicified carbonates of the late Mesoproterozoic to early Neoproterozoic Society Cliffs Formation, Baffin Island, contain distinctive microfabrics and microbenthic assemblages whose paleo-environmental distribution within the formation parallels the distribution of these elements through Proterozoic time. In the Society Cliffs Formation, restricted carbonates--including microdigitate stromatolites, laminated tufa, and tufted microbial mats--consist predominantly of synsedimentary cements; these facies and the cyanobacterial fossils they contain are common in Paleoproterozoic successions but rare in Neoproterozoic and younger rocks. Less restricted tidal-flat facies in the formation are composed of laminated microbialites dominated by micritic carbonate lithified early, yet demonstrably after compaction; these strata contain cyanobacteria that are characteristic in Neoproterozoic rocks. Within the formation, the facies-dependent distribution of microbial populations reflects both the style and timing of carbonate deposition because of the strong substrate specificity of benthic cyanobacteria. A reasonable conclusion is that secular changes in microbenthic assemblages through Proterozoic time reflect a decrease in the overall representation of rapidly lithified carbonate substrates in younger peritidal environments, as well as concomitant changes in the taphonomic window of silicification through which early life is observed.

  4. Molar tooth carbonates and benthic methane fluxes in Proterozoic oceans

    NASA Astrophysics Data System (ADS)

    Shen, Bing; Dong, Lin; Xiao, Shuhai; Lang, Xianguo; Huang, Kangjun; Peng, Yongbo; Zhou, Chuanming; Ke, Shan; Liu, Pengju

    2016-01-01

    Molar tooth structures are ptygmatically folded and microspar-filled structures common in early- and mid-Proterozoic (~2,500-750 million years ago, Ma) subtidal successions, but extremely rare in rocks <750 Ma. Here, on the basis of Mg and S isotopes, we show that molar tooth structures may have formed within sediments where microbial sulphate reduction and methanogenesis converged. The convergence was driven by the abundant production of methyl sulphides (dimethyl sulphide and methanethiol) in euxinic or H2S-rich seawaters that were widespread in Proterozoic continental margins. In this convergence zone, methyl sulphides served as a non-competitive substrate supporting methane generation and methanethiol inhibited anaerobic oxidation of methane, resulting in the buildup of CH4, formation of degassing cracks in sediments and an increase in the benthic methane flux from sediments. Precipitation of crack-filling microspar was driven by methanogenesis-related alkalinity accumulation. Deep ocean ventilation and oxygenation around 750 Ma brought molar tooth structures to an end.

  5. Molar tooth carbonates and benthic methane fluxes in Proterozoic oceans

    PubMed Central

    Shen, Bing; Dong, Lin; Xiao, Shuhai; Lang, Xianguo; Huang, Kangjun; Peng, Yongbo; Zhou, Chuanming; Ke, Shan; Liu, Pengju

    2016-01-01

    Molar tooth structures are ptygmatically folded and microspar-filled structures common in early- and mid-Proterozoic (∼2,500–750 million years ago, Ma) subtidal successions, but extremely rare in rocks <750 Ma. Here, on the basis of Mg and S isotopes, we show that molar tooth structures may have formed within sediments where microbial sulphate reduction and methanogenesis converged. The convergence was driven by the abundant production of methyl sulphides (dimethyl sulphide and methanethiol) in euxinic or H2S-rich seawaters that were widespread in Proterozoic continental margins. In this convergence zone, methyl sulphides served as a non-competitive substrate supporting methane generation and methanethiol inhibited anaerobic oxidation of methane, resulting in the buildup of CH4, formation of degassing cracks in sediments and an increase in the benthic methane flux from sediments. Precipitation of crack-filling microspar was driven by methanogenesis-related alkalinity accumulation. Deep ocean ventilation and oxygenation around 750 Ma brought molar tooth structures to an end. PMID:26739600

  6. Causes and consequences of mid-Proterozoic anoxia

    NASA Astrophysics Data System (ADS)

    Derry, Louis A.

    2015-10-01

    Evidence for low pO2 and a ferruginous ocean characterize the mid-Proterozoic (1.8-0.8 Ga). Considerations of redox sources and sinks imply that generation of O2 via organic carbon (Corg) burial must be low to maintain a low pO2 atmosphere for geologically long intervals, yet low oxygen should result in increased Corg preservation. Marine export production must therefore be low to limit Corg burial and O2 generation. Formation of ferrous phosphate can buffer deepwater phosphate (Pi) to levels an order of magnitude or more below those in the modern ocean, limiting export production. Low deepwater Pi is consistent with the hiatus in sedimentary phosphorite deposits during the mid-Proterozoic, and low pO2 limits formation of sedimentary iron deposits (BIF). We propose that low pO2 was maintained by P limitation resulting from ferrous phosphate buffering. The near-absence of BIF and phosphorite deposition is direct and indirect consequences of the low pO2, respectively.

  7. Specific features of the melt percolation in mantle beneath the kimberlite pipes

    NASA Astrophysics Data System (ADS)

    Ashchepkov, I.

    2004-05-01

    Layered mantle sequences beneath kimberlite pipes influences on melt- fluid percolation. Units recognized from top: 1. Sp- and Sp-garnet facies primitive or Si-enriched. 10-20 kbar (I) heated to 70-90 mv/m2 similar to alkali basalt xenoliths; 2. Upper garnet facie -coarse enriched lherzolites 20-35 kbar (I-II) are diapiric rising from submelted (II) 3.Pyroxenite lens 35-45 kbar (III) 35-55 mv/m2 accumulated water from subducted peridotite dehydration; 4.Layered primary subduction harzburgite-eclogite sequence- 2-4 layers, 45-60 kbar (IV) 35-40 mv/m2; 5. Coarse Ga-dunites, B,C eclogites 60-65 kbar (V) 35 mv/m2 layer(Pokhilenko, Sobolev, 1987) is part of mantle wedge washed by fluids; 6. Lower asthenospheric unit are sheared or melt interacted peridotites resulted from melt impregnation or hot (to1400oC) pyroxenites intrusion 65-75 kbar (VI)or convective motion (Nixon,Boyd,1973); 7. Lower part lithospheric of keel dunites (Pokhilenko et al, 2003) or mixed with the pyroxenites or convective mantle material 75-120 kbar (VII) with subadiabatic PT conditions. Melt types. 1.Anatexic peridotite melts at 35 -40 kbar due to dehydration of slabs (hi- SiO2, Al2O3, CaO -alkalies ) rised to 19-12 kbars enriching diapir roofs and phase boundaries. 2. Subduction- related fluids-melts rising from slabs <45 with Na-Fe or 3.K- LILE metasomatites >70 kbar in continental margins (K- richterites and phlogopites decomposition); deep plum ultramafic kimberlite melts at 200 boundary melt due to density inversion (Agee, 2000), 4. differentiated protokimberlite melts created pre-eruption mantle feeding system; 5. basaltic melts came from 660km and crossing 200 boundary and stopped ~35 40 kbars interacting with pyroxenite lens, next light fluid-rich fractions riching 30-20 asthenospheric trap (Wyllie, 1973) causing Fe- Al enrichment. 7. Various hybrid melts produce by 2 types of plums forming pyroxenites, hydrous metasomatites near phase boundaries and tops of slabs layers. Growth of the

  8. The Homestead kimberlite, central Montana, USA: mineralogy, xenocrysts, and upper-mantle xenoliths

    NASA Astrophysics Data System (ADS)

    Carter Hearn, B.

    2004-09-01

    The Homestead kimberlite was emplaced in lower Cretaceous marine shale and siltstone in the Grassrange area of central Montana. The Grassrange area includes aillikite, alnoite, carbonatite, kimberlite, and monchiquite and is situated within the Archean Wyoming craton. The kimberlite contains 25-30 modal% olivine as xenocrysts and phenocrysts in a matrix of phlogopite, monticellite, diopside, serpentine, chlorite, hydrous Ca-Al-Na silicates, perovskite, and spinel. The rock is kimberlite based on mineralogy, the presence of atoll-textured groundmass spinels, and kimberlitic core-rim zoning of groundmass spinels and groundmass phlogopites. Garnet xenocrysts are mainly Cr-pyropes, of which 2-12% are G10 compositions, crustal almandines are rare and eclogitic garnets are absent. Spinel xenocrysts have MgO and Cr 2O 3 contents ranging into the diamond inclusion field. Mg-ilmenite xenocrysts contain 7-11 wt.% MgO and 0.8-1.9 wt.% Cr 2O 3, with (Fe +3/Fe tot) from 0.17-0.31. Olivine is the only obvious megacryst mineral present. One microdiamond was recovered from caustic fusion of a 45-kg sample. Upper-mantle xenoliths up to 70 cm size are abundant and are some of the largest known garnet peridotite xenoliths in North America. The xenolith suite is dominated by dunites, and harzburgites containing garnet and/or spinel. Granulites are rare and eclogites are absent. Among 153 xenoliths, 7% are lherzolites, 61% are harzburgites, 31% are dunites, and 1% are orthopyroxenites. Three of 30 peridotite xenoliths that were analysed are low-Ca garnet-spinel harzburgites containing G10 garnets. Xenolith textures are mainly coarse granular, and only 5% are porphyroclastic. Xenolith modal mineralogy and mineral compositions indicate ancient major-element depletion as observed in other Wyoming craton xenolith assemblages, followed by younger enrichment events evidenced by tectonized or undeformed veins of orthopyroxenite, clinopyroxenite, websterite, and the presence of phlogopite

  9. The Homestead kimberlite, central Montana, USA: Mineralogy, xenocrysts, and upper-mantle xenoliths

    USGS Publications Warehouse

    Carter, Hearn B.

    2004-01-01

    The Homestead kimberlite was emplaced in lower Cretaceous marine shale and siltstone in the Grassrange area of central Montana. The Grassrange area includes aillikite, alnoite, carbonatite, kimberlite, and monchiquite and is situated within the Archean Wyoming craton. The kimberlite contains 25-30 modal% olivine as xenocrysts and phenocrysts in a matrix of phlogopite, monticellite, diopside, serpentine, chlorite, hydrous Ca-Al-Na silicates, perovskite, and spinel. The rock is kimberlite based on mineralogy, the presence of atoll-textured groundmass spinels, and kimberlitic core-rim zoning of groundmass spinels and groundmass phlogopites. Garnet xenocrysts are mainly Cr-pyropes, of which 2-12% are G10 compositions, crustal almandines are rare and eclogitic garnets are absent. Spinel xenocrysts have MgO and Cr2O3 contents ranging into the diamond inclusion field. Mg-ilmenite xenocrysts contain 7-11 wt.% MgO and 0.8-1.9 wt.% Cr2O3, with (Fe+3/Fetot) from 0.17-0.31. Olivine is the only obvious megacryst mineral present. One microdiamond was recovered from caustic fusion of a 45-kg sample. Upper-mantle xenoliths up to 70 cm size are abundant and are some of the largest known garnet peridotite xenoliths in North America. The xenolith suite is dominated by dunites, and harzburgites containing garnet and/or spinel. Granulites are rare and eclogites are absent. Among 153 xenoliths, 7% are lherzolites, 61% are harzburgites, 31% are dunites, and 1% are orthopyroxenites. Three of 30 peridotite xenoliths that were analysed are low-Ca garnet-spinel harzburgites containing G10 garnets. Xenolith textures are mainly coarse granular, and only 5% are porphyroclastic. Xenolith modal mineralogy and mineral compositions indicate ancient major-element depletion as observed in other Wyoming craton xenolith assemblages, followed by younger enrichment events evidenced by tectonized or undeformed veins of orthopyroxenite, clinopyroxenite, websterite, and the presence of phlogopite

  10. Crystallization of diamond from a silicate melt of kimberlite composition in high-pressure and high-temperature experiments

    SciTech Connect

    Arima, Makoto; Nakayama, Kazuhiro ); Akaishi, Minoru; Yamaoka, Shinobu; Kanda, Hisao )

    1993-11-01

    In high-pressure and high-temperature experiments (1800-2200[degrees]C and 7.0-7.7 GPa), diamond crystallized and grew in a volatile-rich silicate melt of kimberlite composition. This diamond has well-developed [111] faces, and its morphologic characteristics resemble those of natural diamond but differ from those of synthetic diamond grown from metallic solvent-catalysts. The kimberlite melt has a strong solvent-catalytic effect on diamond formation, supporting the view that some natural diamonds crystallized from volatile-rich melts in the upper mantle. 19 refs., 3 figs., 1 tab.

  11. Comparative characteristic of diamonds with olivine inclusions from the Ebelyakh placer and kimberlite pipes of the Yakutian Diamondiferous Province

    NASA Astrophysics Data System (ADS)

    Ugap'eva, S. S.; Pavlushin, A. D.; Goryainov, S. V.; Afanasiev, V. P.; Pokhilenko, N. P.

    2016-05-01

    The results of morphological examination and the character of the structural orientation and estimation of residual pressure calculated from spectra of combination dispersion in olivine inclusions within diamonds of the Ebelyakh placer and kimberlite pipes of the Yakutian Diamondiferous Province are presented. The data analysis aimed at revealing indications of similarity and/or differences between diamonds from the pipes and the placer. Differences in the structural orientation and spectra of combination dispersion of the inclusions of olivine in dodecahedroids of placers of the northeastern part of the Siberian Platform support the assumption of their non-kimberlite nature.

  12. Mn-ilmenites from the Norris kimberlite: metasomatism in the mantle of the south appalachians

    SciTech Connect

    Klobcar, C.L.; Taylor, L.A.

    1985-01-01

    Kimberlites provide petrologists a tantalyzingly diverse sample of the mantle. The Norris kimberlite (30 mile North of Knoxville, Tennessee) contains a unique suite of ilmenite nodules and megacrysts that span a wide compositional range. Some nodules contain the highest MnO contents yet reported (up to 71 mol% MnTiO/sub 3/). These ilmenites reflect redox changes in the upper mantle/lower crust and are our only samples of the mantle underlying the South Appalachians. Ilmenite can be divided into three groups: I high MnO (to 31 wt%), low MgO (<1 wt%); II High Mgo (to 15 wt%), low Cr/sub 2/O/sub 3/ (<1 wt%); and III High Cr/sub 2/O/sub 3/ (to 6.5 wt%) high MgO (to 12 wt%). Ilmenite can also be grouped by Fe/sup 2 +//Fe/sup 2 +/ + Fe/sup 3 +/ (Fe') into low Fe' (<0.8) and high Fe' (>0.8) (cf. Tompkins and Haggerty, 1985). Type II shows no marked increase in MgO from core to rim, common in other kimberlites; Fe' is also constant. Type I occurs in a variety of forms and is secondary to Types II and III. These ilmenites reflect a distinct evolution in the redox conditions which occurred during their formation. Type I (high MnO) formed at some later time and involved a highly-reducing form of metasomatism. Alteration of ilmenites also formed perovskite and spinel. This represents a unique type of Mn metasomatism and emphasizes the heterogeneity of the mantle of the N.A. craton.

  13. On the unusual characteristics of the diamonds from Letšeng-la-Terae kimberlites, Lesotho

    NASA Astrophysics Data System (ADS)

    Bowen, Debbie C.; Ferraris, Ray D.; Palmer, Claire E.; Ward, John D.

    2009-11-01

    The Letšeng-la-Terae kimberlites are situated 3100 m above sea level in the Maloti Mountains of Lesotho, southern Africa. The principal economic bodies are two Late Cretaceous, low grade, 1-3.5 carats/hundred ton (cpht), kimberlite pipes that host high-value diamonds realising US 2000-2500/carat (/ct) in 2008 terms. Locally, the larger kimberlite body is referred to as the Main Pipe (17.2 ha) and the smaller one is called the Satellite Pipe (5.2 ha). These pipes, and their associated eluvial and proximal alluvial deposits, are renowned for yielding large, "D" colour, gem quality diamonds, including + 100 carat (ct) stones. Earlier artisanal effort (1959-1977) and formal mining (1977-1982) produced 335,000 carats (cts), including the 601 ct Lesotho Brown in 1968. In 2003, Letšeng Diamonds Limited re-commenced mining operations and had produced 265,000 cts by the end of July 2008, including 24 + 100 ct diamonds, the largest of which was the 603 ct Lesotho Promise. We report here on the unusual characteristics of the Letšeng diamond population that include: 75% gem quality that is more commonly associated with alluvial diamond deposits, large average stone size of ca. 1 carat/stone (ct/stn) that is also more typical of certain alluvial diamond placers, high-yielding, rounded to flattened irregular, resorbed dodecahedral shapes (Main Pipe 67% and Satellite Pipe 87%) with subordinate dodecahedral macle (Main Pipe 32% and Satellite Pipe 12%) and broken (ca. 1%) forms. In both pipes the octahedral component is virtually absent (< 0.1%), economically favourable colour mix (ca. 33% white colour diamonds in both pipes), abundance of nitrogen-free, "D" colour, Type IIa diamonds that dominate the internationally recognised "special" stone size fraction which covers all diamonds larger than + 10.8 cts (Main Pipe 32% and Satellite Pipe 51%). During 2008, these larger, "special" diamonds commanded prices in excess of US 15,000/ct, contributing ca. 75% of the revenue generated

  14. PGE distribution in deformed lherzolites of the Udachnaya kimberlite pipe (Yakutia)

    NASA Astrophysics Data System (ADS)

    Ilyina, O. V.; Tychkov, N. S.; Agashev, A. M.; Golovin, A. V.; Izokh, A. E.; Kozmenko, O. A.; Pokhilenko, N. P.

    2016-04-01

    The results of the first study of the PGE distribution in deformed lherzolites of the Udachnaya kimberlite pipe (Yakutia) are presented here. The complex character of evolution of the PGE composition in the Deformed lherzolites is assumed to be the result of silicate metasomatism. At the first stage, growth in the amount of clinopyroxene and garnet in the rock is accompanied by a decrease in the concentration of the compatible PGE (Os, Ir). During the final stage, the rock is enriched with incompatible PGE (Pt, Pd) and Re possible due to precipitation of submicron-sized particles of sulfides in the interstitial space of these mantle rocks.

  15. Tychite in mantle xenoliths from kimberlites: The first find and a new genetic type

    NASA Astrophysics Data System (ADS)

    Sharygin, I. S.; Golovin, A. V.; Korsakov, A. V.; Pokhilenko, N. P.

    2016-03-01

    Tychite Na6Mg2(CO3)4(SO3) is a rare natural Na and Mg sulfatocarbonate. It is found only as minor mineral in deposits of saline lakes in the United States, Canada, Uganda, and China. In these continental evaporites tychite has sedimentary genesis. In this study, we report the first occurrence of tychite as a crystal phase in the melt inclusions in olivine from mantle xenoliths of the Udachnaya-East kimberlite pipe. This find provides an evidence for the probability of tychite crystallization from melts; i.e., this rare sulfatocarbonate may have a magmatic origin as well.

  16. Proterozoic orogens in southern Peninsular India: Contiguities and complexities

    NASA Astrophysics Data System (ADS)

    Chetty, T. R. K.; Santosh, M.

    2013-12-01

    The Precambrian terranes of southern Peninsular India have been central to discussions on the history of formation and breakup of supercontinents. Of particular interest are the Proterozoic high grade metamorphic orogens at the southern and eastern margins of the Indian shield, skirting the 3.4 Ga Dharwar craton which not only preserve important records of lower crustal processes and lithospheric geodynamics, but also carry imprints of the tectonic framework related to the assembly of the major Neoproterozoic supercontinents - Rodinia and Gondwana. These Proterozoic orogens are described as Southern Granulite Terrane (SGT) in the southern tip and the Eastern Ghats Mobile Belt (EGMB) in the eastern domains of the peninsula. The contiguity of these orogens is broken for a distance of ˜400 km and disappears in the Bay of Bengal. These orogens expose windows of middle to lower crust with well-preserved rock records displaying multiple tectonothermal events and multiphase exhumation paths.Recent studies in these orogens have led to the recognition of discrete crustal blocks or terranes separated by major shear zone systems, some of which represent collisional sutures. The SGT and EGMB carry several important features such as fold-thrust tectonics, regional granulite facies metamorphism of up to ultrahigh-temperature conditions in some cases, multiple P-T paths, development of lithospheric shear zones, emplacement of ophiolites, presence of alkaline and anorthositic complexes, development of crustal-scale "flower structures", transpressional strains, and reactivation tectonics. A heterogeneous distribution of different metamorphic and magmatic assemblages with distinct spatial and temporal strain variations in shaping the fabric elements in different blocks is identified. Both EGMB and SGT share a common transpressional deformation history during the latest Neoproterozoic characterized by the steepening of the initial low angle crustal scale structures leading to a

  17. Sm-Nd, K-Ar and petrologic study of some kimberlites from eastern United States and their implication for mantle evolution

    USGS Publications Warehouse

    Basu, A.R.; Rubury, E.; Mehnert, H.; Tatsumoto, M.

    1984-01-01

    We provide new data on Sm-Nd systematics, K-Ar dating and the major element chemistry of kimberlites from the eastern United States (mostly from central New York State) and their constituent mineral phases of olivine, clinopyroxene, garnet, phlogopite and perovskite. In addition, we report Nd-isotopes in a few kimberlites from South Africa, Lesotho and from the eastern part of China. The major element compositions of the New York dike rocks and of their constituent minerals including a xenolith of eclogite are comparable with those from the Kimberley area in South Africa. The K-Ar age of emplacement of the New York dikes is further established to be 143 Ma. We have analyzed the Nd-isotopic composition of the following kimberlites and related rocks: Nine kimberlite pipes from South Africa and Lesotho, two from southern India; one from the U.S.S.R., fifteen kimberlite pipes and related dike rocks from eastern and central U.S. and two pipes from the Shandong Province of eastern China. The age of emplacement of these kimberlites ranges from 1300 million years to 90 million years. The initial Nd-isotopic compositions of these kimberlitic rocks expressed as e{open}NdIwith respect to a chondritic bulk-earth growth-curve show a range between 0 and +4, with the majority of the kimberlites being in the range 0 to +2. This range is not matched by any other suite of mantle-derived igneous rocks. This result strengthens our earlier conclusion that kimberlitic liquids are derived from a relatively primeval and unique mantle reservoir with a nearly chondritic Sm/Nd ratio. ?? 1984 Springer-Verlag.

  18. H2O-CO2 solubility in low SiO2-melts and the unique mode of kimberlite degassing and emplacement

    NASA Astrophysics Data System (ADS)

    Moussallam, Yves; Morizet, Yann; Gaillard, Fabrice

    2016-08-01

    Kimberlites are the most deep-seated magmas in the mantle and ascend to the surface at an impressive speed, travelling hundreds of kilometres in just hours while carrying a substantial load of xenolithic material, including diamonds. The ascent dynamics of these melts are buoyancy-controlled and certainly driven by outgassing of volatile species, presumably H2O and CO2, summing to concentration level of ca 15-30 wt.% in kimberlite melts. We provide H2O-CO2 solubility data obtained on quenched glasses that are synthetic analogues of kimberlite melts (SiO2 content ranging from 18 to 28 wt.%). The experiments were conducted in the pressure range 100 to 350 MPa. While the CO2 solubility can reach 20 wt.%, we show that the H2O solubility in these low silica melts is indistinguishable from that found for basalts. Moreover, whereas in typical basalts most of the water exsolves at shallower pressure than the CO2, the opposite relationship is true for the low-SiO2 composition investigated. These data show that kimberlites can rise to depths of the upper crust without suffering significant degassing and must release large quantities of volatiles (>15 wt.%) within the very last few kilometres of ascent. This unconventional degassing path may explain the characteristic pipe, widening-upward from a ≤2.5 km deep root zone, where kimberlites are mined for diamonds. Furthermore, we show that small changes in melt chemistry and original volatile composition (H2O vs. CO2) provide a single mechanism to explain the variety of morphologies of kimberlite pipes found over the world. The cooling associated to such massive degassing must freeze a large quantity of melt explaining the occurrence of hypabyssal kimberlite. Finally, we provide strong constraints on the primary volatile content of kimberlite, showing that the water content reported for kimberlite magma is mostly reflective of secondary alteration.

  19. Proterozoic microfossils revealing the time of algal divergences

    NASA Astrophysics Data System (ADS)

    Moczydlowska-Vidal, Malgorzata

    2010-05-01

    Proterozoic microfossils revealing the time of algal divergences Małgorzata Moczydłowska-Vidal Uppsala University, Department of Earth Sciences, Palaeobiology, Villavägen 16, SE 752 36 Uppsala, Sweden (malgo.vidal@pal.uu.se) Morphological and reproductive features and cell wall ultrastructure and biochemistry of Proterozoic acritarchs are used to determine their affinity to modern algae. The first appearance datum of these microbiota is traced to infer a minimum age of the divergence of the algal classes to which they may belong. The chronological appearance of microfossils that represent phycoma-like and zygotic cysts and vegetative cells and/or aplanospores, respectively interpreted as prasinophyceaen and chlorophyceaen microalgae, is related to the Viridiplantae phylogeny. These divergence times differ from molecular clock estimates, and the palaeontological evidence suggests that they are older. The best examples of unicellular, organic-walled microfossils (acritarchs) from the Mesoproterozoic to Early Ordovician are reviewed to demonstrate features, which are indicative of their affinity to photosynthetic microalgae. The first indication that a microfossil may be algal is a decay- and acid-resistant cell wall, which reflects its biochemistry and ultrastructure, and probably indicates the ability to protect a resting/reproductive cyst. The biopolymers synthesized in the cell walls of algae and in land plants ("plant cells"), such as sporopollenin/algaenan, are diagnostic for photosynthetic taxa and were inherited from early unicellular ancestors. These preservable cell walls are resistant to acetolysis, hydrolysis and acids, and show diagnostic ultrastructures such as the trilaminar sheath structure (TLS). "Plant cell" walls differ in terms of chemical compounds, which give high preservation potential, from fungal and animal cell walls. Fungal and animal cells are fossilized only by syngenetic permineralization, whereas "plant cells" are fossilized as body

  20. Buried Proterozoic foredeep under the Western Canada Sedimentary Basin?

    NASA Astrophysics Data System (ADS)

    Boerner, D. E.; Kurtz, R. D.; Craven, J. A.; Rondenay, S.; Qian, W.

    1995-04-01

    Electromagnetic studies of the Precambrian basement beneath the Western Canada Sedimentary Basin in Alberta indicate a narrow linear conductivity anomaly spatially correlated with a strong positive magnetic feature, the Red Deer high. The conductor is located below sedimentary cover near the top of the crystalline basement and has limited depth extent. We propose that this anomalous feature represents graphitic metasedimentary rocks in the euxinic-flysch facies of a Proterozoic foredeep sequence. The strong magnetic anomaly results from an associated iron formation deposited on the outer ramp of the foredeep. This model explains the geophysical anomalies, has analogues on the exposed shield, and is consistent with the timing, deformation history, and known geology of the Precambrian basement.

  1. Synthesis of Early and Middle Proterozoic orogenies in the southwest

    SciTech Connect

    Karlstrom, K.E. . Dept. of Geology)

    1993-02-01

    Accretion of juvenile crust from 1.8 (Colorado) to 1.72 (Arizona) was accompanied by formation, evolution, and assembly of oceanic island arcs in an Indonesian-style orogenic belt. 2.5--2.0 Ga crust formed basement for arcs in the Mojave Province. The Hualapai block marks a zone where arcs were built across the transition from 2.5--2.0 Ga (NW) to juvenile 1.75--1.72 Ga crust (SE). Early NW-striking low-angle foliations formed between 1.74--1.72 Ga and record arc accretion and outboard collisions. Sutures and major transcurrent boundaries between arcs terranes remain cryptic because of 1.7--1.69 Ga crustal shortening that records welding of terranes to Laurentia. 1.7 Ga quartzite-rhyolite sections from a belt parallel to the southern extent of 1.8--1.72 Ga crust and were syntectonic craton margin and intracratonic basins. Depositions of 1.65 Ga quartzite-rhyolite sections took place during the collision of the Mazatzal province. A major boundary between crustal provinces is marked by the Slate Creek shear zone (AZ) - Jemez lineament (NM) although Mazatzal (1.65 Ga) deformation affected the foreland of the Yavapai Province to the north. The Laurasian supercontinent was assembled by 1.6 Ga and tectonism paused in the Southwest (1.6--1.5 Ga) without appreciable uplift of crust. Middle Proterozoic tectonism (1.5--1.3 Ga) has many of the hallmarks of an orogeny rather than anorogeny'. It involved massive lower crustal melting, granitoid plutonism, regional uplift and unroofing of 10-15 km, regional resetting of Rb-Sr and K-Ar isotopic systems, addition of juvenile crust in the Mid-continent, and locally important deformation and metamorphism around plutons and perhaps over large regions. Pluton generation and ascent were strongly controlled by Early Proterozoic lithospheric structure.

  2. Proterozoic history of the midcontinent region of North America

    NASA Astrophysics Data System (ADS)

    Bickford, M. E.; van, W. R.; Zietz, Isidore

    1986-06-01

    Age and petrographic data from the buried basement of the midcontinent region of North America, integrated with information from exposed rocks and magnetic- and gravity-anomaly maps, allow much of the Proterozoic history of the region to be assembled. The Superior craton may be traced into the subsurface on the basis of characteristic magnetic patterns and limited age data. The region between the Superior craton and the Wyoming craton to the west is evidently underlain by southerly extension of the Trans-Hudson orogen of Canada. The Penokean orogen formed on the southern margin of the Superior craton 1890 1830 Ma, but is not inferred west of northwestern Iowa in the subsurface. Between 1780 and 1720 Ma, a major orogen developed along the southern margin of the continent and is exposed in Arizona and Colorado. These rocks are volcanogenic and, for the most part, juvenile additions to the crust; they can be traced beneath the plains as far as eastern Kansas and Nebraska. Another orogen formed farther to the south about 1700 1630 Ma and is exposed in southern Arizona and New Mexico; rocks of this age and type have beer, traced as far east as central Missouri but may extend as far as central Michigan. A major geophysical feature of the midcontinent is a system of northwest-trending magnetic and gravity anomalies in Missouri, Kansas, and Nebraska; the origin of these is not currently understood. The tectonic history of the midcontinent between 1480 and 1340 Ma was dominated by extensional formation of two widespread granite-rhyolite terranes that evidently were formed from, and overlie, the orogenic provinces. The older, formed 1450 1480 Ma, underlies the eastern midcontinent, whereas the younger, formed 1340 1400 Ma, underlies the southwestern midcontinent. The latest Proterozoic events were the formation of the midcontinent rift system and the collisional Grenville and Llano provinces about 1100 Ma.

  3. Iron isotope composition of some Archean and Proterozoic iron formations

    NASA Astrophysics Data System (ADS)

    Planavsky, Noah; Rouxel, Olivier J.; Bekker, Andrey; Hofmann, Axel; Little, Crispin T. S.; Lyons, Timothy W.

    2012-03-01

    Fe isotopes can provide new insight into redox-dependent biogeochemical processes. Precambrian iron formations (IF) are deserving targets for Fe isotope studies because they are composed predominantly of authigenic Fe phases and record a period of unprecedented iron deposition in Earth's history. We present Fe isotope data for bulk samples from 24 Archean and Proterozoic IF and eight Phanerozoic Fe oxide-rich deposits. These data reveal that many Archean and early Paleoproterozoic iron formations were a sink for isotopically heavy Fe, in contrast to later Proterozoic and Phanerozoic Fe oxide-rich rocks. The positive δ56Fe values in IF are best explained by delivery of particulate ferric oxides formed in the water column to the sediment-water interface. Because IF are a net sink for isotopically heavy Fe, there must be a corresponding pool of isotopically light Fe in the sedimentary record. Earlier work suggested that Archean pyritic black shales were an important part of this light sink before 2.35 billion years ago (Ga). It is therefore likely that the persistently and anomalously low δ56Fe values in shales are linked with the deposition of isotopically heavy Fe in IF in the deeper parts of basins. IF deposition produced a residual isotopically light dissolved Fe pool that was captured by pyritic Fe in shales. Local dissimilatory Fe reduction in porewater and associated diagenetic reactions resulting in pyrite and carbonate precipitation may have further enhanced Fe isotope heterogeneity in marine sediments, and an 'iron shuttle' may have transported isotopically light Fe from shelf sediments to the basin. Nevertheless, water-column processing of hydrothermally delivered Fe likely had the strongest influence on the bulk iron isotope composition of Archean and Paleoproterozoic iron formations and other marine sediments.

  4. [Study on the micro-infrared spectra and origin of polycrystalline diamonds from Mengyin kimberlite pipes].

    PubMed

    Yang, Zhi-Jun; Liang, Rong; Zeng, Xiang-Qing; Ge, Tie-Yan; Ai, Qun; Zheng, Yun-Long; Peng, Ming-Sheng

    2012-06-01

    The natural polycrystalline diamonds from the Mengyin kimberlite pipes can be classified as the euhedral faceted polycrystalline diamonds and anhedral rounded polycrystalline diamonds. The results of micro-FTIR spectra characterization of the polycrystalline diamonds show that the concentration of nitrogen is low, varying from 16.69 to 72.81 microgram per gram and is different among different diamond grains or position in polycrystalline diamonds. The euhedral faceted polycrystalline diamonds are Ia AB type and have higher concentration of A-center defects than B-center defects. Most of the anhedral rounded polycrystalline diamonds are Ia AB type and have higher content of B-center defects. A minority of the anhedral rounded polycrystalline diamonds have C-center, A-center and B-center defects simultaneously. The polycrystalline diamonds probably originated from the relatively deeper mantle and were not formed in diamond nucleation stage, but in the diamond growth period or some special conditions after the diamond grains were formed already. Furthermore, the euhedral faceted polycrystalline diamonds were formed slightly later and the anhedral rounded polycrystalline diamonds were formed obviously earlier than the diamond single crystals from the Mengyin kimberlite pipes.

  5. [Research on the X-ray fluorescence spectrometry method to determine trace elements in kimberlite].

    PubMed

    Zhang, Lei; Yan, Chuan-wei; Lu, Yi

    2003-04-01

    It is very important to detect trace elements for kilmberlite. Through improving the working conditions of X-ray fluorescence spectrometer and optimizing the analytical conditions, the determination method of trace elements, such as Sc, Cr, Ni, Y, Nb, La, in kimberlite was worked out. The method has been successfully applied to the determination of trace elements in over 2 thousand samples of kimberlite from Liaoning province. The detection limits of the method were relatively low (the detection limit of Sc droped from 9.54 to 2.83 micrograms.g-1 and the detection limit of La droped from 21.68 micrograms.g-1 to 9.18 micrograms.g-1), i.e. 2.83, 2.15, 2.20, 1.17, 1.05 and 9.18 micrograms.g-1 for Sc, Cr, Ni, Y, Nb and La, respectively. The precision of the method was very high with 2.10%-7.09% of RSD (n = 20). Compared with ICP spectrometry this method is satisfactory. The method has proven to be simple and rapid with low cost and high efficiency.

  6. Garnet peridotite xenoliths in a Montana, U.S.A., kimberlite

    USGS Publications Warehouse

    Carter, Hearn B.; Boyd, F.R.

    1975-01-01

    Within a swarm of late middle Eocene subsilicic-alkalic diatremes, one diatreme 270 by 370 m and an associated dike contain common xenoliths of granulite and rare xenoliths of spinel peridotite and garnet peridotite. Six garnet lherzolite xenoliths have been found and these show a range of textures. Four are granular, and two are intensely sheared. Phlogopite is absent from the intensely sheared xenoliths and is thought to be primary in part in the granular xenoliths. Estimated temperatures and depths of equilibration of xenolith pyroxenes range from 920??C, 106 km (32 kbar) to 1315??C, 148 km (47 kbar). The xenoliths show increasing amounts of deformation with greater inferred depths of origin. The temperature-depth points suggest a segment of an Eocene geotherm for Montana which is similar in slope to the steep portion of the pyroxene-determined Lesotho geotherm (Boyd and Nixon, this volume) and is considerably steeper than typical calculated shield and continental geotherms at present. The steep trend could be a result of plate-tectonic shearing and magma ascension within an Eocene low-velocity zone. Preservation of intensely sheared textures requires rapid transport of material from about 150 km depth during active deformation of relatively dry rock. The occurrence of monticellite peridotite in this kimberlite diatreme suggests that magmas which crystallized to monticellite peridotite at relatively shallow depth could be one of the primitive types of kimberlite magma. ?? 1975.

  7. Peridotite xenoliths from the Jagersfontein kimberlite pipe: I. Primary and primary-metasomatic mineralogy

    SciTech Connect

    Harte, B. ); Gurney, J.J. ); Winterburn, P.A. Isotope Geochemistry Facility, Pretoria )

    1990-02-01

    The geochemistry and textures of peridotite xenoliths from the Jagersfontein kimberlite pipe are reported. The xenoliths have a primary mineralogy of olivine {plus minus} orthopyroxene {plus minus} clinopyroxene {plus minus} garnet {plus minus} spinel. They are subdivided into coarse and deformed xenoliths corresponding to high- and low-temperature estimates, respectively. Coarse-grained xenoliths are further subdivided into low- and medium-temperature groups. Mineral chemistry of these two groups is distinct, e.g., clinopyroxene 100 Al/(Al + Cr) 24 to 60 and 60 to 70 in the medium- and low-temperature groups, respectively. Low-temperature xenoliths have undergone exsolution of pyroxene, spinel, and garnet in their pyroxenes. Primary modal metasomatism has occurred in the coarse xenoliths with the replacement of orthopyroxene by edenitic amphibole in the low-temperature xenoliths and of clinopyroxene by low-Ti phlogopite in the medium-temperature xenoliths. The amphibole stability limit confines it to shallower depths. Metasomatized xenoliths have been enriched in K, Na, Al, and Ca, and trace incompatible elements. Metasomatism is considered to have occurred at round 1 Ga by the infiltration of, and reaction with, ascending H{sub 2}O-rich fluids with Sr and Nd isotopic characteristics similar to group II kimberlites. The widespread chemical equilibrium seen in metasomatized xenoliths suggests that the particularly distinctive features of the low-temperature Jagersfontein xenoliths, namely exsolution and very low equilibration temperatures, may also be a result of the primary metasomatism.

  8. [Research on the X-ray fluorescence spectrometry method to determine trace elements in kimberlite].

    PubMed

    Zhang, Lei; Yan, Chuan-wei; Lu, Yi

    2003-04-01

    It is very important to detect trace elements for kilmberlite. Through improving the working conditions of X-ray fluorescence spectrometer and optimizing the analytical conditions, the determination method of trace elements, such as Sc, Cr, Ni, Y, Nb, La, in kimberlite was worked out. The method has been successfully applied to the determination of trace elements in over 2 thousand samples of kimberlite from Liaoning province. The detection limits of the method were relatively low (the detection limit of Sc droped from 9.54 to 2.83 micrograms.g-1 and the detection limit of La droped from 21.68 micrograms.g-1 to 9.18 micrograms.g-1), i.e. 2.83, 2.15, 2.20, 1.17, 1.05 and 9.18 micrograms.g-1 for Sc, Cr, Ni, Y, Nb and La, respectively. The precision of the method was very high with 2.10%-7.09% of RSD (n = 20). Compared with ICP spectrometry this method is satisfactory. The method has proven to be simple and rapid with low cost and high efficiency. PMID:12961906

  9. [Study on the micro-infrared spectra and origin of polycrystalline diamonds from Mengyin kimberlite pipes].

    PubMed

    Yang, Zhi-Jun; Liang, Rong; Zeng, Xiang-Qing; Ge, Tie-Yan; Ai, Qun; Zheng, Yun-Long; Peng, Ming-Sheng

    2012-06-01

    The natural polycrystalline diamonds from the Mengyin kimberlite pipes can be classified as the euhedral faceted polycrystalline diamonds and anhedral rounded polycrystalline diamonds. The results of micro-FTIR spectra characterization of the polycrystalline diamonds show that the concentration of nitrogen is low, varying from 16.69 to 72.81 microgram per gram and is different among different diamond grains or position in polycrystalline diamonds. The euhedral faceted polycrystalline diamonds are Ia AB type and have higher concentration of A-center defects than B-center defects. Most of the anhedral rounded polycrystalline diamonds are Ia AB type and have higher content of B-center defects. A minority of the anhedral rounded polycrystalline diamonds have C-center, A-center and B-center defects simultaneously. The polycrystalline diamonds probably originated from the relatively deeper mantle and were not formed in diamond nucleation stage, but in the diamond growth period or some special conditions after the diamond grains were formed already. Furthermore, the euhedral faceted polycrystalline diamonds were formed slightly later and the anhedral rounded polycrystalline diamonds were formed obviously earlier than the diamond single crystals from the Mengyin kimberlite pipes. PMID:22870630

  10. Stable isotope paleoclimatology of the earliest Eocene using kimberlite-hosted mummified wood from the Canadian Subarctic

    NASA Astrophysics Data System (ADS)

    Hook, B. A.; Halfar, J.; Gedalof, Z.; Bollmann, J.; Schulze, D.

    2014-11-01

    The recent discovery of well-preserved mummified wood buried within a subarctic kimberlite diamond mine prompted a paleoclimatic study of the early Eocene "hothouse" (ca. 53.3 Ma). At the time of kimberlite eruption, the Subarctic and Artic were warm and humid producing a temperate rainforest biome well north of the Arctic Circle. Previous studies have estimated mean annual temperatures in this region were 4-20 °C in the early Eocene, using a variety of proxies including leaf margin analysis, and stable isotopes (δ18O) of fossil cellulose. Here, we examine stable isotopes of tree-ring cellulose at subannual to annual scale resolution, using the oldest viable cellulose found to date. We use mechanistic models and transfer functions to estimate earliest Eocene temperatures using mummified cellulose, which was well preserved in the kimberlite. Multiple samples of Piceoxylon wood within the kimberlite were crossdated by tree-ring width. Multiple proxies are used in combination to tease apart likely environmental factors influencing the tree physiology and growth in the unique extinct ecosystem of the Polar rainforest. Calculations of interannual variation in temperature over a multidecadal time-slice in the early Eocene are presented, with a mean temperature estimate of 11.4 °C (1σ = 1.8 °C) based on δ18O. Dual-isotope spectral analysis suggests that multidecadal climate cycles similar to the modern Pacific Decadal Oscillation likely drove temperature and cloudiness trends on 20-30 year timescales.

  11. Stratigraphic relations, kimberlite emplacement and lithospheric thermal evolution, Quiricó Basin, Minas Gerais State, Brazil

    NASA Astrophysics Data System (ADS)

    Read, George; Grutter, Herman; Winter, Stewart; Luckman, Nigel; Gaunt, Frank; Thomsen, Fernando

    2004-09-01

    The Quiricó Basin covers an area of 10,000 km 2 and is situated to the west of the conventionally defined southwestern margin of the Archean São Francisco craton in Minas Gerais State, Brazil. The sedimentary infill of the Quiricó Basin consists of lightly metamorphosed shallow marine clastic bedrock sediments of the Bambuı´ Group (˜650±15 Ma), unconformably overlain by Early Cretaceous terrigenous lacustrine (Quiricó Formation), alluvial fan (Abaeté Formation) and fluvial/aeolian (Três Barras Formation) deposits of the Areado Group. Rare kimberlites and ubiquitous kamafugites of the Alto Paranaı´ba Igneous Province (APIP) erupted through the recently deposited sediments of the Quiricó Basin in the time period 95-61 Ma. The 120-m-thick Mata da Corda Group overlies the Late Cretaceous Areado Group over an area of 8000 km 2 and is composed largely of extrusive kamafugite and related volcanosedimentary material. Unusually large diamonds with proximal surface features and population characteristics are well known to occur in rivers and streams that drain the stratigraphic succession in the Quiricó Basin, prompting the search for their presumably local primary source(s) and a possibly associated Archean basement or cratonic root. Conceptual exploration models for this setting may in part be based on the diamondiferous 120 Ma Canastra and 95 Ma Três Ranchos kimberlites, but require reconciliation with the observed abundance of 85-61 Ma old diamond-free kamafugites. Field relations and carefully controlled stratigraphic samples show that a distinctive mantle-derived indicator mineral suite occurs in the Maxixe Member, a volcaniclastic breccia unit that occurs at the base of the Mata da Corda Group. A detailed thermobarometric comparison of mantle-derived xenocrystic clinopyroxene compositions from this member with clinopyroxene populations derived from kimberlites and kamafugites situated in the Quiricó Basin shows a distinct and abrupt change in

  12. Superplume Under the Siberian Craton: Evidence from Metasomatic Signatures in Kimberlitic Xenoliths

    NASA Astrophysics Data System (ADS)

    Pokhilenko, N.; Taylor, L. A.; Agashev, A. M.; Pokhilenko, L.; Baziotis, I. P.; Carmody, L.; Smith, G. B.

    2012-12-01

    A superplume is proposed to have existed under the Siberian Craton from the mid-Paleozoic to the late Mesozoic. The most dramatic result of this was the eruption of 17 million km3 of the Siberian trap magma at 250 ±2 Ma. However, the effects of this plume were recorded earlier in Devonian (~360 Ma) kimberlitic peridotites, as minor "basaltic" metasomatic overprinting. Indeed, peridotites from later kimberlites (~145 Ma) show evidence for similar, yet extensive, metasomatism, thereby bracketing the major plume climax with possible effects both prior and after this extensive activity. Indeed, a comparative analysis of petrological, mineralogical, and geochemical results from these early and late, Devonian versus Jurassic, peridotites have shown that the thickness of lithosphere of North-Eastern part of Siberian Platform was around 230 km at Middle Paleozoic time and decreased to 140-150 km by Late Mesozoic time. The Devonian Udachnaya kimberlite (central craton), one of the largest diamondiferous pipes in the world, provides a suite of garnet peridotites (>90%), many of which possess a 'sheared' texture. These peridotites possess evidence for a late-stage cryptic metasomatism, particularly in the garnets, exemplified by a notable increase in CaO, with a near constant Cr2O3 content, as the garnet compositions move from the harzburgitic field into that of lherzolites. Conversely, the xenolith suite from the Mesozoic Obnazhennaya kimberlite (north-east craton) is comprised of more pyroxenite xenoliths, with rare peridotite lithologies. The metasomatic over-print on these xenoliths has produced a more fertile chemistry, with the garnets becoming less Cr-rich, following a lherzolitic trend. These two trends in the garnets suggest that a long-lasting metasomatic ultra-event, the same metasomatic processes (and possibly similar sources), operated during both time periods. Changes from harzburgitic in the older Udachnaya peridotite rims to lherzolitic core-rim compositions

  13. The Lake Superior Oronto Group, a middle Proterozoic exploration model for the late Proterozoic Chuar Group of the Grand Canyon

    SciTech Connect

    Dickas, A.B. ); Mudrey, M.G. Jr )

    1992-04-01

    The Lake Superior Oronto Group and the Grand Canyon Chuar Group are the most significant Precambrian hydrocarbon targets within the conterminous United States. These frontier terrains share common Proterozoic age, comparable total organic carbon source rock values, association with Indian-interest properties, plus similarities in reservoir, trap, and maturation characteristics. Extensively studied since 1980, the exploration philosophy applied to the Oronto Group is presented as a model for Chuar Group hydrocarbon evaluation. Hydrocarbon shows have been reported since 1852 from middle Proterozoic rocks of the Lake Superior basin. Occurrences include stains within stromatolitic facies of the Copper Harbor Conglomerate, live subsurface seeps within Nonesuch units in the White Pine copper mine of Michigan, and impsonite-like inclusions within calcite veins of the Freda Formation. These formations compose the Oronto Group, a synrift package infilling the Lake Superior basin of the mid-continent rift system. Seep analyses identify a low sulfur (0.02%), paraffinic (67%), 34 API crude indirectly dated (Rb/Sr) at a minimum of 1047{plus minus}35 Ma. Nonesuch Formation source shales are present within both central horst structures and flank half-grabens. Reservoir-quality criteria are associated with adjacent Copper Harbor and overlying Freda Formation units. Seismically identified traps range from anticlinal and drag folding to onlap, stratigraphic, and unconformity closures. The Lake Superior segment of the mid-continent rift system is subdivided into four structural units (I-IV). Association of oil seeps with stratiform copper deposits (unit III) suggests evidence of geochemical symbiosis. This crude entered oil-window status circa 1.0 Ga due to migrating cupriferous thermal systems.

  14. Difficulties in distinguishing coherent from fragmental kimberlite: A case study of the Muskox pipe (Northern Slave Province, Nunavut, Canada)

    NASA Astrophysics Data System (ADS)

    Hayman, P. C.; Cas, R. A. F.; Johnson, M.

    2008-06-01

    Kimberlite drill core from the Muskox pipe (Northern Slave Province, Nunavut, Canada) highlights the difficulties in distinguishing coherent from fragmental kimberlite and assessing the volcanological implications of the apparent gradational contact between the two facies. Using field log data, petrography, and several methods to quantify crystal and xenolith sizes and abundances, the pipe is divided into two main facies, dark-coloured massive kimberlite (DMK) and light-coloured fragmental kimberlite (LFK). DMK is massive and homogeneous, containing country-rock lithic clasts (~ 10%) and olivine macrocrysts (~ 15%) set in a dark, typically well crystallised, interstitial medium containing abundant microphenocrysts of olivine (~ 15%), opaques and locally monticellite, all of which are enclosed by mostly serpentine. In general, LFK is also massive and structureless, containing ~ 20% country-rock lithic clasts and ~ 12% olivine macrocrysts. These framework components are supported in a matrix of serpentinized olivine microphenocrysts (10%), microlites of clinopyroxene, and phlogopite, all of which are enclosed by serpentine. The contact between DMK and LFK facies is rarely sharp, and more commonly is gradational (from 5 cm to ~ 10 m). The contact divides the pipe roughly in half and is sub-vertical with an irregular shape, locally placing DMK facies both above and below the fragmental rocks. Most features of DMK are consistent with a fragmental origin, particularly the crystal- and xenolith-rich nature (~ 55-65%), but there are some similarities with rocks described as coherent kimberlite in the literature. We discuss possible origins of gradational contacts and consider the significance for understanding the origin of the DMK facies, with an emphasis on the complications of alteration overprinting of primary textures.

  15. Earth history. Low mid-Proterozoic atmospheric oxygen levels and the delayed rise of animals.

    PubMed

    Planavsky, Noah J; Reinhard, Christopher T; Wang, Xiangli; Thomson, Danielle; McGoldrick, Peter; Rainbird, Robert H; Johnson, Thomas; Fischer, Woodward W; Lyons, Timothy W

    2014-10-31

    The oxygenation of Earth's surface fundamentally altered global biogeochemical cycles and ultimately paved the way for the rise of metazoans at the end of the Proterozoic. However, current estimates for atmospheric oxygen (O2) levels during the billion years leading up to this time vary widely. On the basis of chromium (Cr) isotope data from a suite of Proterozoic sediments from China, Australia, and North America, interpreted in the context of data from similar depositional environments from Phanerozoic time, we find evidence for inhibited oxidation of Cr at Earth's surface in the mid-Proterozoic (1.8 to 0.8 billion years ago). These data suggest that atmospheric O2 levels were at most 0.1% of present atmospheric levels. Direct evidence for such low O2 concentrations in the Proterozoic helps explain the late emergence and diversification of metazoans.

  16. Extending the western North American Proterozoic and Paleozoic continental crust through the Mojave Desert

    SciTech Connect

    Martin, M.W.; Walker, J.D. )

    1992-08-01

    Data supporting the existence of Proterozoic basement in the central and western Mojave Desert include U-Pb zircon geochronology and Nd, Sr, and Pb isotopic values of quartzofeldspathic gneisses, detrital zircon provenance ages, and the presence of basement clasts in Paleozoic and Mesozoic conglomerates. These data corroborate existing isotopic data from Mesozoic and Tertiary intrusive rocks that suggest involvement of Proterozoic crust in their genesis. Exposures of Proterozoic basement and Late Proterozoic and Paleozoic transitional miogeoclinal-cratonal facies trends in the central and western Mojave Desert consistently imply that cratonal North America continues westward uninterrupted through this region to the San Andreas fault. These data place geographic limits on the position of several pre-Tertiary tectonic elements speculated to exist in the Mojave Desert.

  17. Low Mid-Proterozoic atmospheric oxygen levels and the delayed rise of animals

    NASA Astrophysics Data System (ADS)

    Planavsky, Noah J.; Reinhard, Christopher T.; Wang, Xiangli; Thomson, Danielle; McGoldrick, Peter; Rainbird, Robert H.; Johnson, Thomas; Fischer, Woodward W.; Lyons, Timothy W.

    2014-10-01

    The oxygenation of Earth’s surface fundamentally altered global biogeochemical cycles and ultimately paved the way for the rise of metazoans at the end of the Proterozoic. However, current estimates for atmospheric oxygen (O2) levels during the billion years leading up to this time vary widely. On the basis of chromium (Cr) isotope data from a suite of Proterozoic sediments from China, Australia, and North America, interpreted in the context of data from similar depositional environments from Phanerozoic time, we find evidence for inhibited oxidation of Cr at Earth’s surface in the mid-Proterozoic (1.8 to 0.8 billion years ago). These data suggest that atmospheric O2 levels were at most 0.1% of present atmospheric levels. Direct evidence for such low O2 concentrations in the Proterozoic helps explain the late emergence and diversification of metazoans.

  18. The Precambrian palaeomagnetic record: the case for the Proterozoic Supercontinent

    NASA Astrophysics Data System (ADS)

    Piper, J. D. A.

    1982-06-01

    The case for a single supercontinent in Proterozoic times is examined in the context of a modified reconstruction with the Precambrian shields collectively comprising a primary lens-shaped body of crust. The palaeomagnetic data base is now sufficient to provide a definitive test of this model and it is shown that palaeopoles from the major shields conform to a single narrow path from ca. 2600 to 570 Ma. The agreement is especially compelling because the model requires no stringent data selection although it is itself highly rigid with adjustments only necessary at 1100 Ma and then applicable only to peripheral shields. There is close correspondence of the predominant field polarities from the constituent shields giving added confidence that the same polarities are being correlated, and the model supports the view that the global palaeoradius has been approximately constant since 2600 Ma. The pre-2200 Ma palaeopoles from Laurentia define "track 6" which is shown to be precisely applicable to the contemporaneous record from the African, the Australian, and possibly the Indian, Shields. The post-2200 Ma palaeopoles record higher rates of apparent polar wander (a.p.w.) movement although the magnitude of these movements is identified within one shield (the Laurentian) alone, and the data from other shields continue to conform to the unique reconstruction. The a.p.w, path is best constrained by the data from anorogenic igneous units over the intervals 2600-1800 and 1500-1000 Ma with second rank data from sedimentary successions in all of the major shields supporting the path defined by the igneous data.The a.p.w, paths over the intervals 1800-1500 and 1000-850 Ma are represented predominantly by uplift magnetisations in metamorphic terrains of the Fennoscandian and Laurentian Shields, although they are matched by magnetisations in supracrustal successions in Africa, South America and Australia, and in Africa and Siberia, respectively. The weakest part of the analysis

  19. Origin of Ti-rich garnets in the groundmass of Wajrakarur field kimberlites, southern India: insights from EPMA and Raman spectroscopy

    NASA Astrophysics Data System (ADS)

    Dongre, Ashish N.; Viljoen, K. S.; Rao, N. V. Chalapathi; Gucsik, A.

