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Sample records for proterozoic diamond-bearing kimberlites

  1. Petrochemical types of kimberlites and their diamond-bearing capacity

    NASA Astrophysics Data System (ADS)

    Kostrovitsky, Sergey

    2010-05-01

    Kimberlite rocks of Yakutian province (belong to 1 group of kimberlites after Smith, 1983) are characterized by wide variations of rock-forming oxides [Ilupin et al., 1986; Milashev, 1965; Kharkiv et al., 1991]. A number of factors could be discussed to explain the variety of chemical compositions of rocks. The first factor, explaining the regional differences in the kimberlite composition with primarily different composition of source kimberlite melt-fluid, is conventionally called «primary». All other factors are connected with the secondary redistribution of chemical components of kimberlites. Irrespective of intensity of secondary factors, the primary composition of kimberlites varies broadly, which is noticeable in kimberlites of some provinces, kimberlites fields, pipe clusters and individual pipes. The petrochemical types are classified based on the contents of such oxides as FeO, TiO2 and K2O, being relatively inert in the secondary processes. In the Yakutian Province we have distinguished 5 petrochemical types of kimberlites (Kostrovitsky et al, 2007); with principal ones - high-Mg, magnesium-ferruginous (Mg-Fe) and ferruginous-titaniferous, their composition: < 6; 6-9; 8-15 % FeOtotal and < 1; 1-2.5; 1.5-5.0 % TiO2). Some petrochemical and mineralogical criteria of diamond-bearing capacity of kimberlites were identified some time before. The essence of petrochemical criterion consists of the inverse correlation dependence between the contents FeOtotal, TiO2 in kimberlite rocks and their diamond-bearing capacity (Milashev, 1965; Krivonos, 1998). The mineralogical criteria of diamond-bearing capacity infer presence of direct dependence of the rate of capacity on the content in kimberlites of low-Ca, high-Cr garnet and chrome spinellids with Cr2O3 > 62% and TiO2 < 0.5%, of dunite-harzburgite paragenesis (Sobolev, 1974; Meyer, 1968). The acquired results are applied to evaluate «efficiency» of criteria of diamond-bearing capacity exemplified by the

  2. 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.

  3. Two diamond-bearing peridotite xenoliths from the finsch kimberlite, South Africa

    NASA Astrophysics Data System (ADS)

    Shee, S. R.; Gurney, J. J.; Robinson, D. N.

    1982-12-01

    Two diamond bearing xenoliths found at Finsch Mine are coarse garnet lherzolites, texturally and chemically similar to the dominant mantle xenoliths in that kimberlite. A total of 46 diamonds weighing 0.053 carats have been recovered from one and 53 diamonds weighing 0.332 carats from the other. The diamonds are less corroded than diamonds recovered from the kimberlite. Geothermobarometric calculations indicate that the xenoliths equilibrated at ˜1,130° C and pressures 50 kb which is within the diamond stability field; this corresponds to depths of 160 km and would place the rocks on a shield geotherm at slightly greater depths than most coarse garnet lherzolites from kimberlite. The primary minerals in the two rocks are very similar to each other but distinctly different to the majority of mineral inclusions in Finsch diamonds. This suggests a different origin for the diamonds in the kimberlite and the diamonds in the xenoliths although the equilibration conditions for both suites are approximately coincident and close to the “wet” peridotite solidus.

  4. 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

  5. Petrography and Mineral Chemistry of the Diamond-Bearing RÉGIS Kimberlite Intrusion (se Brazil)

    NASA Astrophysics Data System (ADS)

    Barbosa, C. T.; Ruberti, E.

    2016-12-01

    The Alto Paranaiba Igneous Province (APIP), located on the northeastern flap of the Paraná Basin, is one of the largest events in volume of alkaline potassic rocks in the world. However, detailed studies regarding the nature of magmas and the relationship between geochemical associations and the occurrence of diamonds in the APIP are still lacking. This work focuses on the petrography and mineral chemistry of the Régis kimberlite (NW of the APIP), one of the main intrusions that occur in the province. Two boreholes (316.4 meters depth) were studied for petrography and mineral chemistry. The mineralogy consists of phlogopite, garnet, augite, diopside, ilmenite, spinel, olivine, carbonate, serpentine, apatite, barite and perovskite. Preliminary microprobe results have shown garnet from peridotite and eclogite. Garnet-peridotites are Cr-pyropes and were classified as garnet-lherzolites that correspond to the G9 group. The garnet-eclogites are almandines with high FeO content and were classified as G0. The most abundant clinopyroxene is augite, sometimes with high Cr202 content (1.0-2.0 wt.%) or MgO (25.0-35.0 wt.%), followed by diopside (CaO 20 wt.%). The olivine crystals consist of magnesian olivine (Fo91,47) with Cr202 > 0.19 wt.%. All micas are magnesian phlogopites (MgO 25.23-29.88 wt.%, Al203 6.29-11.60 wt.% and FeO 4.61-6.68 wt.%). The ilmenites have higher levels of Fe than Ti, and MgO contents rarely exceed 2%. Spinel mostly varies between FeCr2O4-FeTiO4-Fe3O4. These preliminary results suggest characteristics between a kimberlite and lamproite. Type G9 garnets may be associated with the occurrence of diamond and are also in agreement with previous records of microdiamonds in this intrusion. There are significant and peculiar differences in the alteration degree, petrography and mineral chemistry of the Régis intrusion and surrounding alkaline rocks. Intrusions spatially close and so different from each other suggest a complex geodynamic scenario in which

  6. Shock Properties of Kimberlite

    NASA Astrophysics Data System (ADS)

    Willmott, G. R.; Proud, W. G.; Field, J. E.

    2004-07-01

    Plate impact experiments have been performed on the igneous diamond-bearing matrix kimberlite. Longitudinal and lateral stresses were measured in the uniaxial strain regime using manganin stress gauges. The shock Hugoniot of the kimberlite has been characterized at axial stresses between 1 and 9 GPa. The kimberlite has a low impedance response when compared with similar data for other geological materials. The data indicate that the rock behaves inelastically above shock stresses of 1 GPa.

  7. Middle Paleozoic kimberlite magmatism in the northeastern Siberian craton

    NASA Astrophysics Data System (ADS)

    Egorova, E. O.; Afanas'ev, V. P.; Pokhilenko, N. P.