    2016-04-01

    Although Ti-rich garnets are commonly encountered in the groundmass of many alkaline igneous rocks, they are comparatively rare in kimberlites. Here we report on the occurrence of Ti-rich garnets in the groundmass of the P-15 and KL-3 kimberlites from the diamondiferous Wajrakarur field in the Eastern Dharwar craton of southern India. These garnets contain considerable Ti (11.7-23.9 wt.% TiO2), Ca (31.3-35.8 wt.% CaO), Fe (6.8-15.5 wt.% FeOT) and Cr (0.04-9.7 wt.% Cr2O3), but have low Al (0.2-5.7 wt.% Al2O3). In the case of the P-15 kimberlite they display a range in compositions from andradite to schorlomite, with a low proportion of grossular (andradite(17.7-49.9)schorlomite(34.6-49.5)-grossular(3.7-22.8)-pyrope(1.9-10.4)). A few grains also contain significant chromium and represent a solid solution between schorlomite and uvarovite. The Ti-rich garnets in the KL-3 kimberlite, in contrast, are mostly schorlomitic (54.9-90.9 mol %) in composition. The Ti-rich garnets in the groundmass of these two kimberlites are intimately associated with chromian spinels, perhaps suggesting that the garnet formed through the replacement of spinel. From the textural evidence, it appears unlikely that the garnets could have originated through secondary alteration, but rather seem to have formed through a process in which early magmatic spinels have reacted with late circulating, residual fluids in the final stages of crystallization of the kimberlite magma. Raman spectroscopy provides evidence for low crystallinity in the spinels which is likely to be a result of their partial transformation into andradite during their reaction with a late-stage magmatic (kimberlitic) fluid. The close chemical association of these Ti-rich garnets in TiO2-FeO-CaO space with those reported from ultramafic lamprophyres (UML) is also consistent with results predicted by experimental studies, and possibly implies a genetic link between kimberlite and UML magmas. The occurrence of Ti-rich garnets of

  20. Some key issues in reconstructions of Proterozoic supercontinents

    NASA Astrophysics Data System (ADS)

    Zhao, Guochun; Sun, Min; Wilde, Simon A.; Li, Sanzhong; Zhang, Jian

    2006-10-01

    Supercontinents containing most of the earth's continental crust are considered to have existed at least twice in Proterozoic time. The younger one, Rodinia, formed at ˜1.0 Ga by accretion and collision of fragments produced by breakup of the older supercontinent, Columbia, which was assembled by global-scale 2.0-1.8 Ga collisional events. Little consensus has been reached regarding configurations of these supercontinents because of some unresolved issues concerning continental fits. One of these issues concerns how Siberia was related to Laurentia. Previous reconstructions that consider the Aldan Shield of Siberia as a continuation of the Wyoming Province of Laurentia have been largely abandoned in favor of models connecting Siberia to northern Laurentia, but it remains controversial which part of Siberia is contiguous with northern Laurentia. Also at issue is the western Laurentia-Australia-East Antarctica connection. Most Rodinia reconstructions place Australia, together with East Antarctica, adjacent to either western Canada (the SWEAT hypothesis) or the western United States (the AUSWUS hypothesis). However, recent studies combining paleomagnetic and isotopic age data have called into question the validity of SWEAT, AUSWUS and other variants. Another issue is the position of North China in Rodinia/Columbia. Limited paleomagnetic data seem to be consistent with the Paleo-Mesoproterozoic North China-Siberia/Baltica connection, whereas geological data support the recently proposed Archean to Mesoproterozoic North China-India connection. Controversial issues have also been raised about the timing and history of the amalgamation and fragmentation of South America and West Africa. Both geological and paleomagnetic data suggest that South America (São Francisco and Amazonia Cratons) and West Africa (Congo and West African Cratons) coalesced into a single landmass along the 2.1-2.0 Ga Transamazonian/Eburnean orogens. However, whether they were divorced and then re

  1. Microfossils' diversity from the Proterozoic Taoudeni Basin, Mauritania

    NASA Astrophysics Data System (ADS)

    Beghin, Jérémie; Houzay, Jean-Pierre; Blanpied, Christian; Javaux, Emmanuelle

    2014-05-01

    Prokaryotes and microscopic eukaryotes are known to have appeared well before the Cambrian's adaptative radiation which flourished perceptibly as a generalized macroscopic world. What do we know about the trigger events which stimulated eukaryotic diversification during the Proterozoic? Biological innovations or environmental changes, and indeed probably both (Knoll et al., 2006), played a fundamental role controlling this important step of life's evolution on Earth. Javaux (2011), proposed a diversification pattern of early eukaryotes divided into three steps and focusing on different taxonomic levels, from stem group to within crown group, of the domain Eukarya. Here, we present a new, exquisitely preserved and morphologically diverse assemblage of organic-walled microfossils from the 1.1 Ga El Mreiti Group of the Taoudeni Basin (Mauritania). The assemblage includes beautifully preserved microbial mats comprising pyritized filaments, prokaryotic filamentous sheaths and filaments, microfossils of uncertain biological affinity including smooth isolated and colonial sphaeromorphs (eukaryotes and/or prokaryotes), diverse protists (ornamented and process-bearing acritarchs), as well multicellular microfossils interpreted in the literature as possible xanthophyte algae. Several taxa are reported for the first time in Africa, but are known worldwide. This study improves microfossil diversity previously reported by Amard (1986) and shows purported xanthophyte algae contrary to a previous biomarker study suggesting the absence of eukaryotic algae, other than acritarchs, in the basin (Blumenberg et al., 2012). This new microfossil assemblage and others provide, all together, evidences of early and worldwide diversification of eukaryotes. Thereby, those first qualitative results also provide a basis for further and larger quantitative studies on the Taoudeni Basin. To better understand the palaeobiology (stem or crown group, aerobic or anaerobic metabolism) and

  2. Ti in Zircon Megacrysts From Kimberlite: Evidence for low Temperatures of Formation

    NASA Astrophysics Data System (ADS)

    Page, F.; Fu, B.; Kita, N. T.; Fournelle, J.; Spicuzza, M. J.; Schulze, D. J.; Basei, M. A.; Valley, J. W.

    2005-12-01

    The Ti concentration and oxygen isotopic composition of megacrystic zircon hosted by kimberlite is generally homogenous within a given pipe but varies between pipes. These zircons are a distinct suite widely accepted to be of mantle origin and characterized by large (1 - 15 mm, dia), rounded, gemmy, but fractured crystals, low U (<50ppm), and ZrO2 coatings. Titanium in zircon hosted by kimberlite from the Kaapvaal Craton of southern Africa, the Siberian Platform, and Brazil was analyzed on the CAMECA ims-1280 ion microprobe using a 4 nA (O-) primary beam (25 μm spot, 5 μm analysis area). NIST 610 glass (434 ppm Ti) and synthetic Ti-rich (800-1500 ppm Ti, EPMA) zircon (B. Watson) were used as standards. New, precise, laser-fluorination oxygen isotope data were obtained for zircons from Brazil, yielding mantle-like δ18O values (4.8 to 5.8 ‰) similar to published data for Africa (4.9 to 5.9 ‰) and Siberia (4.7 to 5.6 ‰, Valley et al. 1998 CMP 133:1). The zircons analyzed range from 2 to 53 ppm Ti (n=169 analyses, 44 zircons), and most zircons contain less than 20 ppm Ti. The recently calibrated Ti in zircon thermometer (Watson and Harrison, 2005, Science, 308:841) yields an average temperature of 740 ± 64° C (1SD). Independent thermometry of zircon megacrysts based on the associated MARID and other parageneses is poorly constrained, but includes estimates as low as 700° C. Because megacrysts are not generally found within a mineralogical context, the presence of rutile and a(TiO2) are not known. However, megacrysts have been found intergrown with rutile, and the temperature correction for reduced a(TiO2) is likely (<) 50° C. Most megacrysts in this study preserve fine-scale, oscillatory zoning in CL and are generally homogenous in their Ti content and oxygen isotopic composition, consistent with preservation of primary compositions. In samples from Siberia and Africa, Ti concentration appears to vary among kimberlite pipes, but in most cases not within

  3. First data for deep seated xenoliths and mantle geotherms of Zarnitsa kimberlite pipe, Daldyn, Yakutia.

    NASA Astrophysics Data System (ADS)

    Ashchepkov, Igor; Pokhilenko, Nikolai; Vladykin, Nikolai; Spetsius, Zdislav; Logvinova, Alla; Palessky, Stanislav; Khmelnikova, Olga; Shmarov, Gleb

    2014-05-01

    First discovered in Yakutia and the largest in Daldyn region kimberlite pipe is composed from several phases including breccias and porphyric kimberlites. Commonly mantle xenolith from this pipe especially included in the prevailing grey breccia are nearly completely altered. Only relatively fresh material from the brownish - grey breccia from the drilling core and porphyric kimberlites includes material which could be used for the mineral thermobarometry. The picroilmenites from the Zarnitsa pipe are forming three clusters according to the Cr- content: 0.5; 1.0 and 2.5 % Cr2O3 (Ashchepkov,Amshinsky, Pokhilenko, 1980; Amshinsky, Pokhilenko,1984; Alymova et al., 2003) due to the different contamination degree of protokimberlites in mantle peridotites. The ilmenites from porphyric kimberlites are forming stepped trend consisting from three groups of different pressure intervals from 6.5 to 4.0 GPa but more continuous than those determined for the ilmenites from breccia (Ashchepkov et al ., 2010). The relatively low Cr diopsides are corresponding to the deeper part while those containing to 2 -3 of Cr2O3 are from the middle part of the mantle section and are in association with the phlogopites contain the reflecting processes of the protokimbelite differentiation and contamination. Peridotites from the lithosphere base are of Hi temperature type and slightly Fe - enriched and are referred to the porphyroclustic types where garnets contain up to 10% Cr2O3 are they are relatively low in TiO2. But there are alos varieties of reduced Cr and the Fe-enriched which are closer to the deformed type (Agashev et al., 2013). The cold clot in the 60-5.5 GPa (34 mwm-2) are represented by Fe- low peridotites with the garnets of sub-Ca types. The Cr- low and LT eclogites are correspondent to the low 4.5-6.0 GPa interval similar to those from Udachnaya pipe. But near the pyroxenites lens the varieties enriched in Fe and sometimes hybrid pyroxenites appear like in most pf mantle

  4. Spatial, temporal, mineralogical, and compositional variations in Mesozoic kimberlitic magmatism in New York State

    NASA Astrophysics Data System (ADS)

    Bailey, David G.; Lupulescu, Marian V.

    2015-01-01

    Mesozoic kimberlitic magmatism was geographically widespread across central New York State, and nearly 90 distinct intrusions have been discovered since the first "serpentinite body" was described over 175 years ago. Most of the intrusions are narrow (< 30 cm wide), near vertical, north-south oriented dikes, although three larger, irregular diatremes are also known. Previous studies assumed that all of the intrusions were genetically and temporally related, and often examined only a small sub-set of the intrusions. By combining modern samples with historic samples in the collections of the New York State Museum and Hamilton College, we were able to obtain detailed mineralogical and geochemical data on samples from 27 distinct intrusions. The intrusions can be divided into four distinct groups on the basis of both mineralogy and geochemistry, and previously published radiometric age dates suggest that these four groups may also have distinct emplacement ages. Group A intrusions are exposed on the western margin of Cayuga Lake near Ithaca, and are characterized by olivine and phlogopite macrocrysts in a serpentine and phlogopite-rich matrix. These intrusions are relatively Ti-rich and contain abundant perovskite grains in the groundmass that yielded U-Pb crystallization ages of ~ 146 Ma (Heaman and Kjarsgaard, 2000). Group B intrusions are exposed over a relatively large area surrounding Ithaca, and are characterized by having a diverse macrocryst assemblage that includes pyrope, diopside, and spinel in addition to olivine and phlogopite. These intrusions are the most incompatible and REE enriched, and are chemically similar to the Kirkland Lake kimberlites in eastern Ontario. Intrusion ages for this group cluster between 125 and 110 Ma. Group C intrusions are all found within the city of Syracuse, and are similar to the Group B intrusions in both mineralogy and chemistry. They appear to be somewhat older, with intrusion ages of 135-125 Ma. Finally, Group D

  5. Natural occurrence of silicon carbide in a diamondiferous kimberlite from Fuxian

    USGS Publications Warehouse

    Leung, I.; Guo, W.; Friedman, I.; Gleason, J.

    1990-01-01

    Considerable debate surrounds the existence of silicon carbide in nature, mostly owing to the problem of possible contamination by man-made SiC. Recently, Gurney1 reviewed reports of rare SiC inclusions in diamonds, and noted that SiC can only be regarded as a probable rather than proven cogenetic mineral. Here we report our observation of clusters of SiC coexisting with diamond in a kimberlite from Fuxian, China. Macrocrysts of ??-SiC are overgrown epitaxially by ??-SiC, and both polymorphs are structurally well ordered. We have also measured the carbon isotope compositions of SiC and diamonds from Fuxian. We find that SiC is more enriched in 12C than diamond by 20% relative to the PDB standard. Isotope fractionation might have occurred through an isotope exchange reaction in a common carbon reservoir. Silicon carbide may thus ultimately provide information on carbon cycling in the Earth's mantle.

  6. Statistical Characteristics of Xenoliths in the Antioch Kimberlite Pipe, Marshall County, Northeastern Kansas

    USGS Publications Warehouse

    Kotov, S.; Berendsen, P.

    2002-01-01

    Geometrical characteristics of xenoliths in the Antioch kimberlite pipe have been considered in statistical terms. A method of conversion of 2D intersections to 3D dimensions was used. It has been shown that the Rosin-Rammler distribution of mass leads to the Weibull distribution of sizes, whereas a fractal distribution of sizes can be expressed as the Pareto distribution. Lognormal, Weibull, and Pareto distributions have been tested as model distributions. The Pareto distribution could be the most appropriate model for the distribution of xenoliths. This conclusion is in agreement with the general concept that the xenoliths formed as a result of an underground explosion without additional breakage occurring during magma transport. The final distribution maybe shifted from the initial model as a result of processes of redistribution and sorting of xenoliths in liquid-crystalline flows. ?? 2002 International Association for Mathematical Geology.

  7. Helium isotopic variability within single diamonds from the Orapa kimberlite pipe

    NASA Technical Reports Server (NTRS)

    Kurz, Mark D.; Jenkins, William J.; Lott, Dempsey E., III; Gurney, John J.

    1987-01-01

    The possible relationships between diamond mineralogy and helium isotopes were investigated by measuring the distribution and isotopic composition of He in a suite of well-characterized one-carat diamonds from the Orapa kimberlite, Botswana. The results of crushing in vacuo experiments indicated that most of He was contained in the matrix, rather than in the inclusions of the diamonds. Step-heating of individual diamonds at 2000 C released He of He-3/He-4 ratios that differed by up to a factor of 100 among the two heating steps, revealing large isotopic variations within individual diamonds. It is suggested that this internal isotopic variability is the result of stepwise graphitization: the first heating step initiates graphitization which nucleates around defects in a diamond, and the second step graphitizes the relatively defect-free regions of the diamond. This explanation predicts that the highest He-3/He-4 ratios should be found in most perfect crystals.

  8. Inclusions of Cr- and Cr-Nb-Rutile in pyropes from the Internatsionalnaya kimberlite pipe, Yakutia

    NASA Astrophysics Data System (ADS)

    Rezvukhin, D. I.; Malkovets, V. G.; Sharygin, I. S.; Kuzmin, D. V.; Litasov, K. D.; Gibsher, A. A.; Pokhilenko, N. P.; Sobolev, N. V.

    2016-02-01

    The results of study of rutile inclusions in pyrope from the Internatsionalnaya kimberlite pipe are presented. Rutile is characterized by unusually high contents of impurities (up to 25 wt %). The presence of Cr2O3 (up to 9.75 wt %) and Nb2O5 (up to 15.57 wt %) are most typical. Rutile inclusions often occur in assemblage with Ti-rich oxides: picroilmenite and crichtonite group minerals. The Cr-pyropes with inclusions of rutile, picroilmenite, and crichtonite group minerals were formed in the lithospheric mantle beneath the Mirnyi field during their joint crystallization from melts enriched in Fe, Ti, and other incompatible elements as a result of metasomatic enrichment of the depleted lithospheric mantle.

  9. Anoxygenic photosynthesis modulated Proterozoic oxygen and sustained Earth's middle age.

    PubMed

    Johnston, D T; Wolfe-Simon, F; Pearson, A; Knoll, A H

    2009-10-01

    Molecular oxygen (O(2)) began to accumulate in the atmosphere and surface ocean ca. 2,400 million years ago (Ma), but the persistent oxygenation of water masses throughout the oceans developed much later, perhaps beginning as recently as 580-550 Ma. For much of the intervening interval, moderately oxic surface waters lay above an oxygen minimum zone (OMZ) that tended toward euxinia (anoxic and sulfidic). Here we illustrate how contributions to primary production by anoxygenic photoautotrophs (including physiologically versatile cyanobacteria) influenced biogeochemical cycling during Earth's middle age, helping to perpetuate our planet's intermediate redox state by tempering O(2) production. Specifically, the ability to generate organic matter (OM) using sulfide as an electron donor enabled a positive biogeochemical feedback that sustained euxinia in the OMZ. On a geologic time scale, pyrite precipitation and burial governed a second feedback that moderated sulfide availability and water column oxygenation. Thus, we argue that the proportional contribution of anoxygenic photosynthesis to overall primary production would have influenced oceanic redox and the Proterozoic O(2) budget. Later Neoproterozoic collapse of widespread euxinia and a concomitant return to ferruginous (anoxic and Fe(2+) rich) subsurface waters set in motion Earth's transition from its prokaryote-dominated middle age, removing a physiological barrier to eukaryotic diversification (sulfide) and establishing, for the first time in Earth's history, complete dominance of oxygenic photosynthesis in the oceans. This paved the way for the further oxygenation of the oceans and atmosphere and, ultimately, the evolution of complex multicellular organisms.

  10. Anoxygenic photosynthesis modulated Proterozoic oxygen and sustained Earth's middle age

    PubMed Central

    Johnston, D. T.; Wolfe-Simon, F.; Pearson, A.; Knoll, A. H.

    2009-01-01

    Molecular oxygen (O2) began to accumulate in the atmosphere and surface ocean ca. 2,400 million years ago (Ma), but the persistent oxygenation of water masses throughout the oceans developed much later, perhaps beginning as recently as 580–550 Ma. For much of the intervening interval, moderately oxic surface waters lay above an oxygen minimum zone (OMZ) that tended toward euxinia (anoxic and sulfidic). Here we illustrate how contributions to primary production by anoxygenic photoautotrophs (including physiologically versatile cyanobacteria) influenced biogeochemical cycling during Earth's middle age, helping to perpetuate our planet's intermediate redox state by tempering O2 production. Specifically, the ability to generate organic matter (OM) using sulfide as an electron donor enabled a positive biogeochemical feedback that sustained euxinia in the OMZ. On a geologic time scale, pyrite precipitation and burial governed a second feedback that moderated sulfide availability and water column oxygenation. Thus, we argue that the proportional contribution of anoxygenic photosynthesis to overall primary production would have influenced oceanic redox and the Proterozoic O2 budget. Later Neoproterozoic collapse of widespread euxinia and a concomitant return to ferruginous (anoxic and Fe2+ rich) subsurface waters set in motion Earth's transition from its prokaryote-dominated middle age, removing a physiological barrier to eukaryotic diversification (sulfide) and establishing, for the first time in Earth's history, complete dominance of oxygenic photosynthesis in the oceans. This paved the way for the further oxygenation of the oceans and atmosphere and, ultimately, the evolution of complex multicellular organisms. PMID:19805080

  11. Proterozoic to mesozoic mobile-belt geology, Pensacola Mountains, Antarctica

    NASA Astrophysics Data System (ADS)

    Schmidt, D. L.

    The Pensacola Mountains consist of four unconformable sequences of: (1) graywacke (oldest), (2) platform, (3) molasses, and (4) continental (youngest) deposits. The first sequence of Middle Proterozoic graywacke deposits (Patuxent Formation) consists of turbidite quartzbearing sandstone and slate and volcanic rocks. The second sequence consist of extensive platform deposits of Lower Cambrian archaeocyathidbearing limestone and Middle Cambrian trilobitebearing limestone (Nelson Limestone) that are overlain by shale (Wiens Formation), and silicic volcanic rocks (Gambacorta Formation) including rhyolitic ignimbrite of caldera origin. The third sequence, The pre-Devonian Neptune Group consists of of basal orogenic conglomerate and more than 1,500 m of quartz-sandstone molasse that resulted from the erosion of the early Paleozoic mountains of the Ross orogeny. The fourth sequence of continental deposits of the Beacon Supergroup consists of Devonian quartz sanstone (Dover Sandstone), Permian glacial tillite (Gale Mudstone), and Permian siltstone and shale (Pecora Formation) containing glossopterid-bearing coal beds. During Early and Middle Jurassic time, and Transantarctic continental rift extensionally split the East Antarctic craton from West Antarctica as Gondwanaland began to break up. The continental rifting was shortly followed, during Late Jurassic time, by more vigorous extension resulting from major transform faulting.

  12. Continental accretion: contrasting Mesozoic and Early Proterozoic tectonic regimes in North America

    NASA Astrophysics Data System (ADS)

    Condie, Kent C.; Chomiak, Beverly

    1996-11-01

    Juvenile continental crust was accreted to southern and western North America during the Early Proterozoic and Mesozoic, respectively. Graywacke, granite, granodiorite, and basalt comprise most of the accreted Early Proterozoic crust, whereas graywacke, andesite, basalt, and granodiorite comprise most of the Mesozoic crust. In addition, carbonates, ultramafics, pelagic sediments, and tonalite/diorite are minor but important components in the juvenile Mesozoic crust, whereas rhyolites are important in the Early Proterozoic crust. Mesozoic supracrustal rocks vary significantly in chemical composition, while Early Proterozoic supracrustals have a limited compositional range and exhibit a linear relation between many element concentrations suggesting a genetic linkage between accreted terranes. Although SiO 2, Al 2O 3, FeO, and incompatible elements are more enriched in Early Proterozoic than in Mesozoic supracrustal rocks, negative Eu anomalies are typical of rocks of both ages. Early Proterozoic granitoids are enriched in LILE (large ion lithophile elements) compared to Mesozoic granitoids, and granitoids of both ages of are enriched in LILE and have larger Eu anomalies than associated supracrustal rocks. Accreted Mesozoic upper crust is similar to andesite in chemical composition, and the bulk crust is similar to basaltic andesite. In contrast, accreted Early Proterozoic upper crust and bulk crust are similar to granodiorite and andesite, respectively. Incompatible elements are depleted in the Mesozoic compared to the Early Proterozoic crust, but both crustal types have negative NbTa anomalies. Depending on the composition assumed for the lower crust, both ages of crust have either very small or negligible Eu anomalies. Lifespans of the Early Proterozoic terranes (time interval between oldest rocks in a terrane and its collision with North America) are 20-80 My, whereas lifespans of Mesozoic terranes are 50-500 My, with most falling between 50 and 200 My. Within

  13. The x ray microprobe determination of chromium oxidation state in olivine from lunar basalt and kimberlitic diamonds

    NASA Technical Reports Server (NTRS)

    Sutton, S. R.; Bajt, S.; Rivers, M. L.; Smith, J. V.

    1993-01-01

    The synchrotron x-ray microprobe is being used to obtain oxidation state information on planetary materials with high spatial resolution. Initial results on chromium in olivine from various sources including laboratory experiments, lunar basalt, and kimberlitic diamonds are reported. The lunar olivine was dominated by Cr(2+) whereas the diamond inclusions had Cr(2+/Cr(3+) ratios up to about 0.3. The simpliest interpretation is that the terrestrial olivine crystallized in a more oxidizing environment than the lunar olivine.

  14. Earliest Phanerozoic or latest Proterozoic fossils from the Arabian Shield

    USGS Publications Warehouse

    Cloud, P.; Awramik, S.M.; Morrison, K.; Hadley, D.G.

    1979-01-01

    We report here the first biologically definable fossils from pre-Saq (pre-Middle Cambrian) rocks of the Arabian Shield. They include the distinctive helically coiled tubular filaments of the oscillatorialean blue-green alga Obruchevella parva as well as two size classes of spheroidal unicells of uncertain affinity. Also present is the conical stromatolite Conophyton and unidentified stromatolites. All occur in cherty limestones of the Jubaylah Group, northern Saudi Arabia, a nonmarine to locally marine taphrogeosynclinal sequence that fills depressions along the northwest-trending Najd faults. Conophyton has heretofore been found only in strata older than about 680 Ma (except for puzzling records in modern hot springs) while Obruchevella is so far known only from rocks between about 680 and 470 Ma old. Thus it appears that the Jubaylah Group is close to the Proterozoic-Phanerozoic transition. The simple spheroidal nanno-fossils are not diagnostic as to age. Their relationships within what appears to be early diagenetic chert suggest a classical algal-mat association. The brecciated and microchanneled appearance of much of the fossiliferous rock, its locally dolomitic nature, and the prevalence of cryptalgalaminate favors a very shallow, locally turbulent, and perhaps episodically exposed marine or marginal marine setting. The Jubaylah Group lies unconformably beneath the Siq Sandstone (basal member of the Saq Sandstone) of medial Cambrian age, rests nonconformably on crystalline basement, and has yielded a K-Ar whole-rock age (on andesitic basalt) of ???540 Ma. To judge from the fossils, however, that age may be as much as 100 Ma or more too young. ?? 1979.

  15. A Modern Analogue for Proterozoic Inverse Carbon Isotope Signatures

    NASA Astrophysics Data System (ADS)

    Close, H. G.; Diefendorf, A. F.; Freeman, K. H.; Pearson, A.

    2008-12-01

    The carbon isotope distribution preserved in sedimentary lipids changes near the Neoproterozoic-Cambrian boundary. In older samples, n-alkyl lipids contain more 13C than both isoprenoid lipids and kerogen [1]. In younger samples, the opposite prevails. Although extreme heterotrophy has been invoked as a mechanism to explain the enrichment in 13C [2], here we suggest another explanation. The switch may reflect a fundamental transition from an oligotrophic ocean dominated by prokaryotic biomass, to an ocean in which carbon fixation is more intensive and burial is dominated by eukaryotic biomass. An analogue for Proterozoic ordering is found in the modern, oligotrophic Pacific Ocean, where n-alkyl lipids of picoplankton (0.2-0.5 μm particulate matter) contain excess 13C relative to the same lipids found in larger size classes (> 0.5 μm). Picoplanktonic lipids are heavier isotopically (-18 ‰) than both the sterols of eukaryotes (-23 ‰ to -26 ‰) and the total organic matter (-20 ‰; TOM). The 0.2-0.5 μm size class also has a distinct chain-length abundance profile. Although large particles must be the vehicle for total carbon export, paradoxically the lipid component of export production appears to be dominated by the 0.2-0.5 μm source. The picoplanktonic chain lengths and isotopic composition dominate lipids of TOM at 670 meters. When the ratio of prokaryotic to eukaryotic production is high, as in the modern central Pacific Ocean, it appears that exported material has an inverse carbon isotope signature similar to that preserved in Precambrian samples. [1] Logan, G. A. et al., Nature 376:53-56 (1995). [2] Rothman, D. H. et al., PNAS 100:8124-8129 (2003).

  16. Deformation styles in the Proterozoic Pinal schist, Pinal Mountains, Arizona

    SciTech Connect

    Keep, M.; Hansen, V. . Geology Dept.)

    1993-02-01

    A > 3 km thick, ductile, east-trending shear zone in the Pinal Peak map area, near Globe, Arizona, reveals contact metamorphism of the Pinal schist, through intrusion of the Proterozoic Madera granodiorite, and synchronous deformation of both units. The exposed shear zone comprises approximately 40 anastomosing shear zones, ranging in thickness from 10 cm to 40 cm, which cut both the Pinal schist and the Madera granodiorite. The shear zones have sharp boundaries that separate sheared material from massive zones lacking strong foliation, elongation lineation, and ductile deformation fabrics. The shear zones are characterized by well-developed, generally N- and NW- trending elongation lineation, and hand-sample scale S-C fabrics. Microstructures in Pinal schist and Madera granodiorite include S-C fabrics, mica fish', porphyroblast tails, and grain-shape- and lattice-preferred orientation of minerals, most of which indicate top-to-the-south displacement, consistent with field interpretations. Locally microstructures indicate top-to-the-north shear. Quartz c-axis fabrics were measured for 14 samples of Pinal schist and Madera granodiorite. Fabric diagrams show symmetric and symmetric double girdles, and activation of high temperature slip systems is evidenced by clustering of maxima in the center of the stereonet, parallel to the y-axis, in some plots. Symmetric fabrics indicate coaxial flattening. Asymmetric fabrics indicate top-to-the-north displacement, opposite to the shear sense derived from the majority of field and microstructural measurements. This may be indicative of a late-phase of backsliding on the shear zone, which could reset the quartz lattice fabrics. The high temperature slip systems of the quartz may be evidence for intrusion of the Madera granodiorite being early syn-tectonic in nature.

  17. The influence of complex intra- and extra-vent processes on facies characteristics of the Koala Kimberlite, NWT, Canada: volcanology, sedimentology and intrusive processes

    NASA Astrophysics Data System (ADS)

    Porritt, Lucy A.; Cas, Ray A. F.

    2011-08-01

    The Koala kimberlite, Northwest Territories, Canada, is a small pipe-like body that was emplaced into the Archean Koala granodiorite batholith and the overlying Cretaceous to Tertiary sediments at ~53 Ma. Koala is predominantly in-filled by a series of six distinct clastic deposits, the lowermost of which has been intruded by a late stage coherent kimberlite body. The clastic facies are easily distinguished from each other by variations in texture, and in the abundance and distribution of the dominant components. From facies analysis, we infer that the pipe was initially partially filled by a massive, poorly sorted, matrix-supported, olivine-rich lapilli tuff formed from a collapsing eruption column during the waning stage of the pipe-forming eruption. This unit is overlain by a granodiorite cobble-boulder breccia and a massive, poorly sorted, mud-rich pebbly-sandstone. These deposits represent post-eruptive gravitational collapse of the unstable pipe walls and mass wasting of tephra forming the crater rim. The crater then filled with water within which ~20 m of non-kimberlitic, wood-rich, silty sand accumulated, representing up to 47,000 years of quiescence. The upper two units in the Koala pipe are both olivine rich and show distinct grain-size grading. These units are interpreted to have been deposited sub-aqueously, from pyroclastic flows sourced from one or more other kimberlite volcanoes. The uppermost units in the Koala pipe highlight the likelihood that some kimberlite pipes may be only partially filled by their own eruptive products at the cessation of volcanic activity, enabling them to act as depocentres for pyroclastic and sedimentary deposits from the surrounding volcanic landscape. Recognition of these exotic kimberlite deposits has implications for kimberlite eruption and emplacement processes.

  18. Evidence for low sulphate and anoxia in a mid-Proterozoic marine basin.

    PubMed

    Shen, Yanan; Knoll, Andrew H; Walter, Malcolm R

    2003-06-01

    Many independent lines of evidence document a large increase in the Earth's surface oxidation state 2,400 to 2,200 million years ago, and a second biospheric oxygenation 800 to 580 million years ago, just before large animals appear in the fossil record. Such a two-staged oxidation implies a unique ocean chemistry for much of the Proterozoic eon, which would have been neither completely anoxic and iron-rich as hypothesized for Archaean seas, nor fully oxic as supposed for most of the Phanerozoic eon. The redox chemistry of Proterozoic oceans has important implications for evolution, but empirical constraints on competing environmental models are scarce. Here we present an analysis of the iron chemistry of shales deposited in the marine Roper Basin, Australia, between about 1,500 and 1,400 million years ago, which record deep-water anoxia beneath oxidized surface water. The sulphur isotopic compositions of pyrites in the shales show strong variations along a palaeodepth gradient, indicating low sulphate concentrations in mid-Proterozoic oceans. Our data help to integrate a growing body of evidence favouring a long-lived intermediate state of the oceans, generated by the early Proterozoic oxygen revolution and terminated by the environmental transformation late in the Proterozoic eon. PMID:12789336

  19. Evidence for low sulphate and anoxia in a mid-Proterozoic marine basin.

    PubMed

    Shen, Yanan; Knoll, Andrew H; Walter, Malcolm R

    2003-06-01

    Many independent lines of evidence document a large increase in the Earth's surface oxidation state 2,400 to 2,200 million years ago, and a second biospheric oxygenation 800 to 580 million years ago, just before large animals appear in the fossil record. Such a two-staged oxidation implies a unique ocean chemistry for much of the Proterozoic eon, which would have been neither completely anoxic and iron-rich as hypothesized for Archaean seas, nor fully oxic as supposed for most of the Phanerozoic eon. The redox chemistry of Proterozoic oceans has important implications for evolution, but empirical constraints on competing environmental models are scarce. Here we present an analysis of the iron chemistry of shales deposited in the marine Roper Basin, Australia, between about 1,500 and 1,400 million years ago, which record deep-water anoxia beneath oxidized surface water. The sulphur isotopic compositions of pyrites in the shales show strong variations along a palaeodepth gradient, indicating low sulphate concentrations in mid-Proterozoic oceans. Our data help to integrate a growing body of evidence favouring a long-lived intermediate state of the oceans, generated by the early Proterozoic oxygen revolution and terminated by the environmental transformation late in the Proterozoic eon.

  20. Peculiarities of mantle lithosphere beneath the large kimberlite pipes in different regions for Siberian craton

    NASA Astrophysics Data System (ADS)

    Ashchepkov, Igor; Logvinova, Alla; Ntaflos, Theodoros; Vladykin, Nikolai; Spetsius, Zdislav; Kostrovitsky, Sergey; Stegnitsky, Yuri; Prokopyev, Sergey

    2016-04-01

    Comparison of the structure of the mantle columns and mineralogy of the large kimberlite pipes in Yakutia from the different regions, kimberlite fields and mantle terranes in Yakutia allowed several assumptions. 1. The large kimberlite pipes possibly trace the ancient magma feeders occurred in the time of the continent growth. Commonly kimberlites and large pipes are tracing the deep faults and lineaments tracing the ancient sutures, rift zones, trans -lithospheric faults and other permeable structures, which may be parallel to the ancient continental margins. Large pipes locate at the periodic distance like volcanoes in arc settings tracing the "volcanic fronts". 2. Large pipes commonly contain the higher amounts of the sub-calcic garnets representing the dunitic associations (Stachel et al., 2008). In ophiolites dunites veins are representing the channels for the melt transfer (Kelemen et al., 2002). It is likely that ancient large magmatic arc system could have also deep seated roots represented by the (sub calcic) garnet - bearing dunitic systems. 3. Many large pipes including Udachnaya (Pokhilenko et al., 1999) and Mir (Roden et al., 2006) contain in mantle roots high amount of various pyroxenites. The most ancient pyroxenites are supplementary to the dunitic associations. But mostly they represent the materials from the re-melted eclogites and partial and hybrid melts (plume and subduction -related). They are concentrating in the traps in the lithosphere base, in the middle part of mantle section and in the basaltic trap 2.0-3.0 GPa. Pyroxenites in the lithosphere base in some cases are vary abundant but mostly they are protokimberlitic cumulates from of the latest stages of plume activity. Products of the melts crystallization from the earlier stages represent easy melting material at the lithosphere base could be the traps for the later plume melts. 5. Large pipes as a rule reveal contrast layering which is favorite for the capturing of the material from

  1. Peculiarities of mantle lithosphere beneath the large kimberlite pipes in different regions for Siberian craton

    NASA Astrophysics Data System (ADS)

    Ashchepkov, Igor; Logvinova, Alla; Ntaflos, Theodoros; Vladykin, Nikolai; Spetsius, Zdislav; Kostrovitsky, Sergey; Stegnitsky, Yuri; Prokopyev, Sergey

    2016-04-01

    Comparison of the structure of the mantle columns and mineralogy of the large kimberlite pipes in Yakutia from the different regions, kimberlite fields and mantle terranes in Yakutia allowed several assumptions. 1. The large kimberlite pipes possibly trace the ancient magma feeders occurred in the time of the continent growth. Commonly kimberlites and large pipes are tracing the deep faults and lineaments tracing the ancient sutures, rift zones, trans -lithospheric faults and other permeable structures, which may be parallel to the ancient continental margins. Large pipes locate at the periodic distance like volcanoes in arc settings tracing the "volcanic fronts". 2. Large pipes commonly contain the higher amounts of the sub-calcic garnets representing the dunitic associations (Stachel et al., 2008). In ophiolites dunites veins are representing the channels for the melt transfer (Kelemen et al., 2002). It is likely that ancient large magmatic arc system could have also deep seated roots represented by the (sub calcic) garnet - bearing dunitic systems. 3. Many large pipes including Udachnaya (Pokhilenko et al., 1999) and Mir (Roden et al., 2006) contain in mantle roots high amount of various pyroxenites. The most ancient pyroxenites are supplementary to the dunitic associations. But mostly they represent the materials from the re-melted eclogites and partial and hybrid melts (plume and subduction -related). They are concentrating in the traps in the lithosphere base, in the middle part of mantle section and in the basaltic trap 2.0-3.0 GPa. Pyroxenites in the lithosphere base in some cases are vary abundant but mostly they are protokimberlitic cumulates from of the latest stages of plume activity. Products of the melts crystallization from the earlier stages represent easy melting material at the lithosphere base could be the traps for the later plume melts. 5. Large pipes as a rule reveal contrast layering which is favorite for the capturing of the material from

  2. Structure and evolution of the mantle column beneath the Nakyn kimberlite field.

    NASA Astrophysics Data System (ADS)

    Ashchepkov, I.; Stegnitsky, Yu; Minin, V.; Tolstov, A.; Vladykin, N.; Rotman, A.; Palessky, S.; Agashev, A.; Khmelnikova, O.; Skvortsova, M.,

    2012-04-01

    Nakyn kimberlites refer to the main stage of kimberlite magmatism in Yakutia (Agashgev et al., 2004) Upper Devonian (UD) kimberlites. Minerals from the concentrates analyzed by EPMA and LAM ICP from Nyurbinskaya, Botuobinskaya pipes and Maiskaya body and placer were compared to determine mantle structure and compositions. Peridotite garnet population of all pies corresponds the lherzolite field to pyroxenitic 15% Cr2O3 (Pokhilenko, Sobolev, 2004), the subcalsic garnets prevail in (8-11; Cr2O3). Peaks of the TiO2 and Na2O correspond to the same Cr2O3 intervals. (Zinchuk et al, 2003). In chromite trend Cr2O3 ( 65-20% ) lognormal decrease with TiO2 enrichment 3.5% in the beginning. In tuffs containing clinopyroxenes from Nyurbinskaya, Botuobinskaya pipes show decrease of Na, Al Ti and Cr rise with Fe as for Mir pie. The Cp in Nurbinskaya tuffs with Ilm are higher in TiO2 Restricted in TiO2 ilmenite trend show rapid decrease of Na2O, MgO content and V2O5 -FeO rise (Fig. 4). The PTFO2 diagrams determined with new variant of the monomineral thermobarometry (Ashchepkov et al., 2010; 2012) reveal a bit different geotherms for Nyurbinskaya pipe 38 to 43 mv/m2 for garnets and Sp estimates and colder for CPx. The refertillization interval markedby Fe rich Cpx and Ilm is 65-30 kbar. For the Botuobinskaya pipe there are 2 branches 40 and 35 mv/m2 and the heating branch at 65 kbars. Mayskaya is showing the colder geotherm but deviations to the hotter part correspondent to the pyroxenites. The Placer in this area show the wider variations of PT conditions and refertillization interval possibly showing possibility of another source of the disintegration which is much rich in the pyroxenites which amount is highly increase with the depth. The comparisons of the PT conditions show that possibly the Mayskaya and Botuobinskaya pipes represent the earlier stages of the developing of mantle columns while Nyurbinskaya the later one but amount of pyroxenites in Nyurbinskaya is less then

  3. Proterozoic low orbital obliquity and axial-dipolar geomagnetic field from evaporite palaeolatitudes

    NASA Astrophysics Data System (ADS)

    Evans, David A. D.

    2006-11-01

    Palaeomagnetism of climatically sensitive sedimentary rock types, such as glacial deposits and evaporites, can test the uniformitarianism of ancient geomagnetic fields and palaeoclimate zones. Proterozoic glacial deposits laid down in near-equatorial palaeomagnetic latitudes can be explained by `snowball Earth' episodes, high orbital obliquity or markedly non-uniformitarian geomagnetic fields. Here I present a global palaeomagnetic compilation of the Earth's entire basin-scale evaporite record. Magnetic inclinations are consistent with low orbital obliquity and a geocentric-axial-dipole magnetic field for most of the past two billion years, and the snowball Earth hypothesis accordingly remains the most viable model for low-latitude Proterozoic ice ages. Efforts to reconstruct Proterozoic supercontinents are strengthened by this demonstration of a consistently axial and dipolar geomagnetic reference frame, which itself implies stability of geodynamo processes on billion-year timescales.

  4. Proterozoic low orbital obliquity and axial-dipolar geomagnetic field from evaporite palaeolatitudes.

    PubMed

    Evans, David A D

    2006-11-01

    Palaeomagnetism of climatically sensitive sedimentary rock types, such as glacial deposits and evaporites, can test the uniformitarianism of ancient geomagnetic fields and palaeoclimate zones. Proterozoic glacial deposits laid down in near-equatorial palaeomagnetic latitudes can be explained by 'snowball Earth' episodes, high orbital obliquity or markedly non-uniformitarian geomagnetic fields. Here I present a global palaeomagnetic compilation of the Earth's entire basin-scale evaporite record. Magnetic inclinations are consistent with low orbital obliquity and a geocentric-axial-dipole magnetic field for most of the past two billion years, and the snowball Earth hypothesis accordingly remains the most viable model for low-latitude Proterozoic ice ages. Efforts to reconstruct Proterozoic supercontinents are strengthened by this demonstration of a consistently axial and dipolar geomagnetic reference frame, which itself implies stability of geodynamo processes on billion-year timescales.

  5. Proterozoic low orbital obliquity and axial-dipolar geomagnetic field from evaporite palaeolatitudes.

    PubMed

    Evans, David A D

    2006-11-01

    Palaeomagnetism of climatically sensitive sedimentary rock types, such as glacial deposits and evaporites, can test the uniformitarianism of ancient geomagnetic fields and palaeoclimate zones. Proterozoic glacial deposits laid down in near-equatorial palaeomagnetic latitudes can be explained by 'snowball Earth' episodes, high orbital obliquity or markedly non-uniformitarian geomagnetic fields. Here I present a global palaeomagnetic compilation of the Earth's entire basin-scale evaporite record. Magnetic inclinations are consistent with low orbital obliquity and a geocentric-axial-dipole magnetic field for most of the past two billion years, and the snowball Earth hypothesis accordingly remains the most viable model for low-latitude Proterozoic ice ages. Efforts to reconstruct Proterozoic supercontinents are strengthened by this demonstration of a consistently axial and dipolar geomagnetic reference frame, which itself implies stability of geodynamo processes on billion-year timescales. PMID:17080082

  6. Evidence of mantle metasomatism in garnet peridotites from V. Grib kimberlite pipe (Arkhangelsk region, Russia)

    NASA Astrophysics Data System (ADS)

    Shchukina, Elena; Agashev, Alexey; Golovin, Nikolai; Pokhilenko, Nikolai

    2013-04-01

    We have studied 26 samples of garnet peridotite xenoliths from V.Grib pipe and 17 of them are phlogopite bearing. Studied peridotites have features of two types of modal metasomatism: low-temperature (˜ 1100 C°) and high-temperature (˜ 1100 C°). Low-temperature modal metasomatism: 17 samples contain modal phlogopite, which is present in the form of tabular grains (to 3 mm in size) and rims around pyrope grains. Chemical composition of minerals from phlogopite-garnet peridotites and phlogopite free peridotites is distinctly different. Olivine, garnet, orthopyroxene and clinopyroxene have higher concentration of FeO relative to these minerals in phlogopite free peridotites. Occurrence of phlogopite in peridotites indicates the influence of melt enriched in K2O, H2O, FeO and other incompatible elements. Two types of phlogopite have difference in chemical composition that indicates two different sources. High-temperature modal metasomatism: Reconstructed V.Grib pipe peridotite whole-rocks composition and high Mg# of peridotite olivines indicates that these samples are residues after 30-40 % partial melting of primitive mantle. At those high degree of partial melting all clinopyroxene and probably all garnet should be exhausted from residue. Character of REE patterns in garnets and clinopyroxenes indicates that the most garnets and all clinopyroxene in studied peridotites are of metasomatic origin. We used the method of geochemical modeling of fractional crystallization to establish the source's composition for garnets and clinopyroxenes. For geochemical modeling we used the composition of tholeitic basalts, picrites and carbonatites which occurred in Arkhangelsk diamondiferous province (ADP) and have emplacement ages similar to that of kimberlites. Modeling result indicates that garnets could be crystallized from alkali picrite and tholeite basalts compositions. Peridotites containing garnets equilibrated with picritic melt have a different position in lithospheric

  7. Repeated kimberlite magmatism beneath Yakutia and its relationship to Siberian flood volcanism: Insights from in situ U-Pb and Sr-Nd perovskite isotope analysis

    NASA Astrophysics Data System (ADS)

    Sun, Jing; Liu, Chuan-Zhou; Tappe, Sebastian; Kostrovitsky, Sergey I.; Wu, Fu-Yuan; Yakovlev, Dmitry; Yang, Yue-Heng; Yang, Jin-Hui

    2014-10-01

    We report combined U-Pb ages and Sr-Nd isotope compositions of perovskites from 50 kimberlite occurrences, sampled from 9 fields across the Yakutian kimberlite province on the Siberian craton. The new U-Pb ages, together with previously reported geochronological constraints, suggest that kimberlite magmas formed repeatedly during at least 4 episodes: Late Silurian-Early Devonian (419-410 Ma), Late Devonian-Early Carboniferous (376-347 Ma), Late Triassic (231-215 Ma), and Middle/Late Jurassic (171-156 Ma). Recurrent kimberlite melt production beneath the Siberian craton - before and after flood basalt volcanism at 250 Ma - provides a unique opportunity to test existing models for the origin of global kimberlite magmatism. The internally consistent Sr and Nd isotope dataset for perovskites reveals that the Paleozoic and Mesozoic kimberlites of Yakutia have distinctly different initial radiogenic isotope compositions. There exists a notable increase in the initial 143Nd/144Nd ratios through time, with an apparent isotopic evolution that is intermediate between that of Bulk Earth and Depleted MORB Mantle. While the Paleozoic samples range between initial 87Sr/86Sr of 0.7028-0.7034 and 143Nd/144Nd of 0.51229-0.51241, the Mesozoic samples show values between 0.7032-0.7038 and 0.51245-0.51271, respectively. Importantly, perovskites from all studied Yakutian kimberlite fields and age groups have moderately depleted initial εNd values that fall within a relatively narrow range between +1.8 and +5.5. The perovskite isotope systematics of the Yakutian kimberlites are interpreted to reflect magma derivation from the convecting upper mantle, which appears to have a record of continuous melt depletion and crustal recycling throughout the Phanerozoic. The analyzed perovskites neither record highly depleted nor highly enriched isotopic components, which had been previously identified in likely plume-related Siberian Trap basalts. The Siberian craton has frequently been suggested

  8. Proterozoic geochronologic and isotopic boundary in NW Arizona

    SciTech Connect

    Chamberlain, K.R.; Bowring, S.A. )

    1990-05-01

    U-Pb ages determined from zircon, sphene, and apatite in conjunction with Pb isotopic analyses of alkali feldspar establish a regional geochronological framework and constrain the location of a major north-trending Proterozoic crustal boundary in northwestern Arizona. Two regions west of the boundary (Hualapai Mountains and Lost Basin Range-Garnet Mountain) are characterized by complex U-Pb zircon systematics, evidence for inheritance of an older zircon component (1.8-2.3 Ga), and elevated {sup 207}Pb/{sup 204}Pb from feldspars compared to the east. Although the discordia patterns are complex, supracrustal rocks are interpreted to be ca. 1.73 Ga and are intruded by plutonic rocks ca. 1.70 Ga. Deformation is younger than ca. 1.70 Ga foliated granites and older than the 1,682 {plus minus} 4 Ma Garnet Mt. monzogranite. The rocks in one area east of the boundary (Cottonwood Cliffs) are characterized by relatively simple U-Pb zircon systematics, no evidence for inheritance of any older component, and feldspar {sup 207}/{sup 204}Pb near model mantle values. Supracrustal rocks are older than 1.73 Ga, as they are intruded by a 1,730 {plus minus} 9 Ma foliated granodiorite. Timing of deformation is constrained by the late syn-kinematic Valentine granite dated at 1,713 {plus minus} 12 Ma. Mineral ages indicate that the rocks on either side of the boundary had different cooling histories and inferentially, different uplift histories. West of the boundary, the cooling history is inferred from minerals separated from an amphibolite: metamorphic zircon is 1,687 +13/{minus}8 Ma, sphene is 1,660 {plus minus} 5 Ma, hornblende has a {sup 40}Ar/{sup 39}Ar age of 1,552 {plus minus} 5 Ma, and apatite has a U-Pb age of 1,520 {plus minus} 45 Ma. East of the boundary an amphibolite has sphene with an age of 1,670 {plus minus} 11 Ma and apatite with an age of 1,630 {plus minus} 8 Ma.

  9. Geochronologic and isotopic evidence for early Proterozoic crust in the eastern Arabian Shield

    SciTech Connect

    Stacey, J.S.; Hedge, C.E.

    1984-05-01

    The authors report zircon U-Pb, feldspar common Pb, whole-rock Sm-Nd, and Rb-Sr data from sample Z-103, a fine-grained granodiorite from the Jabal Khida region of the Saudi Arabian Shield (lat 21/sup 0/19'N; long 44/sup 0/50'W). The measurements yield conclusive evidence for continental crust of early Proterozoic age (approx.1630 Ma) at that locality. Furthermore, lead-isotope data indicate an even earlier, perhaps Archean, crustal history for the source of the lower Proterozoic rocks. 17 references, 4 figures, 1 table.

  10. Investigating metasomatic effects on the 187Os isotopic signature: A case study on micrometric base metal sulphides in metasomatised peridotite from the Letlhakane kimberlite (Botswana)

    NASA Astrophysics Data System (ADS)

    Wainwright, A. N.; Luguet, A.; Fonseca, R. O. C.; Pearson, D. G.

    2015-09-01

    The peridotite xenoliths of the Letlhakane kimberlite (Botswana), which intrude the Proterozoic Magondi Belt on the western margin of the Zimbabwe craton, represent highly depleted melting residues. These residues suffered subsequent variable metasomatic overprinting, evidenced by cryptic trace element enrichments in the spinel peridotites to modal addition of phlogopite, clinopyroxene and spinel within the garnet peridotites. In order to assess the robustness of the Re-Os chronometer in such highly metasomatised peridotites, detailed investigations of base metal sulphide (BMS) petrography and single-BMS grain 187Os/188Os analyses have been undertaken in three representative peridotites. The BMS occur as < 10 μm-50 μm inclusions and interstitial grains that are associated with metasomatic phases or metasomatised rims of primary silicates or display melt-like morphology, all attesting of their metasomatic origin. Their 187Os isotopic compositions vary from 0.1016 to 0.6109 yielding TRD ages from 3.75 ± 0.54 (2se) to future ones. They vary independently of the cryptic or modal silicate metasomatic overprinting on the peridotites and independently of the BMS-silicate textural habits (e.g. isolated inclusions, pseudo-inclusions, intergranular melt-like pools), contrary to what is commonly assumed. In such highly depleted peridotites that must have been sulphide-free after the partial melting event, the Eoarchean age is likely inherited from residual PGM (platinum group minerals; i.e. laurite and Os-alloys) that formed in response to the exhaustion of the primary BMS and were later redissolved within the metasomatic BMS. In contrast, the younger single grain TRD ages represents an increasing dilution of the residual PGM signals within the metasomatic BMS, with the single grain 187Os/188Os signatures becoming increasingly dominated by Os derived from metasomatic BMS. Taken as a whole, the single BMS grain Eoarchean age suggests a lithospheric stabilisation age in

  11. The Origin and Evolution of Kimberlite Melts: Stabilizing Phlogopite in the CMAS-CO2- H2O-K2O System

    NASA Astrophysics Data System (ADS)

    Buisman, I.; Sparks, S.; Walter, M.

    2008-12-01

    This project aims to investigate the melting phase relations of model lherzolite in the system CMAS- CO2-H2O-K2O to better understand the role of potassium (K) in the evolution and origin of kimberlitic melts. High-pressure multi-anvil and piston cylinder experiments are used to study this system at upper mantle pressures (3-9 GPa). This study aims at constraining the temperature and composition of primary melts at the volatile-rich mantle solidus at which kimberlite melts form. Kimberlites are potassium-rich, ultrabasic magmas (<35% SiO2), have a low viscosity (0.1-1 Pa s), and contain a very high volatile content (CO2 and H2O). A number of models have been suggested for the generation of carbonatite and kimberlite magmas, with the presence of volatiles being particularly important (eg. CO2). Together, H2O and CO2, show a much greater influence on the solidus of mantle lherzolite than when either are present alone. Melts of carbonatitic and kimberlitic composition can be produced under comparable P-T conditions by partial melting of carbonated lherzolite. Petrogenetic links between carbonatites and kimberlites are therefore implied in the CO2-bearing mantle source region (Gudfinnsson, 2005). The isobaric univariant equilibrium for melting of model lherzolite in CMAS-CO2-H2O-K2O is tracked at upper mantle conditions. This is done by constructing a series of bulk compositions that will saturate all phases and yield enough of each phase for EPMA analysis. The compositions of all phases along a portion of the isobaric univariant melting curves will be traced at a series of pressures. In this way, we can rigorously calculate the melting behaviour of lherzolite compositions as a function of pressure, temperature and bulk composition.

  12. Calcified microbes in Neoproterozoic carbonates: implications for our understanding of the Proterozoic/Cambrian transition

    NASA Technical Reports Server (NTRS)

    Knoll, A. H.; Fairchild, I. J.; Swett, K.

    1993-01-01

    Tidal flat and lagoonal dolostones of the Neoproterozoic Draken Formation, Spitsbergen, exhibit excellent preservation of carbonate fabrics, including heavily calcified microfossils. The crust-forming cyanobacterium Polybessurus is preserved locally by carbonate precipitated on and within sheaths in mildly evaporitic upper intertidal to supratidal environments. In contrast, calcified filaments in columnar stromatolites reflect subtidal precipitation. Filament molds in dolomicrites independently document extremely early lithification. The presence of heavily calcified cyanobacteria in Draken and other Proterozoic carbonates constrains potential explanations for the widespread appearance of calcified microorganisms near the Proterozoic-Cambrian boundary. We propose that the rarity of Proterozoic examples principally reflects the abundance and wide distribution of carbonate crystals precipitated on the sea floor or in the water column. Cyanobacterial sheaths would have competed effectively as sites for carbonate nucleation and growth only where calcitic and/or aragonitic nuclei were absent. In this view, the Proterozoic-Cambrian expansion of calcified microfossils primarily reflects the emergence of skeletons as principal agents of carbonate deposition.

  13. Integrated approaches to terminal Proterozoic stratigraphy: an example from the Olenek Uplift, northeastern Siberia

    NASA Technical Reports Server (NTRS)

    Knoll, A. H.; Grotzinger, J. P.; Kaufman, A. J.; Kolosov, P.

    1995-01-01

    In the Olenek Uplift of northeastern Siberia, the Khorbusuonka Group and overlying Kessyusa and Erkeket formations preserve a significant record of terminal Proterozoic and basal Cambrian Earth history. A composite section more than 350 m thick is reconstructed from numerous exposures along the Khorbusuonka River. The Khorbusuonka Group comprises three principal sedimentary sequences: peritidal dolomites of the Mastakh Formation, which are bounded above and below by red beds; the Khatyspyt and most of the overlying Turkut formations, which shallow upward from relatively deep-water carbonaceous micrites to cross-bedded dolomitic grainstones and stromatolites; and a thin upper Turkut sequence bounded by karst surfaces. The overlying Kessyusa Formation is bounded above and below by erosional surfaces and contains additional parasequence boundaries internally. Ediacaran metazoans, simple trace fossils, and vendotaenids occur in the Khatyspyt Formation; small shelly fossils, more complex trace fossils, and acritarchs all appear near the base of the Kessyusa Formation and diversify upward. The carbon-isotopic composition of carbonates varies stratigraphically in a pattern comparable to that determined for other terminal Proterozoic and basal Cambrian successions. In concert, litho-, bio-, and chemostratigraphic data indicate the importance of the Khorbusuonka Group in the global correlation of terminal Proterozoic sedimentary rocks. Stratigraphic data and a recently determined radiometric date on basal Kessyusa volcanic breccias further underscore the significance of the Olenek region in investigations of the Proterozoic-cambrian boundary.

  14. Integrated approaches to terminal Proterozoic stratigraphy: an example from the Olenek Uplift, northeastern Siberia.

    PubMed

    Knoll, A H; Grotzinger, J P; Kaufman, A J; Kolosov, P

    1995-01-01

    In the Olenek Uplift of northeastern Siberia, the Khorbusuonka Group and overlying Kessyusa and Erkeket formations preserve a significant record of terminal Proterozoic and basal Cambrian Earth history. A composite section more than 350 m thick is reconstructed from numerous exposures along the Khorbusuonka River. The Khorbusuonka Group comprises three principal sedimentary sequences: peritidal dolomites of the Mastakh Formation, which are bounded above and below by red beds; the Khatyspyt and most of the overlying Turkut formations, which shallow upward from relatively deep-water carbonaceous micrites to cross-bedded dolomitic grainstones and stromatolites; and a thin upper Turkut sequence bounded by karst surfaces. The overlying Kessyusa Formation is bounded above and below by erosional surfaces and contains additional parasequence boundaries internally. Ediacaran metazoans, simple trace fossils, and vendotaenids occur in the Khatyspyt Formation; small shelly fossils, more complex trace fossils, and acritarchs all appear near the base of the Kessyusa Formation and diversify upward. The carbon-isotopic composition of carbonates varies stratigraphically in a pattern comparable to that determined for other terminal Proterozoic and basal Cambrian successions. In concert, litho-, bio-, and chemostratigraphic data indicate the importance of the Khorbusuonka Group in the global correlation of terminal Proterozoic sedimentary rocks. Stratigraphic data and a recently determined radiometric date on basal Kessyusa volcanic breccias further underscore the significance of the Olenek region in investigations of the Proterozoic-cambrian boundary.

  15. The Diamondiferous Lithospheric Mantle Underlying the Eastern Superior Craton: Evidence From Mantle Xenoliths From the Renard Kimberlites, Quebec

    NASA Astrophysics Data System (ADS)

    Hunt, L.; Stachel, T.; Armstrong, J. P.; Simonetti, A.

    2009-05-01

    The Renard kimberlite cluster consists of nine pipes located within a 2km2 area in the northern Otish Mountains of Quebec. The pipes are named Renards 1 to 10, with subsequent investigation revealing Renards 5 and 6 to join at depth (now Renard 65). The pipes are located within the eastern portion of the Superior craton, emplaced into Archean granitic and gneissic host rocks of the Opinica Subprovince (Percival, 2007). Amphibolite grade metamorphism, locally passing into the granulite facies (Percival et al., 1994) occurred in late Archean time (Moorhead et al., 2003). Radiometric dating of the hypabyssal Renard 1 kimberlite indicates Neoproterozoic emplacement, with a 206Pb/238U model age of 631.6±3.5 Ma (2σ) (Birkett et al., 2004). A later study on the main phases in Renard 2 and 3 gave a similar emplacement, with a 206Pb/238U model age of 640.5±2.8Ma (Fitzgerald et al., 2008). This makes this kimberlite district one of the oldest in Canada, similar in eruption age to the Wemindji kimberlites (629±29Ma: Letendre et al., 2003). These events are broadly coeval with the conversion from subduction magmatism to rifting in northern Laurentia (Birkett et al., 2004). The bodies are part of a late Neoproterozoic to Cambrian kimberlite field in eastern Canada (Girard, 2001; Moorhead et al, 2002; Letendre et al., 2003) and fit into the north-east of the Eocambrian/Cambrian Labrador Sea Province of Heaman et al. (2004). To better understand the diamondiferous lithospheric mantle beneath the Renard kimberlites, 116 microxenoliths and xenocrysts were analysed. The samples were dominantly peridotitic, composed primarily of purple garnet, emerald green clinopyroxene and olivine, with a few pink and red garnets. A minor eclogitic component comprises predominantly orange garnets and lesser amounts of clinopyroxene. A detailed study on the major, minor and trace element composition of xenolith minerals is currently underway. All but three of the clinopyroxenes analysed to date

  16. Dating Kimberlite Eruption and Erosion Phases Using Perovskite, Zircon, and Apatite (U-Th)/He Geochronology to Link Cratonic Lithosphere Evolution and Surface Processes

    NASA Astrophysics Data System (ADS)

    Stanley, J. R.; Flowers, R. M.

    2015-12-01

    In many cases it is difficult to evaluate the synchronicity and thus potential connections between disparate geologic events, such as the links between processes in the mantle lithosphere and at the surface. Developing new geochronologic tools and strategies for integrating existing chronologic data with other information is essential for addressing these problems. Here we use (U-Th)/He dating of multiple kimberlitic minerals to date kimberlite eruption and cratonic erosion phases. This approach permits us to more directly assess the link between unroofing and thermomodification of the lithosphere by tying our results to information obtained from mantle-derived clasts in the same pipes. Kimberlites are rich sources of information about the composition of the cratonic lithosphere and its evolution over time. Their xenoliths and xenocrysts can preserve a snapshot of the entire lithosphere and its sedimentary cover at the time of eruption. Accurate geochronology of these eruptions is crucial for interpreting spatiotemporal trends, but kimberlites can be difficult to date using standard techniques. Here we show that the mid-temperature thermochonometers of the zircon and perovskite (U-Th)/He (ZHe, PHe) systems can be viable tools for dating kimberlite eruption. When combined with the low temperature sensitivity of (U-Th)/He in apatite (AHe), the (U-Th)/He system can be used to date both the emplacement and the erosional cooling history of kimberlites. The southern African shield is an ideal location to test the utility of this approach because the region was repeatedly intruded by kimberlites in the Cretaceous, with two major pulses at ~200-110 Ma and ~100-80 Ma. These kimberlites contain a well-studied suite of mantle xenoliths and xenocrysts that document lithospheric heating and metasomatism over this interval. Our ZHe and PHe dates overlap with published eruption ages and add new ages for undated pipes. Our AHe dates constrain the spatial patterns of Cretaceous

  17. Combining kimberlite (U-Th)/He dating with the mantle xenolith record to decipher elevation change in continental interiors: an example from the southern African Plateau

    NASA Astrophysics Data System (ADS)

    Stanley, J. R.; Flowers, R. M.; Bell, D. R.

    2013-12-01

    Deciphering the patterns and causes of erosion and elevation change histories in continental interiors is commonly not straightforward. Many continental shield regions are repeatedly intruded by small volume kimberlite magmas, which often contain rich xenolith records of the state of the lithosphere and the sedimentary cover at the time of eruption. Here we show that dating kimberlites with apatite (U-Th)/He thermochronometry (AHe), a tool used to constrain thermal histories within the upper 1-3 km of the crust, can tightly bracket the timing of erosion through comparison of cooling dates and eruption ages. Mantle xenoliths from kimberlites erupted at different times also record perturbations to the lithospheric mantle, indicators of changes to the mantle below. The coeval deep and shallow records of kimberlite pipes thus allow the potential to link deep earth processes with the surface response. The southern African Plateau was elevated from sea level to >1000 m elevation in post-Paleozoic time while distal from convergent plate boundaries and with little surface deformation. The timing and mechanisms of surface uplift are debated. AHe data for four kimberlites off the southwestern corner of the Kaapvaal Craton indicate a substantial Mesozoic unroofing episode that was largely completed by 90 Ma. This erosion phase is contemporaneous with significant warming, metasomatism, and thinning of the lithospheric mantle revealed in the peridotite xenoliths and garnet xenocrysts in these same pipes. We suggest that this surface signal is the erosional response to regional, mantle-induced surface uplift. These data also detect a lesser Cenozoic erosion signature in some pipes, focused around a proposed Tertiary paleo-tributary to the Orange River, suggesting that the Cenozoic signal is associated with drainage network evolution rather than long-wavelength surface uplift. Preliminary data from an E-W transect of kimberlites across the Kaapvaal Craton from Kimberley to the

  18. Carbonate deposition during the late Proterozoic Era: an example from Spitsbergen

    NASA Technical Reports Server (NTRS)

    Knoll, A. H.; Swett, K.

    1990-01-01

    Carbonate sediments reflect the physico-chemical and biological circumstances of their formation; thus, features of limestones and dolomites may provide insights into both environmental and evolutionary change through geological time. The Upper Proterozoic (approx 800-700 Ma) Akademikerbreen Group, Spitsbergen, comprises 2000 m of carbonates, with only minor intercalations of quartz arenite and shale. Although Proterozoic carbonates are often seen as predominantly dolomitic, the Akademikerbreen Group is about 45 percent limestone. Stromatolites are conspicuous in outcrop but constitute only 25 percent of the total section. Micrites and coarser intraclastic carbonates derived mainly from micritric precursors comprise 60 percent of the group, while oolites make up the remaining 15 percent. Distinctive sedimentary features of the group include giant (up to 16 mm) ooids, very early diagenetic calcite nodules and cements, micrites containing subaqueous shrinkage cracks filled with equant microspar cement, and strong 13C enrichment in both carbonates and co-occurring organic matter. The principal features of Akademikerbreen carbonates are widely distributed in coeval successions. However, these rocks appear to differ from older limestones and dolomites in their relative abundance of grainstones and, perhaps, micrites, as well as their paucity of tufa-like laminates and columnar or coniform stromatolites that preserve petrographic evidence of in situ precipitation as a dominant means of carbonate accretion. Upper Proterozoic carbonates also differ from Paleozoic accumulations, but the transition is not abrupt. Most changes accompanying the Proterozoic/Phanerozoic transition can be interpreted in terms of the consequences rather than the causes of metazoan and metaphyte evolution, including the evolution of biomineralization. Carbonate sedimentology reinforces data from other sources which indicate the last 200 to 300 Ma of the Proterozoic Eon was a distinctive interval of

  19. Carbonate deposition during the late Proterozoic Era: an example from Spitsbergen.

    PubMed

    Knoll, A H; Swett, K

    1990-01-01

    Carbonate sediments reflect the physico-chemical and biological circumstances of their formation; thus, features of limestones and dolomites may provide insights into both environmental and evolutionary change through geological time. The Upper Proterozoic (approx 800-700 Ma) Akademikerbreen Group, Spitsbergen, comprises 2000 m of carbonates, with only minor intercalations of quartz arenite and shale. Although Proterozoic carbonates are often seen as predominantly dolomitic, the Akademikerbreen Group is about 45 percent limestone. Stromatolites are conspicuous in outcrop but constitute only 25 percent of the total section. Micrites and coarser intraclastic carbonates derived mainly from micritric precursors comprise 60 percent of the group, while oolites make up the remaining 15 percent. Distinctive sedimentary features of the group include giant (up to 16 mm) ooids, very early diagenetic calcite nodules and cements, micrites containing subaqueous shrinkage cracks filled with equant microspar cement, and strong 13C enrichment in both carbonates and co-occurring organic matter. The principal features of Akademikerbreen carbonates are widely distributed in coeval successions. However, these rocks appear to differ from older limestones and dolomites in their relative abundance of grainstones and, perhaps, micrites, as well as their paucity of tufa-like laminates and columnar or coniform stromatolites that preserve petrographic evidence of in situ precipitation as a dominant means of carbonate accretion. Upper Proterozoic carbonates also differ from Paleozoic accumulations, but the transition is not abrupt. Most changes accompanying the Proterozoic/Phanerozoic transition can be interpreted in terms of the consequences rather than the causes of metazoan and metaphyte evolution, including the evolution of biomineralization. Carbonate sedimentology reinforces data from other sources which indicate the last 200 to 300 Ma of the Proterozoic Eon was a distinctive interval of

  20. Proterozoic crustal boundary in the southern part of the Illinois Basin

    USGS Publications Warehouse

    Heigold, P.C.; Kolata, Dennis R.

    1993-01-01

    Recently acquired COCORP and proprietary seismic reflection data in the southern part of the Illinois Basin, combined with other geological and geophysical data, indicate that a WNW-trending Proterozoic terrane boundary (40 km wide) lies within basement. The boundary is characterized by the termination of subhorizontal Proterozoic reflectors and associated diffraction patterns along a line coinciding with the major magnetic lineament in this region (South Central Magnetic Lineament). North of the boundary, where reflectors thought to represent a sequence of layered Proterozoic rocks in the upper crust are widespread and as much as 11 km thick, total magnetic intensity values are relatively high, suggesting layers of rock with high magnetic susceptibility. To the south, the Proterozoic rocks are acoustically transparent on seismic reflection sections and total magnetic intensity values are relatively low. Moreover, relatively high Bouguer gravity anomaly values to the south may be caused by a dense, altered, lower crustal layer similar to that interpreted from deep seismic refraction studies to underlie the northern Mississippi Embayment. The boundary lies along the projected trend of the northern margin of the Early Proterozoic Central Plains orogen and we suggest that it marks the convergent margin of this orogen. Reactivation of the boundary and the associated zone of weakness during late Paleozoic times apparently resulted in structural deformation in the southern part of the Illinois Basin, including movement along the Cottage Grove Fault System and Ste. Genevieve Fault Zone and igneous activity at Hicks Dome. In addition to the role played by this crustal boundary in the evolution of the Illinois Basin, its location between the Wabash Valley Seismic Zone to the northeast and the New Madrid Seismic Zone to the southwest may be a significant factor in present-day seismicity. ?? 1993.

  1. Carbonate deposition during the late Proterozoic Era: an example from Spitsbergen.

    PubMed

    Knoll, A H; Swett, K

    1990-01-01

    Carbonate sediments reflect the physico-chemical and biological circumstances of their formation; thus, features of limestones and dolomites may provide insights into both environmental and evolutionary change through geological time. The Upper Proterozoic (approx 800-700 Ma) Akademikerbreen Group, Spitsbergen, comprises 2000 m of carbonates, with only minor intercalations of quartz arenite and shale. Although Proterozoic carbonates are often seen as predominantly dolomitic, the Akademikerbreen Group is about 45 percent limestone. Stromatolites are conspicuous in outcrop but constitute only 25 percent of the total section. Micrites and coarser intraclastic carbonates derived mainly from micritric precursors comprise 60 percent of the group, while oolites make up the remaining 15 percent. Distinctive sedimentary features of the group include giant (up to 16 mm) ooids, very early diagenetic calcite nodules and cements, micrites containing subaqueous shrinkage cracks filled with equant microspar cement, and strong 13C enrichment in both carbonates and co-occurring organic matter. The principal features of Akademikerbreen carbonates are widely distributed in coeval successions. However, these rocks appear to differ from older limestones and dolomites in their relative abundance of grainstones and, perhaps, micrites, as well as their paucity of tufa-like laminates and columnar or coniform stromatolites that preserve petrographic evidence of in situ precipitation as a dominant means of carbonate accretion. Upper Proterozoic carbonates also differ from Paleozoic accumulations, but the transition is not abrupt. Most changes accompanying the Proterozoic/Phanerozoic transition can be interpreted in terms of the consequences rather than the causes of metazoan and metaphyte evolution, including the evolution of biomineralization. Carbonate sedimentology reinforces data from other sources which indicate the last 200 to 300 Ma of the Proterozoic Eon was a distinctive interval of

  2. The composition of volatile components in olivines from Yakutian kimberlites of various ages: Evidence from gas chromatography-mass spectrometry

    NASA Astrophysics Data System (ADS)

    Tomilenko, A. A.; Bul'bak, T. A.; Khomenko, M. O.; Kuzmin, D. V.; Sobolev, N. V.

    2016-06-01

    The composition of volatiles from fluid and melt inclusions in olivine phenocrysts from Yakutian kimberlite pipes of various ages (Olivinovaya, Malokuonapskaya, and Udachnaya-East) were studied for the first time by gas chromatography-mass spectrometry. It was shown that hydrocarbons and their derivatives, as well as nitrogen-, halogen-, and sulfur-bearing compounds, played a significant role in the mineral formation. The proportion of hydrocarbons and their derivatives in the composition of mantle fluids could reach 99%, including up to 4.9% of chlorineand fluorine-bearing compounds.

  3. Stable isotope paleoclimatology of the earliest Eocene using kimberlite-hosted mummified wood from the Canadian Subarctic

    NASA Astrophysics Data System (ADS)

    Hook, B. A.; Halfar, J.; Gedalof, Z.; Bollmann, J.; Schulze, D. J.

    2015-10-01

    The recent discovery of well-preserved mummified wood buried within a subarctic kimberlite diamond mine prompted a paleoclimatic study of the early Eocene "hothouse" (ca. 53.3 Ma). At the time of kimberlite eruption, the Subarctic was warm and humid producing a temperate rainforest biome well north of the Arctic Circle. Previous studies have estimated that mean annual temperatures in this region were 4-20 °C in the early Eocene, using a variety of proxies including leaf margin analysis and stable isotopes (δ13C and δ18O) of fossil cellulose. Here, we examine stable isotopes of tree-ring cellulose at subannual- to annual-scale resolution, using the oldest viable cellulose found to date. We use mechanistic models and transfer functions to estimate earliest Eocene temperatures using mummified cellulose, which was well preserved in the kimberlite. Multiple samples of Piceoxylon wood within the kimberlite were crossdated by tree-ring width. Multiple proxies are used in combination to tease apart likely environmental factors influencing the tree physiology and growth in the unique extinct ecosystem of the Polar rainforest. Calculations of interannual variation in temperature over a multidecadal time-slice in the early Eocene are presented, with a mean annual temperature (MAT) estimate of 11.4 °C (1 σ = 1.8 °C) based on δ18O, which is 16 °C warmer than the current MAT of the area (-4.6 °C). Early Eocene atmospheric δ13C (δ13Catm) estimates were -5.5 (±0.7) ‰. Isotopic discrimination (Δ) and leaf intercellular pCO2 ratio (ci/ca) were similar to modern values (Δ = 18.7 ± 0.8 ‰; ci/ca = 0.63 ± 0.03 %), but intrinsic water use efficiency (Early Eocene iWUE = 211 ± 20 μmol mol-1) was over twice the level found in modern high-latitude trees. Dual-isotope spectral analysis suggests that multidecadal climate cycles somewhat similar to the modern Pacific Decadal Oscillation likely drove temperature and cloudiness trends on 20-30-year timescales, influencing

  4. Mantle oddities: A sulphate fluid preserved in a MARID xenolith from the Bultfontein kimberlite (Kimberley, South Africa)

    NASA Astrophysics Data System (ADS)

    Giuliani, A.; Phillips, D.; Fiorentini, M. L.; Kendrick, M. A.; Maas, R.; Wing, B. A.; Woodhead, J. D.; Bui, T. H.; Kamenetsky, V. S.

    2013-08-01

    Sulphur in the lithospheric mantle is concentrated in sulphide minerals, with limited evidence for the occurrence of sulphate phases. Here we describe an unusual assemblage of celestine (SrSO4), clinopyroxene and minor phlogopite, pectolite, sphene, apatite, barite (BaSO4) and Ca-Sr carbonates in a MARID mantle xenolith sampled by the Bultfontein kimberlite (Kimberley, South Africa). This assemblage occurs in veins that pervasively traverse the xenolith, indicating that celestine and the other vein minerals crystallised from a fluid. In the MARID host rock, K-richterite is resorbed where in contact with celestine and is overgrown by clinopyroxene. Celestine hosts the other metasomatic vein phases, but also occurs as inclusions in euhedral clinopyroxene, suggesting co-precipitation of these minerals. Celestine was partly replaced by serpentine during alteration by hydrous fluids after kimberlite emplacement in the upper crust. Celestine has relatively radiogenic Sr isotopes (87Sr/86Sr = 0.70677), which overlap those of K-richterite in the MARID host rock and fall within the range of other MARID and phlogopite-K-richterite peridotites sampled by southern African kimberlites. Celestine displays S isotopes (δ34S=+5.9‰) that are slightly heavier than typical mantle values (δ34S∼0‰) and there is no evidence of mass-independent fractionation (Δ33S=-0.01‰). The texture and chemical composition of the metasomatic phases indicate that the MARID rock was infiltrated by a sulphate fluid enriched in Sr, Ba, Na and Ca, with lesser P, Ti, LREE, CO2 and F. The similar Sr/Ba ratios and Sr isotopic compositions of celestine and K-richterite suggest that K-richterite breakdown contributed to the alkali enrichment of the sulphate fluid. A mantle origin for the sulphate fluid is supported by (i) comparisons between the Sr-S isotopic compositions of celestine and the host kimberlite, crustal and mantle lithologies from the area, and (ii) alteration of celestine by late

  5. Structure of the mantle lithosphere beneath the Siberian kimberlite pipes reconstructed by monomineral thermobarometry

    NASA Astrophysics Data System (ADS)

    Ashchepkov, I. V.

    2009-04-01

    columns (10-13) is reconstructed by the stepped TPX trends formed at first by the combinations of subduction and superplume events coinciding with the Re/Os ages (Spetsius, 2007), overprinted by the reactions with the plume and other percolating melts The Fe# increase near the 60 kbar refer to the last superplume events the previous leave similar rhythmic Fe- dunite horizons at 11-12 levels. The comparison of the compositions of minerals and reconstruction of mantle roots for several phases for Yubileinay, Udachnaya and Nyurbinskaya pipes allow to reveal the evolution of the magmatic sources and their interaction with the mantle lithosphere. Reconstruction of the mantle columns beneath 60 pipes allow to make the transsects of the kimberlite fields and the 3D model of the mantle beneath the dense kimberlite clusters with many close located diatrems Mesozoic mantle columns beneath the Anabar, Olenek, Aldan show the HT -Fe# alteration in 60-40 kbar due to interaction with the PT superplume, but relic and LT and low Fe# associations occurs to 60 kbar also. RBRF 05-05-74718, 06-05-65021, 06-05-64416.

  6. H2O content of nominally anhydrous mineral inclusions in diamonds from the Udachnaya kimberlite

    NASA Astrophysics Data System (ADS)

    Novella, D.; Bolfan-Casanova, N.; Nestola, F.; Harris, J. W.

    2015-12-01

    Nominally anhydrous minerals (such as olivine, pyroxene and garnet) present in mantle xenoliths have been found to contain up to hundreds of ppm wt H2O, bonded as H to their mineral structure. However, it is not well understood whether these H2O contents are representative for the hydrous state of the deep mantle where they formed, or if they are the result of interactions between the xenoliths and metasomatic fluids or magmas during their travel to the surface. Given the fact that trace amounts of H2O can alter the physical and chemical properties of mantle materials and therefore affect Earth's dynamics, it is important to accurately determine the H2O content of deep mantle minerals. Natural diamonds can contain mineral inclusions that formed at high depths (>5 GPa) and are representative for the deep and inaccessible portions of the mantle where they originated. This is because the strong and inert diamond prevents the inclusions to react with any fluid or melt that get in contact with it. Therefore, valuable information regarding the H2O content of the deep mantle can be obtained by studying these minerals trapped in diamonds. In this study we measured the H2O contents of 10 olivine and garnet inclusions in diamonds from the Udachnaya kimberlite (Siberian craton) by Fourier Transform Infrared spectroscopy. Olivine crystals contain 1-5 ppm wt H2O while garnets do not show absorption bands indicating the presence of detectable H in their structure and are therefore considered dry. The H2O contents of olivine and garnet inclusions in diamonds presented here are considerably lower than those found in xenoliths or xenocrists from the same locality. Based on these new results, we discuss the presence of H2O in the cratonic mantle and its importance in stabilizing these areas during geological time, as well as the volatile signature of diamond forming melts in the Siberian craton.

  7. Studies of the proterozoic tectonic evolution of the southwestern United States

    NASA Astrophysics Data System (ADS)

    Seeley, John Martin

    This dissertation is comprised of an introductory chapter (1) and four subsequent chapters (2, 3, 4, and 5) which concentrate on the evolution of Proterozoic rocks of the southwestern United States. Chapter 2 utilizes gravity and magnetic geophysical techniques to delineate subsurface occurrences of Mesoproterozoic Unkar and Neoproterozoic Chuar Group rocks in northern Arizona and southern Utah. These rocks were delineated using gravity and aeromagnetic data, in combination with gravity profile modeling, seismic velocity analysis, geologic cross-sections, analysis of surface geologic structures, and analysis of drilling data. Delineation of Proterozoic extensional trends within this region provides insights in to Proterozoic paleogeography along the southwestern margin of Laurentia. Chapter 3 focuses on the geologic mapping and areal distribution of the Mesoproterozoic Apache Group, Troy Quartzite and diabase in central Arizona. Landsat TM imagery was used in combination with existing geologic maps of the region to define the areal distribution of these units. Results were used to construct a regional-scale photo-geologic map of exposed Middle Proterozoic rocks. Chapter 4 introduces a sequence stratigraphic model for siliciclastic rocks of the Mesoproterozoic Lanoria Formation of the Franklin Mountains, Texas, comprised of 700+ m of marine shelf and marginal marine sandstone, siltstone and mudstone. It is divided into six members (L1 through L6). Four facies associations are identified: proximal, distal shelf; tidal flat; incised valley; and, estuarine/deltaic. Six stratigraphic sequences have been interpreted. Sequences LS1, LS2, LS4 and LS6 represent marine shelf deposits. Sequence LS3 comprises incised valley fill and overlying tidal flat deposits. Sequence LS5 comprises incised valley fill overlain by estuarine/deltaic sediments. Sequences, facies, and paleocurrents suggest a northward sloping platform, indicating a southern source. Tidal processes dominated on

  8. Apatite, SiO2, rutile and orthopyroxene precipitates in minerals of eclogite xenoliths from Yakutian kimberlites, Russia

    NASA Astrophysics Data System (ADS)

    Alifirova, T. A.; Pokhilenko, L. N.; Korsakov, A. V.

    2015-06-01

    Eclogite mantle xenoliths from the central part of Siberian craton (Udachnaya and Zarnitsa kimberlite pipes) as well as from the northeastern edge of the craton (Obnazhennaya kimberlite) were studied in detail. Garnet and clinopyroxene show evident exsolution textures. Garnet comprises rutile, ilmenite, apatite, and quartz/coesite oriented inclusions. Clinopyroxene contains rutile (± ilmenite) and apatite precipitates. Granular inclusions of quartz in kyanite and garnet usually retain features of their high-pressure origin. According to thermobarometric calculations, the studied eclogitic suite was equilibrated within lithospheric mantle at 3.2-4.9 GPa and 813-1080 °C. The precursor composition of garnets from Udachnaya and Zarnitsa eclogites suggests their stability at depths 210-260 km. Apatite precipitation in clinopyroxenes of Udachnaya and Zarnitsa allows us to declare that original pyroxenes could have been indicative of their high P-T stability. Raman spectroscopic study of quartz and coesite precipitates in garnet porphyroblasts confirms our hypothesis on the origin of the exsolution textures during pressure-temperature decrease. With respect to mineralogical data, we suppose the rocks to be subjected to stepwise decompression and cooling within mantle reservoir.

  9. He, Ne and Ar in peridotitic and eclogitic paragenesis diamonds from the Jwaneng kimberlite, Botswana—Implications for mantle evolution and diamond formation ages

    NASA Astrophysics Data System (ADS)

    Honda, Masahiko; Phillips, David; Harris, Jeff W.; Matsumoto, Takuya

    2011-01-01

    We have undertaken helium, neon and argon step-heating, isotopic analyses of eleven polycrystalline diamonds of known peridotite/eclogite paragenesis from the Jwaneng kimberlite pipe, Botswana. In contrast to the findings of crustal noble gases in framesites from the same kimberlite pipe (Honda et al., 2004. Unusual noble gas compositions in polycrystalline diamonds: preliminary results from the Jwaneng kimberlite, Botswana. Chem. Geol. 203, 347-358.), the Jwaneng polycrystalline diamonds appear to contain similar noble gas isotopic compositions (particularly Ne) to those representing a mantle source for MORBs. This implies that the Jwaneng polycrystalline diamonds may have formed in recent times, possibly close to the time of kimberlite emplacement at ~ 235 Ma. In contrast, Jwaneng framesites could be as old as gem diamonds (mineral inclusion ages of ~ 2.9 Ga). Furthermore, the data indicate that the sub-continental mantle lithosphere in the region has heterogeneous Ne isotopic compositions, or that these compositions changed over time from crustal Ne (as observed in the framesites) to MORB-like (as observed in the polycrystalline diamonds).

  10. Middle proterozoic tectonic activity in west Texas and eastern New Mexico and analysis of gravity and magnetic anomalies

    SciTech Connect

    Adams, D.C.; Keller, G.R. )

    1994-03-01

    The Precambrian history of west Texas and eastern New Mexico is complex, consisting of four events: Early Proterozoic orogenic activity (16309-1800 Ma), formation of the western granite-rhyolite province (WGRP) (1340-1410 Ma), Grenville age tectonics (1116-1232 Ma), and middle Proterozoic extension possibly related to mid-continent rifting (1086-1109 Ma). Pre-Grenville tectonics, Grenville tectonics, and mid-continent rifting are represented in this area by the Abilene gravity minimum (AGM) and bimodal igneous rocks, which are probably younger. We have used gravity modeling and the comparison of gravity and magnetic anomalies with rock types reported from wells penetrating Precambrian basement to study the AGM and middle Proterozoic extension in this area. The AGM is an east-northeast-trending, 600 km long, gravity low, which extends from the Texas-Oklahoma border through the central basin platform (CBP) to the Delaware basin. This feature appears to predate formation of the mafic body in the CBP (1163 Ma) and is most likely related to Pre-Grenville tectonics, possibly representing a continental margin arc batholith. Evidence of middle Proterozoic extension is found in the form of igneous bodies in the CBP, the Van Horn uplift, the Franklin Mountains, and the Sacramento Mountains. Analysis of gravity and magnetic anomalies shows that paired gravity and magnetic highs are related to mafic intrusions in the upper crust. Mapping of middle Proterozoic igneous rocks and the paired anomalies outlines a 530 km diameter area of distributed east-west-oriented extension. The Debaca-Swisher terrain of shallow marine and clastic sedimentary rocks is age correlative with middle Proterozoic extension. These rocks may represent the lithology of possible Proterozoic exploration targets. Proterozoic structures were reactivated during the Paleozoic, affecting both the structure and deposition in the Permian basin.

  11. Late Proterozoic diabase dikes of the New Jersey Highlands; a remnant of Iapetan rifting in the north-central Appalachians

    USGS Publications Warehouse

    Volkert, R.A.; Puffer, J.H.

    1995-01-01

    Diabase dikes of widespread occurrence intrude only middle Proterozoic rocks in the New Jersey Highlands. These dikes are enriched in TiO2, P2O5, Zr, and light rare earth elements, and have compositions that range from tholeiitic to alkalic. Dike descriptions, field relations, petrography, geochemistry, petrogenesis, and tectonic setting are discussed. The data are consistent with emplacement in a rift-related, within-plate environment and suggest a correlation with other occurrences of late Proterozoic Appalachian basaltic magmatism.

  12. Greenhouse warming by nitrous oxide and methane in the Proterozoic Eon.

    PubMed

    Roberson, A L; Roadt, J; Halevy, I; Kasting, J F

    2011-07-01

    An anoxic, sulfidic ocean that may have existed during the Proterozoic Eon (0.54-2.4 Ga) would have had limited trace metal abundances because of the low solubility of metal sulfides. The lack of copper, in particular, could have had a significant impact on marine denitrification. Copper is needed for the enzyme that controls the final step of denitrification, from N(2) O to N(2) . Today, only about 5-6% of denitrification results in release of N(2) O. If all denitrification stopped at N(2) O during the Proterozoic, the N(2) O flux could have been 15-20 times higher than today, producing N(2) O concentrations of several ppmv, but only if O(2) levels were relatively high (>0.1 PAL). At lower O(2) levels, N(2) O is rapidly photodissociated. Methane concentrations may also have been elevated during this time, as has been previously suggested. A lack of dissolved O(2) and sulfate in the deep ocean could have produced a high methane flux from marine sediments, as much as 10-20 times today's methane flux from land. The photochemical lifetime of CH(4) increases as more CH(4) is added to the atmosphere, so CH(4) concentrations of up to 100 ppmv are possible during this time. The combined greenhouse effect of CH(4) and N(2) O could have provided up to 10° of warming, thereby keeping the surface warm during the Proterozoic without necessitating high CO(2) levels. A second oxygenation event near the end of the Proterozoic would have resulted in a reduction in both atmospheric N(2) O and CH(4) , perhaps triggering the Neoproterozoic "Snowball Earth" glaciations.

  13. Was the Coppermine Homocline of northwestern Canada uplifted as part of a Middle Proterozoic forebulge

    SciTech Connect

    Cook, F.A. )

    1991-01-01

    Application of the principles of elastic plate bending to subsurface geometric information and outcrop in northwestern Canada suggests that Proterozoic crust was loaded and deflected during orogenic activity between about 1.6 and 1.1 Ga. Exposed Hudsonian (ca. 1.8-2.0 Ga) basement separating younger but coeval stratigraphic sequences in the Coppermine Homocline on the west and Bathurst Inlet on the east can be explained as a forebulge associated with the loading.

  14. Characteristics and alteration origins of matrix minerals in volcaniclastic kimberlite of the Muskox pipe (Nunavut, Canada)

    NASA Astrophysics Data System (ADS)

    Hayman, P. C.; Cas, R. A. F.; Johnson, M.

    2009-11-01

    The matrix of volcaniclastic kimberlite (VK) from the Muskox pipe (Northern Slave Province, Nunavut, Canada) is interpreted to represent an overprint of an original clastic matrix. Muskox VK is subdivided into three different matrix mineral assemblages that reflect differences in the proportions of original primary matrix constituents, temperature of formation and nature of the altering fluids. Using whole rock X-ray fluorescence (XRF), whole rock X-ray diffraction (XRD), microprobe analyses, back-scatter electron (BSE) imaging, petrography and core logging, we find that most matrix minerals (serpentine, phlogopite, chlorite, saponite, monticellite, Fe-Ti oxides and calcite) lack either primary igneous or primary clastic textures. The mineralogy and textures are most consistent with formation through alteration overprinting of an original clastic matrix that form by retrograde reactions as the deposit cools, or, in the case of calcite, by precipitation from Ca-bearing fluids into a secondary porosity. The first mineral assemblage consists largely of serpentine, phlogopite, calcite, Fe-Ti oxides and monticellite and occurs in VK with relatively fresh framework clasts. Alteration reactions, driven by deuteric fluids derived from the juvenile constituents, promote the crystallisation of minerals that indicate relatively high temperatures of formation (> 400 °C). Lower-temperature minerals are not present because permeability was occluded before the deposit cooled to low temperatures, thus shielding the facies from further interaction with fluids. The other two matrix mineral assemblages consist largely of serpentine, phlogopite, calcite, +/- diopside, and +/- chlorite. They form in VK that contains more country rock, which may have caused the deposit to be cooler upon emplacement. Most framework components are completely altered, suggesting that larger volumes of fluids drove the alteration reactions. These fluids were likely of meteoric provenance and became heated

  15. Diamonds from the Juina-5 kimberlite provide evidence for crustal volatile recycling into the deep Earth

    NASA Astrophysics Data System (ADS)

    Thomson, Andrew; Walter, Michael; Kohn, Simon; Burnham, Antony; Bulanova, Galina; Smith, Chris; Araujo, Debora

    2014-05-01

    'Superdeep' diamonds originate from a depth range spanning the asthenospheric upper mantle, transition zone and shallowest parts of the lower mantle [1]. Sporadically they entrap small inclusions of pre-existing or co-precipitating minerals during their crystallisation from volatile-rich melts or fluids. Such samples therefore preserve important petrologic, tectonic and geodynamic information about their growth environment together with evidence of the deep volatile cycling. The Juina-5 kimberlite has previously been recognised as a source of 'superdeep' diamonds [2]. Here we present and discuss data from an extended collection of Juina-5 diamonds. This work has revealed that these diamonds are dominantly composed of isotopically light carbon and contain a mineral inclusion cargo mostly of eclogitic affinity consisting of many former Mg- and Ca-perovskite, NAL-phase, CF-phase, stishovite, majoritic garnet, sodic pyroxene, ferropericlase, Fe or Fe-carbide and sulphide minerals. Together these observations suggest that the diamonds form from material of a subducted crustal origin. The high enrichment of the inclusions' trace element compositions implies that they cannot represent trapped fragments of formerly subsolidus mantle material. Geochemical modelling instead allows the compositions of Ca-perovskite and majorite inclusions to be directly linked to formation from a slab-derived carbonate bearing melt. It is suggested that the formation of 'superdeep' diamonds, and their inclusions, is the result of 'redox-freezing' during the interaction of oxidised slab melts and reducing mantle rocks [3]. It is expected that such melts will be produced during slab foundering and thermal equilibration in the upper/lower mantle boundary region, where tomographic evidence suggests slab subduction often stalls [4]. This hypothesis has been tested with experiments performed at transition zone pressures using the multi-anvil apparatus. At 20 GPa the composition of a low degree melt

  16. Composition and thermal structure of the lithospheric mantle beneath kimberlite pipes from the Catoca cluster, Angola

    NASA Astrophysics Data System (ADS)

    Ashchepkov, I. V.; Rotman, A. Y.; Somov, S. V.; Afanasiev, V. P.; Downes, H.; Logvinova, A. M.; Nossyko, S.; Shimupi, J.; Palessky, S. V.; Khmelnikova, O. S.; Vladykin, N. V.

    2012-03-01

    Garnet, clinopyroxene and ilmenite xenocrysts from three Angolan kimberlite pipes belonging to the Catoca cluster (Angola Caquele, Camitongo I and II, and Catoca) from the SW part of the Congo-Kasai craton, reveal similar features which suggest a similarity of mantle structure. PT estimates for pyropes, Cr-diopsides and picroilmenites reveal similar geothermal conditions of ~ 37-40 mW/m2. This is slightly higher than the values determined for the Catoca pipe. Higher temperature conditions ~ 45 mW/m2 were determined for low-Cr pyroxenes and omphacites. The similar general mineralogy and suggested mantle lithology, as well as reconstructed layering of the sub-continental lithospheric mantle (SCLM), are similar for Camitongo I-II as well as for Caquele and Catoca pipes. Heating at depths of 7.5-4.5 GPa (240-140 km) is a general feature of the SCLM beneath the field. The high temperature trend for low-Cr and hybrid pyroxenes from the base of the SCLM up to 30 GPa (100 km) represents the PT path of the protokimberlite melts. PT conditions for ilmenites mainly correspond to colder conditions of crystallization in wall rocks and the outer parts of magmatic channels. Individual geochemical features of the minerals for each SCLM suggest pervasive metasomatism in lower part of the SCLM. Clinopyroxene trace element patterns from the Caquele pipe reveal a lherzolitic affinity; they are LILE-enriched with Ba peaks due to phlogopite melting, while those from Camitongo I-II show Ta-Nb enrichment and Pb troughs. The ilmenite trends trace the mantle column from deep to shallow mantle, evolving to Fe-ilmenites due to advanced AFC of protokimberlite magma that also produced abundant Fe-rich clinopyroxenes. The rise of calculated fO2 correlates with the position of protokimberlites. Comparison with the thermal gradient derived from peridotitic inclusions from Catoca cluster is lower than for Lesotho possibly related to the thicker lithospheric roots beneath the Congo-Kasai craton.

  17. Stromatolites of the Mescal Limestone (Apache Group, middle Proterozoic, central Arizona): taxonomy, biostratigraphy, and paleoenvironments.

    PubMed

    Bertrand-Sarfati, J; Awramik, S M

    1992-09-01

    The 25- to 30-m-thick Algal Member of the Mescal Limestone (middle Proterozoic Apache Group) contains two distinct stromatolitic units: at the base, a 2- to 3-m-thick unit composed of columnar stromatolites and above, a thicker unit of stratiform and pseudocolumnar stromatolites. Columnar forms from the first unit belong to the Group Tungussia, and two new Forms are described: T. mescalita and T. chrysotila. Among the pseudocolumnar stromatolites of the thicker unit, one distinctive new taxon, Apachina henryi, is described. Because of the low stromatolite diversity, the biostratigraphic value of this assemblage is limited. The presence of Tungussia is consistent with the generally accepted isotopic age for the Apache Group of 1200 to 1100 Ma. The Mescal stromatolites do not closely resemble any other known Proterozoic stromatolites in the southwestern United States or northwestern Mexico. Analyses of sedimentary features and stromatolite growth forms suggest deposition on a stable, flat, shallow, subtidal protected platform during phases of Tungussia growth. Current action probably influenced the development of columns, pseudocolumns, and elongate stromatolitic ridges; these conditions alternated with phases of relatively quiet water characterized by nonoriented stromatolitic domes and stratiform stromatolites. Stable conditions favorable for development of the Mescal stromatolites were short-lived and did not permit the development of thick, stromatolite-bearing units such as those characteristic of many Proterozoic sequences elsewhere.

  18. Microfossils from oolites and pisolites of the Upper Proterozoic Eleonore Bay Group, Central East Greenland

    NASA Technical Reports Server (NTRS)

    Green, J. W.; Knoll, A. H.; Swett, K.

    1988-01-01

    Silicified oolites and pisolites from Bed 18 of the Upper Proterozoic (about 700-800 Ma) Limestone-Dolomite "Series" of the Eleonore Bay Group, central East Greenland, contain a diverse suite of organically preserved microfossils that is, for the most part. [Of the] assemblages previously described from Proterozoic cherts and shales. Three principal assemblages occur in these rocks: 1) a class bound assemblage found in detrital carbonate grains (now silicified) that served as nuclei for ooid and pisoid growth, as well as in uncoated mud and mat clasts that were carried into the zone of ooid and pisoid deposition; 2) an epilithic and interstitial assemblage consisting of microorganisms that occurred on top of and between grains; and 3) a euendolithic assemblage composed of microbes that actively bored into coated grains. The Upper Proterozoic euendolithic assemblage closely resembles a community of euendolithic cyanobacteria found today in shallow marine ooid sands of the Bahama Banks. Thirteen species are described, of which eight are new, five representing new genera: Eohyella dichotoma n. sp., Eohyella endoatracta n. sp., Eohyella rectoclada n. sp., Thylacocausticus globorum n. gen. and sp., Cunicularius halleri n. gen. and sp., Graviglomus incrustus n. gen. and sp., Perulagranum obovatum n. gen. and sp., and Parenchymodiscus endolithicus n. gen. and sp.

  19. Stromatolites of the Mescal Limestone (Apache Group, middle Proterozoic, central Arizona): taxonomy, biostratigraphy, and paleoenvironments

    NASA Technical Reports Server (NTRS)

    Bertrand-Sarfati, J.; Awramik, S. M.

    1992-01-01

    The 25- to 30-m-thick Algal Member of the Mescal Limestone (middle Proterozoic Apache Group) contains two distinct stromatolitic units: at the base, a 2- to 3-m-thick unit composed of columnar stromatolites and above, a thicker unit of stratiform and pseudocolumnar stromatolites. Columnar forms from the first unit belong to the Group Tungussia, and two new Forms are described: T. mescalita and T. chrysotila. Among the pseudocolumnar stromatolites of the thicker unit, one distinctive new taxon, Apachina henryi, is described. Because of the low stromatolite diversity, the biostratigraphic value of this assemblage is limited. The presence of Tungussia is consistent with the generally accepted isotopic age for the Apache Group of 1200 to 1100 Ma. The Mescal stromatolites do not closely resemble any other known Proterozoic stromatolites in the southwestern United States or northwestern Mexico. Analyses of sedimentary features and stromatolite growth forms suggest deposition on a stable, flat, shallow, subtidal protected platform during phases of Tungussia growth. Current action probably influenced the development of columns, pseudocolumns, and elongate stromatolitic ridges; these conditions alternated with phases of relatively quiet water characterized by nonoriented stromatolitic domes and stratiform stromatolites. Stable conditions favorable for development of the Mescal stromatolites were short-lived and did not permit the development of thick, stromatolite-bearing units such as those characteristic of many Proterozoic sequences elsewhere.

  20. Sr isotopic variations in Upper Proterozoic carbonates from Svalbard and East Greenland

    NASA Technical Reports Server (NTRS)

    Derry, Louis A.; Keto, Lisette S.; Jacobsen, Stein B.; Knoll, Andrew H.; Swett, Keene

    1989-01-01

    Precambrian Sr isotope stratigraphy was investigated by determining variations in Sr-87/Sr-86 ratios in the Upper Proterozoic carbonate succession from Svalbard and East Greenland. Data from this study were combined with those from literature to construct a curve of Sr-87/Sr-86 versus time for Upper Proterozoic seawater. The curve for the Upper Riphean-Vandian showed that the isotopic composition of Sr in seawater was low (Delta Sr-87 of about -500) between 900 and 650 Ma but rose rapidly to about +30 by 600 Ma (this range of long-term variation exceeds the total Phanerozoic variation). The very low values of Delta Sr-87 inferred for the Riphean require that, for this time, the submarine hydrothermal water flux was a large fraction of the Sr input to the oceans, while the rise in Delta Sr-87 in the Upper Proterozoic seawater reflects both a change in the ratio of hydrothermal and continental fluxes of Sr to the oceans, and a change in the isotopic composition of Sr from continental sources.

  1. Middle Proterozoic age for the Montpelier Anorthosite, Goochland terrane, eastern Piedmont, Virginia

    USGS Publications Warehouse

    Aleinikoff, J.N.; Horton, J.W.; Walter, M.

    1996-01-01

    Uranium-lead dating of zircons from the Montpelier Anorthosite confirms previous interpretations, based on equivocal evidence, that the Goochland terrane in the eastern Piedmont of Virginia contains Grenvillian basement rocks of Middle Proterozoic age. A very few prismatic, elongate, euhedral zircons, which contain 12-29 ppm uranium, are interpreted to be igneous in origin. The vast majority of zircons are more equant, subangular to anhedral, contain 38-52 ppm uranium, and are interpreted to be metamorphic in origin. One fraction of elongate zircon, and four fragments of a very large zircon (occurring in a nelsonite segregation) yield an upper intercept age of 1045 ?? 10 Ma, interpreted as the time of anorthosite crystallization. Irregularly shaped metamorphic zircons are dated at 1011 ?? 2 Ma (weighted average of the 207Pb/206Pb ages). The U-Pb isotopic systematics of metamorphic titanite were reset during the Alleghanian orogeny at 297 ?? 5 Ma. These data provide a minimum age for gneisses of the Goochland terrane that are intruded by the anorthosite. Middle Proterozoic basement rocks of the Goochland terrane may be correlative with those in the Shenandoah massif of the Blue Ridge tectonic province, as suggested by similarities between the Montpelier Anorthosite and the Roseland anorthosite. Although the areal extent of Middle Proterozoic basement and basement-cover relations in the eastern Piedmont remain unresolved, results of this investigation indicate that the Goochland terrane is an internal massif of Laurentian crust rather than an exotic accreted terrane.

  2. The Wisconsin magmatic terrane: An Early Proterozoic greenstone-granite terrane formed by plate tectonic processes

    NASA Technical Reports Server (NTRS)

    Schulz, K. J.; Laberge, G. L.

    1986-01-01

    The Wisconsin magmatic terrane (WMT) is an east trending belt of dominantly volcanic-plutonic complexes of Early Proterozoic age (approx. 1850 m.y.) that lies to the south of the Archean rocks and Early Proterozoic epicratonic sequence (Marquette Range Supergroup) in Michigan. It is separated from the epicratonic Marquette Range Supergroup by the high-angle Niagara fault, is bounded on the south, in central Wisconsin, by Archean gneisses, is truncated on the west by rocks of the Midcontinent rift system, and is intruded on the east by the post-orogenic Wolf river batholith. The overall lithologic, geochemical, metallogenic, metamorphic, and deformational characteristics of the WMT are similar to those observed in recent volcanic arc terranes formed at sites of plate convergence. It is concluded that the WMT represents an evolved oceanic island-arc terrane accreated to the Superior craton in the Early Proterozoic. This conclusion is strengthened by the apparent absence of Archean basement from most of the WMT, and the recent recognition of the passive margin character of the epicratonic Marquette Range Supergroup.

  3. Distribution and diagenesis of microfossils from the lower Proterozoic Duck Creek Dolomite, Western Australia

    NASA Technical Reports Server (NTRS)

    Knoll, A. H.; Strother, P. K.; Rossi, S.

    1988-01-01

    Two distinct generations of microfossils occur in silicified carbonates from a previously undescribed locality of the Lower Proterozoic Duck Creek Dolomite, Western Australia. The earlier generation occurs in discrete organic-rich clasts and clots characterized by microquartz anhedra; it contains a variety of filamentous and coccoidal fossils in varying states of preservation. Second generation microfossils consist almost exclusively of well-preserved Gunflintia minuta filaments that drape clasts or appear to float in clear chalcedony. These filaments appear to represent an ecologically distinct assemblage that colonized a substrate containing the partially degraded remains of the first generation community. The two assemblages differ significantly in taxonomic frequency distribution from previously described Duck Creek florules. Taken together, Duck Creek microfossils exhibit a range of assemblage variability comparable to that found in other Lower Proterozoic iron formations and ferruginous carbonates. With increasing severity of post-mortem alteration, Duck Creek microfossils appear to converge morphologically on assemblages of simple microstructures described from early Archean cherts. Two new species are described: Oscillatoriopsis majuscula and O. cuboides; the former is among the largest septate filamentous fossils described from any Proterozoic formation.

  4. Evidence for two pulses of glaciation during the late Proterozoic in northern Utah and southeastern Idaho.

    USGS Publications Warehouse

    Crittenden, M.D.; Christie-Blick, N.; Link, P.K.

    1983-01-01

    Over much of this area, the glacial deposits and associated rocks thicken westward and form the basal part of a miogeoclinal wedge that accumulated near the late Proterozoic and early Paleozoic continental margin. In the east, such deposits are thin and rest on Archean basement or rocks of Proterozoic Y age; in the west, they are part of thicker sequences in which deposition apparently continued without significant interruption from late Proterozoic into Cambrian time. Recent mapping shows that glacial episodes represented either by diamictite or by dropstones enclosed in fine-grained laminated beds are separated by as much as 1000m of non-glacial deposits, including black slate, alternating graywacke and siltstone, quartzite, and conglomerate. Using reasonable sedimentation rates for such deposits and by comparison with modern analogues, we infer that two episodes of glaciation, each probably consisting of multiple advances and retreats, were separated by a non-glacial interval of a few hundred thousand to a few million years' duration.-from Author

  5. Widespread iron-rich conditions in the mid-Proterozoic ocean.

    PubMed

    Planavsky, Noah J; McGoldrick, Peter; Scott, Clinton T; Li, Chao; Reinhard, Christopher T; Kelly, Amy E; Chu, Xuelei; Bekker, Andrey; Love, Gordon D; Lyons, Timothy W

    2011-09-07

    The chemical composition of the ocean changed markedly with the oxidation of the Earth's surface, and this process has profoundly influenced the evolutionary and ecological history of life. The early Earth was characterized by a reducing ocean-atmosphere system, whereas the Phanerozoic eon (less than 542 million years ago) is known for a stable and oxygenated biosphere conducive to the radiation of animals. The redox characteristics of surface environments during Earth's middle age (1.8-1 billion years ago) are less well known, but it is generally assumed that the mid-Proterozoic was home to a globally sulphidic (euxinic) deep ocean. Here we present iron data from a suite of mid-Proterozoic marine mudstones. Contrary to the popular model, our results indicate that ferruginous (anoxic and Fe(2+)-rich) conditions were both spatially and temporally extensive across diverse palaeogeographic settings in the mid-Proterozoic ocean, inviting new models for the temporal distribution of iron formations and the availability of bioessential trace elements during a critical window for eukaryotic evolution.

  6. Paleobiology of distinctive benthic microfossils from the upper Proterozoic Limestone-Dolomite "Series," central East Greenland

    NASA Technical Reports Server (NTRS)

    Green, J. W.; Knoll, A. H.; Golubic, S.; Swett, K.

    1987-01-01

    Populations of Polybessurus bipartitus Fairchild ex Green et al., a large morphologically distinctive microfossil, occur in silicified carbonates of the Upper Proterozoic (700-800 Ma) Limestone-Dolomite "Series," central East Greenland. Large populations of well-preserved individuals permit reconstruction of P. bipartitus as a coccoidal unicell that "jetted" upward from the sediment by the highly unidirectional secretion of extracellular mucopolysaccharide envelopes. Reproduction by baeocyte formation is inferred on the basis of clustered envelope stalks produced by small cells. Sedimentological evidence indicates that P. bipartitus formed surficial crusts locally within a shallow peritidal carbonate platform. Among living microorganisms a close morphological, reproductive, and behavioral counterpart to Polybessurus is provided by populations of an as yet underscribed cyanobacterium found in coastal Bahamian environments similar to those in which the Proterozoic fossils occur. In general morphology and "jetting" behavior, this population resembles species of the genus Cyanostylon, Geitler (1925), but reproduces via baeocyte formation. Polybessurus is but one of the more than two dozen taxa in the richly fossiliferous biota of the Limestone-Dolomite "Series." This distinctive population, along with co-occurring filamentous cyanobacteria and other microfossils, contributes to an increasingly refined picture of ecological heterogeneity in late Proterozoic oceans.

  7. Magnetotelluric survey to locate the Archean/Proterozoic suture zone north of Wells, Nevada

    USGS Publications Warehouse

    Williams, Jackie M.; Rodriguez, Brian D.