    2016-10-01

    The mineral chemistry and crystal morphology of kimberlite pyropes from the Billyakh River placer in the northeastern Siberian craton are characterised in terms of the placer history. The pyropes bear signatures of chemical weathering (dissolution), presumably in a Middle Paleozoic laterite profile, and therefore were originally hosted by Middle Paleozoic kimberlites. The broad occurrence of placer pyropes with lateritic dissolution signatures points to the presence of Middle Paleozoic diamond-bearing kimberlites in the study area.

  8. 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

  9. Kimberlite-Clan-Rocks in India: Significance of new VGP, T, and GP Observations.

    NASA Astrophysics Data System (ADS)

    Haggerty, S. E.

    2005-05-01

    Although India is acknowledged for the first description of diamond some 2000 BCE, it should also rightfully be credited for the 17th C recognition that diamond is a product of volcanism. With this extraordinary background, it is surprising that the host rocks remain controversial, being neither archetypical kimberlites, nor classic lamproites. Lacking affinities to micro-diamond-bearing UHPT metamorphic rocks and being unequivocally volcanic, the term Kimberlite-Clan-Rock (KCR) is applied. Over 200 KCR pipes and dikes, many of which are richly mineralized, have recently been discovered in the Diamond Corridor (1000 x 200 km) of the Eastern Dharwar Craton, and in adjoining cratons to the N and NE. From 32 absolute age determinations on KCRs, the remaining intrusions, in comparable stratigraphic settings, are assumed to be 1.1 Ga, equivalent to the Argyle lamproite (Australia, highest diamond grade), and the Premier kimberlite (RSA, largest known diamond). India has the largest number of known Proterozoic KCR intrusions that over the centuries have produced a wealth of famous diamonds. The primary source of these extraordinary stones, however, remains unknown, possibly because the unusual host rocks defy conventional exploration protocols. Six new observations make the setting even more unusual: (1) Coeval, large scale magmatism in the Kalahari (>2 m sq km) and Laurentian (>300 k cub km) Cratons at 1085-1112 Ma, during assembly of Rodinia, confirms the relation between and among KCR volcanism, LIPs, and supercontinents; Proterozoic Rodinia, into which the India KCRs, Argyle and Premier were intruded, was constructive, whereas the other, globally-wide, diamond-intrusive event, that occurred during the Mesozoic (80-120 Ma), was related to the breakup of Pangea and the dispersion of Gondwana; both events occurred during long period geomagnetic chrons implying a core relation, and superplume activity from the CMB. (2)The transcontinental Mumbai-Chennai gravity lineament

  10. 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.

  11. Welded Kimberlite?

    NASA Astrophysics Data System (ADS)

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

    2009-05-01

    Welding of pyroclastic deposits generally involves the sintering of hot glassy vesicular particles and requires the presence of a load and/or high temperatures. Welding can occur on various scales as observed in large welded pyroclastic flows, in small-volume agglutinated spatter rims, or as in coalesced clastogenic lava flows. In all these examples welding occurs mainly by reduction or elimination of porosity within the vesicular clasts and/or inter-clast pore space. The end result of welding in pyroclastic deposits is to produce dense, massive, coherent deposits. Here, we present a possible new end-member of the welding process: welding of non- vesicular pyroclasts in intra-crater kimberlite deposits. Kimberlite melt is a low-viscosity liquid carrying abundant crystals. Because of this, kimberlite eruptions generally produce non-vesicular pyroclasts. During welding, these pyroclast cannot deform by volume reduction to form typical fiamme. As a result, welding and compaction in kimberlites proceeds via the reduction of inter-clast pore space alone. The lack of porous pyroclasts limits the maximum amount of volumetric strain within pyroclastic kimberlite deposits to about 30%. This value is substantially lower than the limiting values for welding of more common felsic pyroclastic flows. The lower limit for volumetric strain in welded kimberlite deposits severely restricts the development of a fabric. In addition, pyroclastic kimberlite deposits commonly feature equant-shaped pyroclasts, and equant-shaped crystals. This, in turn, limits the visibility of the results of compaction and pore space reduction, as there are few deformable markers and elongate rigid markers that are able to record the strain during compaction. These features, together with the low viscosity of kimberlite magma and the stratigraphic position of these kimberlite deposits within the upper reaches of the volcanic conduit, call for careful interpretation of coherent-looking rocks in these

  12. 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

  13. Kimberlites: Magmas or mixtures?

    NASA Astrophysics Data System (ADS)

    Patterson, Michael; Francis, Don; McCandless, Tom

    2009-11-01

    Although the presence of xenocrystic olivine is widely recognized in kimberlite, there is little consensus about its contribution to the existing estimates for the composition of kimberlite magma. Whole rock geochemistry is critical to the debate regarding the composition of kimberlite magma, however, it has received little attention as an indicator of diamond grade due to conventional thought that diamonds are xenocrysts unrelated to their host kimberlite. The Foxtrot kimberlite Field in Northern Québec is comprised of at least three distinct kimberlite intrusions exhibiting variation in both diamond grade and geochemistry making it an ideal suite with which to test a possible correlation between diamond grade and whole rock composition. Olivine is ubiquitous (30 to 70%) in the Foxtrot kimberlites and exhibits a restricted composition that overlaps that of olivine in harzburgite xenoliths suggesting that the majority of olivine is xenocrystic. Carbonate is also abundant (8 to 35%) in the Foxtrot kimberlites and exhibits magmatic textures requiring that carbon be considered in any petrogenetic model for the Foxtrot kimberlites. Pearce element ratio analysis assuming P as a conserved element indicates that much of the major element variation in the Foxtrot kimberlites can be explained by variable amounts of olivine and orthopyroxene in proportions (~ 80/20), similar to that of cratonic mantle xenoliths. The xenocrystic nature of olivine requires that the contribution of mantle harzburgite must be removed to constrain the composition of the magma. The calculated magma composition that results from the mathematical removal of olivine and orthopyroxene (80/20) from the whole rock compositions is significantly poorer in MgO (15 wt.%) and silica (~ 24 wt.%), but CO 2 rich (~ 17 wt.%) compared to previous estimates for kimberlite magma. The Foxtrot kimberlites are best modelled as mixtures of harzburgite mantle and a relatively carbonate-rich magma. According to this

  14. Sedimentologic and stratigraphic constraints on emplacement of the Star Kimberlite, east-central Saskatchewan

    NASA Astrophysics Data System (ADS)

    Zonneveld, John-Paul; Kjarsgaard, Bruce A.; Harvey, Shawn E.; Heaman, Larry M.; McNeil, David H.; Marcia, Kirsten Y.