    2006-01-01

    It is important to know whether major mining districts in the Northern Nevada Gold Province are underlain by rocks of the Archean Wyoming craton, which are known to contain orogenic gold deposits, or by accreted rocks of the Paleoproterozoic Mojave province. It is also important to know the location and orientation of the Archean/Proterozoic suture zone between these provinces as well as major basement structures within these terranes because they may influence subsequent patterns of sedimentation, deformation, magmatism, and hydrothermal activity. The Archean was the main gold-mineralization period, and Archean lode-gold deposits were formed at mid-crustal depths along major shear zones. The nature of the crystalline basement below the Northern Nevada Gold Province and the location of major faults within it are relevant to Rodinian reconstructions, crustal development, and ore deposit models (e.g., Hofstra and Cline, 2000; Grauch and others, 2003). According to Whitmeyer and Karlstrom (2004), the Archean cratons of the northwestern United States and Canada had stabilized as continental lithosphere by 2.5 Ga, and were rifted and assembled into a large continental mass by 1.8 Ga, to which the 1.73-1.68 Ga Mohave province was accreted by 1.65 Ga. The Archean/Proterozoic suture zone has a west-southwest strike where it is exposed (Reed, 1993) at the eastern Utah and southwestern Wyoming border (Cheyenne Belt) where it is characterized by an up to 7-km-thick mylonite zone (Smithson and Boyd, 1998). In the Great Basin, the strike of the Archean/Proterozoic suture zone is poorly constrained because it is largely concealed below a Neoproterozoic-Paleozoic miogeocline and basin fill. East-west and southwest-northeast strikes for the Archean/Proterozoic suture zone have been inferred based on Sr, Nd, and Pb isotopic compositions of granitoid intrusions (Tosdal and others, 2000). To better constrain the location and strike of the Archean/Proterozoic suture zone below cover

  8. Controls on Atmospheric O2: The Anoxic Archean and the Suboxic Proterozoic

    NASA Astrophysics Data System (ADS)

    Kasting, J. F.

    2015-12-01

    Geochemists have now reached consensus that the Archean atmosphere was mostly anoxic, that a Great Oxidation Event (GOE) occurred at around 2.5 Ga, and that the ensuing Proterozoic atmosphere was consistently oxidized [1,2]. Evidence for this broad-scale change in atmospheric composition comes from a variety of sources, most importantly from multiple sulfur isotopes [3,4]. The details of both the Archean and Proterozoic environments remain controversial, however, as does the underlying cause of the GOE. Evidence of 'whiffs' of oxygen during the Archean [5] now extend back as far as 3.0 Ga, based on Cr isotopes [6]. This suggests that O2 was being produced by cyanobacteria well before the GOE and that the timing of this event may have been determined by secular changes in O2 sinks. Catling et al. [7] emphasized escape of hydrogen to space, coupled with progressive oxidation of the continents and a concomitant decrease in the flux of reduced gases from metamorphism. But hydrogen produced by serpentinization of seafloor could also have been a controlling factor [8]. Higher mantle temperatures during the Archean should have resulted in thicker, more mafic seafloor and higher H2 production; decreasing mantle temperatures during the Proterozoic should have led to seafloor more like that of today and a corresponding decrease in H2 production, perhaps by enough to trigger the GOE. Once the atmosphere became generally oxidizing, it apparently remained that way during the rest of Earth's history. But O2 levels in the mid-Proterozoic could have been as low at 10-3 times the Present Atmospheric Level (PAL) [9]. The evidence, once again, is based on Cr isotopes. Possible mechanisms for maintaining such a 'suboxic' Proterozoic atmosphere will be discussed. Refs: 1. H. D. Holland, Geochim. Cosmochim. Acta 66, 3811 (2002). 2. H. D. Holland, Philosophical Transactions of the Royal Society B-Biological Sciences 361, 903 (Jun 29, 2006). 3. J. Farquhar, H. Bao, M. Thiemans, Science

  9. P-T Equilibrium Conditions of Xenoliths from the Udachnaya Kimberlite Pipe: Thermal Perturbations in the Lithospheric Mantle

    NASA Astrophysics Data System (ADS)

    Tychkov, Nikolay; Agashev, Alexey; Malygina, Elena; Pokhilenko, Nikolay

    2014-05-01

    Integrated study of 250 peridotite xenoliths from Udachnaya -East pipe show difference in mineral paragenesises and textural-structural peculiarities in the different level of cratonic lithosphere mantle (CLM). The compositions of minerals were determined using EPMA. Thermobarometric parameters (Brey, Kohller, 1990) were determined for all rocks occupying different fields on geothermal curve. The deepest layer (the pressure interval of 5.0-7.0 GPa) contains mostly pophyroclastic lherzolites. Anyway, some rocks of this layer have an idiomorphic texture being also enriched in incompatible components. Higher in the CLM sequence, the interval (4.2-6.3 GPa) is composed of the most depleted rocks: megacristalline ultradepleted harzburgite-dunites and depleted granular harzburgite-dunites, as well as lherzolites in a subordinate amount. They correspond strate to 35 mW/m2 and partly overlap the deeper layer in dapth. It is likely that rocks of this layer are in equilibrium and were not subject to significant secondary changes due to kimberlite magma intrusion. Thus, this interval of the CLM sequence reflects the true (relic) geotherm for the area of the Udachnaya kimberlite pipe. Moreover, it is obvious that this interval was a major supplier of diamonds into kimberlites of the Udachnaya pipe. The interval of 4.2-2.0 GPa in the CLM sequence is also composed of coarse depleted lherzolites and harzburgites. Rocks of this interval are slightly more enriched than those of the underlying interval. This is confirmed by the distinct predominance of lherzolites over harzburgite-dunites. The heat flow in this layer varies in the range of 38-45 mW/m2 and shows a general tendency to increase with decreasing depth. According to occurrence of nonequilibrium mineral assemblages and increased heat flow relative to the major heat flow of 35 mW/m2, this interval is similar to the deepest interval of secondary enriched rocks. Interval of less than 2.0 GPa composed of spinel lherzolites and

  10. High-resolution reconstruction of Arctic paleoclimate derived from 53 million year old kimberlite-hosted Metasequoia wood

    NASA Astrophysics Data System (ADS)

    Walker, J. J.; Halfar, J.; Schulze, D.; Gedalof, Z.; Moore, G. W. K.

    2009-04-01

    The recovery of exceptionally well preserved Eocene wood from kimberlite pipes in the Lac de Gras region of the Northwest Territories, Canada, has allowed for the characterization of the local temperature and hydrologic patterns on an annual scale for this arctic ecosystem (present day latitude: 64 deg N). Wood fragments of ancient Metasequoia sp. trees mixed into the crater facies during kimberlite emplacement were recovered from the Ekati and Diavik diamond mines. Wood recovered from the Ekati "Panda" pipe, which has been dated using Rb-Sr age determination at 53.3 +/- 0.6 million years, is unique in that it exhibits extremely low vitrinite reflectance values, comparable to that of modern near-surface peats. The implication is that the wood has undergone minimal thermal or compressional alteration, thus preserving the geochemical signatures originally formed during growth. Ring width measurements representing over 100 years of climate data show congruence between multiple samples, thus enabling the development of a floating master chronology. Patterns contained in this chronology are of particular interest as the early Eocene was characterized by a period of unusual warming that has been compared to modern day climate change. Wood cellulose does not exchange atoms with its surroundings after formation, making it an ideal candidate for stable isotope analysis once it has been separated from associated whole-wood components. A robotic micromilling device was used to collect whole wood samples of latewood from annual tree rings. The α-cellulose in the samples will be isolated and analyzed for δ13C and δ18O, generating a 34-year floating chronology that can be used to reconstruct decadal scale temperature and humidity patterns. Successful application of this technique suggests that a longer Eocene chronology, using additional wood samples recovered from the Panda pipe as well as samples from neighbouring pipes in the Lac de Gras region, can be developed.

  11. Water-rich carbonatites at low pressures and kimberlites at high pressures

    NASA Astrophysics Data System (ADS)

    Gudfinnsson, G.; Keshav, S.; Presnall, D.

    2008-12-01

    than for water-free carbonate-bearing garnet lherzolite; (2) at these pressures, there is roughly 17-20 wt percent dissolved water in the melts, suggesting that carbonatitic melts can incorporate large amounts of water; (3) from 2.5 to roughly 3.5 GPa, melts coexisting with fo+opx+cpx+gt+dolomite+fluid are highly calcic and partly overlap calciocarbonatites found in nature; (4) a P-T invariant point occurs at 3.7 GPa/1125 C, at which fo+opx+cpx+gt+dolomite+magnesite+melt+fluid coexist, marking the beginning of the stability of magnesite at the hydrous, carbonated peridotite solidus; (5) with increasing pressure starting at this invariant point, the fluid-saturated solidus becomes considerably closer to the water-free, carbonated solidus in the model system CMAS-CO2. For instance, at 7 GPa, it lies only 125 C lower than that of water-free carbonated peridotite. At 6 and 7 GPa, the melt coexisting with the fo+opx+cpx+gt+magnesite+fluid phase assemblage, contains about 5-7 wt percent water, and is more akin to kimberlite (all in wt percent: 20-25 SiO2, 30-32 MgO, 19-20 CaO, 2-3 Al2O3) than carbonatite. At this stage it is not entirely clear what changes in the phase relations cause melts to attain this character.

  12. New model of the mantle lithosphere beneath Kuoyka kimberlite field Yakutia.

    NASA Astrophysics Data System (ADS)

    Ashchepkov, Igor; Kostrovitsky, Sergey; Ovchinnikov, Yury; Tychkov, Nikolai; Khmelnikova, Olga; Palessky, Stanislav

    2013-04-01

    New data for the 11 pipes from Kuoyka field show that high Cr2O3 garnets to 10- 12% as well as high Cr chromites (to 64%Cr2O3) are found in several more pipes Zaozernaya, Seraya, Slyudyanka, Vodorasdelnaya, Titan, Lusya in addition to Djanga pipe. All garnets belong o lherzolite field and not less than 1/3 are TiO rich. The TiO2 rich chromites are dominating in the Cr- rich population. Metasomatic Cr2O3- rich (to 6%) ilmenites pre in the MgO and TiO2- part of the variation diagrams. The Cr- diopside variations show high variations of Fe and Na content to 4 % suggesting the hybridic origin similar to the Cr- pyroxeneis from Obnazhennaya pyroxenites (Taylor et al ., 2003). Omphicites (to 7 % Na2O) are rare. Cr-amphiboles (pargasites and hornblendes) are common in the upper part of the SCLM as well as in the Anabar and Kharamai region. Reconstructions of the mantle sections show the deep lithospheric roots beneath the Zosernaya pipe (7.5 GPa) traced by the PT conditions for Opx, Cpx, Gar, Cr and Ilm. SCLM is divided in to 4 sections and Ilm trace tow intervals in lower and upper part form 4 GPa. Th HT branch is sporadically found from 7 GPa to the Moho. In other pipes ilmenite and garnet PT estimates are more common in the lower part o mantle section while the Cpx trace mainly middle part of SCLM similar to the Obnazhennaya pip. It seems that kimberlites captured mainly the walls of feeders traced by Cr- low garnets and ilmenites in the lower part of SCLM while peridotitic mantle column was captured starting from the middle part of SCLM. The NS transsect of the Kuoyka field show more fertile mantle sections in the NNW part of the field. The TRE determined for the minerals from Kuoyka field show rather rounded patterns for REE of garnets with high variations in HREE part and small elevation in LREE . The depleted compositions reval the inflection in Eu TRE spidergrams well as relatively small Sr minima. Many of them show Ta peak, relatively small Pb elevation and Th

  13. Statistical analysis of iron geochemical data suggests limited late Proterozoic oxygenation

    NASA Astrophysics Data System (ADS)

    Sperling, Erik A.; Wolock, Charles J.; Morgan, Alex S.; Gill, Benjamin C.; Kunzmann, Marcus; Halverson, Galen P.; MacDonald, Francis A.; Knoll, Andrew H.; Johnston, David T.

    2015-07-01

    Sedimentary rocks deposited across the Proterozoic-Phanerozoic transition record extreme climate fluctuations, a potential rise in atmospheric oxygen or re-organization of the seafloor redox landscape, and the initial diversification of animals. It is widely assumed that the inferred redox change facilitated the observed trends in biodiversity. Establishing this palaeoenvironmental context, however, requires that changes in marine redox structure be tracked by means of geochemical proxies and translated into estimates of atmospheric oxygen. Iron-based proxies are among the most effective tools for tracking the redox chemistry of ancient oceans. These proxies are inherently local, but have global implications when analysed collectively and statistically. Here we analyse about 4,700 iron-speciation measurements from shales 2,300 to 360 million years old. Our statistical analyses suggest that subsurface water masses in mid-Proterozoic oceans were predominantly anoxic and ferruginous (depleted in dissolved oxygen and iron-bearing), but with a tendency towards euxinia (sulfide-bearing) that is not observed in the Neoproterozoic era. Analyses further indicate that early animals did not experience appreciable benthic sulfide stress. Finally, unlike proxies based on redox-sensitive trace-metal abundances, iron geochemical data do not show a statistically significant change in oxygen content through the Ediacaran and Cambrian periods, sharply constraining the magnitude of the end-Proterozoic oxygen increase. Indeed, this re-analysis of trace-metal data is consistent with oxygenation continuing well into the Palaeozoic era. Therefore, if changing redox conditions facilitated animal diversification, it did so through a limited rise in oxygen past critical functional and ecological thresholds, as is seen in modern oxygen minimum zone benthic animal communities.

  14. Trace sulfate in mid-Proterozoic carbonates and the sulfur isotope record of biospheric evolution

    NASA Astrophysics Data System (ADS)

    Gellatly, Anne M.; Lyons, Timothy W.

    2005-08-01

    Concentrations of oceanic and atmospheric oxygen have varied over geologic time as a function of sulfur and carbon cycling at or near the Earth's surface. This balance is expressed in the sulfur isotope composition of seawater sulfate. Given the near absence of gypsum in pre-Phanerozoic sediments, trace amounts of carbonate-associated sulfate (CAS) within limestones or dolostones provide the best available constraints on the isotopic composition of sulfate in Precambrian seawater. Although absolute CAS concentrations, which range from those below detection to ˜120 ppm sulfate in this study, may be compromised by diagenesis, the sulfur isotope compositions can be buffered sufficiently to retain primary values. Stratigraphically controlled δ 34S measurements for CAS from three mid-Proterozoic carbonate successions (˜1.2 Ga Mescal Limestone, Apache Group, Arizona, USA; ˜1.45-1.47 Ga Helena and Newland formations, Belt Supergroup, Montana, USA; and ˜1.65 Ga Paradise Creek Formation, McNamara Group, NW Queensland, Australia) show large isotopic variability (+9.1‰ to +18.9‰, -1.1‰ to +27.3‰, and +14.1‰ to +37.3‰, respectively) over stratigraphic intervals of ˜50 to 450 m. This rapid variability, ranging from scattered to highly systematic, and overall low CAS abundances can be linked to sulfate concentrations in the mid-Proterozoic ocean that were substantially lower than those of the Phanerozoic but higher than values inferred for the Archean. Results from the Belt Supergroup specifically corroborate previous arguments for seawater contributions to the basin. Limited sulfate availability that tracks the oxygenation history of the early atmosphere is also consistent with the possibility of extensive deep-ocean sulfate reduction, the scarcity of bedded gypsum, and the stratigraphic δ 34S trends and 34S enrichments commonly observed for iron sulfides of mid-Proterozoic age.

  15. Evidences for multiple remagnetization of Proterozoic dykes from Iguerda inlier (Anti-Atlas Belt, Southern Morocco)

    NASA Astrophysics Data System (ADS)

    Neres, Marta; Silva, Pedro F.; Ikene, Moha; Martins, Sofia; Hafid, Ahmid; Mata, João; Almeida, Francisco; Youbi, Nasrrddine; Boumehdi, Ahmed

    2016-04-01

    Paleomagnetic data able to constrain the paleoposition of the West African Craton (WAC) during Paleo-Mesoproterozoic are absent, mainly due to gaps on the sedimentary record and intense remagnetizations. Dykes that intrude several Proterozoic inliers of WAC in the Anti-Atlas Belt (southern Morocco) have recently been subjected to geochronological studies, which revealed ages between Paleoproterozoic and early Neoproterozoic. Therefore, these dykes represent a window of opportunity for paleomagnetic studies aiming to infer about the paleoposition of WAC during Proterozoic. On this scope we conducted a paleomagnetic study on seven Proterozoic dykes of the Iguerda inlier. We determined the paleomagnetic directions and evaluated their meaning by rock magnetic and mineral analyses, complemented by petrographic observations. Results revealed that these rocks record the presence of a complex history of remagnetization events, mostly assigned to several Phanerozoic thermal/chemical events. In particular, we found components assigned to the late stages of Pan African orogeny (s.l.), to the Late Carboniferous Variscan orogeny, and to more recent events. The recognized remagnetization processes are related to widespread metamorphic events under greenschist facies followed by low-temperature oxidation, both responsible for the formation of new magnetic phases (magnetite and hematite). The primary (magmatic) thermo-remanent magnetization of the dykes was obliterated during these events through multiple thermal and chemical remagnetizations. For only one dyke the presence of primary magnetization is possible to infer, though not to confirm, and would place WAC at an equatorial position around 1750 Ma. The authors wish to acknowledge FCT (Portugal) - CNRST (Morocco) bilateral agreement for its major contribution without which this work wouldn't be possible. Publication supported by project FCT UID/GEO/50019/2013 - Instituto Dom Luiz.

  16. The role of biology in planetary evolution: cyanobacterial primary production in low-oxygen Proterozoic oceans.

    PubMed

    Hamilton, Trinity L; Bryant, Donald A; Macalady, Jennifer L

    2016-02-01

    Understanding the role of biology in planetary evolution remains an outstanding challenge to geobiologists. Progress towards unravelling this puzzle for Earth is hindered by the scarcity of well-preserved rocks from the Archean (4.0 to 2.5 Gyr ago) and Proterozoic (2.5 to 0.5 Gyr ago) Eons. In addition, the microscopic life that dominated Earth's biota for most of its history left a poor fossil record, consisting primarily of lithified microbial mats, rare microbial body fossils and membrane-derived hydrocarbon molecules that are still challenging to interpret. However, it is clear from the sulfur isotope record and other geochemical proxies that the production of oxygen or oxidizing power radically changed Earth's surface and atmosphere during the Proterozoic Eon, pushing it away from the more reducing conditions prevalent during the Archean. In addition to ancient rocks, our reconstruction of Earth's redox evolution is informed by our knowledge of biogeochemical cycles catalysed by extant biota. The emergence of oxygenic photosynthesis in ancient cyanobacteria represents one of the most impressive microbial innovations in Earth's history, and oxygenic photosynthesis is the largest source of O2 in the atmosphere today. Thus the study of microbial metabolisms and evolution provides an important link between extant biota and the clues from the geologic record. Here, we consider the physiology of cyanobacteria (the only microorganisms capable of oxygenic photosynthesis), their co-occurrence with anoxygenic phototrophs in a variety of environments and their persistence in low-oxygen environments, including in water columns as well as mats, throughout much of Earth's history. We examine insights gained from both the rock record and cyanobacteria presently living in early Earth analogue ecosystems and synthesize current knowledge of these ancient microbial mediators in planetary redox evolution. Our analysis supports the hypothesis that anoxygenic photosynthesis

  17. Statistical analysis of iron geochemical data suggests limited late Proterozoic oxygenation.

    PubMed

    Sperling, Erik A; Wolock, Charles J; Morgan, Alex S; Gill, Benjamin C; Kunzmann, Marcus; Halverson, Galen P; Macdonald, Francis A; Knoll, Andrew H; Johnston, David T

    2015-07-23

    Sedimentary rocks deposited across the Proterozoic-Phanerozoic transition record extreme climate fluctuations, a potential rise in atmospheric oxygen or re-organization of the seafloor redox landscape, and the initial diversification of animals. It is widely assumed that the inferred redox change facilitated the observed trends in biodiversity. Establishing this palaeoenvironmental context, however, requires that changes in marine redox structure be tracked by means of geochemical proxies and translated into estimates of atmospheric oxygen. Iron-based proxies are among the most effective tools for tracking the redox chemistry of ancient oceans. These proxies are inherently local, but have global implications when analysed collectively and statistically. Here we analyse about 4,700 iron-speciation measurements from shales 2,300 to 360 million years old. Our statistical analyses suggest that subsurface water masses in mid-Proterozoic oceans were predominantly anoxic and ferruginous (depleted in dissolved oxygen and iron-bearing), but with a tendency towards euxinia (sulfide-bearing) that is not observed in the Neoproterozoic era. Analyses further indicate that early animals did not experience appreciable benthic sulfide stress. Finally, unlike proxies based on redox-sensitive trace-metal abundances, iron geochemical data do not show a statistically significant change in oxygen content through the Ediacaran and Cambrian periods, sharply constraining the magnitude of the end-Proterozoic oxygen increase. Indeed, this re-analysis of trace-metal data is consistent with oxygenation continuing well into the Palaeozoic era. Therefore, if changing redox conditions facilitated animal diversification, it did so through a limited rise in oxygen past critical functional and ecological thresholds, as is seen in modern oxygen minimum zone benthic animal communities. PMID:26201598

  18. The role of biology in planetary evolution: cyanobacterial primary production in low‐oxygen Proterozoic oceans

    PubMed Central

    Bryant, Donald A.; Macalady, Jennifer L.

    2016-01-01

    Summary Understanding the role of biology in planetary evolution remains an outstanding challenge to geobiologists. Progress towards unravelling this puzzle for Earth is hindered by the scarcity of well‐preserved rocks from the Archean (4.0 to 2.5 Gyr ago) and Proterozoic (2.5 to 0.5 Gyr ago) Eons. In addition, the microscopic life that dominated Earth's biota for most of its history left a poor fossil record, consisting primarily of lithified microbial mats, rare microbial body fossils and membrane‐derived hydrocarbon molecules that are still challenging to interpret. However, it is clear from the sulfur isotope record and other geochemical proxies that the production of oxygen or oxidizing power radically changed Earth's surface and atmosphere during the Proterozoic Eon, pushing it away from the more reducing conditions prevalent during the Archean. In addition to ancient rocks, our reconstruction of Earth's redox evolution is informed by our knowledge of biogeochemical cycles catalysed by extant biota. The emergence of oxygenic photosynthesis in ancient cyanobacteria represents one of the most impressive microbial innovations in Earth's history, and oxygenic photosynthesis is the largest source of O 2 in the atmosphere today. Thus the study of microbial metabolisms and evolution provides an important link between extant biota and the clues from the geologic record. Here, we consider the physiology of cyanobacteria (the only microorganisms capable of oxygenic photosynthesis), their co‐occurrence with anoxygenic phototrophs in a variety of environments and their persistence in low‐oxygen environments, including in water columns as well as mats, throughout much of Earth's history. We examine insights gained from both the rock record and cyanobacteria presently living in early Earth analogue ecosystems and synthesize current knowledge of these ancient microbial mediators in planetary redox evolution. Our analysis supports the hypothesis that anoxygenic

  19. Statistical analysis of iron geochemical data suggests limited late Proterozoic oxygenation.

    PubMed

    Sperling, Erik A; Wolock, Charles J; Morgan, Alex S; Gill, Benjamin C; Kunzmann, Marcus; Halverson, Galen P; Macdonald, Francis A; Knoll, Andrew H; Johnston, David T

    2015-07-23

    Sedimentary rocks deposited across the Proterozoic-Phanerozoic transition record extreme climate fluctuations, a potential rise in atmospheric oxygen or re-organization of the seafloor redox landscape, and the initial diversification of animals. It is widely assumed that the inferred redox change facilitated the observed trends in biodiversity. Establishing this palaeoenvironmental context, however, requires that changes in marine redox structure be tracked by means of geochemical proxies and translated into estimates of atmospheric oxygen. Iron-based proxies are among the most effective tools for tracking the redox chemistry of ancient oceans. These proxies are inherently local, but have global implications when analysed collectively and statistically. Here we analyse about 4,700 iron-speciation measurements from shales 2,300 to 360 million years old. Our statistical analyses suggest that subsurface water masses in mid-Proterozoic oceans were predominantly anoxic and ferruginous (depleted in dissolved oxygen and iron-bearing), but with a tendency towards euxinia (sulfide-bearing) that is not observed in the Neoproterozoic era. Analyses further indicate that early animals did not experience appreciable benthic sulfide stress. Finally, unlike proxies based on redox-sensitive trace-metal abundances, iron geochemical data do not show a statistically significant change in oxygen content through the Ediacaran and Cambrian periods, sharply constraining the magnitude of the end-Proterozoic oxygen increase. Indeed, this re-analysis of trace-metal data is consistent with oxygenation continuing well into the Palaeozoic era. Therefore, if changing redox conditions facilitated animal diversification, it did so through a limited rise in oxygen past critical functional and ecological thresholds, as is seen in modern oxygen minimum zone benthic animal communities.

  20. Peculiarities of the composition of volatile components in picroilmenites from Yakutian kimberlites of various ages (by gas chromatography—mass spectrometry)

    NASA Astrophysics Data System (ADS)

    Tomilenko, A. A.; Bul'bak, T. A.; Pokhilenko, L. N.; Kuzmin, D. V.; Sobolev, N. V.

    2016-07-01

    The composition of volatile components in picroilmenites from Yakutian kimberlitic pipes of various ages (the Olivinovaya, Malokuonapskaya, and Udachnaya-East pipes) was studied for the first time by means of gas chromatography-mass spectrometry (GC-MS). It was shown that picroilmenites and olivines from same kimberlitic pipes contained volatile components of close composition, whereas these components were quite different in these minerals from different pipes. These features point to a common source and represent the specificity of the magma chamber formed under the pronounced influence of hydrocarbons with their derivates, as well as nitrogen-, chlorine-, and sulfur-containing compounds. The fraction of hydrocarbons and derivates in the composition of volatile matter is as high as 99%, including 9.7% of chlorine- and fluorinecontaining compounds.

  1. Regularities of spatial association of major endogenous uranium deposits and kimberlitic dykes in the uranium ore regions of the Ukrainian Shield

    NASA Astrophysics Data System (ADS)

    Kalashnyk, Anna

    2015-04-01

    During exploration works we discovered the spatial association and proximity time formation of kimberlite dykes (ages are 1,815 and 1,900 Ga for phlogopite) and major industrial uranium deposits in carbonate-sodium metasomatites (age of the main uranium ore of an albititic formation is 1,85-1,70 Ga according to U-Pb method) in Kirovogradsky, Krivorozhsky and Alekseevsko-Lysogorskiy uranium ore regions of the Ukrainian Shield (UkrSh) [1]. In kimberlites of Kirovogradsky ore region uranium content reaches 18-20 g/t. Carbon dioxide is a major component in the formation of hydrothermal uranium deposits and the formation of the sodium in the process of generating the spectrum of alkaline ultrabasic magmas in the range from picritic to kimberlite and this is the connection between these disparate geochemical processes. For industrial uranium deposits in carbonate-sodium metasomatitics of the Kirovogradsky and Krivorozhsky uranium ore regions are characteristic of uranyl carbonate introduction of uranium, which causes correlation between CO2 content and U in range of "poor - ordinary - rich" uranium ore. In productive areas of uranium-ore fields of the Kirovogradsky ore region for phlogopite-carbonate veinlets of uranium ore albitites deep δ13C values (from -7.9 to -6.9o/oo) are characteristic. Isotope-geochemical investigation of albitites from Novokonstantynovskoe, Dokuchaevskoe, Partyzanskoe uranium deposits allowed obtaining direct evidence of the involvement of mantle material during formation of uranium albitites in Kirovogradsky ore region [2]. Petrological characteristics of kimberlites from uranium ore regions of the UkrSh (presence of nodules of dunite and harzburgite garnet in kimberlites, diamonds of peridotite paragenesis, chemical composition of indicator minerals of kimberlite, in particular Gruzskoy areas pyropes (Cr2O3 = 6,1-7,1%, MgO = 19,33-20,01%, CaO = 4,14-4,38 %, the content of knorringite component of most grains > 50mol%), chromites (Cr2O3 = 45

  2. Shell structure and distribution of Cloudina, a potential index fossil for the terminal Proterozoic

    NASA Technical Reports Server (NTRS)

    Grant, S. W.; Knoll, A. H. (Principal Investigator)

    1990-01-01

    Cloudina-bearing biosparites and biomicrites in the lower part of the Nama Group, Namibia, contain a wide morphological diversity of shell fragments that can all be attributed to the two named species C. hartmannae and C. riemkeae. The curved to sinuous tubular shells of Cloudina were multi-layered. Each shell layer was 8 to 50 micrometers thick and in the form of a slightly flaring tube with one end open and the other closed. Growth appears to have been periodic with successive shell layers forming within older layers. Each added layer was slightly elevated from the previous layer at the proximal end and was asymmetrically placed within the older layer so that only a portion of the new shell layer was fused to the previous layer. This type of growth left a relatively large unminerialized area between the shell layers which was often partially or fully occluded by early marine cements. The thin shell layers exhibit both plastic and brittle deformation and were likely formed of a rigid CaCO3-impregnated organic-rich material. Often the shell layers are preferentially dolomitized suggesting an original mineralogy of high-magnesian calcite. Both species in the Nama Group formed thickets, or perhaps bioherms, and this sedentary and gregarious habit suggests that Cloudina was probably a filter-feeding metazoan of at least a cnidarian grade of organization. The unusual shell structure of Cloudina gives rise to a characteristic suite of taphonomic and diagenetic features that can be used to identify Cloudina-bearing deposits within the Nama Group and in other terminal Proterozoic deposits around the world. Species of Cloudina occur in limestones from Brazil, Spain, China, and Oman in sequences consistent with a latest Proterozoic age assignment. In addition, supposed lower Cambrian, pre-trilobitic, shelly fossils from northwest Mexico and the White-Inyo Mountains in California and Nevada, including Sinotubulites, Nevadatubulus, and Wyattia, are all either closely related

  3. Shell structure and distribution of Cloudina, a potential index fossil for the terminal Proterozoic.

    PubMed

    Grant, S W

    1990-01-01

    Cloudina-bearing biosparites and biomicrites in the lower part of the Nama Group, Namibia, contain a wide morphological diversity of shell fragments that can all be attributed to the two named species C. hartmannae and C. riemkeae. The curved to sinuous tubular shells of Cloudina were multi-layered. Each shell layer was 8 to 50 micrometers thick and in the form of a slightly flaring tube with one end open and the other closed. Growth appears to have been periodic with successive shell layers forming within older layers. Each added layer was slightly elevated from the previous layer at the proximal end and was asymmetrically placed within the older layer so that only a portion of the new shell layer was fused to the previous layer. This type of growth left a relatively large unminerialized area between the shell layers which was often partially or fully occluded by early marine cements. The thin shell layers exhibit both plastic and brittle deformation and were likely formed of a rigid CaCO3-impregnated organic-rich material. Often the shell layers are preferentially dolomitized suggesting an original mineralogy of high-magnesian calcite. Both species in the Nama Group formed thickets, or perhaps bioherms, and this sedentary and gregarious habit suggests that Cloudina was probably a filter-feeding metazoan of at least a cnidarian grade of organization. The unusual shell structure of Cloudina gives rise to a characteristic suite of taphonomic and diagenetic features that can be used to identify Cloudina-bearing deposits within the Nama Group and in other terminal Proterozoic deposits around the world. Species of Cloudina occur in limestones from Brazil, Spain, China, and Oman in sequences consistent with a latest Proterozoic age assignment. In addition, supposed lower Cambrian, pre-trilobitic, shelly fossils from northwest Mexico and the White-Inyo Mountains in California and Nevada, including Sinotubulites, Nevadatubulus, and Wyattia, are all either closely related

  4. Carbon Isotope Evidence for the Stepwise Oxidation of the Proterozoic Environment

    NASA Technical Reports Server (NTRS)

    DesMarais, David J.; Strauss, Harald; Summons, Roger E.; Hayes, J. M.

    1992-01-01

    The oxidation of the Earth's crust and the increase in atmospheric oxygen early in Earth history have been linked to the accumulation of reduced carbon in sedimentary rocks. Trends in the carbon isotope composition of sedimentary organic carbon and carbonate show that during the Proterozoic aeon (2.5-0.54 Gyr ago) the organic carbon reservoir grew in size, relative to the carbonate reservoir. This increase, and the concomitant release of oxidizing power in the environment, occurred mostly during episodes of global rifting and orogeny.

  5. Carbon isotope evidence for the stepwise oxidation of the Proterozoic environment

    NASA Technical Reports Server (NTRS)

    Des Marais, David J.; Strauss, Harald; Summons, Roger E.; Hayes, J. M.

    1992-01-01

    The oxidation of the earth's crust and the increase in atmospheric oxygen early in earth history have been linked to the accumulation of reduced carbon in sedimentary rocks. Trends in the carbon isotope composition of sedimentary organic carbon and carbonate show that during the Proterozoic aeon (2.5-0.54 Gyr ago) the organic carbon reservoir grew in size, relative to the carbonate reservoir. This increase, and the concomitant release of oxidizing power in the environment, occurred mostly during episodes of global rifting and orogeny.

  6. Evolution of the Early Proterozoic Colorado province: Constraints from U-Pb geochronology

    NASA Astrophysics Data System (ADS)

    Reed, John C., Jr.; Bickford, M. E.; Premo, Wayne R.; Aleinikoff, John N.; Pallister, John S.

    1987-09-01

    The Colorado province represents an addition of a belt of rocks more than 500 km wide to the southern margin of the Archean Wyoming craton during the Early Proterozoic, between about 1790 and 1660 Ma. Correspondence in ages between metamorphism, deformation, and plutonism; association of volcanic rocks with comagmatic calc-alkalic plutons; and lack of older basement are all consistent with the interpretation that the rocks of the province are products of arc magmatism and cannibalistic sedimentation along a convergent margin at the southern edge of the craton. Note: Additional material for this article is Supplementary Data 8731, available on request from the GSA Documents Secretary (see footnote 1).

  7. Geochemistry and stratigraphic relations of middle Proterozoic rocks of the New Jersey Highlands

    USGS Publications Warehouse

    Volkert, Richard A.; Drake, Avery Ala

    1999-01-01

    Middle Proterozoic rocks of the New Jersey Highlands consist of a basement of dacitic, tonalitic, trondhjemitic, and charnockitic rocks that constitute the Losee metamorphic suite. These rocks are unconformably overlain by a layered supracrustal sequence of quartzo-feldspathic and calcareous rocks. Abundant sheets of hornblende- and biotite-bearing rocks of the Byram intrusive suite and clinopyroxene-bearing rocks of the Lake Hopatcong intrusive suite were synkinematically emplaced at about 1,090 Ma. These intrusive suites constitute the Vernon Supersuite. The postorogenic Mount Eve Granite has been dated at 1,020?4 Ma and is confined to the extreme northern Highlands.

  8. Early to Middle Proterozoic Construction of the Mojave Province, Southwestern United States

    USGS Publications Warehouse

    Coleman, D.S.; Barth, A.P.; Wooden, J.L.

    2002-01-01

    Zircon and monazite U-Pb geochronology of rocks in the western Mojave province of the southwest US reveals that the Proterozoic arc exposed there shares an intrusive and deformational history with rocks exposed further east in the Yavapai and Mazatzal belts after approximately 1780 Ma. Consequently, it seems likely that the Mojave province was contiguous with these other Laurentian provinces by that time. Isotopic and geochronologic data also suggest that Mojave province plutonic rocks inherit their distinctive isotopic compositions, at least in part, from an enriched lithospheric mantle source and interaction with sedimentary rocks containing Archean detritus.

  9. Carbon isotope evidence for the stepwise oxidation of the Proterozoic environment

    PubMed

    Des Marais, D J; Strauss, H; Summons, R E; Hayes, J M

    1992-10-15

    The oxidation of the Earth's crust and the increase in atmospheric oxygen early in Earth history have been linked to the accumulation of reduced carbon in sedimentary rocks. Trends in the carbon isotope composition of sedimentary organic carbon and carbonate show that during the Proterozoic aeon (2.5-0.54 Gyr ago) the organic carbon reservoir grew in size, relative to the carbonate reservoir. This increase, and the concomitant release of oxidizing power in the environment, occurred mostly during episodes of global rifting and orogeny.

  10. Geochronology of the proterozoic basement of southwesternmost North America, and the origin and evolution of the Mojave crustal province

    USGS Publications Warehouse

    Barth, Andrew P.; Wooden, Joseph L.; Coleman, Drew S.; Fanning, C. Mark

    2000-01-01

    The Proterozoic Baldwin gneiss in the central Transverse Ranges of southern California, a part of the Mojave crustal province, is composed of quartzofeldspathic gneiss and schist, augen and granitic gneiss, trondhjemite gneiss, and minor quartzite, amphibolite, metagabbro, and metapyroxenite. Sensitive high resolution ion microprobe (SHRIMP) data indicate that augen and granitic gneisses comprise a magmatic arc intrusive suite emplaced between 1783 ± 12 and 1675 ± 19 Ma, adjacent to or through thinned Archean crust. High U/Th rims on zircons in most samples suggest an early metamorphic event at ∼1741 Ma, but peak amphibolite facies metamorphism and penetrative, west vergent deformation occurred after 1675 Ma. The Baldwin gneiss is part of a regional allochthon emplaced by west vergent deformation over a Proterozoic shelf-slope sequence (Joshua Tree terrane). We hypothesize that emplacement of this regional allochthon occurred during a late Early or Middle Proterozoic arc-continent collision along the western margin of Laurentia.

  11. Nitrogen cycle feedbacks as a control on euxinia in the mid-Proterozoic ocean

    NASA Astrophysics Data System (ADS)

    Boyle, R. A.; Clark, J. R.; Poulton, S. W.; Shields-Zhou, G.; Canfield, D. E.; Lenton, T. M.

    2013-02-01

    Geochemical evidence invokes anoxic deep oceans until the terminal Neoproterozoic ~0.55 Ma, despite oxygenation of Earth’s atmosphere nearly 2 Gyr earlier. Marine sediments from the intervening period suggest predominantly ferruginous (anoxic Fe(II)-rich) waters, interspersed with euxinia (anoxic H2S-rich conditions) along productive continental margins. Today, sustained biotic H2S production requires NO3- depletion because denitrifiers outcompete sulphate reducers. Thus, euxinia is rare, only occurring concurrently with (steady state) organic carbon availability when N2-fixers dominate the production in the photic zone. Here we use a simple box model of a generic Proterozoic coastal upwelling zone to show how these feedbacks caused the mid-Proterozoic ocean to exhibit a spatial/temporal separation between two states: photic zone NO3- with denitrification in lower anoxic waters, and N2-fixation-driven production overlying euxinia. Interchange between these states likely explains the varying H2S concentration implied by existing data, which persisted until the Neoproterozoic oxygenation event gave rise to modern marine biogeochemistry.

  12. Microfossils from silicified stromatolitic carbonates of the Upper Proterozoic Limestone-Dolomite 'Series', central East Greenland

    NASA Technical Reports Server (NTRS)

    Green, J. W.; Knoll, A. H.; Swett, K.

    1989-01-01

    Silicified flake conglomerates and in situ stratiform stromatolites of the Upper Proterozoic (c. 700-800 Ma) Limestone-Dolomite 'Series', central East Greenland, contain well preserved microfossils. Five stratigraphic horizons within the 1200 m succession contain microbial mat assemblages, providing a broad palaeontological representation of late Proterozoic peritidal mat communities. Comparison of assemblages demonstrates that the taxonomy and diversity of mat builder, dweller, and allochthonous populations all vary considerably within and among horizons. The primary mat builder in most assemblages is Siphonophycus inornatum, a sheath-forming prokaryote of probable but not unequivocally established cyanobacterial affinities. An unusual low diversity unit in Bed 17 is dominated by a different builder, Tenuofilum septatum, while a thin cryptalgal horizon in Bed 18 is built almost exclusively by Siphonophycus kestron. Although variable taphonomic histories contribute to observed assemblage variation, most differences within and among horizons appear to reflect the differential success or failure of individual microbial populations in colonizing different tidal flat microenvironments. Twenty-two taxa are recognized, of which two are described as new: Myxococcoides stragulescens n.sp. and Scissilisphaera gradata n. sp.

  13. Nitrogen cycle feedbacks as a control on euxinia in the mid-Proterozoic ocean.

    PubMed

    Boyle, R A; Clark, J R; Poulton, S W; Shields-Zhou, G; Canfield, D E; Lenton, T M

    2013-01-01

    Geochemical evidence invokes anoxic deep oceans until the terminal Neoproterozoic ~0.55 Ma, despite oxygenation of Earth's atmosphere nearly 2 Gyr earlier. Marine sediments from the intervening period suggest predominantly ferruginous (anoxic Fe(II)-rich) waters, interspersed with euxinia (anoxic H(2)S-rich conditions) along productive continental margins. Today, sustained biotic H(2)S production requires NO(3)(-) depletion because denitrifiers outcompete sulphate reducers. Thus, euxinia is rare, only occurring concurrently with (steady state) organic carbon availability when N(2)-fixers dominate the production in the photic zone. Here we use a simple box model of a generic Proterozoic coastal upwelling zone to show how these feedbacks caused the mid-Proterozoic ocean to exhibit a spatial/temporal separation between two states: photic zone NO(3)(-) with denitrification in lower anoxic waters, and N(2)-fixation-driven production overlying euxinia. Interchange between these states likely explains the varying H(2)S concentration implied by existing data, which persisted until the Neoproterozoic oxygenation event gave rise to modern marine biogeochemistry.

  14. Preliminary digital geologic map of the Penokean (early Proterozoic) continental margin in northern Michigan and Wisconsin

    USGS Publications Warehouse

    Cannon, W.F.; Ottke, Doug

    1999-01-01

    The data on this CD consist of geographic information system (GIS) coverages and tabular data on the geology of Early Proterozoic and Archean rocks in part of the Early Proterozoic Penokean orogeny. The map emphasizes metasedimentary and metavolcanic rocks that were deposited along the southern margin of the Superior craton and were later deformed during continental collision at about 1850 Ma. The area includes the famous iron ranges of the south shore region of the Lake Superior district. Base maps, both as digital raster graphics (DRG) and digital line graphs (DLG) are also provided for the convenience of users. The map has been compiled from many individual studies, mostly by USGS researchers, completed during the past 50 years, including many detailed (1:24,000 scale) geologic maps. Data was compiled at 1:100,000 scale and preserves most of the details of source materials. This product is a preliminary release of the geologic map data bases during ongoing studies of the geology and metallogeny of the Penokean continental margin. Files are provided in three formats: Federal Spatial Data Transfer format (SDTS), Arc export format (.e00) files, and Arc coverages. All files can be accessed directly from the CD-ROM using either ARC/INFO 7.1.2 or later or Arc View 3.0 or later software. ESRI's Arc Explorer, a free GIS data viewer available at the web site: http://www.esri.com/software/arcexplorer/index.html also provides display and querying capability for these files.

  15. Pellet microfossils: Possible evidence for metazoan life in Early Proterozoic time

    PubMed Central

    Robbins, Eleanora Iberall; Porter, Karen Glaus; Haberyan, Kurt A.

    1985-01-01

    Microfossils resembling fecal pellets occur in acid-resistant residues and thin sections of Middle Cambrian to Early Proterozoic shale. The cylindrical microfossils average 50 × 110 μm and are the size and shape of fecal pellets produced by microscopic animals today. Pellets occur in dark gray and black rocks that were deposited in the facies that also preserves sulfide minerals and that represent environments analogous to those that preserve fecal pellets today. Rocks containing pellets and algal microfossils range in age from 0.53 to 1.9 gigayears (Gyr) and include Burgess Shale, Greyson and Newland Formations, Rove Formation, and Gunflint Iron-Formation. Similar rock types of Archean age, ranging from 2.68 to 3.8 Gyr, were barren of pellets. If the Proterozoic microfossils are fossilized fecal pellets, they provide evidence of metazoan life and a complex food chain at 1.9 Gyr ago. This occurrence predates macroscopic metazoan body fossils in the Ediacaran System at 0.67 Gyr, animal trace fossils from 0.9 to 1.3 Gyr, and fossils of unicellular eukaryotic plankton at 1.4 Gyr. Images PMID:16593599

  16. Proterozoic oxygen rise linked to shifting balance between seafloor and terrestrial weathering

    PubMed Central

    Mills, Benjamin; Lenton, Timothy M.; Watson, Andrew J.

    2014-01-01

    A shift toward higher atmospheric oxygen concentration during the late Proterozoic has been inferred from multiple indirect proxies and is seen by many as a prerequisite for the emergence of complex animal life. However, the mechanisms controlling the level of oxygen throughout the Proterozoic and its eventual rise remain uncertain. Here we use a simple biogeochemical model to show that the balance between long-term carbon removal fluxes via terrestrial silicate weathering and ocean crust alteration plays a key role in determining atmospheric oxygen concentration. This balance may be shifted by changes in terrestrial weatherability or in the generation rate of oceanic crust. As a result, the terrestrial chemical weathering flux may be permanently altered—contrasting with the conventional view that the global silicate weathering flux must adjust to equal the volcanic CO2 degassing flux. Changes in chemical weathering flux in turn alter the long-term supply of phosphorus to the ocean, and therefore the flux of organic carbon burial, which is the long-term source of atmospheric oxygen. Hence we propose that increasing solar luminosity and a decrease in seafloor spreading rate over 1,500–500 Ma drove a gradual shift from seafloor weathering to terrestrial weathering, and a corresponding steady rise in atmospheric oxygen. Furthermore, increased terrestrial weatherability during the late Neoproterozoic may explain low temperature, increases in ocean phosphate, ocean sulfate, and atmospheric oxygen concentration at this time. PMID:24927553

  17. Late Proterozoic and Silurian alkaline plutons within the southeastern New England Avalon zone

    SciTech Connect

    Hermes, O.D. ); Zartman, R.E. )

    1992-07-01

    Distinct pulses of quartz-bearing, alkaline plutonism and volcanism are known to have occurred in the Avalon zone of southeastern New England during the Late Ordovician, Early Silurian, Devonian, and Carboniferous. Zircon separates from the Franklin and Dartmouth plutons demonstrate that two additional, previously unrecognized periods of alkaline magmatism occurred. The Franklin pluton yields an age of 417 {plus minus} 6 Ma (Late Silurian), whereas the Dartmouth pluton is Late Proterozoic (595 {plus minus} 5 Ma) and markedly older than the other plutons of alkaline affinity. The new ages further emphasize the episodic nature and long-term duration of such alkaline igneous events within the southeastern New England Avalon zone. The Dartmouth pluton may represent a post-collisional alkaline granite emplaced in the Late Proterozoic, almost immediately after a major period of calcalkaline igneous activity that accompanied plate convergence and continental accretion. The abrupt change from orogenic calcalkaline igneous activity to post-collisional alkaline granite, followed by younger episodes of anorogenic emplacement, is remarkably similar to igneous events reported from pan-African mobile belts widespread throughout Africa. In addition, parts of the Dartmouth pluton exhibit features indicative of mixing and commingling of felsic and mafic melts that are associated with coevally formed mylonitic fabrics. Because these fabrics are conformable to those in adjacent gneisses, but discordant with Alleghanian fabrics in the nearby Carboniferous Narragansett basin, they represent some of the best candidates for pre-Alleghanian structures thus far identified in the southeastern New England Avalon zone.

  18. Middle Proterozoic belt basin syndepositional faults and their influence on Phanerozoic thrusting and extension

    SciTech Connect

    Winston, D.

    1983-08-01

    During the middle Proterozoic, continental crust of the Belt region was cut by nearly east-west and northwest-striking faults that produced a mosaic of large basement blocks. Blocks that subsided formed the Belt basin and were surrounded mostly by uplifted blocks. The Dillon block bounded the basin on the south along the Perry line, and, together with blocks to the south and west, furnished most of the sediment that filled the basin. Great alluvial aprons sloped basinward from the uplifted blocks down to extensive flats that bordered the Belt intracratonic sea. Sediments were deposited in the deeper parts of the sea by underflows and interflows. The graben blocks, including the Helena embayment and the diagonal block to the northwest, received the thickest sediments. Cretaceous to Paleocene compression thrust the Belt rocks and Phanerozoic cover rocks eastward and northeastward, forming first a western, and then an eastern thrust belt. Thrusts on the blocks formed long sheets that deflected and tore along the block boundaries, where depth to basement and tectonic transport distances changed. Where thrust crossed northwest-trending basement faults, they ramped locally. Eocene extension produced fault patterns that change from block to block. Differential extension formed right-lateral strike-slip faults across block boundaries. Proterozoic faults that cut the continental crust, not only formed the framework of the Belt basin, but affected patterns of later compression and extension.

  19. Anorthosites and anorthosites: Contrasting plagioclase-rich rocks in the Archaean and Proterozoic

    SciTech Connect

    Owens, B.E. . Dept. of Earth Planetary Sciences)

    1993-03-01

    Anorthosites -- rocks consisting predominantly of plagioclase feldspar -- have figured prominently in at least two distinct intervals of Earth history: the late-Archaean and mid-Proterozoic. Archaean anorthosites (AA) are a key component of high-grade gneiss terranes, where they typically form laterally extensive deformed sheets or sills up to a km thick. In contrast, Proterozoic anorthosites (PA) form plutons or plutonic complexes, and are most abundant in a quasi-continuous belt across NE N. America. In addition to these temporal and structural contrasts, AA and PA display markedly different mineralogical and geochemical properties, including, respectively: (1) equant plagioclase megacrysts vs. tabular megacrysts; (2) highly calcic compositions vs. intermediate to alkalic compositions; (3) amphibole vs. olivine or orthopyroxene as the dominant mafic mineral; (4) the presence of chromite, locally in ore-grade layers vs. massive, cross-cutting Fe-Ti oxide ores; (5) low levels of Sr and Ba vs. high to extreme levels; (6) high levels of ferromagnesian trace elements vs. low levels; (7) Ga/Al values typical of basaltic plagioclase vs. much lower values; and (8) moderately vs. extremely fractionated REE patterns. Given these contrasts, it appears that the only property AA and PA share is their plag-rich nature, suggesting that there must be more than one process (and probably multiple processes) capable of producing anorthosite.

  20. Hydrocarbon source-rock evaluation - Solor Church Formation (middle Proterozoic, Keweenawan Supergroup), southeastern Minnesota

    USGS Publications Warehouse

    Hatch, J.R.; Morey, G.B.

    1984-01-01

    In the type section (Lonsdale 65-1 core, Rice County, Minnesota) the Solor Church Formation (Middle Proterozoic, Keweenawan Supergroup) consists primarily of reddish-brown mudstone and siltstone and pale reddish-brown sandstone. The sandstone and siltstone are texturally and mineralogically immature. Hydrocarbon source-rock evaluation of bluish-gray, greenish-gray and medium-dark-gray to grayish-black beds, which primarily occur in the lower 104 m (340 ft) of this core, shows: (1) the rocks have low organic carbon contents (<0.5 percent for 22 of 25 samples); (2) the organic matter is thermally very mature (Tmax = 494°C, sample 19) and is probably near the transition between the wet gas phase of catagenesis and metagenesis (dry gas zone); and (3) the rocks have minimal potential for producing additional hydrocarbons (genetic potential <0.30 mgHC/gm rock). Although no direct evidence exists from which to determine maximum depths of burial, the observed thermal maturity of the organic matter requires significantly greater depths of burial and(or) higher geothermal gradients. It is likely, at least on the St. Croix horst, that thermal alteration of the organic matter in the Solor Church took place relatively early, and that any hydrocarbons generated during this early thermal alteration were probably lost prior to deposition of the overlying Fond du Lac Formation (Middle Proterozoic, Keweenawan Supergroup).

  1. Proterozoic oxygen rise linked to shifting balance between seafloor and terrestrial weathering.

    PubMed

    Mills, Benjamin; Lenton, Timothy M; Watson, Andrew J

    2014-06-24

    A shift toward higher atmospheric oxygen concentration during the late Proterozoic has been inferred from multiple indirect proxies and is seen by many as a prerequisite for the emergence of complex animal life. However, the mechanisms controlling the level of oxygen throughout the Proterozoic and its eventual rise remain uncertain. Here we use a simple biogeochemical model to show that the balance between long-term carbon removal fluxes via terrestrial silicate weathering and ocean crust alteration plays a key role in determining atmospheric oxygen concentration. This balance may be shifted by changes in terrestrial weatherability or in the generation rate of oceanic crust. As a result, the terrestrial chemical weathering flux may be permanently altered--contrasting with the conventional view that the global silicate weathering flux must adjust to equal the volcanic CO2 degassing flux. Changes in chemical weathering flux in turn alter the long-term supply of phosphorus to the ocean, and therefore the flux of organic carbon burial, which is the long-term source of atmospheric oxygen. Hence we propose that increasing solar luminosity and a decrease in seafloor spreading rate over 1,500-500 Ma drove a gradual shift from seafloor weathering to terrestrial weathering, and a corresponding steady rise in atmospheric oxygen. Furthermore, increased terrestrial weatherability during the late Neoproterozoic may explain low temperature, increases in ocean phosphate, ocean sulfate, and atmospheric oxygen concentration at this time.

  2. Marine pisolites from Upper Proterozoic carbonates of East Greenland and Spitsbergen.

    PubMed

    Swett, K; Knoll, A H

    1989-01-01

    Upper Proterozoic carbonate successions from central East Greenland (the Limestone-Dolomite 'Series' of the Eleonore Bay Group) and Svalbard (the Backlundtoppen Formation of the Akademikerbreen) Group, Spitsbergen, and the Upper Russo Formation of the Raoldtoppen Group, Nordaustlandet) contain thick sequences dominated by pisolites. These rocks were generated in shallow marine environments, and the pisoids are essentially oversized ooids. A marine environment is supported by the thickness and lateral extent of the carbonates; by a sedimentary association of pisolites with stromatolites, flake-conglomerates, calcarenites, calcilutites, microphytolites, and ooids similar to that found in numerous other Proterozoic carbonate successions; by sedimentary structures, including cross-beds and megaripples that characterize the pisolitic beds; and by microorganisms that inhabit modern marine ooids of the Bahama Banks. Petrographic features and strontium abundances suggest that the pisoids were originally aragonitic, but neomorphism, silicification, calcitization, and dolomitization have extensively modified original mineralogies and fabrics. The East Greenland and Svalbard pisolitic carbonates reflect similar depositional environments and diagenetic histories, reinforcing previous bio-, litho-, and chemostratigraphic interpretations that the two sequences accumulated contiguously in a coastal zone of pisoid genesis which extended for at least 600, and probably 1000 or more, kilometres. PMID:11542187

  3. Application of Radial Basis Functional Link Networks to Exploration for Proterozoic Mineral Deposits in Central Iran

    SciTech Connect

    Behnia, Pouran

    2007-06-15

    The metallogeny of Central Iran is characterized mainly by the presence of several iron, apatite, and uranium deposits of Proterozoic age. Radial Basis Function Link Networks (RBFLN) were used as a data-driven method for GIS-based predictive mapping of Proterozoic mineralization in this area. To generate the input data for RBFLN, the evidential maps comprising stratigraphic, structural, geophysical, and geochemical data were used. Fifty-eight deposits and 58 'nondeposits' were used to train the network. The operations for the application of neural networks employed in this study involve both multiclass and binary representation of evidential maps. Running RBFLN on different input data showed that an increase in the number of evidential maps and classes leads to a larger classification sum of squared error (SSE). As a whole, an increase in the number of iterations resulted in the improvement of training SSE. The results of applying RBFLN showed that a successful classification depends on the existence of spatially well distributed deposits and nondeposits throughout the study area.

  4. Global biogeochemical changes at both ends of the proterozoic: insights from phosphorites.

    PubMed

    Papineau, Dominic

    2010-03-01

    The distribution of major phosphate deposits in the Precambrian sedimentary rock record is restricted to periods that witnessed global biogeochemical changes, but the cause of this distribution is unclear. The oldest known phosphogenic event occurred around 2.0 Ga and was followed, after more than 1.3 billion years, by an even larger phosphogenic event in the Neoproterozoic. Phosphorites (phosphate-rich sedimentary rocks that contain more than 15% P(2)O(5)) preserve a unique record of seawater chemistry, biological activity, and oceanographic changes. In an attempt to emphasize the potentially crucial significance of phosphorites in the evolution of Proterozoic biogeochemical cycles, this contribution provides a review of some important Paleoproterozoic phosphate deposits and of models proposed for their origin. A new model is then presented for the spatial and temporal modes of occurrence of phosphorites along with possible connections to global changes at both ends of the Proterozoic. Central to the new model is that periods of atmospheric oxygenation may have been caused by globally elevated rates of primary productivity stimulated by high fluxes of phosphorus delivery to seawater as a result of increased chemical weathering of continental crust over geological timescales. The striking similarities in biogeochemical evolution between the Paleo- and Neoproterozoic are discussed in light of the two oldest major phosphogenic events and their possible relation to the stepwise rise of atmospheric oxygen that ultimately resulted in significant leaps in biological evolution. PMID:20105035

  5. Proterozoic oxygen rise linked to shifting balance between seafloor and terrestrial weathering.

    PubMed

    Mills, Benjamin; Lenton, Timothy M; Watson, Andrew J

    2014-06-24

    A shift toward higher atmospheric oxygen concentration during the late Proterozoic has been inferred from multiple indirect proxies and is seen by many as a prerequisite for the emergence of complex animal life. However, the mechanisms controlling the level of oxygen throughout the Proterozoic and its eventual rise remain uncertain. Here we use a simple biogeochemical model to show that the balance between long-term carbon removal fluxes via terrestrial silicate weathering and ocean crust alteration plays a key role in determining atmospheric oxygen concentration. This balance may be shifted by changes in terrestrial weatherability or in the generation rate of oceanic crust. As a result, the terrestrial chemical weathering flux may be permanently altered--contrasting with the conventional view that the global silicate weathering flux must adjust to equal the volcanic CO2 degassing flux. Changes in chemical weathering flux in turn alter the long-term supply of phosphorus to the ocean, and therefore the flux of organic carbon burial, which is the long-term source of atmospheric oxygen. Hence we propose that increasing solar luminosity and a decrease in seafloor spreading rate over 1,500-500 Ma drove a gradual shift from seafloor weathering to terrestrial weathering, and a corresponding steady rise in atmospheric oxygen. Furthermore, increased terrestrial weatherability during the late Neoproterozoic may explain low temperature, increases in ocean phosphate, ocean sulfate, and atmospheric oxygen concentration at this time. PMID:24927553

  6. Global biogeochemical changes at both ends of the proterozoic: insights from phosphorites.

    PubMed

    Papineau, Dominic

    2010-03-01

    The distribution of major phosphate deposits in the Precambrian sedimentary rock record is restricted to periods that witnessed global biogeochemical changes, but the cause of this distribution is unclear. The oldest known phosphogenic event occurred around 2.0 Ga and was followed, after more than 1.3 billion years, by an even larger phosphogenic event in the Neoproterozoic. Phosphorites (phosphate-rich sedimentary rocks that contain more than 15% P(2)O(5)) preserve a unique record of seawater chemistry, biological activity, and oceanographic changes. In an attempt to emphasize the potentially crucial significance of phosphorites in the evolution of Proterozoic biogeochemical cycles, this contribution provides a review of some important Paleoproterozoic phosphate deposits and of models proposed for their origin. A new model is then presented for the spatial and temporal modes of occurrence of phosphorites along with possible connections to global changes at both ends of the Proterozoic. Central to the new model is that periods of atmospheric oxygenation may have been caused by globally elevated rates of primary productivity stimulated by high fluxes of phosphorus delivery to seawater as a result of increased chemical weathering of continental crust over geological timescales. The striking similarities in biogeochemical evolution between the Paleo- and Neoproterozoic are discussed in light of the two oldest major phosphogenic events and their possible relation to the stepwise rise of atmospheric oxygen that ultimately resulted in significant leaps in biological evolution.

  7. Marine pisolites from Upper Proterozoic carbonates of East Greenland and Spitsbergen.

    PubMed

    Swett, K; Knoll, A H

    1989-01-01

    Upper Proterozoic carbonate successions from central East Greenland (the Limestone-Dolomite 'Series' of the Eleonore Bay Group) and Svalbard (the Backlundtoppen Formation of the Akademikerbreen) Group, Spitsbergen, and the Upper Russo Formation of the Raoldtoppen Group, Nordaustlandet) contain thick sequences dominated by pisolites. These rocks were generated in shallow marine environments, and the pisoids are essentially oversized ooids. A marine environment is supported by the thickness and lateral extent of the carbonates; by a sedimentary association of pisolites with stromatolites, flake-conglomerates, calcarenites, calcilutites, microphytolites, and ooids similar to that found in numerous other Proterozoic carbonate successions; by sedimentary structures, including cross-beds and megaripples that characterize the pisolitic beds; and by microorganisms that inhabit modern marine ooids of the Bahama Banks. Petrographic features and strontium abundances suggest that the pisoids were originally aragonitic, but neomorphism, silicification, calcitization, and dolomitization have extensively modified original mineralogies and fabrics. The East Greenland and Svalbard pisolitic carbonates reflect similar depositional environments and diagenetic histories, reinforcing previous bio-, litho-, and chemostratigraphic interpretations that the two sequences accumulated contiguously in a coastal zone of pisoid genesis which extended for at least 600, and probably 1000 or more, kilometres.

  8. Marine pisolites from Upper Proterozoic carbonates of East Greenland and Spitsbergen

    NASA Technical Reports Server (NTRS)

    Swett, K.; Knoll, A. H.

    1989-01-01

    Upper Proterozoic carbonate successions from central East Greenland (the Limestone-Dolomite 'Series' of the Eleonore Bay Group) and Svalbard (the Backlundtoppen Formation of the Akademikerbreen) Group, Spitsbergen, and the Upper Russo Formation of the Raoldtoppen Group, Nordaustlandet) contain thick sequences dominated by pisolites. These rocks were generated in shallow marine environments, and the pisoids are essentially oversized ooids. A marine environment is supported by the thickness and lateral extent of the carbonates; by a sedimentary association of pisolites with stromatolites, flake-conglomerates, calcarenites, calcilutites, microphytolites, and ooids similar to that found in numerous other Proterozoic carbonate successions; by sedimentary structures, including cross-beds and megaripples that characterize the pisolitic beds; and by microorganisms that inhabit modern marine ooids of the Bahama Banks. Petrographic features and strontium abundances suggest that the pisoids were originally aragonitic, but neomorphism, silicification, calcitization, and dolomitization have extensively modified original mineralogies and fabrics. The East Greenland and Svalbard pisolitic carbonates reflect similar depositional environments and diagenetic histories, reinforcing previous bio-, litho-, and chemostratigraphic interpretations that the two sequences accumulated contiguously in a coastal zone of pisoid genesis which extended for at least 600, and probably 1000 or more, kilometres.

  9. Global Biogeochemical Changes at Both Ends of the Proterozoic: Insights from Phosphorites

    NASA Astrophysics Data System (ADS)

    Papineau, Dominic

    2010-03-01

    The distribution of major phosphate deposits in the Precambrian sedimentary rock record is restricted to periods that witnessed global biogeochemical changes, but the cause of this distribution is unclear. The oldest known phosphogenic event occurred around 2.0 Ga and was followed, after more than 1.3 billion years, by an even larger phosphogenic event in the Neoproterozoic. Phosphorites (phosphate-rich sedimentary rocks that contain more than 15% P2O5) preserve a unique record of seawater chemistry, biological activity, and oceanographic changes. In an attempt to emphasize the potentially crucial significance of phosphorites in the evolution of Proterozoic biogeochemical cycles, this contribution provides a review of some important Paleoproterozoic phosphate deposits and of models proposed for their origin. A new model is then presented for the spatial and temporal modes of occurrence of phosphorites along with possible connections to global changes at both ends of the Proterozoic. Central to the new model is that periods of atmospheric oxygenation may have been caused by globally elevated rates of primary productivity stimulated by high fluxes of phosphorus delivery to seawater as a result of increased chemical weathering of continental crust over geological timescales. The striking similarities in biogeochemical evolution between the Paleo- and Neoproterozoic are discussed in light of the two oldest major phosphogenic events and their possible relation to the stepwise rise of atmospheric oxygen that ultimately resulted in significant leaps in biological evolution.

  10. PT conditions and trace element variations of picroilmenites and pyropes from placers and kimberlites in the Arkhangelsk region, NW Russia

    NASA Astrophysics Data System (ADS)

    Afanasiev, V. P.; Ashchepkov, I. V.; Verzhak, V. V.; O'Brien, H.; Palessky, S. V.

    2013-07-01

    Compositions of picroilmenite and pyrope concentrates from Carboniferous sandstones in the Arkhangelsk kimberlite province were analyzed by EPMA and LAM ICP MS in Analytic Center of V.S. Sobolev's Institute of Geology and Mineralogy, SD RAS, Novosibirsk. The results from single grain thermobarometry (Ashchepkov et al., 2010, 2011, 2012) for garnet, spinel, ilmenite and clinopyroxene suggest heating of the base of the lithospheric mantle to 1400 °C (45 mw/m2) at 7.0-7.5 GPa and to 900 °C (35 mw/m2) at 3.5-5.5 GPa in an interval corresponding to a lens enriched in chromite and clinopyroxene. The pipes from the eastern fields reveal smoother mantle geotherms and lower temperature PT paths. Mantle columns beneath the kimberlites from northern (Verkhotinskoe field) and western pipes (Kepinskoe field) show heating from the lithosphere base to 5.0 GPa and stepped PT paths shown by chromites probably due to interaction with magmas which caused local Ti-enrichment near 3.0 and 5.5 GPa. The PT paths in the mantle columns beneath the alnöite pipes reveal higher temperature and relatively shallow PT conditions with two major clusters around 3.0 and 5.0 GPa. Trace element patterns for garnets vary from S-type typical of harzburgites to those with a hump in MREE (middle REE) typical for pyroxenites. Lherzolitic garnets with sinusoidal decrease of LREE show distinctive HFSE enrichment. Trace element ratios (Sm/Er)n and (La/Yb)n of garnets correlate positively with pressures estimates by single grain thermobarometry (Ashchepkov et al., 2010, 2011, 2012) but only poorly with Cr2O3 content. Enrichment in HFSE of all garnets is related to metasomatism that accompanied the picroilmenite-forming event. Ilmenites reveal two compositional trends. One corresponds to fractionation within conduits at the lower mantle (6.0-7.0 GPa) without contamination. A second trend at <6.0 GPa, formed due to assimilation fractional crystallization (AFC), is characterized by Fe and Cr increase with

  11. Garnet lherzolite xenoliths in the kimberlites of northern Lesotho: revised P-T equilibration conditions and upper mantle Palaeogeotherm

    NASA Astrophysics Data System (ADS)

    Carswell, D. A.; Gibb, F. G. F.

    1987-12-01

    Evidence is presented that the inflected palaeogeotherm for northern Lesotho, previously highlighted by Boyd (1973), Boyd and Nixon (1973, 1975), Finnerty and Boyd (1984, 1987), is essentially an artifact of the unsatisfactory, over-simplified barometer formulation (based on MacGregor 1974) employed. The absence of an inflection in the palaeogeotherm for Udachnaya, Siberia based on P-T estimates for garnet lherzolite xenoliths calculated with the same barometer, does not prove the reality of an inflected palaeogeotherm for northern Lesotho. Rather, it reflects, at least in part, chemical differences between the equivalent deformed, high- T xenoliths in these two areas — most importantly expressed in the respective contents of Jadeite relative to ureyite in the constituent orthopyroxenes. Accurate estimation of P-T equilibration conditions for garnet lherzolite xenoliths requires both complete and precise mineral analyses and adequate consideration of the influence of minor elements, such as Cr and Na, on the element exchange reaction thermometers and barometers employed. The barometer formulation of Nickel and Green (1985) is judged to be the best currently available. As no single thermometer is entirely satisfactory and dependable throughout the P-T range of interest, equilibration temperatures are currently best assessed as a mean value obtained from application of the most accurate formulations for both the two-pyroxene solvus thermometer (Bertrand and Mercier 1985) and Fe2+-Mg2+ exchange reactions between garnet-clinopyroxene (Powell 1985), garnet-orthopyroxene (Harley 1984a) and garnet-olivine (O'Neill and Wood 1979) mineral pairs. Such ‘best’ P-T estimates for xenoliths in the kimberlites of northern Lesotho indicate a somewhat elevated, non-inflected, upper mantle palaeogeotherm, compatible with a 120 145 km thick thermally conductive lithosphere above a convecting asthenosphere. The common coarse textured, chemically depleted, garnet lherzolite

  12. 40Ar/39Ar evidence for Middle Proterozoic (1300-1500 Ma) slow cooling of the southern Black Hills, South Dakota, midcontinent, North America: Implications for Early Proterozoic P-T evolution and posttectonic magmatism

    NASA Astrophysics Data System (ADS)

    Holm, Daniel K.; Dahl, Peter S.; Lux, Daniel R.

    1997-08-01

    40Ar/39Ar total gas and plateau dates from moscovite and biotite in the southern Black Hills, South Dakota, provide evidence for a period of Middle Proterozoic slow cooling. Early Proterozoic (1600-1650 Ma) mica dates were obtained from metasedimentary rocks located in a synformal structure between the Harney Peak and Bear Mountain domes and also south of Bear Mountain. Metamorphic rocks from the dome areas and undeformed samples of the ˜1710 Ma Harney Peak Granite (HPG) yield Middle Proterozoic mica dates (˜1270-1500 Ma). Two samples collected between the synform and Bear Mountain dome yield intermediate total gas mica dates of ˜1550 Ma. We suggest two end-member interpretations to explain the map pattern of cooling ages: (1) subhorizontal slow cooling of an area which exhibits variation in mica Ar retention intervals or (2) mild folding of a Middle Proterozoic (˜1500 Ma) ˜300°C isotherm. According to the second interpretation, the preservation of older dates between the domes may reflect reactivation of a preexisting synformal structure (and downwarping of relatively cold rocks) during a period of approximately east-west contraction and slow uplift during the Middle Proterozoic. The mica data, together with hornblende data from the Black Hills published elsewhere, indicate that the ambient country-rock temperature at the 3-4 kbar depth of emplacement of the HPG was between 350°C and 500°C, suggesting that the average upper crustal geothermal gradient was 25°-40°C/km prior to intrusion. The thermochronologic data suggest HPG emplacement was followed by a ˜200 m.y. period of stability and tectonic quiescence with little uplift. We propose that crust thickened during the Early Proterozoic was uplifted and erosionally(?) thinned prior to ˜1710 Ma and that the HPG magma was emplaced into isostatically stable crust of relatively normal thickness. We speculate that uplift and crustal thinning prior to HPG intrusion was the result of differential thinning of

  13. Diamond collecting in northern Colorado.

    USGS Publications Warehouse

    Collins, D.S.

    1982-01-01

    The discovery of numerous diamond-bearing kimberlite diatremes in the N Front Range of Colorado and Wyoming is of both scientific and economic interest. Species recovered from heavy-mineral concentrates include Cr-diopside, spinel, Mg-ilmenite, pyrope and diamond. A nodule tentatively identified as a graphite-diamond eclogite was also found. -G.W.R.

  14. Arsenic-induced phosphate limitation under experimental Early Proterozoic oceanic conditions

    NASA Astrophysics Data System (ADS)

    Chi Fru, Ernest; Hemmingsson, Christoffer; Holm, Mikaela; Chiu, Beverly; Iñiguez, Enrique

    2016-01-01

    Comparison of phosphorus concentrations associated with modern hydrothermal Fe(III)(oxyhydr)oxides and ancient Fe(III) oxide-rich iron formations, is used to estimate bioavailable Precambrian marine phosphorus (P) concentrations. This led to the proposition of a low dissolved P budget of ˜10-25% of present-day levels, before ˜1.9 billion years ago. Estimates incorporating ancient marine Si levels ≥ 0.67 mM instead suggested global dissolved P levels greater than today. Here we unite current experimental models that have considered NaCl solutions containing elevated dissolved Fe(II), Si, Ca2+ and Mg2+ ions in the incorporation of P in Precambrian marine Fe(III)(oxyhydr)oxides, in addition to arsenic as a hydrothermal proxy. We show that the coprecipitation of dissolved P and Fe(III)(oxyhydr)oxides from arsenic-rich marine waters produces an average P distribution coefficient of ˜0.072 (± 0.01) μM-1. This is comparable to the ˜ 0.07 μM-1 predicted for Fe(III)(oxyhydr)oxides in modern arsenic-rich, submarine hydrothermal settings, from which the lower Early Proterozoic dissolved marine P concentrations were predicted. As/P molar ratios below modern seawater ratios removed the negative feedback effect high Si impose on P scavenging by Fe(III)(oxyhydr)oxides. The binding of As(III) to Fe(III)(oxyhydr)oxides exhibits a lower competitive influence on P fixation. As(V) that likely became prominent in the surficially oxidized Early Proterozoic oceans induced dissolved P limitation because of preferential P sequestration at the expense of dissolved As(V) enrichment. The control of As on P scavenging by the precipitating Fe(III)(oxyhydr)oxides is strong regardless of common seawater cations (Mg2+ and Ca2+). The data suggest that the application of Si and Fe(III)(oxyhydr)oxides as an ancient seawater P proxy should consider chemical variability between depositional basins, taking into account the rather strong role hydrothermal arsenic has on the distribution of P in

  15. Textural relationship and compositions of ilmenite-corundum exsolutions in rutile from kimberlitic kyanite eclogite xenoliths: microstructural evidence using EBSD

    NASA Astrophysics Data System (ADS)

    Sobolev, N. V.; Schertl, H.; Neuser, R. D.; Lavrentiev, Y. G.; Logvinova, A. M.; Usova, L. V.

    2007-12-01

    Rutile is one of the most common accessory minerals in high pressure (HP) and ultrahigh-pressure (UHP) rocks of crustal and mantle origin. Among those rocks eclogites are very abundant in metamorphic belts and as xenoliths in kimberlite pipes. Some of these xenoliths contain coesite or diamond. Eclogite xenolith rutiles contain high abundance of minor elements reaching up to 0.8 wt.% Al2O3, 2.5 wt.% Fe2O3, 1.4 wt.% Nb2O5, 0.45 wt.% ZrO2 and are mostly heterogeneous with widely varying Al, Fe and Mg contents. These heterogeneities are caused by the presence of closely associated sigmoidal oriented lamellae of ilmenite and corundum which have been detected for the first time in rutile from Roberts Victor mine eclogite xenolith (Sobolev, Yefimova, 2000, Intern. Geol. Rev., v. 42, p. 758-767). We report here on the wide occurrence of such lamellae in rutiles both from diamondiferous kyanite eclogites of Udachnaya mine, Siberia and more samples from Roberts Victor mine confirmed by EMPA. Electron backscatter diffraction (EBSD) measurements on two rutile grains from Roberts Victor mine kyanite eclogite confirm the presence of ilmenite plus corundum exsolutions. We found evidence for respective different crystallographic orientations related to the surrounded rutile host which will be demonstrated in detail in the present study.

  16. Growth, stabilization, and reactivation of Proterozoic lithosphere in the southwestern United States

    SciTech Connect

    Bowring, S.A. ); Karlstrom, K.E. )

    1990-12-01

    Growth of Proterozoic continental lithosphere in the southwestern United States involved assembly of tectonostratigraphic terranes during several pulses of convergent tectonism ca. 1.74, 1.70, and 1.65-1.60 Ga. Prograde metamorphism accompanied orogenic assembly, and peak metamorphic conditions outlasted deformation. Regions now characterized by the highest metamorphic grades underwent slow isobaric cooling and were not uplifted until more than 200 m.y. after assembly. Regions of low metamorphic grade were not uplifted substantially after assembly. The authors suggest that (1) relatively thin lithospheric fragments were assembled into isostatically stable, normal thickness continental lithosphere; (2) assembly did not erase lithospheric-scale heterogeneities; (3) the present juxtaposition of different crustal levels reflects differential uplift related to 1.4-1.1 Ga tectonomagmatic activity; and (4) the boundaries between different lithospheric blocks were repeatedly reactivated from Precambrian through Tertiary time.

  17. Geochemistry and geotectonic setting of Late Proterozoic Katangan basic rocks from Kibambale in central Shaba (Zaire)

    NASA Astrophysics Data System (ADS)

    Kampunzu, A. B.; Kanika, M.; Kapenda, D.; Tshimanga, K.

    1993-12-01

    The Late Proterozoic Katangan basic rocks (dolerites and basalts) outcropping at Kibambale in central Shaba (Zaire) are classified as extension-related tholeiites. The concentrations of immobile major and trace elements resemble those encountered in Phanerozoic enriched oceanic ridge basalts (E-MORB) emplaced in incipient oceanic rifts such as Red Sea and Tadjura Gulf troughs. This conclusion is also sustained by: elemental ratios which are MORB-like; multielement chondrite- and MORB-normalized patterns; sedimentological data, especially the shallow water conditions which prevailed during the deposition of lower Kundelungu tillite in which these basic rocks are interstratified; and the geological and structural framework, notably the emplacement of these igneous rocks in a basin not distant from the Kibaran continental land mass. In this area the geological units containing the igneous rocks are not affected by significant tangential tectonism and so the present structural relationships are relatively undisturbed compared with those which prevailed during the emplacement of the igneous rocks.

  18. Isotopic disequilibrium and lower crustal contamination in slowly ascending magmas: Insights from Proterozoic anorthosites

    NASA Astrophysics Data System (ADS)

    Bybee, G. M.; Ashwal, L. D.

    2015-10-01

    Many Proterozoic anorthosite massifs show crustal isotopic signatures that have, for decades, fuelled debate regarding the source of these temporally-restricted magmas. Are these signatures indicative of lower crustal melting or of significant assimilation of crustal material into mantle-derived magmas? Traditional whole rock isotopic tracers (Sr, Nd, Pb and Os), like other geochemical, petrological and experimental tools, have failed to identify unambiguously the origins of the crust-like signature and resolve the source controversies for these feldspathic, cumulate intrusives. We make use of high precision Sr, Nd and Pb isotopic compositions of mineral phases (plag, opx, mag) and comagmatic, high-pressure orthopyroxene megacrysts as well as whole rock anorthosites/leuconorites from the Mealy Mountains Intrusive Suite (MMIS) and the Nain Plutonic Suite (NPS) to probe the origin of the crustal isotopic signatures and assess the importance of differentiation at lower crustal depths. This selection of samples represents fragments from various stages of the polybaric ascent of the magmas, while the study of the Mealy Mountains Intrusive Suite and the Nain Plutonic Suite is instructive as each is intruded into crust of significantly different age and isotopic composition. We observe marked differences in the whole-rock isotopic composition of Proterozoic anorthosites and high-pressure megacrysts (e.g. εNd;T = +2 to -10) intruded into crustal terranes of different ages and isotopic compositions. Evidence for varying degrees of internal isotopic disequilibrium (ΔNd, ΔSr, ΔPb) in anorthosites from these different terranes reinforces the notion that crustal contamination, and more importantly, the nature of the crustal assimilant, has a profound influence on the chemical signature of Proterozoic anorthosites. While most samples from the MMIS and NPS show significant and measurable ΔNd and ΔPb disequilibrium, ΔSr compositions cluster around zero. This decoupling in

  19. Fractal branching organizations of Ediacaran rangeomorph fronds reveal a lost Proterozoic body plan.

    PubMed

    Hoyal Cuthill, Jennifer F; Conway Morris, Simon

    2014-09-01

    The branching morphology of Ediacaran rangeomorph fronds has no exact counterpart in other complex macroorganisms. As such, these fossils pose major questions as to growth patterns, functional morphology, modes of feeding, and adaptive optimality. Here, using parametric Lindenmayer systems, a formal model of rangeomorph morphologies reveals a fractal body plan characterized by self-similar, axial, apical, alternate branching. Consequent morphological reconstruction for 11 taxa demonstrates an adaptive radiation based on 3D space-filling strategies. The fractal body plan of rangeomorphs is shown to maximize surface area, consistent with diffusive nutrient uptake from the water column (osmotrophy). The enigmas of rangeomorph morphology, evolution, and extinction are resolved by the realization that they were adaptively optimized for unique ecological and geochemical conditions in the late Proterozoic. Changes in ocean conditions associated with the Cambrian explosion sealed their fate. PMID:25114255

  20. Refined Proterozoic evolution of the Gawler Craton, South Australia, through U-Pb zircon geochronology

    USGS Publications Warehouse

    Fanning, C.M.; Flint, R.B.; Parker, A.J.; Ludwig, K. R.; Blissett, A.H.

    1988-01-01

    Through the application of both conventional U-Pb zircon analyses and small-sample U-Pb isotopic analyses, the nature and timing of tectonic events leading to the formation of the Gawler Craton have been defined more precisely. Constraints on deposition of Early Proterozoic iron formation-bearing sediments have been narrowed down to the period 1960-1847 Ma. Deformed acid volcanics, including the economically important Moonta Porphyry, have zircon ages of ??? 1790 and 1740 Ma. The voluminous acid Gawler Range Volcanics and correlatives to the east were erupted over a short interval at 1592 ?? 2 Ma, and were intruded by anorogenic granites at ??? 1575 Ma. Small-sample zircon analyses proved to be an extremely valuable adjunct to conventional analyses, generally yielding more-concordant data which forced a curved discordia through an upper intercept slightly younger than from a conventional straight-line discordia. ?? 1988.

  1. SINOPROBE deep reflection profile reveals a Neo-Proterozoic subduction zone beneath Sichuan Basin

    NASA Astrophysics Data System (ADS)

    Gao, Rui; Chen, Chen; Wang, Haiyan; Lu, Zhanwu; Brown, Larry; Dong, Shuwen; Feng, Shaoying; Li, Qiusheng; Li, Wenhui; Wen, Zhongping; Li, Feng

    2016-11-01

    A new multichannel seismic reflection profile collected across the Sichuan Basin in southern China by SINOPROBE images prominent reflectors that originate within the lower crust and penetrate well into the underlying mantle. The geometry of these mantle reflectors is very similar to those observed on other deep reflection profiles that have been interpreted as relicts of ancient subduction. Considering the geological history of the basement beneath and surrounding Sichuan Basin and ages of granites encountered in nearby wells, we propose that these newly revealed reflectors are the remnants of Neo-Proterozoic subduction that occurred along the NW margin of the Yangtze Craton. This interpretation is consistent with geochemical studies from a gneissic complex at the west margin of the Sichuan Basin. Moreover, preservation of these reflectors supports the idea that the Sichuan lithosphere served as a consolidated tectonic buttress against which the Tibetan Plateau has impinged to produce the Longmenshan orogenic belt.

  2. Atmospheric, Climatic, and Biological Evolution at Both Ends of the Proterozoic Eon

    NASA Astrophysics Data System (ADS)

    Kaufman, A. J.; Farquhar, J.; Wing, B.; Corsetti, F. A.; Bekker, A.

    2006-05-01

    Both ends of the Proterozoic Eon (2.5 to 0.542 Ga) are characterized by inter-related environmental, climatic, and biological events that are likely associated with fluctuations in atmospheric concentrations of oxygen, carbon dioxide, and methane. In each interval there is abundant evidence for widespread glaciation and the alternation of redox sensitive lithologies. Isotopic studies across the Proterozoic Eon reveal the effects of major orogenic and hydrothermal events on ocean chemistry, as well as profound reorganizations of the carbon and sulfur cycles. Detailed stratigraphic studies indicate a close association of the biogeochemical and climatic events with the rise and fall of atmospheric oxygen. In the terminal Proterozoic, carbonates within the Johnnie Formation of Death Valley, CA record a remarkable negative carbon isotope anomaly that accompanies a dramatic rise in trace sulfate abundance and fall of carbonate associated sulfate δ34S values. The 13C-rich alkalinity delivered to the seawater pool was likely associated with a dramatic rise in atmospheric O2, which resulted in 1) the oxidation of exposed continental shelf sediments rich in fossil organic matter and sulfides, and 2) the ventilation of the oceans. The carbon isotope anomaly is recorded in broadly equivalent successions that post-date known Marinoan glacial deposits and pre-date the Precambrian-Cambrian boundary in Oman, India, China, Australia, and Namibia, supporting the view that the oxygenation event was global. Large metazoan fossils (Ediacaran animals) first appear directly above this anomaly, suggesting that a critical threshold with respect to atmospheric O2 had been crossed at this time. In the early Proterozoic, the rise of atmospheric oxygen appears to coincide with Earth's earliest glacial record, and is linked to strong variations in carbon and sulfur isotope compositions of seawater proxies. The glacial interval is notable for the occurrence of widespread diamictite in at least

  3. River Valley pluton, Ontario - A late-Archean/early-Proterozoic anorthositic intrusion in the Grenville Province

    NASA Technical Reports Server (NTRS)

    Ashwal, Lewis D.; Wooden, Joseph L.

    1989-01-01

    This paper presents Nd, Sr, and Pb isotopic data indicating a late-Archean/early-Proterozoic age for the River Valley anorthositic pluton of the southwestern Grenville Province of Sudbury, Ontario. Pb-Pb isotopic data on 10 whole-rock samples ranging in composition from anorthosite to gabbro yield an age of 2560 + or - 155 Ma. The River Valley pluton is thus the oldest anorthositic intrusive yet recognized within the Grenville Province. The Sm-Nd isotopic system records an age of 2377 + or - 68 Ma. High Pb-208/Pb-204 of deformed samples relative to igneous-textured rocks implies Th introduction and/or U loss during metamorphism in the River Valley area. Rb-Sr data from igneous-textured and deformed samples and from mineral separates give an age of 2185 + or - 105 Ma, indicating substantial disturbance of the Rb-Sr isotopic system.

  4. Late proterozoic and paleozoic tides, retreat of the moon, and rotation of the earth

    USGS Publications Warehouse

    Sonett, C.P.; Kvale, E.P.; Zakharian, A.; Chan, M.A.; Demko, T.M.

    1996-01-01

    The tidal rhythmites in the Proterozoic Big Cottonwood Formation (Utah, United States), the Neoproterozoic Elatina Formation of the Flinders Range (southern Australia), and the Lower Pennsylvanian Pottsville Formation (Alabama, United States) and Mansfield Formation (Indiana, United States) indicate that the rate of retreat of the lunar orbit is d??/dt k2 sin(2??) (where ?? is the Earth-moon radius vector, k2 is the tidal Love number, and ?? is the tidal lag angle) and that this rate has been approximately constant since the late Precambrian. When the contribution to tidal friction from the sun is taken into account, these data imply that the length of the terrestrial day 900 million years ago was -18 hours.

  5. Fractal branching organizations of Ediacaran rangeomorph fronds reveal a lost Proterozoic body plan

    PubMed Central

    Hoyal Cuthill, Jennifer F.; Conway Morris, Simon

    2014-01-01

    The branching morphology of Ediacaran rangeomorph fronds has no exact counterpart in other complex macroorganisms. As such, these fossils pose major questions as to growth patterns, functional morphology, modes of feeding, and adaptive optimality. Here, using parametric Lindenmayer systems, a formal model of rangeomorph morphologies reveals a fractal body plan characterized by self-similar, axial, apical, alternate branching. Consequent morphological reconstruction for 11 taxa demonstrates an adaptive radiation based on 3D space-filling strategies. The fractal body plan of rangeomorphs is shown to maximize surface area, consistent with diffusive nutrient uptake from the water column (osmotrophy). The enigmas of rangeomorph morphology, evolution, and extinction are resolved by the realization that they were adaptively optimized for unique ecological and geochemical conditions in the late Proterozoic. Changes in ocean conditions associated with the Cambrian explosion sealed their fate. PMID:25114255

  6. A Late Proterozoic Early Paleozoic magmatic cycle in Sierra de la Ventana, Argentina

    NASA Astrophysics Data System (ADS)

    Gregori, D. A.; López, V. L.; Grecco, L. E.

    2005-06-01

    Late Proterozoic-Early Paleozoic intrusive and volcanic rocks of Sierra de la Ventana can be grouped into two magmatic assemblages: the Meyer and Cochenleufú suites. The older (700-570 Ma) is composed of S-type quartz-monzodiorites, synogranites, and monzogranites associated with andesites and rhyolites and related to volcanic-arc and postcollisional settings. The younger (540-470 Ma) corresponds to highly fractionated homogeneous A-type monzogranites, linked to final plutonic events during postorogenic extension in collisional belts. Strong similarities between Sierra de la Ventana magmatic rocks and the S- and A-type granites of the Cape granite suite in South Africa allow positive correlation. In both areas, primitive volcanic arcs or collisional orogens are recognized. Continuous transpressional shearing between the Swartland and Tygerberg terranes in the Saldania belt may have triggered the generation and emplacement of both suites.

  7. Tectonics of an early Proterozoic geosuture: The Halls Creek orogenic sub-province, northern Australia

    NASA Astrophysics Data System (ADS)

    Hancock, S. L.; Rutland, R. W. R.

    1984-12-01

    The exposed elements of the Lower Proterozoic orogenic belts of the Halls Creek sub-province, Northern Australia, lie in fault zones which have suffered repeated tectonic activity at various times through the Proterozoic and Phanerozoic. The Halls Creek and King Leopold orogenic domains subtend an angle of 80° and are characterized by linear late tectonic batholithic complexes several hundred kilometres long but only a few tens of kilometres wide, reminiscent of those in Phanerozoic Cordilleran orogenies. The associated superposed folding and high temperature metamorphism are more akin to those in Phanerozoic collision orogenies. The sub-province is analyzed in the wider context of the North Australian orogenic province which was deformed, metamorphosed and intruded by granitic plutons approximately 1900-1800 Ma ago. In this province the Archaen basement was extended and broken into a mosaic of blocks, some of which (now largely concealed by younger Kimberley and McArthur basin sediments) retained a more positive character and fed sediment to intervening regions (such as the Pine Creek Geosyncline) which suffered greater extension and subsidence, but which retained a thinned Archaean basement. The Halls Creek Group was deposited in a trough to the south-east of the Kimberley island continent, and deposition was probably broadly contemporaneous with, and continuous with, that in the Pine Creek geosyncline. A volcanic—fine grained clastic—carbonate phase of marine deposition, following basin formation, is represented by the Biscay Formation. During the later phase of basin evolution widespread flysch facies (Olympio Formation), partly derived from the island continent, was deposited and is now preserved in low grade zones on both sides of the main belt of high strain and upper amphibolite to lower granulite facies metamorphism which displays recumbent folding and nappe tectonics with fold axes oblique to the major faults. No island arc compex or paired

  8. Geochronological and isotopic evidence for early Proterozoic crust in the eastern Arabian Shield.

    USGS Publications Warehouse

    Stacey, J.S.; Hedge, C.E.

    1984-01-01

    Zircon U/Pb, feldspar common Pb, whole-rock Sm/Nd, and Rb/Sr data indicate that the fine-grained granodiorite (Z103) has yielded conclusive evidence for rocks of early Proterozoic age in the eastern Arabian Shield (21o19' N, 44o50' W). Z103 may have been emplaced approx 1630 m.y. ago and subsequently was severely deformed or perhaps even remobilized at approx 660 m.y. Furthermore, lead isotope data, along with other evidence, show that the 1630 m.y. crustal rocks inherited material from an older, probably Archaean, source at the time of their formation. At that time addition of mantle material considerably modified the Rb-Sr and Sm-Nd systems so that they now yield similar, or only slightly older apparent ages (1600-1800 m.y.).-L.diH.

  9. Early Proterozoic activity on Archean faults in the western Superior province - evidence from pseudotachylite

    USGS Publications Warehouse

    Peterman, Z.E.; Day, W.

    1989-01-01

    Major transcurrent faults in the Superior province developed in the Late Archean at the close of the Kenoran orogeny. Reactivation of some of these faults late in the Early Proterozoic is indicated by Rb-Sr analyses of pseudotachylite from the Rainy Lake-Seine River and Quetico faults in the Rainy Lake region of Minnesota and Ontario. Fault veins of pseudotachylite and immediately adjacent country rock at two localities yielded subparallel isochrons that are pooled for an age of 1947??23 Ma. K-Ar and Rb-Sr biotite ages register earlier regional cooling of the terrane at about 2500 Ma with no evidence of younger thermal overprinting at temperatures exceeding 300??C. Accordingly, the 1947??23 Ma age is interpreted as dating the formation of the pseudotachylite. Reactivation of existing faults at this time was caused by stresses transmitted from margins of the Superior province where compressional tectonic events were occurring. -Authors

  10. The global record of local iron geochemical data from Proterozoic through Paleozoic basins

    NASA Astrophysics Data System (ADS)

    Sperling, E. A.; Wolock, C.; Johnston, D. T.; Knoll, A. H.

    2013-12-01

    Iron-based redox proxies represent one of the most mature tools available to sedimentary geochemists. These techniques, which benefit from decades of refinement, are based on the fact that rocks deposited under anoxic conditions tend to be enriched in highly-reactive iron. However, there are myriad local controls on the development of anoxia, and no local section is an exemplar for the global ocean. The global signal must thus be determined using techniques like those developed to solve an analogous problem in paleobiology: the inference of global diversity patterns through time from faunas seen in local stratigraphic sections. Here we analyze a dataset of over 4000 iron speciation measurements (including over 600 de novo analyses) to better understand redox changes from the Proterozoic through the Paleozoic Era. Preliminary database analyses yield interesting observations. We find that although anoxic water columns in the middle Proterozoic were dominantly ferruginous, there was a statistical tendency towards euxinia not seen in early Neoproterozoic or Ediacaran data. Also, we find that in the Neoproterozoic oceans, oxic depositional environments-the likely home for early animals-have exceptionally low pyrite contents, and by inference low levels of porewater sulfide. This runs contrary to notions of sulfide stress on early metazoans. Finally, the current database of iron speciation data does not support an Ediacaran or Cambrian oxygenation event. This conclusion is of course only as sharp as the ability of the Fe-proxy database to track dissolved oxygen and does not rule out the possibility of a small-magnitude change in oxygen. It does suggest, however, that if changing pO2 facilitated animal diversification it did so by a limited rise past critical ecological thresholds, such as seen in the modern Oxygen Minimum Zones benthos. Oxygen increase to modern levels thus becomes a Paleozoic problem, and one in need of better sampling if a database approach is to be

  11. Chromium Isotopes in Carbonate Rocks: New Insights into Proterozoic Atmospheric Oxygenation

    NASA Astrophysics Data System (ADS)

    Kah, L. C.; Gilleaudeau, G. J.; Frei, R.; Kaufman, A. J.; Azmy, K.; Bartley, J. K.; Chernyavskiy, P.; Knoll, A. H.

    2015-12-01

    There has been a long-standing debate in geobiology about the role that Earth's oxygenation played in the evolution of complex life. Temporal linkages exist between the Great Oxidation Event (GOE) and the evolution of eukaryotes, as well as Neoproterozoic rise in oxygen and the diversification of metazoans. Further advances have been hampered, however, by the lack of direct proxies that mark specific levels of atmospheric pO2 in the geologic past. Chromium (Cr) isotopes show promise in this regard because the oxidation of Cr during terrestrial weathering—which results in isotopic fractionation—is dependent on a specific threshold of atmospheric pO2 (0.1-1% of the present atmospheric level [PAL]). This threshold value broadly coincides with recent estimates of the oxygen requirements of early animals. Here we report new Cr-isotope data from four late Mesoproterozoic carbonate-dominated successions. Samples were collected from the Turukhansk Uplift (Siberia), the El Mreiti Group (Mauritania), the Vazante Group (Brazil), and the Angmaat Formation (Canada). We emphasize the application of Cr-isotopes to carbonate rocks because the broad temporal range of this lithology in the geologic record provides an opportunity to significantly expand our understanding of Proterozoic oxygenation on shorter time scales. Our data indicate that pO2 levels required to support early animals were attained long before Neoproterozoic metazoan diversification, although the large degree of isotopic heterogeneity in our dataset may indicate that pO2 > 0.1-1% PAL was only a transient phenomenon in the Mesoproterozoic. This study demonstrates the utility of Cr-isotopes as an atmospheric redox proxy in carbonate rocks and helps inform future avenues of research on Proterozoic pO2 thresholds.

  12. Proterozoic metamorphism and uplift history of the north-central Laramie Mountains, Wyoming, USA

    USGS Publications Warehouse

    Patel, S.C.; Frost, B.R.; Chamberlain, K.R.; Snyder, G.L.

    1999-01-01

    The Laramie Mountains of south-eastern Wyoming contain two metamorphic domains that are separated by the 1.76 Ga. Laramie Peak shear zone (LPSZ). South of the LPSZ lies the Palmer Canyon block, where apatite U-Pb ages are c. 1745 Ma and the rocks have undergone Proterozoic kyanite-grade Barrovian metamorphism. In contrast, in the Laramie Peak block, north of the shear zone, the U-Pb apatite ages are 2.4-2.1 Ga, the granitic rocks are unmetamorphosed and supracrustal rocks record only low-T amphibolite facies metamorphism that is Archean in age. Peak mineral assemblages in the Palmer Canyon block include (a) quartz-biotite-plagioclase-garnet-staurolite-kyanite in the pelitic schists; (b) quartz-biotite-plagioclase-low-Ca amphiboles-kyanite in Mg-Al-rich schists, and locally (c) hornblende-plagioclase-garnet in amphibolites. All rock types show abundant textural evidence of decompression and retrograde re-equilibration. Notable among the texturally late minerals are cordierite and sapphirine, which occur in coronas around kyanite in Mg-Al-rich schists. Thermobarometry from texturally early and late assemblages for samples from different areas within the Palmer Canyon block define decompression from > 7 kbar to < 3 kbar. The high-pressure regional metamorphism is interpreted to be a response to thrusting associated with the Medicine Bow orogeny at c. 1.78-1.76 Ga. At this time, the north-central Laramie Range was tectonically thickened by as much as 12 km. This crustal thickening extended for more than 60 km north of the Cheyenne belt in southern Wyoming. Late in the orogenic cycle, rocks of the Palmer Canyon block were uplifted and unroofed as the result of transpression along the Laramie Peak shear zone to produce the widespread decompression textures. The Proterozoic tectonic history of the central Laramie Range is similar to exhumation that accompanied late-orogenic oblique convergence in many Phanerozoic orogenic belts.

  13. Evidence for a lower crustal origin of high-Al orthopyroxene megacrysts in Proterozoic anorthosites

    SciTech Connect

    Wiebe, R.A.

    1985-01-01

    Nodules and xenocrysts dominated by high-Al orthopyroxene (HAO) occur in strongly chilled Proterozoic basaltic dikes which cut the Nain anorthosite complex, Labrador. HAO (En 73-68, Al/sub 2/O/sub 3/ = 6.5-4.5) lacks exsolution; it occurs both as anhedral xenocrysts up to 10 cm in diameter and with euhedral plagioclase (An55) in ophitic nodules. Rarely, olivine occurs with HAO and Al-spinel with plagioclase. Scarce Fe-rich nodules contain: (1) opx + pig, (2) aug + pig, and (3) coarsely exsolved ulvospinel. Pyroxene pairs yield T's of 1250 to 1170/degree/C, whereas coexisting lamellae in exsolved ulvospinel yield T's between 1145 and 1120/degree/C, with fO/sub 2/ near the WM buffer. If all nodules came from a similar depth, the rare occurrence of olivine with plagioclase suggests a maximum pressure of about 11 kb. The high subsolidus T's of the nodules contrasts with the low T of the host anorthosites at the time of dike emplacement and hence indicates a deep source for the nodules. HAO is nearly identical in composition to the high-Al orthopyroxene megacrysts with exsolved plagioclase (HAOM) found in most Proterozoic anorthosites. Many nodules of plagioclase and HAO also have textures comparable to ophitic occurrences of HAOM in anorthosite. Rafting of cotectic nodules from the lower crust could explain occurrences of HAOM in shallow-level anorthosites. The nodules and xenocrysts are samples of lower crustal cumulates. Their compositions suggest that they were produced by magmas similar to those that were parental to the anorthosites. They lend support to models which derive anorthosites by fractional crystallization of basaltic magma.

  14. Stable isotope, chemical, and mineral compositions of the Middle Proterozoic Lijiaying Mn deposit, Shaanxi Province, China

    USGS Publications Warehouse

    Yeh, Hsueh-Wen; Hein, James R.; Ye, Jie; Fan, Delian

    1999-01-01

    The Lijiaying Mn deposit, located about 250 km southwest of Xian, is a high-quality ore characterized by low P and Fe contents and a mean Mn content of about 23%. The ore deposit occurs in shallow-water marine sedimentary rocks of probable Middle Proterozoic age. Carbonate minerals in the ore deposit include kutnahorite, calcite, Mn calcite, and Mg calcite. Carbon (−0.4 to −4.0‰) and oxygen (−3.7 to −12.9‰) isotopes show that, with a few exceptions, those carbonate minerals are not pristine low-temperature marine precipitates. All samples are depleted in rare earth elements (REEs) relative to shale and have negative Eu and positive Ce anomalies on chondrite-normalized plots. The Fe/Mn ratios of representative ore samples range from about 0.034 to <0.008 and P/Mn from 0.0023 to <0.001. Based on mineralogical data, the low ends of those ranges of ratios are probably close to ratios for the pure Mn minerals. Manganese contents have a strong positive correlation with Ce anomaly values and a moderate correlation with total REE contents. Compositional data indicate that kutnahorite is a metamorphic mineral and that most calcites formed as low-temperature marine carbonates that were subsequently metamorphosed. The braunite ore precursor mineral was probably a Mn oxyhydroxide, similar to those that formed on the deep ocean-floor during the Cenozoic. Because the Lijiaying precursor mineral formed in a shallow-water marine environment, the atmospheric oxygen content during the Middle Proterozoic may have been lower than it has been during the Cenozoic.

  15. Empirical Records of Environmental Change across the Archean-Proterozoic Transition

    NASA Astrophysics Data System (ADS)

    Kaufman, A. J.

    2011-12-01

    Time-series geochemical analyses of scientific drill cores intersecting the Archean-Proterozoic transition suggest a coupling of environmental and biological change that culminated in the pervasive oxygenation of Earth's atmosphere and oceans. Elemental and multiple isotope measurements of sedimentary archives, including carbonate, shale, and banded iron-formation from Western Australia, South Africa, Brazil, and southern Canada, indicate important changes in the carbon, sulfur, and nitrogen cycles that monitor the redox state of the oceans and the cyanobacterial buildup of atmospheric oxygen and ozone. In response, continental weathering would have increased, resulting in the enhanced delivery of sulfate and nutrients to seawater, further stimulating photoautotrophic fluxes of oxygen to surface environments. The positive feedback may additionally be responsible for the decline of atmospheric methane and surface refrigeration, represented by a series of discrete ice ages beginning around 2.4 billion years ago, due to the loss of greenhouse capacity during a time of lower solar luminosity. While speculative, the linkage of surface oxidation with enhanced nutrient supply and development of stratospheric sunscreen soon after the Archean-Proterozoic boundary suggests that the earliest perturbation in the carbon cycle may be associated with the rapid expansion of single-celled eukaryotes. Both sterol synthesis in eukaryotes and aerobic respiration require significant levels of oxygen in the ambient environment. Hence, Earth's earliest ice age(s) and onset of a modern and far more energetic carbon cycle may have been directly related to the global expansion of cyanobacteria that released oxygen to the environment, and of eukaryotes that respired it.

  16. Stratigraphy of the Proterozoic Revett Formation, Coeur d'Alene District, Idaho

    USGS Publications Warehouse

    Mauk, Jeffrey L.

    2002-01-01

    The Proterozoic Revett Formation of the Belt Supergroup contains three informal members that can be identified throughout the Coeur d'Alene mining district of northern Idaho. The lower Revett Formation is dominated by quartzite, but also contains intervals of siltite. The middle Revett consists predominantly of siltite, though quartzite and argillite locally form significant intervals. The upper Revett consists of intervals of quartzite that alternate with intervals of siltite and/or thin-bedded argillite. These units show dramatic changes in thickness and sedimentary facies within the Coeur d'Alene mining district; changes that are more abrupt and extreme than seen elsewhere in the Belt basin. The regionally significant Osburn fault bisects the district, with 20 to 30 km of post-mineralization right-lateral strike-slip offset. South of this fault, the upper Revett is 640 m thick at the Bunker Hill mine in the west, 450 to 500 m thick in the centrally located Silver Belt, and over 550 m thick at the Reindeer Queen deposit to the east. North of the Osburn fault, the upper Revett is approximately 120 m thick in the vicinity of the Lucky Friday mine, but abruptly thins to 45 to 90 m to the north and northeast, in the southern end of the western Montana copper sulfide belt. The middle Revett Formation south of the Osburn fault appears to be 400 to 450 m thick. North of the Osburn Fault, the middle Revett thins to approximately 120 m in the Lucky Friday area, and to approximately 60 m at Military Gulch. The lower Revett Formation is approximately 1650 m thick south of the Osburn fault, but thins to 400 to 450 m thick to the north of the Osburn fault. Observed thickness changes support previous hypotheses that the current Osburn fault coincides with a Proterozoic synsedimentary fault that controlled sedimentation in this region.

  17. Geochemical and oxygen isotope signatures of mantle corundum megacrysts from the Mbuji-Mayi kimberlite, Democratic Republic of Congo, and the Changle alkali basalt, China

    NASA Astrophysics Data System (ADS)

    Giuliani, Gaston; Pivin, Marjorie; Fallick, Anthony E.; Ohnenstetter, Daniel; Song, Yucai; Demaiffe, Daniel

    2015-01-01

    Oxygen isotope signatures of ruby and sapphire megacrysts, combined with trace-element analysis, from the Mbuji-Mayi kimberlite, Democratic Republic of Congo, and the Changle alkali basalt, China, provide clues to specify their origin in the deep Earth. At Mbuji-Mayi, pink sapphires have δ18O values in the range 4.3 to 5.4‰ (N = 10) with a mean of 4.9 ± 0.4‰, and rubies from 5.5 to 5.6‰ (N = 3). The Ga/Mg ratio of pink sapphires is between 1.9 and 3.9, and in rubies, between 0.6 and 2.6. The blue or yellow sapphires from Changle have δ18O values from 4.6 to 5.2 ‰, with a mean of 4.9 ± 0.2‰ (N = 9). The Ga/Mg ratio is between 5.7 and 11.3. The homogenous isotopic composition of ruby suggests a derivation from upper mantle xenoliths (garnet lherzolite, pyroxenite) or metagabbros and/or lower crustal garnet clinopyroxenite eclogite-type xenoliths included in kimberlites. Data from the pink sapphires from Mbuji-Mayi suggest a mantle origin, but different probable protoliths: either subducted oceanic protolith transformed into eclogite with δ18O values buffered to the mantle value, or clinopyroxenite protoliths in peridotite. The Changle sapphires have a mantle O-isotope signature. They probably formed in syenitic magmas produced by low degree partial melting of a spinel lherzolite source. The kimberlite and the alkali basalt acted as gem conveyors from the upper mantle up to the surface.

  18. Microbial Carbon Processing in Gas-Rich Ultrabasic Wells from Groundwaters Hosted in Serpentinizing Diamondiferous Kimberlite Pipes

    NASA Astrophysics Data System (ADS)

    Schrenk, M. O.; Hamilton, S.; Esen, B.; Brisco, T.; Lacrampe Couloume, G.; Sherwood Lollar, B.

    2014-12-01

    To date, a majority of studies relating serpentinization to microbial activities have focused upon the aqueous alteration of ophiolites, or upon hydrothermal systems near mid-ocean ridges. The alteration of ultramafic intrusions in continental settings represents an additional, distinct type of environment that may sustain biogeochemical process related to serpentinization and play important roles in controlling the exchange of carbon and reducing power between the deep Earth and the surface biosphere. Serpentinizing diamondiferous kimberlite intrusions represent a geographically and contextually distinct system to explore the relationship between serpentinization, carbon, and life. A series of deep, ultrabasic wells were sampled near Kirkland Lake, Ontario, Canada in 2012 and 2013 for coupled geochemical and microbiological analyses. The wells were high pH (between 9 and 12) and contained elevated concentrations of H2 and hydrocarbon gases. However, the wells exhibited methane isotopic signatures distinct from one another, which reflect different sources and/or processing mechanisms for the resulting gas. Parallel analyses of microbial abundances and community composition provided insight into the biogeochemistry of these ecosystems. While hydrogen-driven chemolithoautotrophy appeared to be common across all of the wells, based upon the predominance of Betaproteobacteria in tag sequencing analyses of 16S rRNA genes, sequences associated with known methanogens and anaerobic methanotrophs were partitioned between the wells. These results are in stark contrast to existing data on continental serpentinites, which harbor relatively scant evidence for biological methane cycling. Putative biogeochemical roles for the subsurface microbial communities will be discussed and compared in a biogeographic sense to other known serpentinizing ecosystems.

  19. Volatile composition of microinclusions in diamonds from the Panda kimberlite, Canada: Implications for chemical and isotopic heterogeneity in the mantle

    NASA Astrophysics Data System (ADS)

    Burgess, Ray; Cartigny, Pierre; Harrison, Darrell; Hobson, Emily; Harris, Jeff

    2009-03-01

    In order to better investigate the compositions and the origins of fluids associated with diamond growth, we have carried-out combined noble gas (He and Ar), C and N isotope, K, Ca and halogen (Cl, Br, I) determinations on fragments of individual microinclusion-bearing diamonds from the Panda kimberlite, North West Territories, Canada. The fluid concentrations of halogens and noble gases in Panda diamonds are enriched by several orders of magnitude over typical upper mantle abundances. However, noble gas, C and N isotopic ratios ( 3He/ 4He = 4-6 Ra, 40Ar/ 36Ar = 20,000-30,000, δ 13C = -4.5‰ to -6.9‰ and δ 15N = -1.2‰ to -8.8‰) are within the worldwide range determined for fibrous diamonds and similar to the mid ocean ridge basalt (MORB) source value. The high 36Ar content of the diamonds (>1 × 10 -9 cm 3/g) is at least an order of magnitude higher than any previously reported mantle sample and enables the 36Ar content of the subcontinental lithospheric mantle to be estimated at ˜0.6 × 10 -12 cm 3/g, again similar to estimates for the MORB source. Three fluid types distinguished on the basis of Ca-K-Cl compositions are consistent with carbonatitic, silicic and saline end-members identified in previous studies of diamonds from worldwide sources. These fluid end-members also have distinct halogen ratios (Br/Cl and I/Cl). The role of subducted seawater-derived halogens, originally invoked to explain some of the halogen ratio variations in diamonds, is not considered an essential component in the formation of the fluids. In contrast, it is considered that large halogen fractionation of a primitive mantle ratio occurs during fluid-melt partitioning in forming silicic fluids, and during separation of an immiscible saline fluid.