    2004-09-01

    Diamond-bearing kimberlites in the Fort à la Corne region, east-central Saskatchewan, consist primarily of extra-crater pyroclastic deposits which are interstratified with Lower Cretaceous (Albian and Cenomanian) marine, marginal marine and continental sediments. Approximately 70 individual kimberlite occurrences have been documented. The Star Kimberlite, occurring at the southeastern end of the main Fort à la Corne trend, has been identified as being of economic interest, and is characterized by an excellent drill core database. Integration of multi-disciplinary data-sets has helped to refine and resolve models for emplacement of the Star Kimberlite. Detailed core logging has provided the foundation for sedimentological and volcanological studies and for construction of a regionally consistent stratigraphic and architectural framework for the kimberlite complex. Micropaleontologic and biostratigraphic analysis of selected sedimentary rocks, and U-Pb perovskite geochronology on kimberlite samples have been integrated to define periods of kimberlite emplacement. Radiometric age determination and micropaleontologic evidence support the hypothesis that multiple kimberlite eruptive phases occurred at Star. The oldest kimberlite in the Star body erupted during deposition of the predominantly continental strata of the lower Mannville Group (Cantuar Formation). Kimberlites within the Cantuar Formation include terrestrial airfall deposits as well as fluvially transported kimberlitic sandstone and conglomerate. Successive eruptive events occurred contemporaneous with deposition of the marginal marine upper Mannville Group (Pense Formation). Kimberlites within the Pense Formation consist primarily of terrestrial airfall deposits. Fine- to medium-grained cross-stratified kimberlitic (olivine-dominated) sandstone in this interval reflects reworking of airfall deposits during a regional marine transgression. The location of the source feeder vents of the Cantuar and Pense

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

    NASA Astrophysics Data System (ADS)

    Schertl, H.-P.; Sobolev, N. V.

    2013-02-01

    After the discovery of metamorphic coesite in crustal rocks from the Western Alps (Italy) and the Western gneiss region (Norway) in the mid 1980s of the last century, metamorphic diamond was observed only a few years later "in situ" in the Kokchetav Massif (Kazakhstan). Findings of such coesite- and diamond-bearing ultrahigh pressure metamorphic (UHP) rocks with protoliths formed or embedded in crustal levels and subsequently experienced PT-conditions within or even higher than the coesite stability field have dramatically changed our geodynamic view of orogenetic processes. These occurrences provide evidence that crustal rocks were subducted into mantle depths and exhumed to the surface. Recent studies even suggest continental subduction to depths exceeding 300 km. These rocks have been extensively studied and many new and important observations have been made. Thus far, more than 350 papers have been published on various aspects of Kokchetav UHP rocks. The Kokchetav Massif of northern Kazakhstan is part of one of the largest suture zones in Central Asia and contains slices of HP and UHP metamorphic rocks. Classical UHP rocks mainly occur in the Kumdy Kol, Barchi Kol and Kulet areas, and include a large variety of lithologies such as calcsilicate rocks, eclogite, gneisses, schists, marbles of various compositions, garnet-pyroxene-quartz rocks, and garnet peridotite. Most of them contain microdiamonds; some of which reach a grain size of 200 μm. Most diamond grains show cuboid shapes but in rare cases, diamonds within clinozoisite gneiss from Barchi Kol occur as octahhedral form. Microdiamonds contain highly potassic fluid inclusions, as well as solid inclusions like carbonates, silicates and metal sulfides, which favour the idea of diamond formation from a C-O-H bearing fluid. Nitrogen isotope data and negative δ13C values of Kokchetav diamonds indicate a metasedimentary origin. PT-estimates of Kokchetav UHP rocks yield peak metamorphic conditions of at least 43

  16. Tectonic setting of kimberlites

    NASA Astrophysics Data System (ADS)

    Jelsma, Hielke; Barnett, Wayne; Richards, Simon; Lister, Gordon

    2009-11-01

    Kimberlites can be viewed as time capsules in a global plate tectonic framework. Their distribution illustrates clustering in time and space. Kimberlite ages span the assembly and break-up of a number of supercontinents, such as Rodinia and Gondwana. These supercontinents show time lines with (i) broad periods devoid of kimberlite magmatism corresponding to times of continent stability, and (ii) narrow kimberlite emplacement windows corresponding to times of fundamental plate reorganizations. This episodicity implies that kimberlite emplacement events are intrinsically related to particular stages in the life cycle of supercontinents. The onset of kimberlite magmatism is closely associated with thermal perturbations (thermal insulation, mantle upwelling?) beneath a stagnant or sluggish supercontinent. These perturbations may have caused uplift and the onset of continental break-up through fracture zones propagating into the supercontinent. Subsequent spreading and ocean floor development is marked by apparent cusps and jogs in plate motion paths. Resultant strain is accommodated along trans-lithospheric corridors with episodic uplift and erosion and focused kimberlite melt migration. The corridors are manifest as discontinuities in the lithosphere mantle, measured as geophysical gradients and as changes in mantle lithosphere composition. Within the crust, these corridors are expressed as (a) terrane boundaries, (b) incipient continental rifts, (c) fracture zones, or (d) major dyke swarms. Some kimberlite populations are clustered along parallel sets of corridors widely distributed across a large part of a subcontinent and repeated magmatism is seen within many of the clusters. The association of kimberlite occurrences with discontinuities may be ascribed to favorable conditions for melt production and to resultant melt focusing along high strain zones that contain fractures and faults. Such conditions may be attained during different stages in the evolution of

  17. 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

  18. Redox preconditioning deep cratonic lithosphere for kimberlite genesis - evidence from the central Slave Craton.

    PubMed

    Yaxley, G M; Berry, A J; Rosenthal, A; Woodland, A B; Paterson, D

    2017-12-01

    We present the first oxygen fugacity (fO2) profile through the cratonic lithospheric mantle under the Panda kimberlite (Ekati Diamond Mine) in the Lac de Gras kimberlite field, central Slave Craton, northern Canada. Combining this data with new and existing data from garnet peridotite xenoliths from an almost coeval kimberlite (A154-N) at the nearby Diavik Diamond Mine demonstrates that the oxygen fugacity of the Slave cratonic mantle varies by several orders of magnitude as a function of depth and over short lateral distances. The lower part of the diamond-bearing Slave lithosphere (>120-130 km deep) has been oxidized by up to 4 log units in fO2, and this is clearly linked to metasomatic enrichment. Such coupled enrichment and oxidation was likely caused by infiltrating carbonate-bearing, hydrous, silicate melts in the presence of diamond, a process proposed to be critical for "pre-conditioning" deep lithospheric mantle and rendering it suitable for later generation of kimberlites and other SiO2-undersaturated magmas.

  19. Changes in the content and crystal morphology of diamonds from Paleozoic and Mesozoic kimberlits in the northeastern Yakutian kimberlite province

    NASA Astrophysics Data System (ADS)

    Biller, Anastasia; Smelov, Alexander; Zaitsev, Albert

    2010-05-01

    The Yakutian kimberlite province combines more than 1000 kimberlite bodies. It is subdivided into two parts: southern and northeastern. The southern part contains highly diamondiferous kimberlite pipes of Middle Paleozoic age. In the northeastern part, weakly diamondiferous or barren kimberlite pipes of Paleozoic and Mesozoic age are found. The content of diamond-bearing pipes in the southern part is 27 %, and in the northeastern part - 0.06 %. The kimberlite pipes from the northeastern part are characterized by kimberlite- and Brazilian-type diamonds present in different proportions. We have made a statistical analyses of the content of morphologically different diamonds in these kimberlites with regard to their age. The most representative information was obtained for the kimberlite pipes Zapolyarnaya (360 Ma), Komsomol'skaya (382 Ma), Novinka (355 Ma), Leninrgad (380 Ma), Aerologicheskaya (409 Ma), Djanga (243 Ma), Malokuonapskaya (170 Ma), Grenada and Nadezhda (159 Ma) as well as for the Luchekan field kimberlites with an average age of 197 Ma. Diamonds from the Paleozoic and Mesozoic kimberlites reveal a reverse relationship between the contents of octahedral and rounded crystals (r = - 0.969). Such relationship is characteristic of a single geologic body. The younger kimberlites contain higher amounts of Brazilian-type diamonds. In the age interval of 400-160 Ma, the average degree of diamond content in kimberlites decreases by about 85 %, and average weight of crystals by 55 %. More complex relationships are established in the systems: octahedral crystals - kimberlite age and rounded crystals - kimberlite age. The first system is characterized by a decrease in the amount of octahedra in the 400-250 Ma interval (r = 0.848,) and their increase in the interval from 250 to 160 Ma (r = - 0.901). Characteristic of the second system is the growing content of rounded diamonds in the interval 400-250 Ma (r = - 0.835) and their decline in the interval from 250 to 160

  20. Palaeomagnetic Emplacement Temperature Determinations of Pyroclastic and Volcaniclastic Deposits in Southern African Kimberlite Pipes

    NASA Astrophysics Data System (ADS)

    Fontana, G.; Mac Niocaill, C.; Brown, R.; Sparks, R. S.; Matthew, F.; Gernon, T. M.

    2009-12-01

    Kimberlites are complex, ultramafic and diamond-bearing volcanic rocks preserved in volcanic pipes, dykes and craters. The formation of kimberlite pipes is a strongly debated issue and two principal theories have been proposed to explain pipe formation: (1) the explosive degassing of magma, and (2) the interaction of rising magma with groundwater (phreatomagmatism). Progressive thermal demagnetization studies are a powerful tool for determining the emplacement temperatures of ancient volcanic deposits and we present the first application of such techniques to kimberlite deposits. Lithic clasts were sampled from a variety of lithofacies, from three pipes for which the internal geology is well constrained (A/K1 pipe, Orapa Mine, Botswana and the K1 and K2 pipes, Venetia Mine, South Africa). The sampled deposits included massive and layered vent-filling breccias with varying abundances of lithic inclusions and layered crater-filling pyroclastic deposits, talus breccias and volcaniclastic breccias. Lithic clasts sampled from layered and massive vent-filling pyroclastic deposits in A/K1 were emplaced at >590° C. Results from K1 and K2 provide a maximum emplacement temperature limit for vent-filling breccias of 420-460° C; and constrain equilibrium deposit temperatures at 300-340° C. Crater-filling volcaniclastic kimberlite breccias and talus deposits from A/K1 were emplaced at ambient temperatures, consistent with infilling of the pipe by post-eruption epiclastic processes. Identified within the epiclastic crater-fill succession is a laterally extensive 15-20 metre thick kimberlite pyroclastic flow deposit emplaced at temperatures of 220-440° C. It overlies the post-eruption epiclastic units and is considered an extraneous pyroclastic kimberlite deposit erupted from another kimberlite vent. The results provide important constraints on kimberlite emplacement mechanisms and eruption dynamics. Emplacement temperatures of >590°C for pipe-filling pyroclastic deposits

  1. Kimberlite degassing and paleoclimate

    NASA Astrophysics Data System (ADS)

    Patterson, M.; Francis, D.

    2009-12-01

    The late Paleocene to mid Eocene period experienced one of the warmest periods in Earth’s history. Although the cause of the Paleocene-Eocene thermal maximum (PETM ~ 56 Ma) is best explained by a massive injection of CO2 (~5000 GtC) into the atmosphere, the source of that CO2 is still unknown (Zeebe et al., 2009). This period was also one of prolific kimberlite eruption in the Lac de Gras region of northern Canada suggesting that CO2 degassing during kimberlite emplacement may have provided the trigger for ocean warming events such as the PETM. A corresponding emplacement age of ~56 Ma for kimberlites in the Lac de Gras region of the Northwest Territories, Canada (Creaser et al., 2004) suggests that kimberlite-derived CO2 may have provided the positive forcing for the warming event. Associated with the PETM is a sharp and negative carbon isotopic excursion (CIE) of ~3 ‰ in the δ13C record and is attributed to the release of isotopically light carbon (-60‰ δ13C) from the dissolution of methane hydrates (Dickens et al., 1997). Although the PETM has been attributed to a massive injection of CO2 into the atmosphere, the mass of isotopically light carbon from methane hydrate required to cause the CIE is estimated to be ~1500 GtC, much less than the 5000 GtC required to achieve the 5-9 °C temperature during the PETM (Bowen et al., 2004). Furthermore, the CIE lags approximately 30 ky behind the onset of the warming event, thus appears to a consequence of the warming event, not the cause (Thomas et al., 2002). The methane hydrate hypothesis is plausible as the cause of the CIE, but the data clearly indicates that the dissolution of methane hydrates does not adequately explain the warming event that preceded it. The Lac de Gras kimberlite field has at least 220 kimberlite intrusions that were emplaced over the period 75 to 45 Ma with an average age of ~56 Ma (Creaser et al., 2004). Studies of hypabyssal kimberlite indicate that primary kimberlite magma is CO2 rich

  2. 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.