  20. The molecular structure of melts along the carbonatite-kimberlite-basalt compositional joint: CO2 and polymerisation

    NASA Astrophysics Data System (ADS)

    Moussallam, Yves; Florian, Pierre; Corradini, Dario; Morizet, Yann; Sator, Nicolas; Vuilleumier, Rodolphe; Guillot, Bertrand; Iacono-Marziano, Giada; Schmidt, Burkhard C.; Gaillard, Fabrice

    2016-01-01

    Transitional melts, intermediate in composition between silicate and carbonate melts, form by low degree partial melting of mantle peridotite and might be the most abundant type of melt in the asthenosphere. Their role in the transport of volatile elements and in metasomatic processes at the planetary scale might be significant yet they have remained largely unstudied. Their molecular structure has remained elusive in part because these melts are difficult to quench to glass. Here we use FTIR, Raman, 13C and 29Si NMR spectroscopy together with First Principle Molecular Dynamic (FPMD) simulations to investigate the molecular structure of transitional melts and in particular to assess the effect of CO2 on their structure. We found that carbon in these glasses forms free ionic carbonate groups attracting cations away from their usual 'depolymerising' role in breaking up the covalent silicate network. Solution of CO2 in these melts strongly modifies their structure resulting in a significant polymerisation of the aluminosilicate network with a decrease in NBO/Si of about 0.2 for every 5 mol% CO2 dissolved. This polymerisation effect is expected to influence the physical and transport properties of transitional melts. An increase in viscosity is expected with increasing CO2 content, potentially leading to melt ponding at certain levels in the mantle such as at the lithosphere-asthenosphere boundary. Conversely an ascending and degassing transitional melt such as a kimberlite would become increasingly fluid during ascent hence potentially accelerate. Carbon-rich transitional melts are effectively composed of two sub-networks: a carbonate and a silicate one leading to peculiar physical and transport properties.

  1. Principal stages in evolution of precambrian organic world: Communication 2. The late proterozoic

    NASA Astrophysics Data System (ADS)

    Sergeev, V. N.; Semikhatov, M. A.; Fedonkin, M. A.; Vorob'eva, N. G.

    2010-12-01

    A new suggested model outlining the evolution of the organic world from the mid-Early Proterozoic (˜2.0 Ga) to the Early Cambrian is based on data characterizing the relevant chert-embedded and compression-preserved organic-walled microbiotas, impressions of soft-bodied multicellular organisms, and biomarkers. Critical analysis of overall paleontological data resulted in the distinguishing of seven successive assemblages of Proterozoic micro- and macrofossils. Being of global geographic range, the assemblages correspond to the major stages in evolution of the organic world and typify global units which are termed the Labradorian (˜2.0-1.65 Ga), Anabarian (1.65-1.2 Ga), Turukhanian (1.2-1.03 Ga), Uchuromayan (1.03-0.85 Ga), Yuzhnouralian (0.85-0.635 Ga), Amadeusian (0.635-0.56 Ga), and Belomorian (0.56-0.535 Ga). Characteristic of the Labradorian unit are microfossil assemblages of the Gunflint type including remains of morphologically bizarre prokaryotic microorganisms: star-like Eoastrion, umbrella-shaped Kakabekia, dumbbell-shaped Xenothrix, and some others. Fine-grained siliciclastic deposits of the same age yield the oldest remains of millimeter-sized eukaryotes: spherical to ribbon-like Chuaria and Tawuia. Microfossils prevailing in shallow-water carbonate facies of the Anabarian unit are akinetes of nostocalean cyanophyceae Archaeoellipsoides and entophysalidacean cyanobacteria Eoentophysalis, whereas acanthomorphic acritarchs Tappania and Shuiyousphaeridium dominate the assemblages of open-shelf facies, where they are associated with the first-found rare macroscopic multicellular fossils Horodyskia. The distinguishing feature of the next Turukhanian unit is the first occurrence of filamentous red alga Bangiomorpha and the stalked cyanobacterium Polybessurus. The Uchuromayan unit is characterized by the appearance and worldwide radiation of structurally complicated eukaryotic microorganisms, primarily of acanthomorphic acritarchs Trachyhystrichosphaera and

  2. Gold metallogeny and the copper-gold association of the Australian Proterozoic

    NASA Astrophysics Data System (ADS)

    Davidson, G. J.; Large, R. R.

    1994-07-01

    Australian Proterozoic gold-producing deposits, emplaced mainly at 1.55 2.00 Ga, are divided into the following categories: (1) iron oxide-dominated, brecciahosted, Cu-U±Au replacement deposits spatially associated with felsic intrusions (273t Au); (2) stratabound Au±Cu-bearing iron formations (152.4t Au); (3) unconformity-style U ±Cu/PGM/Au deposits (53t Au); (4) Iron oxide-dominated Au±Cu mineralisation hosted within elements of ductile deformation (146.7t Au); (5) Broken Hill and volcanic-hosted massive sulphides (150t Au); (6) iron-sulphide-dominated veins and replacement zones spatially related to felsic intrusions (150.7t Au), and (7) iron-sulphide-dominated veins and replacement zones spatially related to elements of regional deformation (159.9t Au). Categories (1) to (4) are mainly confined to Proterozoic rocks, constituting an association in which Au and Cu are commonly present together, with variable amounts of U, Bi, Co, W, Se, Te and REE. Most examples in categories 1 4 fall into either of two groups: Cu-Aumagnetite ±hematite types formed at relatively high temperature (300 450 °C), and Cu-U±Au-hematite types formed at 150 300 °C. We postulate that these ores formed from a common high salinity (15 35 wt. % NaCl equiv.), low total sulphur (aΣ S = 10-3 to 10-2), high fO2 fluid-type, in which metal transport was dominated by chloride-complexing. The most effective method of metal deposition was fluid mixing, achieving a synchronous decrease in fO2 and temperature. This unusual oxidised fluid association was favoured in high heat-flow extensional settings containing oxidised and/or oxidised-evaporitic sedimentary sequences. The intrusion of oxidised fractionated granites, which are commonly temporally associated with metal emplacement, acted in some places to heat and focus basinal fluids, and in others was the ultimate source of metals.

  3. Plumbotectonic aspects of polymetallic vein mineralization in Paleozoic sediments and Proterozoic basement of Moravia (Czech Republic)

    NASA Astrophysics Data System (ADS)

    Slobodník, Marek; Jacher-Śliwczyńska, Katarzyna; Taylor, Matthew C.; Schneider, Jens; Dolníček, Zdeněk

    2008-02-01

    A regional isotopic study of Pb and S in hydrothermal galenas and U-Pb and S in potential source rocks was carried out for part of Moravia, Czech Republic. Two major generations of veins, (syn-) Variscan and post-Variscan, are defined based on the Pb-isotope system together with structural constraints (local structures and regional trends). The Pb-isotopic compositions of galena plot in two distinct populations with outliers in 206Pb/204Pb-207Pb/204Pb space. Galena from veins hosted in greywackes provides a cluster with the lowest Pb-Pb ratios: 206Pb/204Pb = 18.15-18.27, 207Pb/204Pb = 15.59-15.61, 208Pb/204Pb = 38.11-38.23. Those hosted in both limestones and greywackes provide the second cluster: 206Pb/204Pb = 18.37-18.44, 207Pb/204Pb = 15.60-15.63, 208Pb/204Pb = 38.14-38.32. These clusters suggest model Pb ages as Early Carboniferous and Triassic-Jurassic, the latter associated with MVT-like deposits. Two samples from veins hosted in Proterozoic rocks lie outside the two clusters: in metagranitoid (206Pb/204Pb = 18.55, 207Pb/204Pb = 15.64, 208Pb/204Pb = 38.29) and in orthogneiss (206Pb/204Pb = 18.79, 207Pb/204Pb = 15.73, 208Pb/204Pb = 38.54). The results from these two samples suggest an interaction of mineralizing fluids with the radiogenic Pb-rich source (basement?). The values of δ34S suggest the Paleozoic host rocks (mostly -6.7 to +5.2‰ CDT) as the source of S for hydrothermal sulfides (mostly -4.8 to +2.5‰ CDT). U-Pb data and Pb isotope evolutionary curves indicate that Late Devonian and Early Carboniferous sediments, especially siliciclastics, are the general dominant contributor of Pb for galena mineralization developed in sedimentary rocks. Plumbotectonic mixing occurred, it is deduced, only between the lower and the upper crust (the latter involving Proterozoic basement containing heterogeneous radiogenic Pb), without any significant input from the mantle. It is concluded that in the Moravo-Silesian and Rhenohercynian zones (including proximal

  4. P wave velocity of Proterozoic upper mantle beneath central and southern Asia

    NASA Astrophysics Data System (ADS)

    Nyblade, Andrew A.; Vogfjord, Kristin S.; Langston, Charles A.

    1996-05-01

    P wave velocity structure of Proterozoic upper mantle beneath central and southern Africa was investigated by forward modeling of Pnl waveforms from four moderate size earthquakes. The source-receiver path of one event crosses central Africa and lies outside the African superswell while the source-receiver paths for the other events cross Proterozoic lithosphere within southern Africa, inside the African superswell. Three observables (Pn waveshape, PL-Pn time, and Pn/PL amplitude ratio) from the Pnl waveform were used to constrain upper mantle velocity models in a grid search procedure. For central Africa, synthetic seismograms were computed for 5880 upper mantle models using the generalized ray method and wavenumber integration; synthetic seismograms for 216 models were computed for southern Africa. Successful models were taken as those whose synthetic seismograms had similar waveshapes to the observed waveforms, as well as PL-Pn times within 3 s of the observed times and Pn/PL amplitude ratios within 30% of the observed ratio. Successful models for central Africa yield a range of uppermost mantle velocity between 7.9 and 8.3 km s-1, velocities between 8.3 and 8.5 km s-1 at a depth of 200 km, and velocity gradients that are constant or slightly positive. For southern Africa, successful models yield uppermost mantle velocities between 8.1 and 8.3 km s-1, velocities between 7.9 and 8.4 km s-1 at a depth of 130 km, and velocity gradients between -0.001 and 0.001 s-1. Because velocity gradients are controlled strongly by structure at the bottoming depths for Pn waves, it is not easy to compare the velocity gradients obtained for central and southern Africa. For central Africa, Pn waves turn at depths of about 150-200 km, whereas for southern Africa they bottom at ˜100-150 km depth. With regard to the origin of the African superswell, our results do not have sufficient resolution to test hypotheses that invoke simple lithospheric reheating. However, our models are not

  5. How Strong is the Case for Proterozoic Low-Latitude Glaciation?

    NASA Astrophysics Data System (ADS)

    Evans, D. A.

    2004-05-01

    The most recent global compilations of paleomagnetic depositional latitudes for Proterozoic glaciogenic formations indicate a dominant mode near the paleo-equator (Evans 2000 AJS; Evans 2003 Tectonophysics). This result would therefore support either the snowball Earth or the large-obliquity hypotheses for Precambrian ice ages, but would reject the uniformitarian comparison to polar-temperate-restricted Phanerozoic glaciogenic deposits. The most reliable low-latitude results come from the Australian Marinoan succession, but a recent summary of these units has suggested that a glaciogenic origin is not yet demonstrated (Eyles and Januszczak 2004 Earth-Sci Reviews). It becomes useful, then, to review the global evidence for Proterozoic low-latitude glaciation. Eyles and Januszczak (ibid.) identified 13 Neoproterozoic deposits with "demonstrated" glacial influence. Among these, poor age constraints and lack of paleomagnetic data prohibit estimation of depositional paleolatitudes for the Fiq, Sturtian, Vreeland, Taoudeni, East Greenland, Port Askaig, and Zhengmuguan units. Moderate paleolatitudes are reasonably well supported for the South China, Gaskiers, Smalfjord, and Moelv units. Among the three remaining units, the Rapitan Group can be assigned a near-equatorial paleolatitude indirectly through use of the Galeros and Franklin-Natkusiak paleomagnetic results, as long as the Rapitan age lies within 750-720 Ma as generally expected. The Moonlight Valley Formation in northern Australia may be assigned a tropical paleolatitude according to high-quality paleomagnetic results from compellingly correlated Marinoan strata in southern Australia. Those strata, including the famous Elatina Formation, have yielded a robust paleomagnetic signature that is commonly interpreted to imply frigid climate (manifest in part by frost-wedge polygons) at near-equatorial latitudes. Concerns that the Neoproterozoic geomagnetic field was either nonaxial or nondipolar are valid in principle

  6. Isotopic Disequilibrium and High-Crystallinity Magma Ascent: Clues to the Temporal Restriction of Proterozoic Anorthosites

    NASA Astrophysics Data System (ADS)

    Bybee, G. M.

    2014-12-01

    Many Proterozoic anorthosite massifs show crustal isotopic signatures that have fuelled debate regarding the source (mantle vs. lower crust) of these temporally restricted magmas. The models advocating a mantle derivation for these rocks suggest that lower crustal assimilation plays an important role in developing the isotopic signature of the massifs, but no evidence exists to support this. We make use of Sr, Nd and Pb isotopic compositions of anorthosites from the Mealy Mountains Intrusive Suite (MMIS), the Nain Plutonic Suite (NPS) and the Rogaland Anorthosite Province (RAP), their internal mineral phases and comagmatic, high-pressure pyroxene megacrysts, which represent samples from various stages of the polybaric ascent of the magmas, to probe the origin of the crustal isotopic signatures and assess the importance of differentiation at lower crustal depths. Study of the MMIS and NPS is instructive as each is intruded into crust of significantly different age and isotopic composition. We observe varying degrees of internal isotopic disequilibrium, enforcing the notion that the nature of the crustal assimilant has a profound influence on the chemical signature of the magmas (Fig. 1). We also find unexpected patterns of internal isotopic disequilibrium, such as isotopically depleted orthopyroxene relative to plagioclase (Fig. 1), which suggests that anorthosite petrogenesis is not a "simple" case of progressive crustal contamination during polybaric magma ascent, but is more likely to involve significant differentiation and solidification at lower crust depths. The 100 m.y. magmatic timescales observed in these anorthosite systems may be caused by significant magmatic differentiation at Moho/lower crustal levels, as well as formation in long-lived arc environments. These long-lived magmatic timescales contrast with recent observations suggesting that the duration of magma ascent from the Moho to surface in arc environments is on the order of months to years. Such

  7. Geologic and Geochronologic Studies of the Early Proterozoic Kanektok Metamorphic Complex of Southwestern Alaska

    USGS Publications Warehouse

    Turner, Donald L.; Forbes, Robert B.; Aleinikoff, John N.; McDougall, Ian; Hedge, Carl E.; Preface by: Wilson, Frederic H.; Layer, Paul W.; Hults, Chad P.

    2009-01-01

    The Kanektok complex of southwestern Alaska appears to be a rootless terrane of early Proterozoic sedimentary, volcanic, and intrusive rocks which were metamorphosed to amphibolite and granulite facies and later underwent a pervasive late Mesozoic thermal event accompanied by granitic plutonism and greenschist facies metamorphism of overlying sediments. The terrane is structurally complex and exhibits characteristics generally attributed to mantled gneiss domes. U-Th-Pb analyses of zircon and sphene from a core zone granitic orthogneiss indicate that the orthogneiss protolith crystallized about 2.05 b.y. ago and that the protolithic sedimentary, volcanic and granitic intrusive rocks of the core zone were metamorphosed to granulite and amphibolite facies about 1.77 b.y. ago. A Rb-Sr study of 13 whole-rock samples also suggests metamorphism of an early Proterozoic [Paleoproterozoic] protolith at 1.77 Ga, although the data are scattered and difficult to interpret. Seventy-seven conventional 40K/40Ar mineral ages were determined for 58 rocks distributed throughout the outcrop area of the complex. Analysis of the K-Ar data indicate that nearly all of these ages have been totally or partially reset by a pervasive late Mesozoic thermal event accompanied by granitic plutonism and greenschist facies metamorphism. Several biotites gave apparent K-Ar ages over 2 Ga. These ages appear to be controlled by excess radiogenic 40Ar produced by the degassing protolith during the 1.77 Ga metamorphism and incorporated by the biotites when they were at temperatures at which Ar could diffuse through the lattice. Five amphibolites yielded apparent Precambrian 40K/40Ar hornblende ages. There is no evidence that these hornblende ages have been increased by excess argon. The oldest 40K/40Ar hornblende age of 1.77 Ga is identical to the sphene 207Pb/206Pb orthogneiss age and to the Rb-Sr 'isochron' age for six of the 13 whole-rock samples. The younger hornblende ages are interpreted as

  8. SIMS and NanoSIMS analyses of Mesoproterozoic individual microfossils indicating continuous oxygen-producing photosynthesis in Proterozoic Ocean

    NASA Astrophysics Data System (ADS)

    Peng, X.; Guo, Z.; House, C. H.; Chen, S.; Ta, K.

    2015-12-01

    Well-preserved microfossils in the stromatolites from the Gaoyuzhuang Formation (~1500Ma), which is younger than the Gunflint Formation (~1880Ma) and older than the Bitter Springs Formation (~850Ma), may play key roles in systematizing information about the evolution of early life and environmental changes in the Proterozoic Ocean. Here, a combination of light microscopy (LM), scanning electron microscopy (SEM), focused ion beam (FIB), nano-scale secondary ion mass spectrometry (NanoSIMS) and secondary ion mass spectrometry (SIMS) were employed to characterize the morphology, elemental distributions and carbon isotope values of individual microfossils in the stromatolites from the Gaoyuahzuang Formation. Light microscopy analyses show that abundant filamentous and coccoid microfossils are exceptionally well preserved in chert. NanoSIMS analyses show that metabolically important elements such as 12C-, 13C-, 12C14N-, 32S-, and 34S- are concentrated in these microfossils and that the variations in the concentrations of these elements are similar, establishing the elemental distributions in incontestably biogenic microstructures. Carbon isotope (δ13C) values of individual microfossils range from -32.2‰ ± 0.9‰ to -23.3‰ ± 1.0‰ (weighted mean= -28.9‰ ± 0.1‰), consistent with carbon fixation via the Calvin cycle. The elevated δ13C values of the microfossils from Early-, Meso- to Late Proterozoic Era, possibly indicate decreasing CO2 and increasing O2 concentrations in the Proterozoic atmosphere. Our results, for the first time, provided the element distributions and cell specific carbon isotope values on convincing Mesoproterozoic cyanobacterial fossils, supporting continuous oxygen-producing photosynthesis in the Proterozoic Ocean.

  9. A hypothesis for Proterozoic-Phanerozoic supercontinent cyclicity, with implications for mantle convection, plate tectonics and Earth system evolution

    NASA Astrophysics Data System (ADS)

    Grenholm, Mikael; Scherstén, Anders

    2015-11-01

    We present a conceptual model for supercontinent cycles in the Proterozoic-Phanerozoic Eons. It is based on the repetitive behavior of C and Sr isotopes in marine carbonates and U-Pb ages and εHf of detrital zircons seen during the Neoproterozoic-Paleozoic and Paleoproterozoic Eras, respectively. These records are considered to reflect secular changes in global tectonics, and it is hypothesized that the repetitive pattern is caused by the same type of changes in global tectonics. The fundamental premise of this paper is that such repetitive changes should also be recorded in orogenic belts worldwide. This carries the implication that Neoproterozoic-Paleozoic orogenic belts should have Paleoproterozoic equivalents. It is proposed that this is the case for the East African, Uralides and Ouachita-Alleghanian orogens, which have Paleoproterozoic analogs in the West African-Amazon, Laurentian and East European cratons, respectively. The Neoproterozoic-Paleozoic orogenic belts are not isolated features but occur in a specific global context, which correspond to the relatively well-constrained Neoproterozoic break-up of Rodinia, and the subsequent Late Paleozoic assembly of Pangea. The existence of Paleoproterozoic equivalents to Neoproterozoic-Paleozoic orogens requires that the same cycle defined the Paleoproterozoic. We therefore hypothesize that there were Paleoproterozoic supercontinents equivalent to Rodinia and Pangea, and that Proterozoic-Phanerozoic supercontinents are comprised of two basic types of configurations, equivalent to Rodinia (R-type) and Pangea (P-type). The Paleoproterozoic equivalent of Rodinia is likely the first supercontinent to have formed, and Proterozoic-Phanerozoic supercontinent cycles are therefore defined by R- to R-type cycles, each lasting approximately 1.5 Gyr. We use this cyclic pattern as a framework to develop a conceptual model that predicts the configuration and cycles of Proterozoic-Phanerozoic supercontinents, and their

  10. Partitioning of H2O between olivine and carbonate-silicate melts at 6.3 GPa and 1400 °C: Implications for kimberlite formation

    NASA Astrophysics Data System (ADS)

    Sokol, Alexander G.; Kupriyanov, Igor N.; Palyanov, Yuri N.

    2013-12-01

    Partitioning of H2O between olivine and carbonate-silicate melts has been studied at 6.3 GPa and 1400 °C using a split-sphere multianvil apparatus. Olivine was synthesized in equilibrium with hydrous silicate and hydrous carbonate-silicate±chloride melts saturated with respect to one of Opx, Grt, Ms or a harzburgitic (Ol+Opx+Grt) residue and had CO2/(CO2+SiO2) molar ratios from 0 to 0.8. The concentration of H2O in olivine was determined using FTIR spectroscopy. We found that depending on the melt carbonation and saturation in equilibrium silicate phases the H2O content in olivine varied from 100 to 1500 ppm. The obtained results and data reported in Sokol et al. (2013) indicate that H2O content in olivine becomes approximately two times lower as CO2/(CO2+SiO2) molar ratios in the equilibrium melt increases from 0 to 0.4-0.8 and the crystallization media transform from hydrous silicate to hydrous carbonate-silicate (kimberlite like) melt. The estimated water partitioning between carbonate-silicate melt and nominally anhydrous mantle minerals indicates that carbonatitic melt can effectively extract water once it invades H2O-poore the peridotite. We suggest that extraction of H2O owing to the freezing point depression may provide the necessary melting degree of metasomatized peridotite source and formation of kimberlitic magma.

  11. The influence of volcanological and sedimentological processes on diamond grade distribution in kimberlites: examples from the EKATI Diamond Mine, NWT, Canada

    NASA Astrophysics Data System (ADS)

    Porritt, Lucy A.; Cas, R. A. F.; Ailleres, L.; Oshust, P.

    2011-10-01

    The distribution of diamonds within individual kimberlite pipes is poorly documented in the public domain due to the proprietary nature of the data. The study of the diamond distribution within two pipes, Fox and Koala, from the EKATI Diamond Mine, NWT, Canada, in conjunction with detailed facies models has shown several distinct relationships of deposit type and grade distribution. In both pipes, the lithological facies represent grade units which can be distinguished from each other in terms of relative size and abundance of diamonds. A positive relationship between olivine grain size and abundance with diamond size and abundance is observed, indicating that sorting of fragmental kimberlites influences diamond distribution. Though surface geological processes do not control the diamond potential of the erupting magma, they can be responsible for concentrating diamonds into economically significant proportions. A good understanding of the eruption, transport and depositional processes responsible for the individual lithological units and the diamond distribution within them is important for successful resource estimation. This may lead to recognition of areas suitable for selective mining, making a marginal deposit economic.

  12. Constraints on the development of Proterozoic basins in central India from 40Ar/39Ar analysis of authigenic glauconitic minerals

    USGS Publications Warehouse

    Conrad, J.E.; Hein, J.R.; Chaudhuri, A.K.; Patranabis-Deb, S.; Mukhopadhyay, J.; Deb, G.K.; Beukes, N.J.

    2011-01-01

    Ages of some key stratigraphic sequences in central Indian Proterozoic basins are based predominantly on lithostratigraphic relationships that have been constrained by only a few radioisotopic dates. To help improve age constraints, single grains of glauconitic minerals taken from sandstone and limestone in two Proterozoic sequences in the Pranhita-Godavari Valley and the Chattisgarh basin were analyzed by the 40Ar/39Ar incremental heating method. Analysis of the age spectra distinguishes between ages that are interpreted to reflect the time of glauconite formation, and anomalous ages that result from inherited argon or postcrystallization heating. The analyses indicate an age of 1686 ± 6 Ma for the Pandikunta Limestone and 1566 ± 6 Ma for the Ramgundam Sandstone, two units in the western belt of Proterozoic sequences in Pranhita-Godavari Valley. Glauconite from the Chanda Limestone, in the upper part of this sequence, contains inherited 40Ar but is interpreted to reflect an age of ca. 1200 Ma. Glauconite from the Somanpalli Group in the eastern belt of the Pranhita-Godavari Valley gives an age of 1620 ± 6 Ma. In the Chattisgarh basin, glauconite from two units gives disturbed ages that suggest a period of regional heating in the Chattisgarh basin at ca. 960–1000 Ma. These new ages indicate that these sequences are 200–400 m.y. older than previously recognized, which has important implications for geochemical studies of Mesoproterozoic ocean redox conditions in addition to providing important constraints on regional tectonics and lithostratigraphy.

  13. Concepts for diamond exploration in "on/off craton" areas—British Columbia, Canada

    NASA Astrophysics Data System (ADS)

    Simandl, George J.

    2004-09-01

    The tectonic setting of British Columbia (BC) differs from classic diamond-bearing intracratonic regions such as the Northwest Territories and South Africa. Nevertheless, several diamond occurrences have been reported in BC. It is also known that parts of the province are underlain by Proterozoic and possibly Archean basement. Because the continents of today are composites of fragments of ancient continents, it is possible that some of the regions underlain by old crystalline basement in eastern British Columbia were associated with a deep crustal keel. The keel may have predated the break-up of the early Neoproterozoic supercontinent called Rodinia and was preserved possibly until the Triassic. Some of these old continental fragments may have been displaced relative to their position of origin and dissociated from their keel, or the keel may have since been destroyed. Such fragments represent favourable exploration grounds in terms of the "Diamondiferous Mantle Root" model (DMR model) if they were intersected by kimberlites or lamproites prior to displacement or destruction of their underlying deep keel. Therefore, extrapolation of fragments of the diamond-bearing Precambrian basement from the Northwest Territories or Alberta to BC provides a sufficient reason for initiating reconnaissance indicator mineral surveys. The "Eclogite Subduction Zone" model (ES model) predicts formation of diamonds at lower pressure (i.e., depth) than required by the DMR model in convergent tectonic settings. Although not proven, this model is supported by thermal modeling of cold subduction zones and recent discoveries of diamonds in areas characterized by convergent tectonic settings. If the ES model is correct, then the parts of BC with a geological history similar to today's "cold" subduction zones, such as Honshu (Japan), or to continental collision zones, such as Kokchetav massif (Kazakhstan) and the Dabie-Sulu Terrane (east central China), may be diamondiferous. The terranes

  14. Evolution of diamond resorption in a silicic aqueous fluid at 1-3 GPa: Application to kimberlite emplacement and mantle metasomatism

    NASA Astrophysics Data System (ADS)

    Zhang, Zhihai; Fedortchouk, Yana; Hanley, Jacob J.

    2015-06-01

    Natural diamonds grow and partially dissolve during mantle metasomatism and undergo further resorption during the ascent to the Earth's surface in kimberlite magmas. This study uses atomic force microscopy (AFM) for quantitative characterization of diamond resorption morphology in order to provide robust constraints of the composition of kimberlitic and mantle metasomatic fluids. We performed experiments in a piston-cylinder apparatus at pressures (P) of 1-3 GPa and temperatures (T) of 1150-1400 °C to examine the impact of P, T, and silica content of an aqueous fluid on diamond dissolution. Petrographic observation and microthermometry of synthetic fluid inclusions trapped in olivine at the run conditions provide constraints on the composition and density of the fluid reacting with the diamond. Our results confirm an inverse relationship between P and T on diamond dissolution kinetics. A P increase of 1 GPa suppresses diamond oxidation rates by the same value as a T decrease by 50 °C, while the transformation rate of diamond crystal morphology from octahedron to tetrahexahedron increases with both P and T. All dissolved diamonds develop glossy surfaces, ditrigonal {111} faces, sheaf striations, and negative trigons, while circular pits only occur in aqueous fluids with low silica content (≤ 4.2 mol/kg) at 1 GPa. We identify five distinct morphological groups of trigons: two types of point-bottomed (p/b) (trumpet- and V-shaped) and three types of flat-bottomed (f/b) (trumpet-shaped, trapezoid-shaped and rounded). AFM measurements of trigons from two successive runs showed three stages of their evolution. Etch pits nucleate at defects as trumpet p/b trigons with the vertical dissolution rate (Vd) faster than the dissolution rates at the surface free of defects; they further develop by growth of the bottoms in (111) plane to create trumpet-shaped f/b trigons accompanied by decrease in Vd; and finally form trapezoid-shaped f/b trigon with constant wall angles. The

  15. Mechanisms of sandstone deposition in a late Proterozoic submarine canyon, Adelaide geosyncline, South Australia

    SciTech Connect

    von der Borch, C.C.; Grady, A.E.

    1984-06-01

    Late Proterozoic submarine canyon fills the Adelaide Supergroup (Flinders Range, South Australia) are asymmetrical in terms of their facies. Coarse breccia units, commonly associated with coarse-grained channelized turbidite sandstone units, generally occur adjacent to north walls of all the east-west-trending canyon incisions. In contrast, fine-grained sandstones and mudstones within the canyon fill are generally associated with south walls. In one canyon (Patsy-Springs canyon), an additional element of asymmetry is associated with the prevalence of northward-climbing sets of climbing ripples (southward-dipping stoss sides) within channelized turbidites, in what are interpreted to be major thalweg channels and their associated levees. Flute casts at the bases of individual turbidite sandstones invariably indicate initial turbidity current flow to the west throughout the vertical sequences of the channel fills. Parallel laminations above the flute casts in each flow pass upward into climbing ripples with south-dipping stoss sides, implying southward lateral accretion across the channel of a levee or bar as each turbidity current decelerated. The asymmetries outlined could be explained by: Coriolis force acting on the turbidity currents, or the existence on a steep slope of a meandering canyon gorge, confining a thalweg channel developed within subsequent canyon fill. In such a situation, roller-coastering turbidity currents would seek outer bends of the meandering primary gorge.

  16. Geochemistry and petrogenesis of proterozoic mafic dykes in north Kerala, southwestern Indian Shield - Preliminary results

    NASA Technical Reports Server (NTRS)

    Radhakrishna, T.; Gopakumar, K.; Murali, A. V.; Mitchell, J. G.

    1991-01-01

    Mafic dyke intrusions occur in three distinct orientations ( NNW-SSE and NW-SE and NE-SW) in north Kerala regions, the southwestern part of the Indian Shield. Dating of two NNW-SSE trending dykes by K-Ar method has yielded Middle Proterozoic ages (ca. 1660 Ma and ca. 1420 Ma respectively). Our initial geochemical results on these dyke rocks (0.65-0.15 wt pct K2O, 0.37-0.38 wt pct P2O5, 3.30-1.00 wt pct TiO2, 11-1 p.p.m. Rb, 250-90 p.p.m. Sr, 230-40 p.p.m. Ba, 160-40 p.p.m. Zr, and 30-10 x chondrite rare earth elemental abundances) indicate a transitional character between abyssal and plateau tholeiites. Petrogenetic modelling suggests that the dyke compositions have been derived by different degrees of partial melting of a heterogenous source mantle. The mantle sources with accessory amphibole and/or garnet, variably enriched in LREE and LIL elements, are compatible with the observed geochemical data.

  17. Temporal Trends in Syngenetic Lipid Biomarker Signals from Proterozoic Sedimentary Organic Matter

    NASA Astrophysics Data System (ADS)

    Love, G. D.; Li, C.; Summons, R. E.

    2008-12-01

    The development of continuous-flow catalytic hydropyrolysis (HyPy) for reproducible recovery of biomarker lipid skeletons covalently-bound within kerogen has proved to be an important analytical breakthrough for ancient lipid biomarker research. The parallel analyses of free (solvent-extractable) and kerogen-bound biomarkers affords more confidence that we have correctly identified syngenetic compounds. Combining HyPy with detailed biomarker product analyzes using metastable reaction monitoring-gas chromatography mass spectrometry (MRM-GC-MS) allows detection of a large suite of biomarker compounds which are usually too low in abundance to be analyzed in detail using conventional GC-MS. Here we compare free and bound lipid biomarker records generated from Paleoproterozoic (ca. 1.7 Ga) to Late Ediacaran age (ca. 542 Ma) strata from marine basins from North and South China, Australia and Oman. Fundamental changes in eukaryotic community structure are evident after the Sturtian glaciation (ca. 713 Ma) from distinctive sterane distributions. In particular, radiations in basal animals (sponges) and chlorophyte microalgae are first apparent in Huqf sedimentary rocks from South Oman Salt Basin. Marine microbial communities were not globally homogenous in contemporaneous Proterozoic settings from comparison of biomarker profiles and this could reflect differences in ocean chemistry, affecting nutrient supply, from basin to basin.

  18. Sporulation and ultrastructure in a late Proterozoic cyanophyte - Some implications for taxonomy and plant phylogeny

    NASA Technical Reports Server (NTRS)

    Cloud, P.; Moorman, M.; Pierce, D.

    1975-01-01

    Electron microscopical studies of a morphologically diverse, coccoid, presumably late Proterozoic blue-green alga are here reported. They show, together with light microscopy, that the form studied is widespread in the Cordilleran geosyncline, extend the record of well-defined endosporangia perhaps 700 million years into the past, and reveal previously unrecorded ultrastructural details. Coming from northeastern Utah, southwestern Alberta, and east central Alaska, these minute fossils belong to the recently described, morphologically diverse taxon Sphaerocongregus variabilis Moorman, are related to living entophysalidaceans, and have affinities with both the chroococcalean and chamaesiphonalean cyanophytes. Included in the morphological modes displayed by this alga are individual unicells, coenobial clusters of unicells, and a range of endosporangia comparable to those described for living entophysalidaceans. Scanning and transmission electron microscopy reveal that the endospores are commonly embedded in a vesicular matrix, that some of them show what appears to be a bilaminate or perhaps locally multilaminate wall structure, and that some remain together to mature as coenobial clones or 'colonies'. Taxonomic classification and phylogeny are discussed.

  19. Root zone of the late Proterozoic Salma caldera, northeastern Arabian Shield, Kingdom of Saudi Arabia.

    USGS Publications Warehouse

    Kellogg, K.S.

    1985-01-01

    The eroded root of the late Proterozoic Salma caldera crops out in a striking, roughly elliptical feature, about 27 km long and 22 km wide, near the NE edge of the Arabian Shield, The caldera is genetically part of an elongate alkalic granitic massif (Jabal Salma) that extends 35 km from the caldera to the SW. Comenditic ash flow tuff and lava(?) of the caldera fill, probably more than 1 km thick, are the oldest recognized rocks of the caldera complex. These rocks were erupted during caldera collapse associated with the rapid evacuation of the upper, mildly peralkalic part of a zoned magma reservoir. Within the caldera fill, a massive, lithic-rich intracaldera rhyolite, probably a lava in excess of 1 km thick, is overlain by a layered ash flow sequence. Numerous megabreccia blocks, probably derived from the caldera wall, occur in the massive rhyolite. No apparent structural doming of the exposed volcanic rocks along the E side of the caldera took place; the layered ash flows commonly dip steeply toward the center of the caldera. Postemplacement deformation and metamorphism of the caldera are mimimal. Small-displacement strike-slip faults cut the complex, which is tilted to the NE by no more than about 2o.-from Author

  20. Probable calcified metaphytes in the latest Proterozoic Nama Group, Namibia: origin, diagenesis, and implications

    NASA Technical Reports Server (NTRS)

    Grant, S. W.; Knoll, A. H.; Germs, G. J.

    1991-01-01

    Samples from the Huns Limestone Member, Urusis Formation, Nama Group, at two adjacent localities in southern Namibia contain thin foliose to arched, sheet-like carbonate crusts that are 100-500 micrometers thick and up to 5 cm in lateral dimension. Morphologic, petrographic, and geochemical evidence supports the interpretation of these delicate crusts as biogenic, most likely the remains of calcified encrusting metaphytes. The original sediments of the fossiliferous samples contained aragonitic encrusting algae, botryoidal aragonite cements, and an aragonite mud groundmass. Spherulites within the precursor mud could represent bacterially induced mineral growths or the concretions of marine rivularian cyanobacteria. Original textures were severely disrupted during the diagenetic transition of aragonite to low-magnesian calcite, but some primary structures remain discernible as ghosts in the neomorphic mosaic. Gross morphology, original aragonite mineralogy, and hypobasal calcification indicate that the crusts are similar to late Paleozoic phylloid algae and extant peyssonnelid red algae. Structures interpreted as possible conceptacles also suggest possible affinities with the Corallinaceae. Two species of Cloudina, interpreted as the remains of a shelly metazoan, are also known from limestones in the Nama Group. It is possible, therefore, that skeletalization in metaphytes and animals arose nearly simultaneously near the end of the Proterozoic Eon.

  1. Organically preserved microbial endoliths from the late Proterozoic of East Greenland

    NASA Technical Reports Server (NTRS)

    Knoll, A. H.; Golubic, S.; Green, J.; Swett, K.

    1986-01-01

    Diverse microorganisms ranging from cyanobacteria to eukaryotic algae and fungi live endolithically within ooids, hardgrounds and invertebrate shells on the present-day sea floor. These organisms are involved in the mechanical destruction of carbonates, and are useful ecological indicators of water depth and pollution. The Phanerozoic history of microbial endoliths has been elucidated through the study of microborings (the trace fossils of endolithic microorganisms) and rare cellularly preserved individuals, but nothing was known of the possible Precambrian evolution of comparable microorganisms until Campbell documented the occurrence of microborings in late Proterozoic ooids from central East Greenland. We now report the discovery of large populations of organically preserved endolithic microorganisms in silicified pisolites from 700-800-Myr-old Limestone-Dolomite Series of East Greenland. This fossil assemblage is significant for three reasons: (1) It confirms the prediction that oolites, pisolites and hardgrounds--the substrates for pre-Phanerozoic endoliths--provide a hitherto poorly explored but rewarding set of environments into which the search for early microfossils must be broadened; (2) the assemblage is diverse, containing about 12 taxa of morphologically distinct and previously unknown endolithic cyanobacteria, plus associated epilithic and interstitial populations; and (3) at least six of the fossil populations are indistinguishable in morphology, pattern of development, reproductive biology and inferred ecology from distinctive cyanobacterial species that bore ooids today in the Bahama Banks.

  2. Origin of the Eumetazoa: testing ecological predictions of molecular clocks against the Proterozoic fossil record

    NASA Technical Reports Server (NTRS)

    Peterson, Kevin J.; Butterfield, Nicholas J.

    2005-01-01

    Molecular clocks have the potential to shed light on the timing of early metazoan divergences, but differing algorithms and calibration points yield conspicuously discordant results. We argue here that competing molecular clock hypotheses should be testable in the fossil record, on the principle that fundamentally new grades of animal organization will have ecosystem-wide impacts. Using a set of seven nuclear-encoded protein sequences, we demonstrate the paraphyly of Porifera and calculate sponge/eumetazoan and cnidarian/bilaterian divergence times by using both distance [minimum evolution (ME)] and maximum likelihood (ML) molecular clocks; ME brackets the appearance of Eumetazoa between 634 and 604 Ma, whereas ML suggests it was between 867 and 748 Ma. Significantly, the ME, but not the ML, estimate is coincident with a major regime change in the Proterozoic acritarch record, including: (i) disappearance of low-diversity, evolutionarily static, pre-Ediacaran acanthomorphs; (ii) radiation of the high-diversity, short-lived Doushantuo-Pertatataka microbiota; and (iii) an order-of-magnitude increase in evolutionary turnover rate. We interpret this turnover as a consequence of the novel ecological challenges accompanying the evolution of the eumetazoan nervous system and gut. Thus, the more readily preserved microfossil record provides positive evidence for the absence of pre-Ediacaran eumetazoans and strongly supports the veracity, and therefore more general application, of the ME molecular clock.

  3. Isotopic geochemistry of a mid-Proterozoic evaporite basin: Balmat, New York

    USGS Publications Warehouse

    Whelan, J.F.; Rye, R.O.; De Lorraine, W.; Ohmoto, H.

    1990-01-01

    Lenses of Proterozoic meta-evaporitic anhydrite as much as 60 m thick occur in a 1150 to 1300 Ma Grenville Series marble sequence near Balmat, New York. ??34Sanhy values in the oldest lens average 8.2 permil; progressively younger lenses average 19.6, 27.2, 26.7, and 19.1 permil. The thickest and isotopically heaviest lens displays coincident upsection increases in ??34Sanhy (24.1-30.2 permil) and ??18Oanhy (19.8-22.5 permil) values. These increases and the overall 34S enrichment of the anhydrite indicate that anhydrite precipitation was concurrent with large-scale bacterial sulfate reduction in the depositional basin. The arid climate and high evaporation rates implied by evaporite deposition result in 18O enrichment of the basin waters and the carbonates precipitated from them. The evidence of arid climate and the variability and 34S-enrichment of the anhydrite imply that the Balmat environment was a closed or restricted basin in which evaporation and bacterial sulfate reduction combined to produce a 34S-enriched, sulfate-rich brine. -from Authors

  4. Geochemistry and petrogenesis of Proterozoic granitic rocks from northern margin of the Chotanagpur Gneissic Complex (CGC)

    NASA Astrophysics Data System (ADS)

    Yadav, Bhupendra S.; Wanjari, Nishchal; Ahmad, Talat; Chaturvedi, Rajesh

    2016-07-01

    This study presents the geochemical characteristics of granitic rocks located on the northern margin of Chotanagpur Gneissic Complex (CGC), exposed in parts of Gaya district, Bihar and discusses the possible petrogenetic process and source characteristics. These granites are associated with Barabar Anorthosite Complex and Neo-proterozoic Munger-Rajgir group of rocks. The granitic litho-units identified in the field are grey, pink and porphyritic granites. On the basis of geochemical and petrographic characteristics, the grey and pink granites were grouped together as GPG while the porphyritic granites were named as PG. Both GPG and PG are enriched in SiO2, K2O, Na2O, REE (except Eu), Rb, Ba, HFSE (Nb, Y, Zr), depleted in MgO, CaO, Sr and are characterised by high Fe* values, Ga/Al ratios and high Zr saturation temperatures (GPGavg˜ 861 ∘C and PGavg˜ 835 ∘C). The REE patterns for GPG are moderately fractionated with an average (La/Yb)N˜ 4.55 and Eu/Eu* ˜ 0.58, than PG which are strongly fractionated with an average (La/Yb)N˜ 31.86 and Eu/Eu* ˜ 0.75. These features indicate that the granites have an A-type character. On the basis of geochemical data, we conclude that the granites are probably derived from a predominant crustal source with variable mantle involvement in a post-collisional setting.

  5. Juvenile Middle Proterozoic crust in the Adirondack Highlands, Grenville province, northeastern North America

    SciTech Connect

    Daly, J.S. ); McLelland, J.M. )

    1991-02-01

    Nd isotope data indicate that minimal amounts of significantly older crust have contributed to the genesis of the oldest (ca. 1.3-13.5 Ga) plutons in the Adirondack Highlands. These are magmatic arc tonalites with positive initial {epsilon}{sub Nd} values and Sm-Nd depleted mantle model ages (t{sub DM}) that are within 70 m.y. of the time of their crystallization. Granitoids of the anorthosite-mangerite-charnockite-granite suite, dated at 1,156-1,134 Ma, as well as the 1,100-1,050 Ma plutons, associated with the Ottawan phase of the Grenvillian orogenic cycle, also have positive initial {epsilon}{sub Nd} values and t{sub DM} ages similar to the tonalites. Derivation of both groups of granitoids by crustal melting of the magmatic arc is consistent with the available isotopic and geochemical data. Juvenile late Middle Proterozoic crust that formed during or just prior to the Grenville cycle appears to dominate the southwestern Grenville province as well as the Grenville inliers to the south. In contrast, most of the contiguous Grenville province in Canada comprises largely reworked older crust.

  6. Isotopic evidence from the eastern Canadian shield for geochemical discontinuity in the proterozoic mantle

    USGS Publications Warehouse

    Ashwal, L.D.; Wooden, J.L.

    1983-01-01

    Most workers agree that Proterozoic anorthosite massifs represent the crystallization products of mantle-derived magmas1,2, although the composition of the parental melts is a major unsolved petrological problem 3. As mantle-derived rocks, the massifs can be used as geochemical probes of their late Precambrian upper mantle sources. We report here Nd and Sr isotopic compositions of anorthosites and related rocks from the Grenville and Nain Provinces of the eastern Canadian shield. Here 75% of the Earth's known anorthosite is found in a 1,600-km belt from the Adirondack Mountains of northern New York State to the eastern coast of Labrador4 (Fig. 1). The results indicate that the massifs were derived from at least two distinct mantle source regions which were established before 1,650 Myr ago, and were episodically involved in magmatism over ???500 Myr. One reservoir, below the Grenville Province, and probably below much of the eastern Superior Province, was isotopically similar to the depleted, modern-day mid-ocean ridge basalt (MORB) source. The other reservoir was chondritic to moderately enriched, and is most easily identified in the Nain Province, but may have occurred scattered throughout the Superior Province. ?? 1983 Nature Publishing Group.

  7. Using SHRIMP Zircon Geochronology to Characterise the Evolution of the Proterozoic Mount Isa Inlier, Australia

    NASA Astrophysics Data System (ADS)

    Neumann, N. L.; Southgate, P. N.; Gibson, G. M.

    2008-12-01

    The Proterozoic Mount Isa Inlier of northern Australia records an extensive record of basin evolution between 1800 Ma and 1575 Ma, and contains a number of world-class Pb-Zn-Ag, U and Iron Oxide Cu-Au deposits. Understanding the timing and nature of basin development is a critical component in understanding these mineral systems. The integration of U-Pb zircon SHIRMP geochronology with structural and facies analysis has allowed basin packages across this area to be divided into three superbasins; the Leichhardt, Calvert and Isa Superbasins. Detrital zircon geochronology of stratigraphic units within these basins has been used in conjunction with syn-sedimentary volcanics to constrain depositional ages, and to identify and characterise changes in provenance through time. Sedimentation between 1790 Ma and 1740 Ma associated with the Leichhardt Superbasin is characterised by fluvial to shallow marine sandstones deposited in half-grabens. Between 1690 Ma and 1670 Ma, deep-water turbidites in the eastern-most parts of the inlier were deposited during an interval of missing rock record on the platform to the west, and are coincident with the initiation of a break-up unconformity. Sedimentation between 1790 Ma and 1670 Ma is also associated with voluminous felsic and mafic magmatism, and mafic rocks emplaced during this time period record a change in geochemical signature from continental flood basalts to oceanic tholeiites. We interpret these changes to be consistent with an evolution in tectonic setting from intercontinental rifting to near passive margin development.

  8. Implications for ancient terrestrial conditions from widespread Middle Proterozoic saprolite, Northwest Territories, Canada

    SciTech Connect

    Chiarenzelli, J.R.; Donaldson, J.A.

    1985-01-01

    Beneath the undeformed and unmetamorphosed fluvial sedimentary rocks of Thelon Basin, a saprolitic paleoweathering profile 1.8 billion years old is developed in Aphebian and Archean basement rocks. The saprolite is up to 50m thick and has a laterally continuous, vertical color zonation from red downwards to green. The thickness of the profile, upward geochemical trends, and the kaolinite-hematite assemblage in the uppermost hematite zone are suggestive of intense chemical weathering. Potential modern analogs include tropical or lateritic weathering profiles. Compared to these, the Proterozoic saprolites have a different profile zonation and mineralogy, similar but less developed geochemical trends, and lack overlying soil horizons. These differences can be explained by the downward superpositioning of geochemical zones during erosional truncation, and burial diagenesis. The Thelon Basin saprolites, represent a period of intense chemical weathering which played a key role in the planation of the NW Canadian Shield. Similar extensive planation surfaces are known from the Tertiary rock record of Australia, and may suggest a parallelism of major paleoclimatic, paleogeographic, and tectonic factors. The strong influence of the sapprolite on subsequent sedimentation in the Thelon Basin is shown in the paired chemical maturity/textural immaturity of arenites in the overlying Thelon Formation.

  9. Proterozoic tectonostratigraphy and paleogeography of central Madagascar derived from detrital zircon U-Pb age populations

    USGS Publications Warehouse

    Cox, R.; Coleman, D.S.; Chokel, C.B.; DeOreo, S.B.; Wooden, J.L.; Collins, A.S.; De Waele, B.; Kroner, A.

    2004-01-01

    Detrital zircon U-Pb ages determined by SHRIMP distinguish two clastic sequences among Proterozoic metasedimentary rocks from central Madagascar. The Itremo Group is older: zircon data, stromatolite characteristics, and carbon isotope data all point to a depositional age around 1500-1700 Ma. The Molo Group is younger, deposited between ???620 Ma (the age of the youngest zircon) and ???560 Ma (the age of metamorphic overgrowths on detrital cores). Geochronologic provenance analysis of the Itremo Group points to sources in East Africa as well as local sources in central and southern Madagascar but provides no evidence for a detrital contribution from northern and eastern Madagascar nor from southern India. Detrital zircon and sedimentologic similarities between rocks of the Itremo Group and the Zambian Muva Supergroup suggest a lithostratigraphic correlation between the two. The Molo Group has a strong 1000-1100 Ma detrital signature that also indicates an east African provenance and suggests a Neoproterozoic geographic connection with Sri Lanka but shows no indication of input from the Dharwar craton and eastern Madagascar. Central Madagascar was probably juxtaposed with the Tanzanian craton in the Paleo- and Mesoproterozoic, whereas northern and eastern Madagascar were connected to India. Internal assembly of Madagascar postdates Neoproterozoic Molo Group sedimentation and is likely to have occurred at about 560 Ma. ?? 2004 by The University of Chicago. All rights reserved.

  10. Lithology, age and structure of early proterozoic greenstone belts, West African shield

    NASA Technical Reports Server (NTRS)

    Attoh, K.

    1986-01-01

    Lithologic and chemical data have been compiled for belts in the Proterozoic terrane. Available stratigraphic information from geologic maps of these areas indicate that a typical sequence is comprised of predominately mafic lava flows (basalt-andesite) at the base, which are overlain by felsic volcanic rocks including pyroclastic rocks and lavas. Lithostratigraphic data indicate the volcanic succession is 6-8 km thick. This is followed by 3-4 km of basaltic lava flows which are locally pillowed, the top of the unit is marked by a distinctive manganese formation (MF) consisting of Mn-Fe rich cherts up to 200 m thick. The youngest volcanic unit consists of mafic tuffs and breccia with a distinctive fragmental texture. Of about 100 chemical analyses reported calc-alkaline rocks constitute 55% and tholeiites 45%. Quartz-normative basalt constitutes 99% of the rock type in the tholeiitic suite. In the calc-alkaline suite, 9% of the analyses is basalt, 45% andesite and the rest is dacite and rhyodacite. The available data lead to the conclusion that the minimum age for the volcanic activity must be between 2200 and 2100 million years. It is significant that Archean ages have not been reported from any of the volcanic belts (1-10).

  11. Terminal Proterozoic mid-shelf benthic microbial mats in the Centralian Superbasin and their environmental significance.

    PubMed

    Logan, G A; Calver, C R; Gorjan, P; Summons, R E; Hayes, J M; Walter, M R

    1999-05-01

    A combined sedimentological and biogeochemical study has been conducted on several Terminal Proterozoic mid-shelf microbial mat facies from the Centralian Super-basin. Isotopic and organic geochemical analysis of the bitumen and kerogen indicated that two sources of organic matter from 'planktonic' and 'benthic microbial-mat' populations contributed to the sediment. The 'planktonic' source provided a suite of n-alkanes with C20, whereas, the 'benthic' source contributed an overlay of n-alkanes >C20 with a strong even preference, together with mid-chain methyl alkanes. Kerogen and biomarkers derived from the microbial mat were found to be depleted in 13C relative to planktonic material. Pyrite in the microbial mats was also found to be depleted in 34S compared to surrounding facies. The combination of these observations suggested that the mats may have been at least partly composed of sulfide oxidising bacteria. These organisms have specific environmental tolerances that set limits on palaeo-environment. Their requirement for oxygen indicates that the water column above the mid-shelf could not have been anoxic. Accordingly, from the results and age determinations reported here, it would appear that mid-shelf environments of the Centralian Superbasin of Australia were seeing significant levels of oxygen through the Ediacarian. PMID:11543353

  12. The Presence of a Stable Block bounded by Active Zones (Mobile Belts) in the southwestern North American Proterozoic craton

    NASA Astrophysics Data System (ADS)

    Goodell, P.; Martinez P, C.; Mahar, M. A.

    2014-12-01

    Bouguer gravity data, initial Sr isotope values, zircon U-Pb, and multiple occurrences of felsic Proterozoic rocks, have revealed an elevated, less deformed, felsic cratonic block in the northern Mexico. The block is situated in western Chihuahua and is bounded by active zones or mobile belts on three sides, and is here referred to as the Western Chihuahua Cratonic Block (WCCB). Bouguer gravity data clearly indicate a region of a highly negative anomaly (< -200 mgal) in contrast to adjoining areas. The region is large and the anomaly is relatively smooth over broad areas; the WCCB appears as a smaller version of the Colorado Plateau. The block is characterized by high initial Sr isotope ratios (<0.706). Several occurrences of Proterozoic rocks are located within or next to the WCCB, and they reveal the character of the Bouguer anomaly. On the east, at Los Filtros, Proterozoic rocks crop out in a basement cored uplift interpreted to having been derived from the WCCB during the Ouachita orogeny. At Sierra La Mojina boulders of 1.1 Ga granites are found in Permian conglomerates. And at Basasiachic, xenoliths of 1.1 Ga granites are present in ash flow tuffs. Establishment of the Precambrian character of the WCCB is of importance, and these multiple occurrences are evidence. Prior studies of the Sierra Madre Occidental suggest that the region was uplifted because of a vast Cenozoic batholith presumed to lie under the SLIP (Silicic Large Igneous Province), the Upper Volcanic Series. The present study challenges that conclusion and maintains the SMO is underlain by Proterozoic silicic crust. The geology of age dated samples supports this. The WCCB is surrounded on three sides by Active Zones or Mobile Belts, which have been active extensional and translational zones periodically over a long period of time. On the east are the Paleozoic Pedrogosa Basin, Mesozoic Chihuahua Trough and Cenozoic Rio Grande Rift, the first two of which also continue around the northern border

  13. Infrared spectral and carbon isotopic characteristics of micro- and macro-diamonds from the Panda kimberlite (Central Slave Craton, Canada)

    NASA Astrophysics Data System (ADS)

    Melton, G. L.; Stachel, T.; Stern, R. A.; Carlson, J.; Harris, J. W.