  3. The geology of kimberlite pipes of the Ekati property, Northwest Territories, Canada

    NASA Astrophysics Data System (ADS)

    Nowicki, Tom; Crawford, Barbara; Dyck, Darren; Carlson, Jon; McElroy, Ross; Oshust, Peter; Helmstaedt, Herb

    2004-09-01

    This paper reviews key characteristics of kimberlites on the Ekati property, NWT, Canada. To date 150 kimberlites have been discovered on the property, five of which are mined for diamonds. The kimberlites intrude Archean basement of the central Slave craton. Numerous Proterozoic diabase dykes intrude the area. The Precambrian rocks are overlain by Quaternary glacial sediments. No Phanerozoic rocks are present. However, mudstone xenoliths and disaggregated sediment within the kimberlites indicate that late-Cretaceous and Tertiary cover (likely <200 m) was present at the time of emplacement. The Ekati kimberlites range in age from 45 to 75 Ma. They are mostly small pipe-like bodies (surface area mostly <3 ha but up to 20 ha) that typically extend to projected depths of 400-600 m below current surface. Pipe morphologies are strongly controlled by joints and faults. The kimberlites consist primarily of variably bedded volcaniclastic kimberlite (VK). This is dominated by juvenile constituents (olivine and lesser kimberlitic ash) and variable amounts of exotic sediment (primarily mud), with minor amounts of xenolithic wall-rock material (generally <5%). Kimberlite types include: mud-rich resedimented VK (mRVK); olivine-rich VK (oVK); sedimentary kimberlite; primary VK (PVK); tuffisitic kimberlite (TK) and magmatic kimberlite (MK). The presence and arrangement of these rock types varies widely. The majority of bodies are dominated by oVK and mRVK, but PVK is prominent in the lower portions of certain kimberlites. TK is rare. MK occurs primarily as precursor dykes but, in a few cases, forms pipe-filling intrusions. The internal geology of the kimberlites ranges from simple single-phase pipes (RVK or MK), to complex bodies with multiple, distinct units of VK. The latter include pipes infilled with steep, irregular VK blocks/wedges and at least one case in which the pipe is occupied by well-defined sub-horizontal VK phases, including a unique, 100-m-thick graded sequence

  4. 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.

  5. 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

  6. 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.

  7. Relation of slab-derived carbonate melts to kimberlite magma genesis and diamond formation

    NASA Astrophysics Data System (ADS)

    Golubkova, A.; Schmidt, M. W.

    2012-12-01

    kimberlite magma formation near the base of the continental lithosphere. At 8 GPa we also performed experiments with H2O, as phlogopite is a substantial phase in the kimberlite source. At oxygen fugacities near the CCO buffer, both peridotites were transformed into carbonated wherlites after the reaction with carbonatite melt. The "apparent" solidi of K2O- and CO2-rich wherlites lie below the mantle adiabat and are depressed to 1000°C in the presence of H2O at 8 GPa. Melts at the solidus differ from kimberlites; they are dolomitic (XCa 0.3-0.4) with low SiO2 content and high K2O (up to 30 wt% K2O at 8 GPa) and resemble high-Mg fluid inclusions in diamonds [Klein-BenDavid et al., 2009, Lithos]. Such melts could be a source of C for diamond growth at conditions when metal-saturated mantle dominates over carbonate melt and melts become "redox frozen". A second set of experiments was buffered close to the IW equilibrium to replicate this mechanism. In these experiments diamond/graphite precipitated and coexists with periclase, olivine, clinopyroxene, and K-bearing silicates in those mantle portions which become infested by alkali-rich slab-derived melts. These experiments demonstrate that slab derived carbonatite melts form diamond-bearing metasomatised mantle source regions at temperatures reigning at the base of the cold subcontinental lithosphere.

  8. The Carolina kimberlite, Brazil — Insights into an unconventional diamond deposit

    NASA Astrophysics Data System (ADS)

    Hunt, Lucy; Stachel, Thomas; Morton, Roger; Grütter, Herman; Creaser, Robert A.

    2009-11-01

    The diamondiferous Carolina kimberlite (Rondônia State, Brazil) is located within Proterozoic basement rocks (1.8 to 1.2 Ga) of the Amazon Craton. This "unconventional" post-Archean setting is consistent with a lack of harzburgitic (G10) garnets in heavy media concentrate from the kimberlite. Diamonds from Carolina have high nitrogen contents and in part highly negative carbon isotopic values suggesting derivation predominantly from eclogitic portions of the underlying lithospheric mantle. This is consistent with the abundance and chemistry of eclogitic garnet xenocrysts, which make up 13% of the garnets analysed: just over half of the eclogitic garnets classify as Group I (> 0.07 wt.% Na 2O), which is considered to be an indication of good diamond potential. Based on nitrogen contents and aggregation states, the majority of the Carolina diamonds indicate time averaged residence temperatures between 1100 and 1150 °C (at 1.5 Ga mantle residence). Platelet degradation was noted in the majority of diamonds, suggesting that their mantle source was affected by a transient heating event. Geothermobarometry on clinopyroxene grains derived from both surficial samples and kimberlite core indicates two distinct model geotherms: a hot "Somerset Island type" geotherm (44 mW/m 2), and a colder "Slave type" geotherm (38 mW/m 2). Grains from the kimberlite drill core exclusively reflect the lower model geotherm, whereas clinopyroxenes from surficial samples depict both gradients. Given the Triassic age (230 Ma, Rb-Sr model age on phlogopite) of the Carolina kimberlite, it is speculated that a younger generation of Cretaceous-Tertiary kimberlites in the Pimenta Bueno area may represent the source of "hotter" mantle xenocrysts seen in surficial samples. The implied change in geotherm reflects a large scale, possibly plume related, heating episode occurring between the two kimberlite events (i.e. between the Jurassic and Cretaceous) that may relate to the opening of the South

  9. 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

  10. 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

  11. The temporal evolution of North American kimberlites

    NASA Astrophysics Data System (ADS)

    Heaman, Larry M.; Kjarsgaard, Bruce A.; Creaser, Robert A.

    2004-09-01

    North American kimberlite magmatism spans a period of time in excess of 1 billion years from Mesoproterozoic kimberlites in the Lake Superior and James Bay Lowlands region of Ontario to Eocene kimberlites in the Lac de Gras field, N.W.T. Based on a compilation of more than 150 robust radiometric age determinations, several distinct kimberlite emplacement patterns are recognized. In general, the temporal pattern of kimberlite emplacement in North America can be broadly subdivided into five domains: (1) a Mesoproterozoic kimberlite province in central Ontario, (2) an Eocambrian/Cambrian Labrador Sea Province in northern Québec and Labrador, (3) an eastern Jurassic Province, (4) a central Cretaceous corridor and (5) a western mixed domain that includes two Type-3 kimberlite provinces (i.e. multiple periods of kimberlite emplacement preserved in the Slave and Wyoming cratons). For some provinces the origin of kimberlite magmatism can be linked to known mantle heat sources such as mantle plume hotspots and upwelling asthenosphere attendant with continental rifting. For example, the timing and location of Mesoproterozoic kimberlites in North America coincides with and slightly precedes the timing of 1.1 Ga intracontinental rifting that culminated in the Midcontinent Rift centered in the Lake Superior region. Many of the kimberlites in the Eocambrian/Cambrian Labrador Sea province were emplaced soon after the opening of the Iapetus Ocean at about 615 Ma and may also be linked to mantle upwelling associated with continental rifting. The eastern Jurassic kimberlites record an age progression where magmatism youngs in a southeast direction from the ˜200 Ma Rankin Inlet kimberlites to the 155-126 Ma Timiskaming kimberlites. The location of several kimberlite fields and clusters in Ontario and Québec lie along a continental extension of the Great Meteor hotspot track and represents one of the best examples in the world of kimberlite magmatism triggered by mantle plumes. The

  12. 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

  13. 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?

  14. 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.

  15. Kimberlites of the Man craton, West Africa

    NASA Astrophysics Data System (ADS)

    Skinner, E. M. W.; Apter, D. B.; Morelli, C.; Smithson, N. K.

    2004-09-01

    The Man craton in West Africa is an Archaean craton formerly joined to the Guyana craton (South America) that was rifted apart in the Mesozoic. Kimberlites of the Man craton include three Jurassic-aged clusters in Guinea, two Jurassic-aged clusters in Sierra Leone, and in Liberia two clusters of unknown age and one Neoproterozoic cluster recently dated at ˜800 Ma. All of the kimberlites irrespective of age occur as small pipes and prolific dykes. Some of the Banankoro cluster pipes in Guinea, the Koidu pipes in Sierra Leone and small pipes in the Weasua cluster in Liberia contain hypabyssal-facies kimberlite and remnants of the so-called transitional-facies and diatreme-facies kimberlite. Most of the Man craton kimberlites are mineralogically classified as phlogopite kimberlites, although potassium contents are relatively low. They are chemically similar to mica-poor Group 1A Southern African examples. The Jurassic kimberlites are considered to represent one province of kimberlites that track from older bodies in Guinea (Droujba 153 Ma) to progressively younger kimberlites in Sierra Leone (Koidu, 146 Ma and Tongo, 140 Ma). The scarcity of diatreme-facies kimberlites relative to hypabyssal-facies kimberlites and the presence of the so-called transitional-facies indicate that the pipes have been eroded down to the interface between the root and diatreme zones. From this observation, it is concluded that extensive erosion (1-2 km) has occurred since the Jurassic. In addition to erosion, the presence of abundant early crystallizing phlogopite is considered to have had an effect on the relatively small sizes of the Man craton kimberlites.

  16. 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.

  17. 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

  18. Petrology, geochemistry and genesis of newly discovered Mesoproterozoic highly magnesian, calcite-rich kimberlites from Siddanpalli, Eastern Dharwar Craton, Southern India: products of subduction-related magmatic sources?

    NASA Astrophysics Data System (ADS)

    Chalapathi Rao, N. V.; Dongre, A.; Kamde, G.; Srivastava, Rajesh K.; Sridhar, M.; Kaminsky, F. V.

    2010-03-01

    The Siddanpalli kimberlites constitute a newly discovered cluster (SKC) of Mesoproterozoic (1090 Ma) dykes occurring in the granite-greenstone terrain of the Gadwal area in the Eastern Dharwar Craton (EDC), Southern India. They belong to coherent facies and contain serpentinized olivines (two generations), phlogopite, spinel, perovskite, ilmenite, apatite, carbonate and garnet xenocrysts. A peculiar feature of these kimberlites is the abundance of carbonate and limestone xenoliths of the eroded platformal Proterozoic (Purana) sedimentary cover of Kurnool/Bhima age. Chemically, the Siddanpalli dykes are the most magnesium-rich (up to 35 wt.% MgO) and silica-undersaturated (SiO2 < 35 wt.%) of all kimberlites described so far from the Eastern Dharwar Craton. The La/Yb ratio in the Siddanpalli kimberlites (64-105) is considerably lower than that in the other EDC kimberlites (108-145), primarily owing to their much higher HREE abundances. Since there is no evidence of any crustal contamination by granitic rocks we infer this to be a specific character of the magmatic source. A comparison of the REE geochemistry of the Siddanpalli kimberlites with petrogenetic models for southern African kimberlites suggests that they display involvement of a wide range in the degree of melting in their genesis. The different geochemical signatures of the SKC compared to the other known kimberlites in the EDC can be explained by a combination of factors involving: (i) higher degrees of partial melting; (ii) relatively shallower depths of derivation; (iii) possible involvement of subducted component in their mantle source region; and (iv) previous extraction of boninitic magmas from their geological domain.

  19. 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.

  20. Applicability of aeromagnetics for kimberlite exploration in the hard rock terrain of the Eastern Dharwar Craton, south India

    NASA Astrophysics Data System (ADS)

    M, P.; Ram Babu, H. V.