    2013-09-01

    One hundred and twenty-one micro-diamonds (< 1 mm) and 90 macro-diamonds (2.5 mm to 3.4 mm) from the Panda kimberlite (Ekati mine, Central Slave Craton, Canada) were analyzed for nitrogen content, nitrogen aggregation state (%B) and platelet and hydrogen peak areas (cm- 2). Micro-diamond nitrogen concentrations range from < 10 at. ppm to 1696 at. ppm (median = 805 at. ppm) and the median aggregation state is 23%B. Macro-diamonds range from < 10 at. ppm to 1260 at. ppm (median = 187 at. ppm) nitrogen and have a median nitrogen aggregation of 26%B. Platelet and hydrogen peaks were observed in 37% and 79% of the micro-diamonds and 79% and 56% of the macro-diamonds, respectively. Nitrogen based time averaged residence temperatures indicate that micro- and macro-diamonds experienced similar thermal mantle residence histories, both populations displaying bimodal residence temperature distributions with a gap between 1130 °C and 1160 °C (at 3.5 Ga residence). In addition, SIMS carbon isotopic analyses for the micro-diamonds were obtained: δ13C compositions range from - 6.9‰ to + 1.8‰ (median = - 4.3‰). CL imaging reveals distinct growth layers that in some samples differ by > 2‰, but mostly vary by < 0.5‰. Comparison of only the “gem-quality” samples (n = 49 micro- and 90 macro-diamonds) between the two diamond sets, indicates a statistically significant shift of + 1.3‰ in average δ13C from macro- to micro-diamonds and this shift documents distinct diamond forming fluids, fractionation process or growth histories. A broad transition to heavier isotopic values is also observed in connection to decreasing mantle residence temperatures. The bimodal mantle residence temperature distribution may coincide with the transition from highly depleted shallow to more fertile deep lithospheric mantle observed beneath the Central Slave Craton. The increase in δ13C with decreasing residence temperature (proxy for decreasing depth) is interpreted to reflect diamond

  14. Evidence for the mantle metasomatism at the formation of polycrystalline diamond aggregates from Mir kimberlite, Yakutia, Russia

    NASA Astrophysics Data System (ADS)

    Sobolev, N. V.; Shatsky, V. S.; Zedgenizov, D.; Ragozin, A.; Reutsky, V.

    2013-12-01

    Polycrystalline diamond aggregates (boart, framesites, diamondites) have been widely studied but their origin is poorly understood. We report the results of a study of two polished fragments of fine-grained (40-400 μm size of individual diamond grains) dense polycrystalline diamond aggregates from the Mir pipe containing visible multiple interstitial garnet inclusions. They were analyzed for major and trace elements of inclusions and for δ13C and N abundance of host diamonds. These aggregates are classified as variety IX by Orlov (1977). No cavities were observed in these samples. Sixty two irregular garnet grains and one clinopyroxene inclusion were detected and analyzed in sample MR-832. Garnets are homogeneous within single grains but variable in mg# [100Mg/(Mg+Fe)] from 60 up to 87 and CaO contents (3.3-5.3 wt.%) among grains with a trend to negative correlation. Low Cr (550-640 ppm) confirms eclogitic (E-type) paragenesis. High Na2O contents (5.2 wt.%) of a single pyroxene inclusion is additional evidence of eclogitic nature of this sample. Wide variations in trace elements (ppm) are characteristic for garnet grains: Sr (2.7-25.6), Y (9.7-14.1), Zr (15.6-38.7) and positive Eu anomaly is present. The δ13C of diamonds within studied sample is relatively constant (-7.1÷-7.8 ‰ PDB) but N abundance is variable (620-1150 ppm). The second peridotitic (U/P-type) sample MR-838 contains eight inclusions of Mg-rich Cr-pyropes (mg# ~85, Cr2O3 3.2-3.4 wt.%) and magnesite inclusion with 4.35 wt.% FeO and 1.73 wt.% CaO. Trace elements content in pyropes is relatively uniform (ppm): Sr (0.4-1.6), Y (13.2-13.4) and Zr (13.0). The δ13C is between -5.6 and -9.8 ‰ PDB and N abundance is low and relatively constant (75-98 ppm). We conclude that heterogeneous distribution of the trace elements among garnet grains and magnesite presence are indicative of the effects of mantle metasomatism and rapid crystallization shortly before the eruption of the kimberlite (e.g. Shimizu

  15. Eclogites and the Metasomatism of Eclogites from the Jagersfontein Kimberlite: Punctuated Transport and Implications for Alkali Magmatism

    NASA Astrophysics Data System (ADS)

    Pyle, Joseph M.; Haggerty, Stephen E.

    1998-04-01

    The first detailed petrographic and electron microbeam study of eclogite xenoliths and associated metasomatism from the Jagersfontein Kimberlite are presented with the objectives of determining depths of origin and of establishing possible links to cratonic magmatism. Jagersfontein eclogites are primarily bimineralic garnet + clinopyroxene with wide variations in color, size, and modal abundances, and with initial equilibration in the diamond stability field; five of seventy-nine eclogites derive from P > 8 Gpa. Phlogopite, amphibole, and calcite are the dominant metasomatic minerals, with accessory spinel, feldspar, and serpentine. Garnets are py 39 to py 77 in composition, and zoned garnets are observed as overgrowth rims on early garnet, adjacent to phlogopite, and as relicts in spinel-bearing subassemblages. Phlogopite and amphibole are enriched in Ti, and serpentine contains noticeable Al and Fe. Pleonaste has limited magnetite and minor Cr; and a strong correlation exists between MgO and Cr 2O 3 contents in spinel and in coexisting garnet. Pyroxene is compositionally diverse, and three distinct varieties exist: (1) primary omphacite (jd 7-jd 55) with distinct high-Al [VI] and low-Al [VI] groups; (2) secondary symplectitic diopsidic omphacite (jd 2-jd 25) with slightly greater Al [IV] content than primary omphacite; and (3) spinel-associated pyroxene with little or no jadeite component (max 0.96 wt% Na 2O), variable Ca/(Ca + Mg + Fe) (0.03-0.47), and high Tschermak content (up to ˜0.3 cations Al [IV] pfu). Feldspar compositions are variable and controlled by associated minerals (garnet, pyroxene, phlogopite) and composition of reacting phases. Pressure-T estimates for metasomatism are <2 Gpa and <900 C, in the field of graphite stability. From mass balance considerations, the metasomatic melts contained K, Ti, Cr, H 2O, and CO 2, but also Na in amphibole and Nb in rutile. Garnet:pyroxene ratios control metasomatic end-product minerals. Hydrous metasomatism of

  16. Comparison of the mantle modification of the mantle column between two phases of kimberlite intrusion in Dalnyaya pipe, Yakutia

    NASA Astrophysics Data System (ADS)

    Ashchepkov, Igor; Spetsius, Zdislav; Salikhov, Ravil; Khmelnikova, Olga

    2013-04-01

    Dalnaya pipe is one of the largest in Daldyn field, Yakutia is composed of autolite breccia (AKB) and porphyric kimberlite (PK). Minerals from concentrates of both phases were compared and with the peridotite xenoliths minerals. Cpx from Dalnyaya are showing common tendencies Fe--Ti rise and Cr, Al, Na decrease. Garnets belong to lherzolite field with more deviation to harzbuirgitic one for (PK) . The chromites show two subtrends for Cr Fe, Ni vs TiO2. In general the variations of the AKB minerals and dispersion are higher but amount of depleted varieties is higher in PK. We used >50 xenoliths and ~1200 concentrate minerals for PT reconstructions. Combine PTX diagram show deep SCLM root beneath Dalnyaya with the main heating ~7 GPa. The HT 45mwm-2 branch is traced by some xenoliths from base to 2GPa. Essential inflection and heating detected by PT for OPx ~3GPa referring to Ca- enriched pyroxenitic garnets . Small Fe enrichment for Cpx and Gar found near 6 GPa referring to heated porhyroclastic varieties. Continuous and irregular growth of Fe# for Gar and low Fe Cpx Fe# 6 to 12# suggest that primary mantle layering beneath this pipe was smoothed by the high scale interaction with melts. The refertilization trend with Fe#9-15% rising upward in two branches refer to the Ilm and Cpx parental melt evolutions produced the intergrowth sometimes with garnets. In the PFO2 diagrams garnets and Cpx show continuous reduction to the lithosphere base to 4QMF higher for Cpx. Ilm - garnet trend is rising upward between -2 -0 QMF. The PT diagram for the AKB minerals from Dalnyaya pipe is nearly the same with the high dispersion to Fe rich varieties and smaller amount o f Mg rich minerals. Since the diamond grade is often determined by the amount of depleted varieties it is higher for the PK. Trace elements determined for Gar and Cpx from 13 xenoliths from the middle part of mantle section reveal very similar patterns in general. Supported by RBRF grant 11-05-00060.

  17. Identification of /sup 13/C depleted mantle carbon in diamonds from the Roberts Victor Kimberlite, South Africa

    SciTech Connect

    Deines, P.

    1985-01-01

    The Roberts Victor Kimberlite is known for the abundance of eclogite xenoliths, some of which show an unusual depletion in /sup 18/O. The question whether the observed oxygen isotope variations can be related to carbon isotopic composition variations has been investigated. Peridotite-suite diamons (X = -5.4 per thousand vs. PDB, s = +/-0.9 per thousand, n = 65) and sulfide containing diamonds (X = -4.9, s = +/-0.9, n = 20) do not differ in their /sup 13/C content. For these samples, delta/sup 13/C is not related to diamond shape, color, minerals occluded, or the inclusion chemistry. Eclogite suite diamonds (11) can be subdivided into two groups, GI and GII, based on delta/sup 13/C : GI = (X = -15.4, s = +/-0.4, n = 8); GII = (X = -5.9, s = +/-0.4, n = 3). The composition of the gt and cpx inclusions of the two groups is distinct; e.g. cpx of GI is significantly depleted in SiO/sub 2/, MgO, and CaO, and significantly enriched in Al/sub 2/O/sub 3/, FeO and MnO, compared to cpx of GII. Comparison of the chemical composition of the inclusions in E-type diamonds with those of eclogite xenoliths showing /sup 18/O depletion suggests that /sup 13/C and /sup 18/O depletion are not likely to be related. Evaluation of compositional trends of gt and cpx in eclogite xenoliths indicates that GI and GII are not related by a single fractionation event, but represent products from different reservoirs. Equilibration conditions deduced from coexisting gt and cpx demonstrate that GI diamonds come from larger depths than eclogite xenoliths and by inference GII diamonds. The high FeO and MnO content of a gt inclusion in cpx of an eclogite xenolith is used to argue for the existence of two separate events responsible for the formation of GI and GII diamonds.

  18. Origin of sub-lithospheric diamonds from the Juina-5 kimberlite (Brazil): constraints from carbon isotopes and inclusion compositions

    NASA Astrophysics Data System (ADS)

    Thomson, A. R.; Kohn, S. C.; Bulanova, G. P.; Smith, C. B.; Araujo, D.; Walter, M. J.

    2014-12-01

    Forty-one diamonds sourced from the Juina-5 kimberlite pipe in Southern Brazil, which contain optically identifiable inclusions, have been studied using an integrated approach. The diamonds contain <20 ppm nitrogen (N) that is fully aggregated as B centres. Internal structures in several diamonds revealed using cathodoluminescence (CL) are unlike those normally observed in lithospheric samples. The majority of the diamonds are composed of isotopically light carbon, and the collection has a unimodal distribution heavily skewed towards δ13C ~ -25 ‰. Individual diamonds can display large carbon isotope heterogeneity of up to ~15 ‰ and predominantly have isotopically lighter cores displaying blue CL, and heavier rims with green CL. The light carbon isotopic compositions are interpreted as evidence of diamond growth from abiotic organic carbon added to the oceanic crust during hydrothermal alteration. The bulk isotopic composition of the oceanic crust, carbonates plus organics, is equal to the composition of mantle carbon (-5 ‰), and we suggest that recycling/mixing of subducted material will replenish this reservoir over geological time. Several exposed, syngenetic inclusions have bulk compositions consistent with former eclogitic magnesium silicate perovskite, calcium silicate perovskite and NAL or CF phases that have re-equilibrated during their exhumation to the surface. There are multiple occurrences of majoritic garnet with pyroxene exsolution, coesite with and without kyanite exsolution, clinopyroxene, Fe or Fe-carbide and sulphide minerals alongside single occurrences of olivine and ferropericlase. As a group, the inclusions have eclogitic affinity and provide evidence for diamond formation at pressures extending to Earth's deep transition zone and possibly the lower mantle. It is observed that the major element composition of inclusions and isotopic compositions of host Juina-5 diamonds are not correlated. The diamond and inclusion compositions are

  19. Nd, Sr, Pb, Ar, and O isotopic systematics of Sturgeon Lake kimberlite, Saskatchewan, Canada: constraints on emplacement age, alteration, and source composition

    NASA Astrophysics Data System (ADS)

    Hegner, E.; Roddick, J. C.; Fortier, S. M.; Hulbert, L.

    1995-06-01

    Rb-Sr isotopic dating of phlogopite megacryst samples separated from Sturgeon Lake kimberlite, Saskatchewan, yields a crystallization age of 98±1 Ma (2 σ, MSWD=1.2; 87Sr/86Sr( t)=0.7059). The 40Ar/39Ar analyses of a phlogopite megacryst sample indicate the presence of large amounts of excess 40Ar and yield an excessively old age of ˜410 Ma. Assessment of the Ar data using isotope correlation plots indicates clustering of the data points about a mixing line between the radiogenic 40Ar component at 98 Ma and a trapped component with uniform 36Ar/40Ar and Cl/40Ar. Values of δ 18O as high as +20‰ (VSMOW) for calcite from the groundmass and a whole-rock sample indicate pervasive low-temperature alteration. The δ 13C of matrix carbonate is -11.3‰ (PDB), slightly lighter than typical values from the literature. The δ 18O values of about +5‰ (VSMOW) for brown phlogopite megacrysts may be primary, green phlogopites are interpreted to be an alteration product of the brown variety and are 2‰ heavier. Initial Nd-Sr-Pb isotopic ratios for a whole-rock sample ( ɛ Nd=+0.8; 87Sr/86Sr=0.7063, 206Pb/204Pb=18.67, 207Pb/204Pb=15.54, 208Pb/204Pb=38.97) suggest an affinity with group I kimberlites. Initial ɛ Nd values of +1.7 and +0.5 (87Sr/86Sr( t)=0.7053 and 0.7050) for eclogitic and lherzolitic garnet megacryst samples, and values of 0.0 for two phlogopite megacryst samples reflect an origin from an isotopically evolving melt due to assimilation of heterogeneous mantle. Lilac high-Cr lherzolitic garnet megacrysts give an unusually high ɛ Nd(98. Ma) of +28.6 (87Sr/86Sr=0.7046) indicating a xenocrystic origin probably from the lithospheric mantle. The very radiogenic 87Sr/86Sr and 206Pb/204Pb ratios of the kimberlite are consistent with melting of EM II (enriched) mantle components.

  20. Stable isotopic studies of mafic sills and proterozoic metasedimentary rocks located beneath the Duluth Complex, Minnesota

    NASA Astrophysics Data System (ADS)

    Park, Young-Rok; Ripley, Edward M.; Severson, Mark; Hauck, Steven

    1999-03-01

    Thin (3-20 m), picritic to noritic sills are found in the Proterozoic metasedimentary footwall rocks of the Duluth Complex, Minnesota. The sills were emplaced prior to the major intrusions that comprise the Duluth Complex, and underwent contact metamorphism along with the country rocks. Oxygen, hydrogen, and sulfur isotopic compositions of the sills indicate a varied history of isotopic exchange between minerals, melts, and hydrothermal fluids in the high temperature environment below the major plutonic bodies of the Midcontinent Rift system. The pre-Duluth Complex sills exhibit a range in δ 18O values from 4.9‰ to 14.8‰, with values between 6‰ and 7‰ generally found in sill interiors. High δ 18O values near sill contacts with high 18O metasedimentary rocks of the pelitic Virginia Formation or Biwabik Iron Formation, coupled with a smooth sigmoidal isotopic profile centered at the contact, suggest that oxygen diffusion was an important exchange mechanism. The elevated δ 18O values near the center of the thickest sills are thought to reflect the emplacement of isotopically contaminated basaltic magma. Dehydration reactions in the pelitic rocks of the contact aureole liberated high 18O fluids that enhanced subsolidus diffusive exchange. Advective displacement of the diffusion profiles toward the sill interior is less than 40 cm, and suggests that layer parallel flow dominated in the dehydration of the contact aureole. Elevated, but uniform δ 18O values (9.7‰ to 10.5‰) in thinner sills suggests that oxygen diffusivity was increased relative to country rocks due to enhanced porosity, perhaps related to extensive development of microcracks. Although δD values of the pelitic country rocks record a history of dehydration, systematic variations of δD (-64‰ to -143‰) and H 2O (0.15 to 5.40 wt.%) content are not found in the sills. δ 18O values of coexisting plagioclase and pyroxene from the sills indicate a close approach to isotopic equilibrium, and

  1. Paleomagnetism of Proterozoic mafic dikes from the Tobacco Root Mountains, southwest Montana

    USGS Publications Warehouse

    Harlan, S.S.; Geissman, J. Wm; Snee, L.W.

    2008-01-01

    Paleomagnetic data from Proterozoic mafic dikes in southwestern Montana provides evidence for two distinct episodes of subparallel dike emplacement at ca. 1450 and 780 Ma. Published geochemical data from dikes in the southern Tobacco Root Mountains has identified three distinct compositional groups, termed groups A, B, and C. Geochronological data from the group A dikes yielded a Sm-Nd age of 1448 ?? 49 Ma. Emplacement of these dikes is thought to reflect mafic magmatism associated with extension accompanying development of the adjacent Mesoproterozoic Belt Basin. Paleomagnetic results from these dikes and a group C dike yield antipodal magnetizations with a group-mean direction of D = 225.0??, I = 61.8?? (k = 27.9, ??95 = 7.7??, N = 14 independent means/24 sites). The average paleomagnetic pole (8.7??N, 216.1??E, A95 = 10.3??) is considered to be primary on the basis of positive baked contact tests and similarity to poles of ca. 1.45-1.4 Ga from intrusions elsewhere in North America, but is discordant with respect to poles from age equivalent sedimentary rocks of the Meosoproterozoic Belt Supergroup. 40Ar/39Ar dates from geochemical group B dikes are consistent with published U-Pb dates that demonstrate dike emplacement at 780 Ma as part of the regional Gunbarrel magmatic event. Hornblende concentrates from the group B dikes yield 40Ar/39Ar apparent ages of 778-772 Ma, whereas biotite from a baked contact zone yielded a plateau date of 788 Ma. Paleomagnetic results from the group B dikes yield a mean direction of D = 301.5??, I = -17.1?? (k = 65.7, ??95 = 4.0??, N = 12 independent means/23 sites) with a paleomagnetic pole at 14.6??N, 127.0??E (A95 = 3.2??). The combination of geochronologic data, results of a baked contact test, and spatial agreement of the paleomagnetic poles with poles of similar age elsewhere in North America indicates that this is also a primary magnetization associated with dike emplacement. Paleomagnetic data from some of the Tobacco Root

  2. Vent Complexes above Dolerite Sills in Phanerozoic LIPs: Implications for Proterozoic LIPs and IOCG Deposits

    NASA Astrophysics Data System (ADS)

    Ernst, R. E.; Bleeker, W.; Svensen, H.; Planke, S.; Polozov, A. G.

    2009-05-01

    New insights into the origin of IOCG (iron oxide copper gold) deposits [e.g., 1, 2, 3] follow from recent studies of Phanerozoic Large Igneous Provinces (LIPs). Detailed seismic studies of the 62-55 Ma North Atlantic Igneous Province and complementary studies in the 183 Ma Karoo and 250 Ma Siberian LIPs reveal thousands of hydrothermal vent complexes (HVCs). Up to 5-10 km across at the paleosurface, these vents connect to underlying dolerite sills at paleodepths of up to 8 km [4, 5, 6, 7]. They originate from explosive release of gases generated when thick sills (>50 m) are emplaced into volatile-rich but low-permeability sedimentary strata. HVCs are phreatomagmatic in origin. Their architecture, economic potential for IOCG-type deposits, and effects on climate strongly depend on the type of host rocks (black shales at Karoo and evaporites at Siberian LIPs) and its fluid (brines) saturation at the time of emplacement. About 250 HVCs associated with the Siberian LIP are mineralized having magnetite in the matrix. Some are being mined for Fe (Korshunovskoe and Rudnogorskoe), but their economic potential for copper and gold mineralization is understudied. These observations from the Phanerozoic LIP record suggest that HVCs should also be an essential component of sill provinces associated with Proterozoic LIPs, with a potential for causing major climatic shifts and IOCG-type deposits, particularly if the host sediments include substantial evaporites. Two examples are discussed here. The 725 Ma Franklin LIP covers 1.1 Mkm2 in northern Canada [8]; in the Minto Inlier of Victoria Island, this event comprises volcanics, sills, and breccia pipes [9, 10]. The breccia pipes appear identical to HVCs and, furthermore, the presence of evaporites in the host sediments of the Shaler Supergroup suggests (based on the Siberian trap example) the potential for IOCG-type mineralization. Could 1.59 Ga sills, as exemplified by the exposed Western Channel Diabase sills on the eastern

  3. Fluid inclusion geochemistry of the early Proterozoic Pirilä gold deposit in Rantasalmi, Southeastern Finland

    NASA Astrophysics Data System (ADS)

    Poutiainen, M.

    1993-04-01

    The small Pirilä gold deposit, which is located in the southeastern part of the Svecofennian complex near the Archean/Proterozoic boundary, is hosted by quartz veins and lenses occurring in mica schist. The rocks of the area were metamorphosed under conditions of amphibolite facies. Gold is invariably associated with sulphides. Microthermometry of fluid inclusions in quartz indicates four types of inclusions: (1) weakly saline H2O-CO2 (< 4.0 eq.wt% NaCl) with small amounts of CH4 (< 10 mole% CH4); (2) CO2 (< 10 mole% CH4); (3) CH4; and (4) H2O (< 25 eq.wt% NaCl) with less than 0.85 mole% CO2 in the vapour phase. Texturally these inclusion types are classified as primary (H2O-CO2) and secondary (H2O, CO2 and CH4). Leachate analysis shows that, in addition to Na, the aqueous fluids contain Ca and Fe with minor amounts of K and Mg. The primary H2O-CO2 and the secondary H2O inclusions contain sulphide and unidentified opaque grains, respectively. The secondary CH4 inclusions are often associated with short trails of arsenopyrite grains. Fluid inclusion and geological data suggest ore mineral mobilization, crystallization of host quartz, and deposition of sulphides controlled by the D2 and D3 structures in the presence of a H2O-CO2 fluid mainly during the plastic D3 deformation and during the amphibolite facies metamorphism (i.e. 3.4 kbars/540 670°C). During ductile-brittle deformation (probably D4), precipitation of tectonic remobilized gold from sulphides in fractures occurred in the presence of CH4 and H2O fluids at lowered temperature (< 440°C) and pressure (< 2 kbars).

  4. The structure and chemical layering of Proterozoic stromatolites in the Mojave Desert

    NASA Astrophysics Data System (ADS)

    Douglas, Susanne; Perry, Meredith E.; Abbey, William J.; Tanaka, Zuki; Chen, Bin; McKay, Christopher P.

    2015-07-01

    The Proterozoic carbonate stromatolites of the Pahrump Group from the Crystal Spring formation exhibit interesting layering patterns. In continuous vertical formations, there are sections of chevron-shaped stromatolites alternating with sections of simple horizontal layering. This apparent cycle of stromatolite formation and lack of formation repeats several times over a vertical distance of at least 30 m at the locality investigated. Small representative samples from each layer were taken and analysed using X-ray diffraction (XRD), X-ray fluorescence (XRF), environmental scanning electron microscopy - energy dispersive X-ray spectrometry, and were optically analysed in thin section. Optical and spectroscopic analyses of stromatolite and of non-stromatolite samples were undertaken with the objective of determining the differences between them. Elemental analysis of samples from within each of the four stromatolite layers and the four intervening layers shows that the two types of layers are chemically and mineralogically distinct. In the layers that contain stromatolites the Ca/Si ratio is high; in layers without stromatolites the Ca/Si ratio is low. In the high Si layers, both K and Al are positively correlated with the presence and levels of Si. This, together with XRD analysis, suggested a high K-feldspar (microcline) content in the non-stromatolitic layers. This variation between these two types of rocks could be due to changes in biological growth rates in an otherwise uniform environment or variations in detrital influx and the resultant impact on biology. The current analysis does not allow us to choose between these two alternatives. A Mars rover would have adequate resolution to image these structures and instrumentation capable of conducting a similar elemental analysis.

  5. Paleomagnetism of the Middle Proterozoic Electra Lake Gabbro, Needle Mountains, southwestern Colorado

    USGS Publications Warehouse

    Harlan, S.S.; Geissman, J.W.

    1998-01-01

    The Electra Lake Gabbro is a small 1.435 Ga pluton that intrudes 1.7 to 1.6 Ga gneisses and schists of the Needle Mountains in southwestern Colorado. Paleomagnetic samples were collected from the main phases of the gabbro, diabase dikes, granite, and alaskite dikes that cut the gabbro and from a partially melted zone in gneiss along the southern margin of the pluton. Gabbro, diabase, and some melt zone samples have a single-polarity characteristic magnetization of northeast declination (D) and moderate negative inclination (I). Demagnetization behavior and rock magnetic characteristics indicate that the remanence is carried by nearly pure magnetite. After correction for the minor west dip of overlying Paleozoic strata, we obtain a mean direction of D = 32.1??, I = -41.9?? (k = 94, ??95 = 3.3??, N = 21 sites) and a paleomagnetic pole at 21.1?? S, 221.1 ??E, (K= 89, A95 = 3.4??). This pole is similar to poles from the Middle Proterozoic Belt Supergroup but is located at a higher southerly latitude than poles from other 1.47-1.44 Ga plutons from North America, most of which plot at equatorial latitudes. The reason for this discrepancy is not clear but may result from a combination of factors, including unrecognized tilting of the gabbro, the failure of this relatively small pluton to fully average paleosecular variation, and uncertainties in the overall reliability of other 1.5-1.4 Ga poles of the North American apparent polar wander path.

  6. Middle to Late Proterozoic tectonic history of Grenville province at Van Horn, Trans-Pecos Texas

    SciTech Connect

    Soegaard, K.; Nielsen, K.C.; Manton, W.I. . Programs in Geosciences)

    1993-02-01

    Middle to Late Proterozoic volcanic and sedimentary sequences near Van Horn in west Texas record the pre- and syn-tectonic history of deformation from shallow crustal levels. Pre-deformational sequences include the Carrizo Mountain Group, the Allamoore and Tumbledown formations. The Carrizo Mountain Group is thrust northward, along the Streeruwitz fault, over a para-autochthonous sequence of unmetamorphosed carbonate and polymodal volcanic rocks of Allamoore and Tumbledown formations. Carbonates in the Allamoore Formation were deposited in shallow water and contain diverse stromatolites, desiccation cracks, and halite pseudomorphs. The Tumbledown formation unconformably overlies the Allamoore carbonate succession and consists of volcanic sandstones, agglomerates, pillow breccias, and mafic volcanic flows. Deformation of the pre-orogenic sequences led to deposition of more than 2,500 meters of immature alluvial fan conglomerates and eolian sandstones of the Hazel Formation. Based on clast composition of boulder conglomerates in the Hazel Formation, the principal source rocks were the Allamoore and Tumbledown formations and an unknown rhyolite terrain, with Carrizo Mountain Group lithologies notably absent. Alluvial megasequences in the alluvial fan conglomerates are aggradational and indicate at least two episodes of faulting accompanied Hazel sedimentation. A mismatch between source terrain and corresponding sediment composition, in addition to aggradation of alluvial fan megasequences, along the southern fault-bounded basin margin supports a transpressive regime for this orogenic belt in favor of a purely convergent margin. The Hazel orogenic sequence post-dated a 1,126--1,070 Ma old regional igneous event, whereas the Grenville deformation in the Llano uplift of central Texas pre-dated the igneous event.

  7. Geochemistry and zircon geochronology of Late Proterozoic leucogranites north of Boston, eastern Massachusetts

    SciTech Connect

    Markus, R.; Hon, R. . Geology and Geophysics); Dunning, G. . Dept. of Earth Sciences)

    1993-03-01

    An igneous sequence that includes Late Precambrian volcanics (Lynn Volcanics) and granites, granodiorites, tonalites, plus diorites of the Dedham North suite, is located in a tectonic block bounded by the Walden Pond and Northern Boundary Faults north of the Boston Basin. Within the block between the rhyolites and granodiorites is a several hundred foot wide zone of leucogranites that contains frequent roof pendants in various stages of partial melting. The migmatitic nature of the pendants suggests that the leucogranites were locally derived by melt extraction from the partially melted pendant xenoliths. U-Pb zircon ages were obtained from samples of the leucogranite, granodiorite and diorite. Their crystallization ages are: leucogranite 609 [+-] 4 Ma, granodiorite 607 [+-] 4 Ma, and diorite 606 [+-] 3 Ma. All three samples yield ages that are identical within their statistical error limits and all three samples contain inherited component with average mid-Proterozoic ages. Major and trace element geochemistry of 43 samples of all representative types show that the predominantly pelitic'' layers underwent extensive partial melting and that the leucogranites represent a minimum granite melt at 0.5 to 2.0 kb of P(H2O). Geochemical modeling also supports the origin by partial melting of the pendant inclusions. Once formed, the leucogranite melts were then mixed with mafic magmas which must have also been the provider of the necessary heat to sustain the partial melting process. The data indicate that the Dedham North plutonic suite was formed at shallow crustal levels and that its compositional range is a result of magma mixing of varying proportions between the leucogranite and mafic melts.

  8. Timing of Proterozoic regional deformation in the southern Manzano Mountains, central New Mexico

    SciTech Connect

    Bauer, P.W. ); Bowring, S.A. . Dept. of Earth, Atmospheric, and Planetary Sciences); Karlstrom, K.E. . Dept. of Geology)

    1992-01-01

    Early Proterozoic supracrustal and plutonic rocks of the Manzano Mtns have sustained a remarkably complex history of ductile deformation, metamorphism, and plutonism. A comparison of field relations and deformational features between the two southernmost plutons suggests that they differ greatly in timing of intrusion with respect to regional deformation. The Monte Largo pluton consists of medium-grained granodiorite and quartz monzonite that is bounded on three sides by strongly deformed quartzite and phyllite. An S1 foliation is folded by upright, N-trending folds (F2). S2, axial planar to F2, is mylonitic along the E pluton margin. The degree of deformation in the pluton is comparable to that in the country rock. The Monte Largo pluton has a U-Pb zircon age of ca. 1.66 Ga. The Priest pluton is a 10-km-long, N-S elongate, megacrystic quartz monzonite that is intrusive into quartzite sand schists. Large microcline crystals define a magmatic foliation. The body contains a weakly to moderately well-developed NE-striking tectonic foliation defined by flattened quartz grains, best developed along the W margin. On the N end of the pluton, map-scale folds in quartzite and schist have been cross-cut, and a contact metamorphic aureole cross-cuts country rock structures. The degree of deformation in the pluton is significantly less than that of country rock quartzites, some of which are mylonitic. The Priest Pluton has a U-Pb zircon age of ca. 1.45 Ga. These data suggest that the ca. 1.66 Ga Monte Largo pluton is syntectonic with respect to regional deformation, whereas the ca. 1.45 Ga priest pluton is post-tectonic with respect to the regional deformation.

  9. Polyphase Proterozoic metamorphism and deformation at Four Peaks, southern Mazatzal Mountains, Arizona

    SciTech Connect

    Powicki, D.A.; Williams, M.L. . Dept. of Geology and Geography)

    1993-02-01

    Proterozoic rocks at Four Peaks may have been deformed in the 1.65 Ga. Mazatzal Orogeny and again at ca. 1.4 Ga. Metamorphic and ductile deformation events occurred before and after emplacement of the ca. 1.4 Ga. Four Peaks granite. The granite cuts a metamorphosed supracrustal package tentatively correlated with the rocks of the Alder and Mazatzal Groups. Exposed rocks include (from oldest to youngest): rhyolite, massive quartzite (30m), a thinly-bedded sequence of schist and immature micaceous quartzite (300m), vitreous orthoquartzite (300m), and a green-black slate of unknown thickness. In addition to the K-feldspar megacrystic ca. 1.4 Ga. Granite, plutonic rocks include a large body of coarse-grained hornblende granodiorite, and several smaller, fine grained granitoids. The syncline folds an early foliation (S1) and is cut by a heterogeneous axial plane cleavage (S2). South of the syncline is a major NE-striking shear zone, more than 500 meters wide. North, west, and east of the syncline the 1.4 Ga. granite cuts the syncline and has no deformational fabric, indicating that folding and thrusting are older. Adjacent to the shear zone, the generally unfoliated. Four Peaks granite has an anastomosing steeply SE-dipping foliation, parallel to, but apparently less intense than that in the shear zone, suggesting a reactivation of the shear zone after emplacement. The two phase history is supported by microfabric evidence and phase relationship. Regionally, porphyroblasts that preserve S1 are present in rocks dominated by a younger foliation (S2). Rocks proximal to the 1.4 Ga. granite contain pseudomorphs after earlier porphyroblasts and new porphyroblasts that overgrow both foliations. The authors interpret the older deformation and metamorphism to be related to the ca 1.6 Ga.

  10. Timing of Proterozoic deformation, plutonism, and metamorphism in the Los Pinos Mountains, Central New Mexico

    SciTech Connect

    Shastri, L.L. . Dept. of Geology); Bowring, S.A. )

    1992-01-01

    Geochronologic, structural, and metamorphic studies within the Los Pinos Mountains (LPM), central NM provide new insights into the Proterozoic geologic history of this area. The LPM consist of a NE-trending, NW-dipping sequence of complexly deformed amphibolites and felsic schists. These have been intruded by a pervasively deformed granitic pluton. Two predominant deformational fabrics exist in the LPM. S1 is an early northwest-trending foliation, commonly parallel to compositional layering, which is folded about S2. S2 is axial planar foliation to tight to isoclinal folds and is the regional NE-trending fabric. Other fabrics and complex fold interference patterns may be related to localized strain partitioning around granitic bodies. A network of granitic dikes associated with the pluton crosscuts S2 but contains a weak foliation parallel to S2, suggesting synkinematic intrusion of the dikes. Regional metamorphism in the LPM took place at upper greenschist to lower amphibolite facies. Electron microprobe traverses of garnets show compositional variation indicative of growth zoning. No abrupt changes in composition representative of multiple metamorphic events are observed. Garnet-biotite geothermometry yields average rim temperatures of 454 [+-] 50 C. U-Pb geochronology of zircons from amphibolite, granite, and a granite dike indicates essentially the same age for all three units (1.66 Ga). The amphibolite contains abundant zircons which have complex morphologies typical of metamorphic growth; however, an igneous origin cannot yet be precluded. Spheres from the same amphibolite yield a near concordant age of 1.62 Ga. Thus, deformation, plutonism, and possibly the peak of metamorphism, were coeval at ca. 1.66 Ga, with metamorphism cooling through the blocking temperature of sphene at 1.62 Ga. The LPM are similar to other mountain ranges in south-central New Mexico where 1.66 Ga ages have been reported.

  11. Preliminary paleomagnetic poles and correlation of the Proterozoic Uinta Mountain Group, Utah and Colorado

    NASA Astrophysics Data System (ADS)

    Bressler, Stephen L.

    1981-09-01

    Stable paleomagnetic directions have been obtained from seven sites spanning much of the 7-km-thick middle Proterozoic Uinta Mountain Group. The characteristic magnetization is carried partly by detrital(?) and secondary specularite and partly by hematite pigment. Preliminary paleomagnetic poles for six of the seven sites are closely grouped and their mean pole is 0.6°N, 157.2°E ( N = 6, α 95 = 5.4°). The pole for the seventh and stratigraphically highest site lies to the west, 4.6°N, 140.1°E, and ( n = 8, α 95= 15.9°). On the basis of the poles, revisions to correlations of the Uinta Mountain Group with other sequences are proposed. The Uinta Mountain Group appears to correlate with the Chuar Group and perhaps the upper member of the underlying Nankoweap Formation, and post-dates the Unkar Group of the Grand Canyon Supergroup. In addition, the upper part of the Uinta Mountain Group appears to correlate with the Little Dal Group of the Northwest Territories, Canada. The Uinta Mountain Group appears to entirely post-date the Belt Supergroup of Montana and Idaho. Correlation of the Uinta Mountain Group with the Chuar and Little Dal Groups is supported by the presence of the fossil Chuaria and by apparently overlapping isotopic ages. A maximum age range for the Uinta Mountain Group, extrapolated from the Grand Canyon Supergroup and the Little Dal Group, extends from <1100 to > 770 m.y.

  12. Preliminary petroleum source rock assessment of upper Proterozoic Chuar group, Grand Canyon, Arizona

    SciTech Connect

    Palacas, J.G.; Reynolds, M.W.

    1989-03-01

    Strata in the Chuar Group, Grand Canyon, Arizona, are potential petroleum source rocks. This group, divided into the Galeros Formation below and the Kwagunt Formaton above, consists predominantly of very fine-grained siliciclastic rocks and thin sequences of sandstones and stromatolites and cryptalgal carbonate rocks. Over half the succession consists of organic-rich, gray to black mudstone and siltstone. Geochemical analyses indicate that the 281-m thick Walcott Member, the uppermost unit of the Kwagunt, has good to excellent petroleum source rock potential. The lower half of the Walcott is characterized by total organic carbon (TOC) contents as much as 7.0% (average /approximately/ 3.0%), hydrogen indices as much as 204 mg HC/g TOC (average 135 mg HC/g TOC), genetic potentials (S/sub 1/ + S/sub 2/) of nearly 16,000 ppm (average /approximately/ 6000 ppm), and extractable organic matter (EOM) as much as 4000 ppm. Data for the upper Walcott are incomplete but suggest that these rocks are as rich or richer than the lower Walcott. Maturity assessment indicates that source rocks of the Walcott are within the oil generation window. Strata of the thermally mature underlying Awatubi Member of the Kwagunt and the thermally mature to overmature Galeros Formation are, in general, rated as poor oil sources with genetic potentials generally less than 1000 ppm, but they are possible gas sources. Several thin sequences in these units, however, display good oil source characteristics, with EOM nearly 2000 ppm and genetic potentials nearly 7000 ppm. Chuar Group strata may be potential sources for economical accumulations of petroleum in upper Proterozoic or lower Paleozoic reservoir rocks in northwest Arizona and southwest Utah.

  13. The Early Proterozoic structural and tectonic history of the south central Lake Superior Region

    NASA Astrophysics Data System (ADS)

    Ueng, Wen-Long C.; Larue, Dave K.

    1988-06-01

    The early Proterozoic tectonic evolution of the south central Lake Superior region is complex, owing to the presence of four tectonostratigraphic terranes, which were affected by six phases of deformation. The four terranes are the passive margin of the Superior craton, two paraautochthonous passive margin terranes (Crystal Falls and Florence-Niagara terranes), and a southern magmatic arc complex which is probably allochthonous with respect to the other terranes. Four of the six deformational episodes accompanied subhorizontal shortening, while two were caused by subvertical shortening. The first and the most penetrative phase of deformation is marked by subhorizontal shortening in a NNE-SSW direction. The second and fourth deformations were characterized by subvertical shortening and did not significantly modify the structural orientations from previous events in the study area. The third, fifth, and sixth deformations mostly caused open folding, and shortening directions were NW, NE, and W, respectively. Because all the terranes in the south central Lake Superior region share parallel deformational histories, it is suggested that the accretion of these terranes occurred during the first deformational episode. After removal of younger deformational effects, including open folding of the suture zone, the tectonostratigraphic assemblages in this region show the following sequence from NNE to SSW: a platformal assemblage overlying sialic basement, a basinal assemblage of tholeiitic volcanic rocks overlain by deep-water turbidites, an assemblage of basin floor deposits (Crystal Falls terrane) with apparently no demonstratably underlying crystalline basement, a fault-bounded terrane with highly strained passive margin strata (Florence-Niagara terrane), and a calc-alkaline magmatic arc assemblage. Such an arrangement of tectonostratigraphic assemblages is comparable with cross sections through Phanerozoic accretionary continental margins and therefore supports an arc

  14. Integrated chronostratigraphy of Proterozoic-Cambrian boundary beds in the western Anabar region, northern Siberia.

    PubMed

    Kaufman, A J; Knoll, A H; Semikhatov, M A; Grotzinger, J P; Jacobsen, S B; Adams, W

    1996-01-01

    Carbonate-rich sedimentary rocks of the western Anabar region, northern Siberia, preserve an exceptional record of evolutionary and biogeochemical events near the Proterozoic/Cambrian boundary. Sedimentologically, the boundary succession can be divided into three sequences representing successive episodes of late transgressive to early highstand deposition; four parasequences are recognized in the sequence corresponding lithostratigraphically to the Manykal Formation. Small shelly fossils are abundant and include many taxa that also occur in standard sections of southeastern Siberia. Despite this coincidence of faunal elements, biostratigraphic correlations between the two regions have been controversial because numerous species that first appear at or immediately above the basal Tommotian boundary in southeastern sections have first appearances scattered through more than thirty metres of section in the western Anabar. Carbon- and Sr-isotopic data on petrographically and geochemically screened samples collected at one- to two-metre intervals in a section along the Kotuikan River, favour correlation of the Staraya Reckha Formation and most of the overlying Manykai Formation with sub-Tommotian carbonates in southeastern Siberia. In contrast, isotopic data suggest that the uppermost Manykai Formation and the basal 26 m of the unconformably overlying Medvezhya Formation may have no equivalent in the southeast; they appear to provide a sedimentary and palaeontological record of an evolutionarily significant time interval represented in southeastern Siberia only by the sub-Tommotian unconformity. Correlations with radiometrically dated horizons in the Olenek and Kharaulakh regions of northern Siberia suggest that this interval lasted approximately three to six million years, during which essentially all 'basal Tommotian' small shelly fossils evolved.

  15. Proterozoic structure, cambrian rifting, and younger faulting as revealed by a regional seismic reflection network in the Southern Illinois Basin

    USGS Publications Warehouse

    Potter, C.J.; Drahovzal, J.A.; Sargent, M.L.; McBride, J.H.

    1997-01-01

    Four high-quality seismic reflection profiles through the southern Illinois Basin, totaling 245 km in length, provide an excellent regional subsurface stratigraphic and structural framework for evaluation of seismic risk, hydrocarbon occurrence, and other regional geologic studies. These data provide extensive subsurface information on the geometry of the intersection of the Cambrian Reelfoot and Rough Creek rifts, on extensive Proterozoic reflection sequences, and on structures (including the Fluorspar Area Fault Complex and Hicks Dome) that underlie a transitional area between the well-defined New Madrid seismic zone (to the southwest) and a more diffuse area of seismicity in the southern Illinois Basin. Our principal interpretations from these data are listed here in order of geologic age, from oldest to youngest: 1. Prominent Proterozoic layering, possibly equivalent to Proterozoic (???1 Ga) Middle Run Formation clastic strata and underlying (1.3-1.5 Ga) volcanic rocks of the East Continent rift basin, has been strongly deformed, probably as part of the Grenville foreland fold and thrust belt. 2. A well-defined angular unconformity is seen in many places between Proterozoic and Cambrian strata; a post-Grenville Proterozoic sequence is also apparent locally, directly beneath the base of the Cambrian. 3. We infer a major reversal in Cambrian rift polarity (accommodation zone) in the Rough Creek Graben in western Kentucky. 4. Seismic facies analysis suggests the presence of basin-floor fan complexes at and near the base of the Cambrian interval and within parts of a Proterozoic post-Grenville sequence in several parts of the Rough Creek Graben. 5. There is an abrupt pinchout of the Mount Simon Sandstone against crystalline basement beneath the Dale Dome (near the Texaco no. 1 Cuppy well, Hamilton County) in southeastern Illinois, and a more gradual Mount Simon pinchout to the southeast. 6. Where crossed by the seismic reflection line in southeast Illinois, some

  16. Carbonate- and silicate-rich globules in the kimberlitic rocks of northwestern Tarim large igneous province, NW China: Evidence for carbonated mantle source

    NASA Astrophysics Data System (ADS)

    Cheng, Zhiguo; Zhang, Zhaochong; Santosh, M.; Hou, Tong; Zhang, Dongyang

    2014-12-01

    We report carbonate- and silicate-rich globules and andradite from the Wajilitage kimberlitic rocks in the northwestern Tarim large igneous province, NW China. The carbonate-rich globules vary in size from 1 to 3 mm, and most have ellipsoidal or round shape, and are composed of nearly pure calcite. The silicate-rich globules are elliptical to round in shape and are typically larger than the carbonate-rich globules ranging from 2 to several centimeters in diameter. They are characterized by clear reaction rims and contain several silicate minerals such as garnet, diopside and phlogopite. The silicate-rich globules, reported here for the first time, are suggested to be related to the origin of andradite within the kimberlitic rocks. Our results show that calcite in the carbonate-rich globules has a high XCa (>0.97) and is characterized by extremely high concentrations of the total rare earth elements (up to 1500 ppm), enrichment in Sr (8521-10,645 ppm) and LREE, and remarkable depletion in Nd, Ta, Zr, Hf and Ti. The calcite in the silicate-rich globules is geochemically similar to those in the carbonate-rich globules except the lower trace element contents. Garnet is dominantly andradite (And59.56-92.32Grs5.67-36.03Pyr0.36-4.61Spe0-0.33) and is enriched in light rare earth elements (LREEs) and relatively depleted in Rb, Ba, Th, Pb, Sr, Zr and Hf. Phlogopite in the silicate-rich globules has a high Mg# ranging from 0.93 to 0.97. The composition of the diopside is Wo45.82-51.39En39.81-49.09Fs0.88-0.95 with a high Mg# ranging from 0.88 to 0.95. Diopside in the silicate-rich globules has low total rare earth element (REE) contents (14-31 ppm) and shows middle REE- (Eu to Gd), slight light REE- and heavy REE-enrichment with elevated Zr, Hf and Sr contents and a negative Nb anomaly in the normalized diagram. The matrix of the kimberlitic rocks are silica undersaturated (27.92-29.31 wt.% SiO2) with low Al2O3 (4.51-5.15 wt.%) and high CaO (17.29-17.77 wt.%) contents. The

  17. Ti-rich Silicate Perovskite: A New Lower Mantle Phase and the Possible Source of Unradiogenic Hf in Kimberlites and Carbonatites

    NASA Astrophysics Data System (ADS)

    Collerson, K. D.; Terasaki, H.; Ohtani, E.; Suzuki, A.; Kondo, T.

    2005-12-01

    In an attempt to synthesize the pre-exsolution homogeneous phase proposed by [1] as the protolith of exsolution-textured cpx-ilm xenoliths in kimberlite, we conducted a subsolidus MA experiment at 25 GPa and 1800°C using a natural cpx-ilm xenolith from Monastery kimberlite containing 17% TiO2 as the starting composition. Phases identified (EPMA, Raman &XRD) were Ti-rich MgSi perovskite, Ti-rich CaSi perovskite and stishovite. TiO2 contents ranged from 16-18% in the CaTiSiPv to between 12.5 and 25% in the MgTiSiPv. This indicates that an extensive field of solid solution exists in the system MgSiPv - CaSiPv - CaTiPv at pressures greater than 24 GPa [cf. 2,3]. Furthermore, a multi-phase system was observed using XRD in a DAC experiment at 30 GPa and 1800°C. By contrast, the maximum TiO2 in majorite garnet in this composition is only 5-6% at 18 GPa [4]. Raman spectra for CaTiSiPv vary systematically with Ti content. This observation could have application for interpretation of spectra obtained in subsequent DA experiments on the stability of CaTiSiPv. The protolith of the cpx-ilm xenoliths does not exist as a single homogeneous Ti-rich silicate phase in the upper mantle. However, presence of a Ti-bearing phase in the lower mantle (LM) is inferred from crystals of CaSiPv and CaTiPv that occur in contact with each other in LM diamonds [5]. These were interpreted as reversion products, formed from CaSiPv and CaTiPv solid solution during ascent in kimberlite magma of at P < 9 GPa [2]. However, following [2] these phases must have existed as a single phase at higher pressure. Using the mean composition of CaTiSiPv produced in our experiments, we calculated that this solid solution is likely to involve 0.7 CaSiPv and 0.3 CaTiPv. Ti-rich SiPv in the LM phase may explain the "hidden" low Lu/Hf reservoir required by unradiogenic Hf isotopic compositions in kimberlites and carbonatites [6,7]. [1] Ringwood & Lovering (1970) EPSL,7, 371. [2] Kubo et al., (1997) PCM 24: 488

  18. Late Cretaceous remagnetization of Proterozoic mafic dikes, southern Highland Mountains, southwestern Montana: A paleomagnetic and 40Ar/39Ar study

    USGS Publications Warehouse

    Harlan, S.S.; Geissman, J.W.; Snee, L.W.; Reynolds, R.L.

    1996-01-01

    Paleomagnetic results from Early Proterozoic metabasite sills and Middle Proterozoic diabase dikes from the southern Highland Mountains of southwestern Montana give well-defined, dual-polarity magnetizations that are statistically identical to those from a small Late Cretaceous pluton that cuts the dikes. The concordance of paleomagnetic directions from rocks of three widely separated ages indicates that the Proterozoic rocks were remagnetized, probably during Late Cretaceous time. Paleomagnetic, rock magnetic, and petrographic observations from the metabasite and diabase samples indicate that remanence is carried primarily by low-Ti magnetite. Combining virtual geomagnetic poles from metabasite sills, diabase dikes, and the Late Cretaceous pluton, we obtain a paleomagnetic pole at 85.5??N, 310.7??E (K = 19.9, A95 = 9.1??, N = 14 sites) that is similar to a reference pole from the 74 Ma Adel Mountain Volcanics of western Montana. Biotite and hornblende 40Ar/39Ar isotopic dates from host basement geneiss and a hornblende from a remagnetized metabasite sill yield ages of ca. 1800 Ma; these dates probably record cooling of the southern Highland Mountains following high-grade metamorphism at 1.9-1.8 Ga. The gneiss and metabasite age spectra show virtually no evidence of disturbance, indicating that the basement rocks were never heated to temperatures sufficient to cause even partial resetting of their argon systems. Thus, the overprint magnetization of the Highland Mountains rocks is not a thermoremanent magnetization acquired during conductive cooling of nearby Late Cretaceous plutons. Remagnetization of the metabasite sills and diabase dikes was probably caused by localized thermochemical and thermoviscous effects during circulation of Late Cretaceous hydrothermal fluids related to epithermal mineralization. The absence of significant disturbance to the 40Ar/39Ar age spectrum from the remagnetized metabasite hornblende indicates that some secondary magnetizations may

  19. Subduction-related origin of eclogite xenoliths from the Wajrakarur kimberlite field, Eastern Dharwar craton, Southern India: Constraints from petrology and geochemistry

    NASA Astrophysics Data System (ADS)

    Dongre, A. N.; Jacob, D. E.; Stern, R. A.

    2015-10-01

    Major and trace elements as well as the first oxygen isotopes are reported on eclogite xenoliths from the Mesoproterozoic KL2 and P3 kimberlite pipes of the Wajrakarur kimberlite field, Eastern Dharwar craton, Southern India. Garnets in kyanite-bearing samples are rich in grossular, whereas they are predominantly pyrope-almandines in the bimineralic (kyanite-free) samples. The kyanite eclogite from the P3 pipe is more Mg-rich than those from KL2 pipe. Equilibration temperatures indicate derivation from 4.5 to 5.3 GPa and 1060 to 1220 °C for the KL2 samples and 3.6 GPa, 918 °C for the P3 sample. Garnet rare earth element patterns show two characteristic types, one with relatively low and flat heavy rare earth element patterns: Wajrakarur Group 1 and a second with lower light to heavy rare earth element ratios: Wajrakarur Group 2. Most samples in Wajrakarur Group 1 show pronounced positive Eu anomalies in garnet and positive Eu and Sr anomalies in the reconstructed whole rock trace element patterns; these are among the strongest anomalies in eclogite xenoliths worldwide. In contrast, Wajrakarur Group 2 samples show only subtle positive Eu anomalies. Oxygen isotopic ratios of garnets range between +5.3‰ and +7.8‰ δ18O. This range extends significantly beyond the range for unchanged mantle. Similar to many other eclogite suites worldwide, the Wajrakarur Group 1 and Group 2 eclogite suites shows evidence for an origin as crustal gabbroic material, likely once part of the oceanic crust, which was subducted and imbricated under the Eastern Dharwar craton. Their surface origin therefore lends support to geodynamic models that favor amalgamation of the Dharwar craton by subduction.

  20. Linking petrology and seismology of the southwest Greenland lithosphere

    NASA Astrophysics Data System (ADS)

    Lesher, C. E.; Vestergaard, C.; Brown, E.; Schutt, D.

    2015-12-01

    Mantle xenoliths from late-Proterozoic diamond-bearing kimberlitic dikes in the Kangerlussuaq, Sarfartoq and Maniitsoq areas of southwestern Greenland provide constraints on the composition and thermal state of lithospheric mantle beneath Greenland to depths of ~200 km [1]. Similarly, surface wave tomography studies carried out as part of the GLATIS project use a range of Rayleigh wave periods sensitive to structures at a similar depth interval within southwestern Greenland lithospheric mantle [2]. Here we link petrologic and seismologic constraints on the mantle lithosphere beneath Greenland utilizing methods of [3] that show that inferred chemical and mineralogical stratification inferred from petrology, showing mantle peridotite transitioning from garnet-free harzburgite to garnet lherzolite between ~70 and 180 km, cannot readily be resolved with fundamental mode Rayleigh waves. On the other hand, comparing phase velocities predicted from xenolith compositions, mineralogy and last equilibration temperatures and pressures, defining the continental geotherm during late-Proterozoic time, with those for the present-day mantle lithosphere suggest significant cooling of the cratonic mantle to a modern geotherm characterized by a heat flux of 30 mW/m2 and average crustal heat production of 0.3 mW/m3 [4]. These preliminary findings point to the weak dependence of shear wave velocities on mantle peridotite composition and mineralogy, and further illustrate its strong temperature dependence. Comparison of ancient and modern continental geotherms made possible by combining petrologic and seismological data, as shown here for southwest Greenland, provide additional constraints on secular cooling of cratonic regions linked to large-scale tectonic processes. [1] Bizzarro et al., 2003, CMP, 146; Sand et al., Lithos, 112. [2] Darbyshire et al., 2004, GJI, 158. [3] Schutt and Lesher, 2006, JGR, 111. [4] Meirerbachtol et al., 2015, JGR/ES, 120.

  1. Eclogite xenoliths from the Lace kimberlite, Kaapvaal craton: From convecting mantle source to palaeo-ocean floor and back

    NASA Astrophysics Data System (ADS)

    Aulbach, S.; Viljoen, K. S.

    2015-12-01

    Major- and trace-element compositions of eclogite and pyroxenite xenoliths of ≥2.5 Ga age (in situ Pb-Pb data on clinopyroxene) from the Lace kimberlite on the Kaapvaal craton were investigated in order to constrain: (1) the nature and evolution of their protoliths; (2) the extent to which they preserve information on the state of the asthenospheric mantle source that gave rise to their low-pressure protoliths; and (3) the effect of their deep recycling on the radiogenic isotope evolution of the convecting mantle. Their elemental relationships are consistent with low-pressure fractionation of olivine ± plagioclase and clinopyroxene during oceanic crust formation, whereby the residual melt was enriched in rare-earth elements (REE), high field-strength elements and Y, producing inverse correlations of ΣREE with the size of Eu- and Sr-anomalies. LREE-depletion may indicate loss of on average 20% of a partial melt upon subduction and metamorphism (eclogitisation) of oceanic crust, which did not, however, contribute to juvenile growth of continental crust. The eclogites have median Sm/Nd (0.40) and Lu/Hf (0.27) similar to Depleted Mantle, and lower U/Pb (0.02) and Th/Pb (0.02). If deeply subducted, these rocks cannot explain unradiogenic Nd and Hf, and radiogenic Pb isotope compositions in the sources of some modern ocean island basalts. Low incompatible trace-element contents similar to picrites, and Yb concentrations at a given TiO2 content similar to modern MORB, indicate derivation of the protoliths by average melt fractions of ∼ 0.20- 0.25 that left a spinel peridotite residue at pressures ≤2.5 to 3.0 GPa. This shallow intersection of the peridotite solidus suggests moderate Archaean ambient mantle potential temperatures of ≤1420 to 1470 °C. Samples filtered for clinopyroxene fractionation and metasomatism have V/Sc (4.7 ± 1.2; n = 11) indicating lower fO2 (-1.9 relative to the fayalite-magnetite-quartz buffer = ΔFMQ) than modern MORB. This is in part

  2. In-situ Analysis of Diamonds and Their Mineral Inclusions From the Lynx Kimberlite Dyke Complex, Central Quebec

    NASA Astrophysics Data System (ADS)

    van Rythoven, A.; McCandless, T. E.; Schulze, D. J.; Bellis, A.; Taylor, L. A.; Liu, Y.