    2011-12-01

    In south India, the Eastern Dharwar Craton (EDC) is one of the most promising geological entities for Kimberlite exploration. World famous diamonds like Kohinoor were discovered in this region that is known as "Home of Diamonds" since ages. The eastern most part of this craton is occupied by the mid-upper proterozoic rocks of the Cuddapah basin and the western part is the archean terrain intruded by a number of kimberilte pipes. In recent times, atleast 60 kimberlite pipes are discovered by various geological and geophysical methods and the search continues further. As the area is extremely large, airborne geophysical survey is considered to be the best option to scan large areas in a limited time frame. Since 1980's aeromagnetic surveys have been conducted on various scales in EDC for kimberlite exploration with the available technology at that point of time. These aeromagnetic data sets helped in narrowing down the prospective zones through identifying favourable geological environments, though, highly variable physical property contrast made direct detection of kimberlite pipes in this region a challenging task. More recently, in the year 2004, about 5000 square kilometers have been covered by high resolution aeromagnetic survey with a sensor height of 40 m at 200m line spacing. This area in the EDC, specifically known as Kalyandurg area is traversed by many doleritic and amphibole dykes in ENE, WNW directions with a number of shear zones in ENE and N-S directions. The dikes and the host rock produces large amplitude high frequency magnetic anomalies that are much similar to those of kimberlites. Detailed interpretation of aeromagnetic data could lead to reveal a large number of anomalies that could be attributed to kimberlites. The success of the aeromagnetic method in a highly noisy environment such as this depends only on the interpretation skills and the data processing tools that are discussed in this paper

  1. 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 progressively

  2. 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.

  3. 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

  4. 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.

  5. Proterozoic Geomagnetic Field Geometry

    NASA Astrophysics Data System (ADS)

    Panzik, J. E.; Evans, D. A.

    2011-12-01

    Pre-Mesozoic continental reconstructions and paleoclimatic inferences from paleomagnetism rely critically upon the assumption of a time-averaged geocentric axial dipole (GAD) magnetic field. We have been testing the GAD assumption and localized non-dipole components in a different manner, by observing directional variations within the Matachewan, Mackenzie and Franklin dyke swarms. Large dyke swarms, commonly emplaced within a few million years, provide the necessary broad areal coverage to perform a test of global geomagnetic field geometry. Our analysis varies the quadrupole and octupole values of the generalized paleolatitude equation to determine a minimal angular dispersion and maximum precision of paleopoles from each dyke swarm. As a control, paleomagnetic data from the central Atlantic magmatic province (CAMP) show the sensitivities of our method to non-GAD contributions to the ancient geomagnetic field. Within the uncertainties, CAMP data are consistent with independent estimates of non-GAD contributions derived from global tectonic reconstructions (Torsvik & Van der Voo, 2002). Current results from the three Proterozoic dyke swarms all have best fits that are non-dipolar, but they differ in their optimal quadrupole/ octupole components. Treated together under the hypothesis of a static Proterozoic field geometry, the data allow a pure GAD geodynamo within the uncertainty of the method. Current results were performed using Fisherian statistics, but Bingham statistics will be included to account for the ellipticity of data.

  6. 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

  7. 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.

  8. Age of UHP metamorphism in the Western Mediterranean: Insight from rutile and minute zircon inclusions in a diamond-bearing garnet megacryst (Edough Massif, NE Algeria)

    NASA Astrophysics Data System (ADS)

    Bruguier, Olivier; Bosch, Delphine; Caby, Renaud; Vitale-Brovarone, Alberto; Fernandez, Laure; Hammor, Dalila; Laouar, Rabah; Ouabadi, Aziouz; Abdallah, Nachida; Mechati, Mehdi

    2017-09-01

    Diamond-bearing UHP metamorphic rocks witness for subduction of lithospheric slabs into the mantle and their return to shallow levels. In this study we present U-Pb and trace elements analyses of zircon and rutile inclusions from a diamond-bearing garnet megacryst collected in a mélange unit exposed on the northern margin of Africa (Edough Massif, NE Algeria). Large rutile crystals (up to 300 μm in size) analyzed in situ provide a U-Pb age of 32.4 ± 3.3 Ma interpreted as dating the prograde to peak subduction stage of the mafic protolith. Trace element analyses of minute zircons (≤30 μm) indicate that they formed in equilibrium with the garnet megacryst at a temperature of 740-810 °C, most likely during HP retrograde metamorphism. U-Pb analyses provide a significantly younger age of 20.7 ± 2.3 Ma attributed to exhumation of the UHP units. This study allows bracketing the age of UHP metamorphism in the Western Mediterranean Orogen to the Oligocene/early Miocene, thus unambiguously relating UHP metamorphism to the Alpine history. Exhumation of these UHP units is coeval with the counterclockwise rotation of the Corsica-Sardinia block and most likely resulted from subduction rollback that was driven by slab pull.

  9. 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.

  10. 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.

  11. 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

  12. Latest Proterozoic stratigraphy and Earth history.

    PubMed

    Knoll, A H; Walter, M R

    1992-04-23

    The end of the Proterozoic Eon was a time of pronounced biological, biogeochemical, climatic and tectonic change. New bio- and chemostratigraphic data provide an improved framework for stratigraphic correlation, making possible a deeper understanding of latest Proterozoic Earth history and providing tools for a chronostratigraphic division of late Proterozoic time.

  13. 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 CO2-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 three

  14. Methane-related diamond crystallization in the Earth's mantle: Stable isotope evidences from a single diamond-bearing xenolith

    NASA Astrophysics Data System (ADS)

    Thomassot, E.; Cartigny, P.; Harris, J. W.; (Fanus) Viljoen, K. S.