    2009-05-01

    Twenty diamonds from the 522 Ma Lynx kimberlite dyke complex were selected from 442 stones in the 1.47- 3.45mm (+3 to +11 DTC) sieve class on the basis of visible inclusions. The 442 diamonds are part of a larger population of 6598 stones produced from 34 t and 494 t bulk samples taken in 2005 and 2007, respectively. The twenty diamonds all have octahedral primary growth forms. Three macles occur, as does one example of two intergrown octahedra connected along their {111} faces. Two samples are coarse intergrowths of octahedra. Most of the diamonds display a significant degree of resorption and range from octahedra with rounded corners and edges to tetrahexahedroida. Shield and serrate laminae, and hillocks are the most common resorption-related surface features. Nineteen of the samples have light brown to brown colouration. After their external morphology was examined, the diamonds were cut and polished along a single plane to expose included mineral grains for compositional analysis and to image internal structure. Cathodoluminescence imaging reveals deformation lamellae in the majority of the diamonds. A subset of these stones show deformation lamellae truncated by growth/resorption zones and in some cases intersection of planes of different orientation. Oscillatory planar growth patterns are the most common. However, examples of simple homogeneous, complex planar, and complex undulating growth zones occur. Inclusions, particularly olivine, typically occur in core/early growth regions of the diamonds. Of the twenty diamonds, sixteen have primary inclusions. The inclusion suite is largely peridotitic. Seventeen forsteritic olivine inclusions occur in ten diamonds and have molar Mg/(Mg+Fe)= 0.916-0.933. Seven Cr-diopside inclusions occur in one diamond (2.2-2.3 wt. % Cr2O3). Four Cr-pyropes (Cr/(Cr+Al) = 0.28-0.41) occur in three diamonds. Two enstatite inclusions (Mg/(Mg+Fe) = 0.938-0.94) occur in two diamonds. One heterogeneous inclusion of monosulfide solid

  3. Analysis of single oil-bearing fluid inclusions in mid-Proterozoic sandstones (Roper Group, Australia)

    NASA Astrophysics Data System (ADS)

    Siljeström, Sandra; Volk, Herbert; George, Simon C.; Lausmaa, Jukka; Sjövall, Peter; Dutkiewicz, Adriana; Hode, Tomas

    2013-12-01

    Hydrocarbons and organic biomarkers extracted from black shales and other carbonaceous sedimentary rocks are valuable sources of information on the biodiversity and environment of early Earth. However, many Precambrian hydrocarbons including biomarkers are suspected of being younger contamination. An alternative approach is to study biomarkers trapped in oil-bearing fluid inclusions by bulk crushing samples and subsequently analysing the extracted hydrocarbons with gas chromatography-mass spectrometry. However, this method does not constrain the hydrocarbons to one particular oil inclusion, which means that if several different generations of oil inclusions are present in the sample, a mix of the content from these oil inclusions will be analysed. In addition, samples with few and/or small inclusions are often below the detection limit. Recently, we showed that it is possible to detect organic biomarkers in single oil-bearing fluid inclusions using time-of-flight secondary ion mass spectrometry (ToF-SIMS). In the present study, single fluid inclusion analysis has been performed on Proterozoic samples for the first time. Four individual oil-bearing fluid inclusions, found in 1430 Ma sandstone from the Roper Superbasin in Northern Australia, were analysed with ToF-SIMS. The ToF-SIMS spectra of the oil in the different inclusions are very similar to each other and are consistent with the presence of n-alkanes/branched alkanes, monocyclic alkanes, bicyclic alkanes, aromatic hydrocarbons, and tetracyclic and pentacyclic hydrocarbons. These results are in agreement with those obtained from bulk crushing of inclusions trapped in the same samples. The capability to analyse the hydrocarbon and biomarker composition of single oil-bearing fluid inclusions is a major breakthrough, as it opens up a way of obtaining molecular compositional data on ancient oils without the ambiguity of the origin of these hydrocarbons. Additionally, this finding suggests that it will be possible

  4. New Paleomagnetic Constraints on the Proterozoic Supercontinent Evolution: A view from the South

    NASA Astrophysics Data System (ADS)

    Trindade, R. I.

    2013-05-01

    The assembly and disruption of supercontinents is thought to have impacted the long-term evolution of different envelopes of the Earth throughout Precambrian times, from mantle convection dynamics to feedback mechanisms leading to the stepwise change in atmospheric oxygenation. But the timing, duration, the size and the paleogeographic configuration of Precambrian supercontinents is still a matter of discussion. Large South American cratonic units (>30,000 km2) such as the Amazon, Rio de la Plata and São Francisco are usually represented as key pieces of different supercontinental assemblies but their paleomagnetic database is still scarce. The most important advances in the Precambrian paleomagnetic database concerns the Amazon Craton. Recent paleomagnetic studies allows one to track the participation of the Amazon Craton in several supercontinent assemblies from 2.0 Ga up to the end of the Proterozoic era. Amazonia was definitely part of the Columbia Supercontinent as attested by 1.78-1.79 Ga key poles. This supercontinent also comprised Laurentia, Baltica, North China, and Amazonia, forming a long and continuous landmass, linked by Paleo- to Mesoproterozoic mobile belts. Paleomagnetic data for Amazonia support a long-lived connection between Laurentia and Baltica at least until 1.26 Ga ago. However, new paleomagnetic poles from the same craton suggest that Columbia was in fact ephemerous, indicating a changing configuration between Amazonia and Baltica between 1.78 and 1.44 Ma. At the end of the Mesoproterozoic, the Amazon craton is part of Rodinia based on its record of Grenvillian events with overlapping ages with similar orogenic belts in eastern Laurentia. But its relative position in that supercontinent is still intensively debated. Presently only four poles for the Amazonian craton are available for the 1,200-900 Ma interval. Based on these results a dynamic model for the Amazonian craton was envisaged, which considers its oblique collision with southern

  5. The West African Mauritanid metamorphic suite of Proterozoic age in the subsurface of peninsular Florida and environs

    SciTech Connect

    Winston, G.O.

    1993-03-01

    A high and low-grade Gondwanan metamorphic terrane is revealed by 14 wells in Florida and environs. Two high-grade metamorphics (gneiss and schist) are located in central Florida and are probably Early Proterozoic in age. The 12 other wells contain low-grade metamorphic suites, principally composed of inter-bedded argillites, acid volcanics and quartzites belonging to the Mauritanid sequence of West Africa. These suites are present in south Georgia, north Florida and offshore; a 3,975-foot section was penetrated in one well. These widespread metamorphic rocks are probably the terrane into which the Cambrian Osceola granite of central Florida was intruded. The two grades of metamorphics represent Early and Late Proterozoic episodes of sedimentation, each followed by metamorphism and erosion. Lower Ordovician to Devonian sediments were deposited on this terrane in southern Georgia and northern Florida. In the Early Jurassic, volcanics completely covered southern Florida, concealing the nature of the old underlying surface. Younger Mesozoic sediments eventually buried the entire Pre-Cambrian-Lower Jurassic terrane.

  6. Constraining the location of the Archean--Proterozoic suture in the Great Basin based on magnetotelluric soundings

    USGS Publications Warehouse

    Rodriguez, Brian D.; Sampson, Jay A.

    2012-01-01

    It is important to understand whether major mining districts in north-central Nevada are underlain by Archean crust, known to contain major orogenic gold deposits, or, alternatively, by accreted crust of the Paleoproterozoic Mojave province. Determining the location and orientation of the Archean-Proterozoic suture zone between the Archean crust and Mojave province is also critical because it may influence subsequent patterns of sedimentation, deformation, magmatism, and hydrothermal activity. In the Great Basin, the attitude of the suture zone is unknown because it is concealed below cover. A regional magnetotelluric sounding profile along the Utah-Nevada State line reveals a deeply penetrating, broad electrical conductor that may be the Archean-Proterozoic suture zone in the northwest corner of Utah. This major crustal conductor's strike direction is northwest, where it broadens to about 80 km wide below about 3-km depth. These results suggest that the southwestern limit of intact Archean crust in this part of the Great Basin is farther north than previously reported. These results also suggest that the major gold belts in north-central Nevada are located over the Paleoproterozoic Mojave province, and the Archean terrain lies northeast in the northwest corner of Utah. Rifted Archean crust segments south and west of the suture suggest that future mineral exploration northeast of current mineral trends may yield additional gold deposits.

  7. Chronologic and isotopic framework for early Proterozoic crustal evolution in the eastern Mojave Desert region, SE California

    USGS Publications Warehouse

    Wooden, J.L.; Miller, D.M.

    1990-01-01

    The Early Proterozoic geologic evolution of the region, as defined by characteristics of its supracrustal rocks, granitoids, metamorphism, structural history, and Pb and Nd isotopic signature, contrasts sharply with other Proterozoic provinces of the southwestern US. The oldest supracrustal rocks contain zircons over 2.0 Ga, corroborating Nd isotopic evidence for a much older crust here. Granitoids widely emplaced within these supracrustal rocks range from 1.76 to 1.64 Ga. The earlier plutons and surrounding supracrustal rocks were metamorphosed to granulite and high amphibolite facies throughout the province at about 1705 Ma in a migmatite-producing event that we term (informally) the Ivanpah orogeny. Subsequent granitoids, emplaced from 1.69 to 1.67 Ga, were voluminous along a north trending belt in the middle of the Mojave province. Younger plutons were emplaced at about 1.66 Ga in several places and at about 1.64 Ga along the extreme southern part of the province. -from Authors

  8. Diamonds, Eclogites, and the Oxidation State of the Earth's Mantle.

    PubMed

    Luth, R W

    1993-07-01

    The reaction dolomite + 2 coesite --><-- diopside + 2 diamond + 2O(2) defines the coexistence of diamond and carbonate in mantle eclogites. The oxygen fugacity of this reaction is approximately 1 log unit higher at a given temperature and pressure than the oxygen fugacities of the analogous reactions that govern the stability of diamond in peridotite. This difference allows diamond-bearing eclogite to coexist with peridotite containing carbonate or carbonate + diamond. This potential coexistence of diamond-bearing eclogite and carbonate-bearing peridotite can explain the presence of carbon-free peridotite interlayered with garnet pyroxenites that contain graphitized diamond in the Moroccan Beni Bousera massif at the Earth's surface and the preferential preservation of diamond-bearing eclogitic relative to peridotitic xenoliths in the Roberts Victor kimberlite. PMID:17750546

  9. Diamonds, Eclogites, and the Oxidation State of the Earth's Mantle.

    PubMed

    Luth, R W

    1993-07-01

    The reaction dolomite + 2 coesite --><-- diopside + 2 diamond + 2O(2) defines the coexistence of diamond and carbonate in mantle eclogites. The oxygen fugacity of this reaction is approximately 1 log unit higher at a given temperature and pressure than the oxygen fugacities of the analogous reactions that govern the stability of diamond in peridotite. This difference allows diamond-bearing eclogite to coexist with peridotite containing carbonate or carbonate + diamond. This potential coexistence of diamond-bearing eclogite and carbonate-bearing peridotite can explain the presence of carbon-free peridotite interlayered with garnet pyroxenites that contain graphitized diamond in the Moroccan Beni Bousera massif at the Earth's surface and the preferential preservation of diamond-bearing eclogitic relative to peridotitic xenoliths in the Roberts Victor kimberlite.

  10. Lithospheric reworking at the Proterozoic-Phanerozoic transition of Australia imaged using AusLAMP Magnetotelluric data

    NASA Astrophysics Data System (ADS)

    Robertson, Kate; Heinson, Graham; Thiel, Stephan

    2016-10-01

    Seventy-four stations from the long-period Australia-wide AusLAMP (Australian Lithospheric Architecture Magnetotelluric Project) dataset were used to image the electrical resistivity beneath the Neoproterozoic Ikara-Flinders Ranges and adjacent Palaeo-Mesoproterozoic Curnamona Province. Results from 3D inversions using ModEM software show a relatively resistive Ikara-Flinders Ranges, with two parallel arcuate conductors at 20 to 80 km depth in the Nackara Arc. There is a good correlation of diamondiferous kimberlites occurring over conductors, which we interpret as evidence for these conductors to be residing on large lithospheric structures that have been conduits for partial melt and volatile movement in the Jurassic. The Curnamona Province is remarkably conductive for a region that is thought to have a cratonic core, with Delamerian reworking only at its edges. The conductor covers most of the province at depths of 10-40 km, and its presence at lower crustal depths suggests that conductive sediments can not entirely explain it. Fluids associated with subduction may have pervasively modified the crust in the past, resulting in an enrichment of carbon, enhancing the conductivity. Additionally, we conclude that the notion of a single continuous arcuate Flinders Conductivity Anomaly is unlikely and that the anomalous response observed is instead a result of the combined response of three separate anomalies; the Curnamona Province Conductor and the two Nackara Arc Conductors.

  11. Geochemistry and Rb-sr geochronology of associated proterozoic peralkaline and subalkaline anorogenic granites from Labrador

    NASA Astrophysics Data System (ADS)

    Collerson, Kenneth D.

    1982-12-01

    Anorogenic granites of middle to late Proterozoic age in the Davis Inlet — Flowers Bay area of Labrador are subdivided on the basis of petrology and geochemistry into three coeval suites. Two of these are high-temperature anhydrous hypersolvus granites: a peralkaline aegirine-sodic-calcic to sodic amphibole-bearing suite and a non-alkaline fayalite-pyroxene-bearing suite. The third is a group of non-alkaline subsolvus hornblende-biotite-bearing granites. Associated with the hypersolvus peralkaline suite is a group of genetically related syenites and quartz syenites. The granites cut ca. 3,000 Ma old Archaean gneisses as well as Elsonian layered basic intrusions of the Nain Complex. One of these, a crudely layered mass which ranges in composition from gabbro to diorite and monzonite, appears to be related to the syenites. The peralkaline granites and some of the syenites are extremely enriched in the high field-strength elements such as Y, Zr, Nd, as well as Rb, Ga and Zn, and have low abundances of Ba, Sr and most of the transition elements. In contrast, the non-alkaline hypersolvus and subsolvus granites do not show the same degree of enrichment. Concentration of the highly charged cations in the peralkaline suite is believed to be the result of halogen-rich fluid activity during fractionation of the magma. The sodic evolution trend in the peralkaline suite is reflected mineralogically by the development of aegirine and aegirine-hedenbergite solid solutions, and by a spectacular amphibole compositional range from katophorite through winchite, richterite, riebeckite to arfvedsonite and ferro eckermannite. Accessory phases which are ubiquitous in these rocks include aenigmatite, astrophyllite, fluorite, monazite and zircon. The non-alkaline hypersolvus granites typically contain iron-rich phases such as fayalite, eulite, ferrosilite-hedenbergite, and annite rich biotite. In the subsolvus granites, amphiboles range in composition from edenite through common

  12. Clastic metasediments of the Early Proterozoic Broken Hill Group, New South Wales, Australia: Geochemistry, provenance, and metallogenic significance

    USGS Publications Warehouse

    Slack, J.F.; Stevens, B.P.J.

    1994-01-01

    Whole-rock analyses of samples of pelite, psammite, and psammopelite from the Early Proterozoic Broken Hill Group (Willyama Supergroup) in the Broken Hill Block, New South Wales, Australia, reveal distinctive geochemical signatures. Major-element data show high Al2O3 and K2O, low MgO and Na2O, and relatively high Fe2O3T MgO ratios, compared to average Early Proterozoic clastic metasediments. High field strength elements (HFSE) are especially abundant, including Nb (most 15-27 ppm), Ta (most 1.0-2.2 ppm), Th (17-36 ppm), Hf (4-15 ppm), and Zr (most 170-400 ppm); Y (33-74 ppm) is also high. Concentrations of ferromagnesian elements are generally low (Sc = < 20 ppm, Ni = ??? 62 ppm, Co = <26 ppm; Cr = most < 100 ppm). Data for rare earth elements (REEs) show high abundances of light REEs (LaCN = 116-250 ?? chondrite; LaCN = 437 in one sample), high LaCN YbCN ratios (5.6-13.9), and large negative Eu anomalies ( Eu Eu* = 0.32-0.57). The geochemical data indicate derivation of the metasedimentary rocks of the Broken Hill Group by the erosion mainly of felsic igneous (or meta-igneous) rocks. High concentrations of HFSE, Y, and REEs in the metasediments suggest a provenance dominanted by anorogenic granites and(or) rhyolites, including those with A-type chemistry. Likely sources of the metasediments were the rhyolitic to rhyodacitic protoliths of local quartz + feldspar ?? biotite ?? garnet gneisses (e.g., Potosi-type gneiss) that occur within the lower part of the Willyama Supergroup, or chemically similar basement rocks in the region; alternative sources may have included Early Proterozoic anorogenic granites and(or) rhyolites in the Mount Isa and(or) Pine Creek Blocks of northern Australia, or in the Gawler craton of South Australia. Metallogenic considerations suggest that the metasediments of the Broken Hill Block formed enriched source rocks during the generation of pegmatite-hosted deposits and concentrations of La, Ce, Nb, Ta, Th, and Sn in the region. Li, Be, B, W

  13. Seismic evidence for a mantle source for mid-Proterozoic anorthosites and implications for models of crustal growth

    USGS Publications Warehouse

    Musacchio, G.; Mooney, W.D.

    2002-01-01

    Voluminous anorthosite intrusions are common in mid-Proterozoic crust. Historically, two end-member models have been proposed for the origin of these anorthosites. In the first model anorthosites derive from fractionation of a mantle source leaving a residue of metagabbro in the lower crust; in the second model anorthosites are the product of partial melting of the lower crust with residual pyroxene and high-grade minerals (i.e. a pyroxenitic and/or metapelitic lower crust). Although a general consensus has developed that the first model provides the best fit to petrological and geochemical constraints, the sparse evidence for mafic and ultramafic counterparts to the anorthosites leaves the issue still unresolved. We use the absolute P-wave velocity and the ratio between P- and S-wave velocities (VP/VS) to infer the composition of the lower crust beneath the Marcy Anorthosite (New York State, USA). Seismic refraction data reveal a lower crust 20 km thick, where VP and VP/VS range from top to bottom between 7.0 km s-1 and 7.2 ?? 0.1 and 1.84 km s-1 and 1.81 ?? 0.02, respectively. Laboratory measurements on rock samples indicate that these seismic properties are typical of plagioclase-rich rocks. Magmatic underplating of basaltic melts is a mechanism to form plagioclase-rich bulk composition for the Grenville crust. At the bottom of the lower crust, increase of P-wave velocity, slight decrease of VP/VS ratios and the presence of a low-reflective seismic Moho are additional observations supporting crust-mantle interactions related to magmatic underplating. High P-wave velocity (8.6 km s-1) in the upper mantle may indicate that the ultramafic portion (e.g. pyroxenites) of the underplated magma has become eclogite. High average P-wave velocity (6.7 km s-1) and VP/VS (1.81), and the exceptional abundance of anorthosites-norites-troctolites among the rocks exposed at the surface, indicate that the Grenville Proterozoic crust may have a unique plagioclase-rich bulk

  14. Hydrogeologic and water-quality characteristics of the crystalline-rock aquifers of Archean and Proterozoic age, Minnesota

    USGS Publications Warehouse

    Anderson, H.W.

    1986-01-01

    Five aquifers in crystalline rocks of Archean and Proterozoic age in Minnesota include in descending order the North Shore Volcanic, Sioux Quartzite, Proterozoic metasedimentary, Biwabik Iron formation and undifferentiated Precambrian aquifers. The North Shore Volcanic aquifer generally yields < 15 gal/min to wells from interflow sediments and fractures in the basaltic lava flows along the northern shore of Lake Superior and along the upper St. Croix River. Dissolved solids concentrations range from 91 to 74,300 mg/L, and the water is of several chemical types. The Sioux Quartzite aquifer yields from 1 to 450 gal/min to wells open to joints and fractures and loose sand zones in the predominantly pink orthoquartzite in southwestern Minnesota. Dissolved solids concentrations range from 237 mg/L in water from wells in outcrop areas to 2,300 mg/L from wells where the Sioux Quartzite aquifer underlies Cretaceous rocks or thick Des Moines drift. The water generally is a calcium sulfate type. The Proterozoic metasedimentary aquifer generally yields < 20 gal/min to wells in weathered regolith and fractures in thin-bedded gray to black argillite in north-central Minnesota. Dissolved solids concentrations generally range from 126 to 340 mg/L, and the water is a calcium magnesium bicarbonate type. The Biwabik Iron formation aquifer yields 1,000 gal/min to wells in leached zones in the ferruginous chert and interbedded hematite and magnitite iron ore in north-central Minnesota. Dissolved solids range from 157 to 388 mg/L in water that is a calcium magnesium bicarbonate type. The undifferentiated Precambrian aquifer generally yields < 25 gal/min to wells from fractures and the weathered regolith developed on a variety of crystalline-rock types. Wells have been developed in parts of the aquifer throughout the State except in the southeast where it is too deeply buried. Dissolved solids concentrations average < 400 mg/L in central and northeastern Minnesota, but average about 700

  15. Mid-Proterozoic detrital zircons and the depositional history of the Jack Hills (Narryer Gneiss Complex, Western Australia)

    NASA Astrophysics Data System (ADS)

    Bell, E. A.; Harrison, T. M.; Mojzsis, S. J.

    2012-12-01

    Quartz-biotite schists ("metaconglomerates" s.l.) in the Jack Hills outcrops of the Narryer Terrane of Western Australia host detrital >4 Ga zircons. These rocks are presumed to have been deposited at about 3 Ga and subsequently experienced metamorphism and granite intrusion at 2.45-2.6 Ga. Protoliths of the oldest detrital zircon population (>3.8 Ga) have not yet been identified, and the post-2.6 Ga history of the Jack Hills remains murky. Rare metavolcanic units are documented at ca. 1.8-1.9 Ga. Some metasediments near the Hadean zircon "discovery" site that are apparently young enough to have incorporated mid-Proterozoic detrital zircons are evidence of further geologic activity, although it is possible that these rocks were tectonically juxtaposed later. To further resolve the Proterozoic history of the Jack Hills, we report U-Pb age data for 60 detrital zircons extracted from two quartzites from the eastern Jack Hills (~50 km distant from the "discovery" site of Hadean zircons); in this sample suite 26 concordant zircons range in age from 0.168-3.49 Ga, with an age peak at ca. 2.6 Ga and a scattering of ages from 1.06-2.0 Ga. Zircon sources <1.8 Ga have not so far been identified in the Narryer Terrane and may point to far-field sediment transport. Several ca. 1.07 Ga zircons may derive from the Warakurna Large Igneous Province of the same age, which extends through much of central and Western Australia. One concordant zircon with a 206Pb/238U age of 0.166±6 Ga displays a patchy internal texture indicative of alteration; all other concordant grains are >1.06 Ga. Only 2 concordant zircons display ages >3 Ga, unlike the dominant mid-Archean signature from other reported Proterozoic sediments in the Jack Hills. All told, these quartzites display a significantly different provenance than other previously studied units from the terrane, with decreased importance of mid-Archean zircon sources and significantly younger (<1.06 Ga) ages of sedimentation than

  16. 40Ar/ 39Ar-ages of phlogopite in mantle xenoliths from South African kimberlites: Evidence for metasomatic mantle impregnation during the Kibaran orogenic cycle

    NASA Astrophysics Data System (ADS)

    Hopp, Jens; Trieloff, M.; Brey, G. P.; Woodland, A. B.; Simon, N. S. C.; Wijbrans, J. R.; Siebel, W.; Reitter, E.

    2008-12-01

    We applied the 40Ar/ 39Ar dating method to an extensive suite of phlogopites from kimberlite-hosted mantle xenoliths (dominantly garnet bearing) from the mines of Bultfontein (South Africa), Letseng-la-Terae and Liqhobong (Lesotho). Argon extraction was performed by conventional high resolution stepwise heating technique, laser incremental heating technique and laser spot analysis. All age spectra obtained by conventional analysis indicate various degrees of 40Ar loss during kimberlite emplacement, but never resulted in a total reset of the argon system. Most intriguingly, the sample-specific maximum apparent ages cluster between 1.0 and 1.22 Ga for the phlogopites with the least disturbed age spectra. A maximum apparent age of 1.02 Ga was observed during laser heating analysis. Individual grains tend to yield older ages in their cores, with successively younger ages at their rims. The range in age obtained via the laser fusion technique and with conventional stepwise heating technique agrees with each other, as well as with literature data. The often inferred presence of excess 40Ar in those phlogopites cannot explain the coherent age pattern in the large suite of samples. Hence, the age constraint of 1.0-1.25 Ga is regarded as geologically meaningful and assigned to metasomatism of the local cratonic mantle during the advent of Kibaran orogenesis (1.00-1.25 Ga). The major consequences of our findings are: (i) The argon system of phlogopite can remain closed for long time scales, even at ambient temperatures of 800-1200 °C within the mantle, most likely because the solid/solid partitioning behaviour of Ar between phlogopite and other major phases in the mantle strongly favours phlogopite, or because conventionally inferred diffusivity of argon in phlogopite is seriously overestimated. Thus, the 40Ar/ 39Ar phlogopite system appears to be a valuable tool for deciphering ancient metasomatic events affecting the lithospheric mantle. (ii) The cratonic lithospheric

  17. The age and history of the lithospheric mantle of the Siberian craton: Re-Os and PGE study of peridotite xenoliths from the Obnazhennaya kimberlite

    NASA Astrophysics Data System (ADS)

    Ionov, Dmitri A.; Carlson, Richard W.; Doucet, Luc S.; Golovin, Alexander V.; Oleinikov, Oleg B.

    2015-10-01

    The formation age of the lithospheric mantle of the Siberian craton (one of the largest on Earth) is not well established; nearly all published whole-rock Re-Os data are for mantle xenoliths from a single kimberlite in the center of the craton (Udachnaya). We report Re-Os isotope and PGE concentration data for 19 spinel and garnet peridotite xenoliths from the Obnazhennaya kimberlite in the northeastern portion of the craton. Most samples in this study, and many Obnazhennaya peridotites in general, show a combination of relatively low Al2O3 (0.1-2%) with high CaO (1.4-4%) concentrations. Only four dunites and harzburgites in our sample suite have low contents of both Al2O3 and CaO (0.1-0.8%), but their relatively low Mg# (0.888-0.919) and highly variable Os concentrations (0.6-35 ppb) suggest they may have formed in melt migration channels rather than as residues of partial melt extraction. A group of six Ca-rich (2.0-3.2% CaO) peridotites yields the highest Re-Os model ages (mean TRD = 2.8 Ga, mean TMA = 3.5 Ga). Eight peridotites with low to moderate Al2O3 (<2%) and Mg# ≥0.91, including three low-Ca harzburgites, yield lower Re-Os model ages (mean TRD = 1.9 Ga, mean TMA = 2.2 Ga). The remainder of the samples may not yield meaningful TRD ages because they are not refractory (Al2O3 >2.6% and/or Mg# ≤0.90). We interpret these results as evidence for a two-stage formation of the lithospheric mantle. The peridotites formed at the two stages show very similar chemical compositions. The enrichment in Ca, which we attribute to widespread post-melting metasomatism by carbonate-rich melts, may have taken place either at the end of the Archean melting event, when at least one Ca-Al-rich peridotite was formed, or later. The combined Re-Os age data on xenoliths from Obnazhennaya and Udachnaya suggest that the lithospheric mantle beneath the Siberian craton was not formed in a single event, but grew in at least two events, one in the late Archean and the other in the

  18. Post-Archean formation of the lithospheric mantle in the central Siberian craton: Re-Os and PGE study of peridotite xenoliths from the Udachnaya kimberlite

    NASA Astrophysics Data System (ADS)

    Ionov, Dmitri A.; Doucet, Luc S.; Carlson, Richard W.; Golovin, Alexander V.; Korsakov, Andrey V.

    2015-09-01

    The formation age of the Siberian cratonic mantle is not well established. Re-Os data on various mantle-derived materials brought up by kimberlite magmas have shown that it contains Archean components, but the reported ages range broadly (3.4 to <1 Ga). We report Re-Os isotope and PGE concentration data for a suite of 29 fresh, well-characterized xenoliths from the Udachnaya-East kimberlite representing all major peridotite rock types and a large part of the cratonic mantle profile. Several xenoliths with very low Os contents (<0.3 ppb) and/or high Re/Os ratios are not suitable for age estimates. The Os (and Ir) depletions are common in cpx-bearing spinel harzburgites and coarse garnet harzburgites, but are not found in deformed, high-T peridotites. Twenty refractory (Al2O3 0.1-1.6%) peridotites yield TRD ages from 0.9 to 2.2 Ga. TRD for a subset of six high-Mg# (0.92-0.93), low-T (⩽930 °C) spinel harzburgites and a single garnet harzburgite yield a narrow range from 2.0 to 2.2 Ga with an average of 2.1 ± 0.1 Ga, which we consider the best estimate for the age of the melting event that initially formed the lithospheric mantle beneath Udachnaya. The TRD estimates for less refractory (Mg# 0.907-0.919) deformed garnet peridotites show a greater range and are generally lower (0.9-2.0 Ga; average 1.54 ± 0.28 Ga) apparently due to the effects of melt metasomatism on the initial melting residues. The predominant part of the mantle in the central Siberian craton formed in the Paleoproterozoic and not in the Archean, unlike cratons in southern Africa and North America. Minor older components reported earlier from Udachnaya may be fragments of pre-existing lithosphere trapped during stacking of melting residues formed about 2 Ga ago. We argue that the formation of cratonic lithospheric mantle, with common high-Mg# (⩾0.92) and opx-enriched peridotites, was not limited to the Archean as previously thought, but continued in the Paleoproterozoic, i.e. that asthenospheric

  19. Trace element enrichment in off-craton peridotites: comparison of off-craton Proterozoic and Pan-African mantle beneath southern Africa

    NASA Astrophysics Data System (ADS)

    le Roex, Anton; Class, Cornelia

    2014-05-01

    Trace element abundances in constituent clinopyroxene, garnet and phlogopite from off-craton mantle beneath the Proterozoic Namaqua-Natal belt (South Africa) and in clinopyroxene and amphibole from the Pan African Damara belt (Namibia) have been determined by laser ablation ICP-MS. Chondrite normalised rare earth element abundances in clinopyroxenes show a wide range of patterns from LREE depleted, through sinusoidal patterns to strongly LREE enriched. In garnets, the patterns range from strongly LREE depleted with flat HREE, through sinusoidal patterns to 'humped' sinusoidal patterns. Numerical modelling shows that the range of REE enrichment patterns can be generated via reactive porous flow mechanisms involving an alkaline silicate melt and where, in the Proterozoic mantle, there is a high fluid/solid ratio (i.e. all clinopyroxene and garnet are products of modal metasomatism and/or near fully equilibrated with a metasomatic fluid). In contrast, clinopyroxene in the Pan African mantle shows evidence for cryptic metasomatism, where enrichment of pre-existing clinopyroxene by alkaline silicate melts has occurred at the distal regions of an advancing metasomatic front. Extreme fluid/melt compositions are not required to account for the enrichment patterns Numerical modelling of primitive mantle normalised trace element abundances in clinopyroxenes show that the commonly observed strong relative depletions in Nb-Ta and Rb-Ba present in clinopyroxenes can be accounted for by crystallisation of accessory phlogopite (in Proterozoic mantle) and amphibole (in Pan African mantle) from the metasomatising melt/fluid. In contrast, strong relative depletion in Zr-Hf in some clinopyroxenes in both Proterozoic and Pan African mantle regions requires either metasomatic crystallisation of zircon, or involvement of a carbonatitic melt as metasomatic agent. Trace element enrichment patterns seen in clinopyroxene and garnet in the Proterozoic off-craton regions of southern Africa

  20. Neodymium, strontium, and oxygen isotopic variations in the crust of the western United States: Origin of Proterozoic continental crust and tectonic implications

    SciTech Connect

    Bennett, V.C.

    1989-01-01

    Initial Nd isotopic ratios of crystalline rocks from an area of about 1.5 {times} 10{sup 6} km{sup 2} of the western United States have been determined in order to map Precambrian age province boundaries and thus document the growth and modification of the North American continent in the Proterozoic. Three age provinces have been delineated. It is demonstrated that large regions of Early Proterozoic continental crust were formed with anomalous isotopic compositions ({sup 143}Nd/{sup 144}Nd ratios lower than Early Proterozoic depleted-mantle). The variations in the initial {epsilon}{sub Nd} and {delta}{sup 18}O values correlate with each other, and correspond to the previously determined Nd isotopic provinces. The Pelona, Rand, Chocolate Mountain and Orocopia Schists are represented by 15 lithologically and structurally similar schist bodies exposed along the San Andreas and Garlock faults in southern California. The grayschists have measured {epsilon}{sub Nd} values from -1.7 to -11.7 with depleted-mantle model ages of 0.9 to 1.7 Ga. The Nd isotopic compositions can be modeled as variable mixtures of Early Proterozoic continental crust with a Mesozoic are component. The measured {sup 87}Sr/{sup 86}Sr ratios are from 0.7087 to 0.7129 and reflect the presence of an old continental source. Independent of age, the high initial {epsilon}{sub Nd} values ({sup +}9 {plus minus} 1.5) are consistent with derivation at an oceanic spreading center, either at a MORB or in a back-arc basin environment. The presence of both Early Proterozoic continental detritus and a younger sedimentary component in the grayschist protolith, and the MORB affinity of the metabasalts are compatible with formation of the protoliths of the Pelona and related schists in a Mesozoic basin adjacent to the southwestern United States continental margin.

  1. Paleomagnetism and geochronology of an Early Proterozoic quartz diorite in the southern Wind River Range, Wyoming, USA

    USGS Publications Warehouse

    Harlan, S.S.; Geisman, J.W.; Premo, W.R.

    2003-01-01

    We present geochronologic and paleomagnetic data from a north-trending quartz diorite intrusion that cuts Archean metasedimentary and metaigneous rocks of the South Pass Greenstone Belt of the Wyoming craton. The quartz diorite was previously thought to be either Archean or Early Proterozoic (?) in age and is cut by north and northeast-trending Proterozoic diabase dikes of uncertain age, for which we also report paleomagnetic data. New U-Pb analyses of baddeleyite and zircon from the quartz diorite yield a concordia upper intercept age of 2170 ?? 8 Ma (95% confidence). An 40Ar/39Ar amphibole date from the same sample yields a similar apparent age of about 2124 ?? 30 Ma (2??), thus confirming that the intrusion is Early Proterozoic in age and that it has probably not been thermally disturbed since emplacement. A magmatic event at ca. 2.17 Ga has not previously been documented in the Wyoming craton. The quartz diorite and one of the crosscutting diabase dikes yield essentially identical, well-defined characteristic remanent magnetizations. Results from eight sites in the quartz diorite yield an in situ mean direction of north declination and moderate to steep positive inclination (Dec.=355??, Inc.=65??, k=145, ??95=5??) with a paleomagnetic pole at 84??N, 215??E (??m=6??, ??p=7??). Data from other diabase dike sites are inconsistent with the quartz diorite results, but the importance of these results is uncertain because the age of the dikes is not well known. Interpretation of the quartz diorite remanent magnetization is problematic. The in situ direction is similar to expected directions for magnetizations of Late Cretaceous/early Tertiary age. However, there is no compelling evidence to suggest that these rocks were remagnetized during the late Mesozoic or Cenozoic. Assuming this magnetization to be primary, then the in situ paleomagnetic pole is strongly discordant with poles of 2167, 2214, and 2217 Ma from the Canadian Shield, and is consistent with proposed

  2. Yardea Dacite -large-volume, high-temperature felsic volcanism from the Middle Proterozoic of South Australia

    SciTech Connect

    Creaser, R.A.; White, A.J.R. )

    1991-01-01

    The Yardea Dacite is a large-volume felsic volcanic unit from the Middle Proterozoic Gawler Range Volcanics of South Australia; it has been previously described as an ignimbrite. However, some samples contain no petrographic evidence for a pyroclastic origin, but have characteristics compatible with final crystallization from a nonfragmented magma. These samples may have erupted as lavas, but others are likely to be extremely densely welded ignimbrites, suggesting a compound nature for the unit. Geothermometry and phase equilibria indicate that the Yardea Dacite originated from a high-temperature ({approximately}1,000{degree}C) felsic magma with a low water content ({le}2%). The Yardea Dacite is not associated with a known caldera of the Valles type, and shares many characteristics of recently described Cenozoic felsic volcanic rocks from the western United States, interpreted as rheoignimbrites or as unusually extensive lavas.

  3. HYDROCARBON SOURCE ROCK EVALUATION OF MIDDLE PROTEROZOIC SOLOR CHURCH FORMATION, NORTH AMERICAN MID-CONTINENT RIFT SYSTEM, RICE COUNTY, MINNESOTA.

    USGS Publications Warehouse

    Hatch, J.R.; Morey, G.B.

    1985-01-01

    Hydrocarbon source rock evaluation of the Middle Proterozoic Solor Church Formation (Keweenawan Supergroup) as sampled in the Lonsdale 65-1 well, Rice County, shows that: the rocks are organic matter lean; the organic matter is thermally post-mature, probably near the transition between the wet gas phase of catagenesis and metagenesis; and the rocks have minimal potential for producing additional hydrocarbons. The observed thermal maturity of the organic matter requires significantly greater burial depths, a higher geothermal gradient, or both. It is likely, that thermal maturation of the organic matter in the Solor Church took place relatively early, and that any hydrocarbons generated during this early phase were probably lost prior to deposition of the overlying formation.

  4. Identification of 2. 0 to 2. 4 Ga Nd model age crustal material in the Cheyenne belt, southeastern Wyoming: Implications for Proterozoic accretionary tectonics at the southern margin of the Wyoming craton

    SciTech Connect

    Ball, T.T.; Farmer, G.L. )

    1991-04-01

    A detailed survey was conducted of the Nd-isotope compositions of Precambrian metamorphic rocks both within and adjacent to the Cheyenne belt, a metamorphosed crustal segment in southeastern Wyoming that defines the Proterozoic suture zone between the Archean Wyoming craton and accreted Proterozoic island arc(s). The results of the study indicate that Proterozoic miogeoclinal sedimentary rocks exposed north of the Cheyenne belt and deposited at the southern margin of the Wyoming craton contain only Archean crust derived material and no detritus from Proterozoic island arcs approaching from the south. Similarly, Proterozoic crust exposed south of the Cheyenne belt contains no crustal material derived from Archean sources. However, within the Cheyenne belt, paragneiss of the Barber Lake block has 2.0 to 2.4 Ga Nd model ages and is interpreted as metamorphosed intermixed sedimentary detritus from both Archean and Proterozoic sources. The paragneiss is thought to represent remnants of oceanic trench or foredeep sediments deposited in close proximity to both the Archean craton and the Proterozoic island arcs, similar to sedimentary rocks with 2.0 to 2.4 Ga Nd model ages exposed in the Penokean orogen in Wisconsin. The overall lack of 2.0 to 2.4 Ga modal age crustal material in the vicinity of the Cheyenne belt may reflect the fact that much of this crustal material was overthrust onto the Wyoming craton during accretion and then eroded during postorogenic rebound of the continental margin.

  5. Relationships between oxygen fugacity and metasomatism in the Kaapvaal subcratonic mantle, represented by garnet peridotite xenoliths in the Wesselton kimberlite, South Africa

    NASA Astrophysics Data System (ADS)

    Hanger, Brendan J.; Yaxley, Gregory M.; Berry, Andrew J.; Kamenetsky, Vadim S.

    2015-01-01

    A suite of 12 peridotite xenoliths from the Wesselton kimberlite was studied and found to sample the subcratonic lithospheric mantle over a pressure range from 3.6 to 4.7 GPa and a temperature range of 880 to 1120 °C. Major, minor and trace element compositions indicate that both metasomatised and un-metasomatised samples are present over this pressure range. Fe3 +/∑ Fe in garnet from four xenoliths was determined using Fe K-edge XANES spectroscopy, enabling the redox state of the sampled subcratonic mantle to be determined for three garnet bearing samples. ΔlogfO2[FMQ] varied from 0 to - 3.3 over the sampled pressure interval, with the un-metasomatised samples falling within the global trend of decreasing ΔlogfO2[FMQ] with increasing depth. Superimposed on this was an oxidation trend, at higher pressures (≥ 4.5 GPa), with ΔlogfO2 increasing by 1.5 to 2 units in the metasomatically enriched samples, indicating a clear link between metasomatism and oxidation. One potential source of this oxidation is a carbonated silicate melt, which will increase in carbonate content as ΔlogfO2 increases. Mantle minerals affected by such a melt have the potential to shift from the field of diamond stability into that of carbonate, threatening the stability of diamond.

  6. Structural analysis and implicit 3D modelling of high-grade host rocks to the Venetia kimberlite diatremes, Central Zone, Limpopo Belt, South Africa

    NASA Astrophysics Data System (ADS)

    Basson, I. J.; Creus, P. K.; Anthonissen, C. J.; Stoch, B.; Ekkerd, J.

    2016-05-01

    The Beit Bridge Complex of the Central Zone (CZ) of the Limpopo Belt hosts the 519 ± 6 Ma Venetia kimberlite diatremes. Deformed shelf- or platform-type supracrustal sequences include the Mount Dowe, Malala Drift and Gumbu Groups, comprising quartzofeldspathic units, biotite-bearing gneiss, quartzite, metapelite, metacalcsilicate and ortho- and para-amphibolite. Previous studies define tectonometamorphic events at 3.3-3.1 Ga, 2.7-2.5 Ga and 2.04 Ga. Detailed structural mapping over 10 years highlights four deformation events at Venetia. Rules-based implicit 3D modelling in Leapfrog Geo™ provides an unprecedented insight into CZ ductile deformation and sheath folding. D1 juxtaposed gneisses against metasediments. D2 produced a pervasive axial planar foliation (S2) to isoclinal F2 folds. Sheared lithological contacts and S2 were refolded into regional, open, predominantly southward-verging, E-W trending F3 folds. Intrusion of a hornblendite protolith occurred at high angles to incipient S2. Constrictional-prolate D4 shows moderately NE-plunging azimuths defined by elongated hornblendite lenses, andalusite crystals in metapelite, crenulations in fuchsitic quartzite and sheath folding. D4 overlaps with a: 1) 2.03-2.01 Ga regional M3 metamorphic overprint; b) transpressional deformation at 2.2-1.9 Ga and c) 2.03 Ga transpressional, dextral shearing and thrusting around the CZ and d) formation of the Avoca, Bellavue and Baklykraal sheath folds and parallel lineations.

  7. The Amazon-Laurentian connection as viewed from the Middle Proterozoic rocks in the central Andes, western Bolivia and northern Chile

    USGS Publications Warehouse

    Tosdal, R.M.

    1996-01-01

    Middle Proterozoic rocks underlying the Andes in western Bolivia, western Argentina, and northern Chile and Early Proterozoic rocks of the Arequipa massif in southern Peru?? from the Arequipa-Antofalla craton. These rocks are discontinuously exposed beneath Mesozoic and Cenozoic rocks, but abundant crystalline clasts in Tertiary sedimentary rocks in the western altiplano allow indirect samples of the craton. Near Berenguela, western Bolivia, the Oligocene and Miocene Mauri Formation contains boulders of granodiorite augen gneiss (1171??20 Ma and 1158??12 Ma; U-Pb zircon), quartzose gneiss and granofels that are inferred to have arkosic protoliths (1100 Ma source region; U-Pb zircon), quartzofeldspathic and mafic orthogneisses that have amphibolite- and granulite-facies metamorphic mineral assemblages (???1080 Ma metamorphism; U-Pb zircon), and undeformed granitic rocks of Phanerozoic(?) age. The Middle Proterozoic crystalline rocks from Berenguela and elsewhere in western Bolivia and from the Middle Proterozoic Bele??n Schist in northern Chile generally have present-day low 206Pb/204Pb ( 15.57), and elevated 208Pb/204Pb (37.2 to 50.7) indicative of high time-averaged Th/U values. The Middle Proterozoic rocks in general have higher presentday 206Pb/204Pb values than those of the Early Proterozoic rocks of the Arequipa massif (206Pb/204Pb between 16.1 and 17.1) but lower than rocks of the southern Arequipa-Antofalla craton (206Pb/204Pb> 18.5), a difference inferred to reflect Grenvillian granulite metamorphism. The Pb isotopic compositions for the various Proterozoic rocks lie on common Pb isotopic growth curves, implying that Pb incorporated in rocks composing the Arequipa-Antofalla craton was extracted from a similar evolving Pb isotopic reservoir. Evidently, the craton has been a coherent terrane since the Middle Proterozoic. Moreover, the Pb isotopic compositions for the Arequipa-Antofalla craton overlap those of the Amazon craton, thereby supporting a link

  8. Mineral inclusions and geochemical characteristics of microdiamonds from the DO27, A154, A21, A418, DO18, DD17 and Ranch Lake kimberlites at Lac de Gras, Slave Craton, Canada

    NASA Astrophysics Data System (ADS)

    Davies, Rondi M.; Griffin, William L.; O'Reilly, Suzanne Y.; Doyle, Buddy J.

    2004-09-01

    A mineral inclusion, carbon isotope, nitrogen content, nitrogen aggregation state and morphological study of 576 microdiamonds from the DO27, A154, A21, A418, DO18, DD17 and Ranch Lake kimberlites at Lac de Gras, Slave Craton, was conducted. Mineral inclusion data show the diamonds are largely eclogitic (64%), followed by peridotitic (25%) and ultradeep (11%). The paragenetic abundances are similar to macrodiamonds from the DO27 kimberlite (Davies, R.M., Griffin, W.L., O'Reilly, S.Y., 1999. Diamonds from the deep: pipe DO27, Slave craton, Canada. In: Gurney, J.J., Gurney, J.L., Pascoe, M.D., Richardson, S.H. (Eds.), The J. B. Dawson Vol., Proc. 7th Internat. Kimberlite Conf., Red Roof Designs, Cape Town, pp. 148-155) but differ to diamonds from nearby kimberlites at Ekati (e.g., Lithos (2004); Tappert, R., Stachel, T., Harris, J.W., Brey, G.P., 2004. Mineral Inclusions in Diamonds from the Panda Kimberlite, S. P., Canada. 8th International Kimberlite Conference, extended abstracts) and Snap Lake to the south (Dokl. Earth Sci. 380 (7) (2001) 806), that are dominated by peridotitic stones. Eclogitic diamonds with variable inclusion compositions and temperatures of formation (1040-1300 °C) crystallised at variable lithospheric depths sometimes in changing chemical environments. A large range to very 13C-depleted C-isotope compositions ( δ13C=-35.8‰ to -2.2‰) and an NMORB bulk composition, calculated from trace elements in garnet and clinopyroxene inclusions, are consistent with an origin from subducted oceanic crust and sediments. Carbon isotopes in the peridotitic diamonds have mantle compositions ( δ13C mode -4.0‰). Mineral inclusion compositions are largely harzburgitic. Variable temperatures of formation (garnet TNi=800-1300 °C) suggest the peridotitic diamonds originate from the shallow ultra-depleted and deeper less depleted layers of the central Slave lithosphere. Carbon isotopes ( δ13C av.=-5.1‰) and mineral inclusions in the ultradeep diamonds

  9. Speculations on nature and extent of Archean basement in Labrador as indicated by SR, ND and PB isotopic systematics of proterozoic intrusives

    NASA Technical Reports Server (NTRS)

    Ashwal, L. D.; Wooden, J. L.; Emslie, R. F.

    1986-01-01

    The Sm-Nd and Rb-Sr isotopic compositions of mid to late Proterozoic (approximately 1.6 to 1.1 Ga) massif-type anorthosites and mafic intrusives in the eastern Canadian shield are correlated with geographic location. Complexes in the Grenville province have positive epsilon sub Nd values and initial Sr-87/Sr-86 (I sub Sr) generally less than 0.703, suggesting derivation from depleted mantle. In Labrador, similar complexes close to or northwest of a line roughly corresponding to the Grenville Front have negative epsilon sub Nd values and I sub Sr 0.703. This contrast was intrepreted as reflecting either enriched mantle under the Nain Province, or contamination of the Nain intrusives with older crustal components. Lead isotopic compositions, however, favor the latter. The possibility of using these Proterozoic intrusives as tracers to characterize the nature and extent of older basement types in Labrador is discussed.

  10. Properties of the proterozoic geomagnetic field and geological applications of paleomagnetic data from rocks of the North American Midcontinent rift

    NASA Astrophysics Data System (ADS)

    Kulakov, Evgeniy V.

    Rocks of the North American Midcontinent rift (MCR) exposed in the Lake Superior area provide an excellent opportunity to use paleomagnetism as a means of studying the characteristics of the Proterozoic geomagnetic field and the history of the rift itself. Detailed paleomagnetic and paleointensity studies of different rock units associated with the MCR, including the 1108 Ma alkaline Coldwell Complex (Ontario, Canada), the basaltic lava flows of the Geordie Lake (Ontario, Canada) and Silver Mountain (Upper Michigan, USA) that are assumed to be 1107-1108 Ma, the ˜1095 Ma lava flows of the Portage Lake Volcanics (PLV) (Keweenaw Peninsula, Michigan), and the ˜1088 Ma flows of the Lake Shore Traps (LST) (Keweenaw Peninsula, Michigan) are presented. Paleomagnetic data from the Coldwell Complex indicate that the apparent asymmetry of geomagnetic reversal, recorded by many Keweenawan rocks is an artifact due to fast equator-ward motion of the North American plate during the MCR evolution. The Coldwell Complex data support the validity of the geocentric axial dipole assumption for the ˜1.1 Ga. Extrusive rocks exposed on the Keweenaw Peninsula reveal similar to that of the present day geomagnetic field paleosecular variation. Samples from the ˜1088 Ma Lake Shore Traps yielded consistent paleofield values with a mean value of 26.3 +/- 4.7 μT, which corresponds to a virtual dipole moment of 5.9 +/- 1.1 x 10 22 Am2. The mean and range of paleofield values are similar to those of the recent Earth's magnetic field and incompatible with a "Proterozoic dipole low". These results are consistent with a modern type compositionally-driven geodynamo operating by the end of Mesoproterozoic. New high-quality paleomagnetic poles calculated for the ˜1108 Ma Coldwell Complex and coeval extrusive rocks, and ˜ 1094 Ma PLV indicate that North America was moving directly equator-ward with an approximately 20-25 cm/year rate between 1108 and 1094 Ma with a significant slowdown in motion

  11. New paleomagnetic results from Middle and Late Proterozoic intrusive rocks of the central and southern Rocky Mountains

    SciTech Connect

    Harlan, S.S. )

    1993-04-01

    Paleomagnetic results from Proterozoic intrusive rocks from the central and southern Rocky Mountains yield data that provide definition to the North American APW path. Results from the ca. 1420 Ma Laramie anorthosite (LA) and Sherman Granite (SG) yield dual polarity magnetizations; a combined pole (7[degree]S, 215[degree]E, A[sub 95] = 4[degree]) is essentially identical to 1400 Ma poles from elsewhere in North America. Group A dikes from the Tobacco Root (TR) Mtns, MT, along the southern margin of the Belt Basin, have a dual-polarity remanence with a pole at 10[degree]N, 222[degree]E (A[sub 95] = 11[degree]) that plots slightly north of the LA/SG and other poles of ca. 1450 Ma. Comparison of Belt Supergroup poles, assuming coherence of the Belt terrane and interior Laurentia, with 1400 Ma poles and those of the well-defined western arm of the Logan Loop of the N. American APW path, would seem to indicate that the age of most Belt poles are in the range of 1300--1400 Ma, although this age assignment conflicts with available geochronologic data. Results from 1100 Ma diabase sheets from central AZ yield two distinct, normal and reverse polarity magnetizations: ADn and ADr. ADn gives a pole at 23[degree]N, 181[degree]E (A[sub 95] = 8[degree]) that overlaps poles of ca. 1100 Ma from the midcontinent rift (MR). Pole ADr is located at 38[degree]N, 248[degree]E (A[sub 95] = 39[degree]); the large uncertainty of this pole precludes its use in defining the APW path. All dikes are of WNW trend and several from the TR Mtns and the Christmas Lake dike from the Beartooth uplift give hornblende [sup 40]Ar/[sup 39]Ar ages of 760--770 Ma. The data are interpreted to provide evidence for Late Proterozoic mafic magmatism along the W and SW margin of the Wyoming Province, possibly related to crustal extension accompanying deposition of the Windermere Group in the northern part of the Cordillera and volcanism and sedimentation in SE Idaho and NE Washington.

  12. A deposit model for magmatic iron-titanium-oxide deposits related to Proterozoic massif anorthosite plutonic suites

    USGS Publications Warehouse

    Woodruff, Laurel G.; Nicholson, Suzanne W.; Fey, David L.

    2013-01-01

    This descriptive model for magmatic iron-titanium-oxide (Fe-Ti-oxide) deposits hosted by Proterozoic age massif-type anorthosite and related rock types presents their geological, mineralogical, geochemical, and geoenvironmental attributes. Although these Proterozoic rocks are found worldwide, the majority of known deposits are found within exposed rocks of the Grenville Province, stretching from southwestern United States through eastern Canada; its extension into Norway is termed the Rogaland Anorthosite Province. This type of Fe-Ti-oxide deposit dominated by ilmenite rarely contains more than 300 million tons of ore, with between 10- to 45-percent titanium dioxide (TiO2), 32- to 45-percent iron oxide (FeO), and less than 0.2-percent vanadium (V). The origin of these typically discordant ore deposits remains as enigmatic as the magmatic evolution of their host rocks. The deposits clearly have a magmatic origin, hosted by an age-constrained unique suite of rocks that likely are the consequence of a particular combination of tectonic circumstances, rather than any a priori temporal control. Principal ore minerals are ilmenite and hemo-ilmenite (ilmenite with extensive hematite exsolution lamellae); occurrences of titanomagnetite, magnetite, and apatite that are related to this deposit type are currently of less economic importance. Ore-mineral paragenesis is somewhat obscured by complicated solid solution and oxidation behavior within the Fe-Ti-oxide system. Anorthosite suites hosting these deposits require an extensive history of voluminous plagioclase crystallization to develop plagioclase-melt diapirs with entrained Fe-Ti-rich melt rising from the base of the lithosphere to mid- and upper-crustal levels. Timing and style of oxide mineralization are related to magmatic and dynamic evolution of these diapiric systems and to development and movement of oxide cumulates and related melts. Active mines have developed large open pits with extensive waste-