    2007-05-01

    Mineralogical studies of deep-seated xenoliths and mineral inclusions in diamonds indicate that there is significant variability in oxygen fugacity within the Earth's upper mantle. This variability is consistent both with the occurrence of reduced (methane-bearing) or oxidized (CO 2/carbonate-bearing) fluids. Invariably, direct sampling of reduced deep fluids is not possible as they are unquenchable and re-equilibrate with either the surrounding mantle or are affected by degassing. Key information about the nature of such fluids might be found in diamond if it were possible to study a population related to a single source. Usually, diamonds within a kimberlite pipe have different parageneses and can be shown to have formed at different times and depths. We studied 59 diamonds extracted from a single diamondiferous peridotite xenolith (with a volume of only 27 cm 3), from the Cullinan mine (formerly called the Premier mine) in South Africa. Diamond sizes range from 0.0005 to 0.169 carats (0.1 to 33.8 mg). A correlation between the nitrogen contents of the diamonds (range 40 to 1430 ppm) and their nitrogen aggregation state (varying from 10 to 85% of IaB defects) is compatible with a single growth event. δ 13C-values range from - 4.2‰ to - 0.1‰, with slight internal variability measured in the largest diamonds. Nitrogen isotope measurements show δ 15N ranging from - 1.2‰ to + 7.2‰. On the centimeter scale of this upper mantle rock, the variations for nitrogen content, nitrogen aggregation state, carbon and nitrogen isotopic compositions, respectively, cover 64%, 75%, 15% and 23% of the ranges known for peridotitic diamonds. In spite of such large ranges, N-content, δ 13C and δ 15N within this diamond population are distinctly coupled. These relationships do not support a mixing of carbon sources, but are best explained by a Rayleigh distillation within the sub-continental mantle at depths > 150 km and T > 1200 °C, which precipitates diamonds from methane

  15. Sub-volcanic development of kimberlite pipes: Evidence from the Lace and Voorspoed (Group II) kimberlites, South Africa

    NASA Astrophysics Data System (ADS)

    Howarth, Geoffrey H.; Skinner, E. Michael W.

    2013-12-01

    The Lace and Voorspoed kimberlites occur on the Kaapvaal Craton (South Africa), and form part of the Kroonstad Group II kimberlite (orangeite) cluster. The Lace kimberlite is composed of a main pipe and a satellite blind pipe, the latter of which does not reach the current land surface (~ 30 m below the current land surface), and is not observed connecting with the main pipe at depth. The main pipe increases in size from ~ 100 m to ~ 250 m in diameter at depth. The Voorspoed kimberlite pipe is the largest of the cluster and is dominantly infilled with massive layers (up to 200 m thick) of resedimented volcaniclastic kimberlite (RVK). Coherent kimberlite (CK), identified at all three pipes, is described here in order to constrain their formation.

  16. 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.

  17. 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

  18. 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.

  19. 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

  20. Variations of the Fe# of garnet, olivine and other peridotite minerals in the mantle columns beneath the Yakutian kimberlites.

    NASA Astrophysics Data System (ADS)

    Ashchepkov, I.; Ntaflos, T.; Logvinova, A.; Smelov, A.; Vladykin, N.; Kostrovitsky, S.; Lelyukh, S.; Afanasiev, V.; Kuligin, S.; Minin, V.; Downes, H.; Khmelnikova, O.; Nigmatulina, E.; Tychkov, N.; Skvortsova, M.; Rotman, A.

    2012-04-01

    . The reasons of variation issinterction with the mantle melts. The fluid rich melts produced essential depletions, the basaltic ascending trap basaltic melts create the increasing Fe within the extended interval from SCLM base to top. The alkaline basalts mainly interacts within the upper part of mantle sections. The protokimberlites mainly affect the form the bas e to 41 kbars. And the lamproites whci ar more Fe# rich may produce the decreasing Fe trends from the base to top. The temporal changes of mantle Fe# 8 in Archean 9 Proterozoic and 10 (Griffin , O 'Reillly, 2003) show in he reality even Proterozoic Permier SCLM there are three separate P-Fe# trends 5, 7 and 9 % increasing upward. Similar trend for Dharvar kimberlites have Fe# 7- 10% variations. Paleozoic kimberlites Baltica. are similar to Sibrian. The Wyoming SCLM is rather Mg rich. The SCLM for Mz kimbelrites in Africa and Canada show many trends to 15 % spreading upward. RBRF grant 11-05-00060.

  1. Crystal-Chemical Correlations in Chromites from Kimberlitic and Non-Kimberlitic Sources.

    NASA Astrophysics Data System (ADS)

    Freckelton, C. N.; Flemming, R. L.

    2009-05-01

    This study explores the utility of micro X-ray diffraction (μXRD) as a tool for diamond exploration, as a compliment to current industry-standard techniques such as electron probe microanalysis (EPMA). Here we examine chromite. As one of the first phases to crystallize in mantle rocks, it is a useful indicator of upper mantle magmatic conditions in rocks that have been sampled by kimberlites. In addition, chromite does not alter easily from chemical and physical weathering processes. As such, chromite is a useful kimberlite indicator mineral in diamond exploration. We present correlations between crystal structure (unit cell) and chemical composition of chromite, (Fe,Mg)[Cr, Al]2O4, using correlated μXRD and EPMA data for 133 chromites from a three source locations: Two kimberlite sources and one non-kimberlitic source from an Archean granite/greenstone terrain. Quantitative analysis was performed using Electron Probe Microanalysis (EPMA) at Mineral Services, South Africa, prior to the loan of the samples. Randomly-oriented chromite grains, approximately 500 μm in diameter, were analyzed as previously mounted for EPMA. Micro X-ray-diffraction was performed using a Bruker D8-Discover Diffractometer, with θ-θ geometry, with CuKα radiation, operating at 40 kV and 40 mA, with nominal beam diameter of 500 μm. The data were collected in omega scan mode. Two dimensional General Area Detector Diffraction System (GADDS) images were collected for 20 minutes per image, and integrated to produce one-dimensional plots of intensity versus 2θ, for subsequent unit cell refinement using CELREF. Although all samples in this study were considered to be 'chromite', a plot of Cr/(Cr+Al) versus Fe2+/(Fe2++Mg) shows extensive substitution among four dominant members: chromite (FeCr2O4), magnesio-chromite (MgCr2O4), spinel (MgAl2O4), and hercynite (FeAl2O4), where Mg and Fe2+ substitute for one another on the tetrahedral site, and Cr and Al substitute for one another on the

  2. 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.

  3. 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.

  4. 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.

  5. Oxygen isotope ratios in eclogites from kimberlites.

    PubMed

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

    1971-06-04

    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.

  6. An infrared investigation of inclusion-bearing diamonds from the Venetia kimberlite, Northern Province, South Africa : implications for diamonds from craton-margin settings

    NASA Astrophysics Data System (ADS)

    Viljoen, (Fanus)

    2002-08-01

    The Venetia kimberlites in the Northern Province of South Africa sampled diamonds from the lithosphere underlying the Central Zone of the Limpopo Belt. Given the general correlation of diamond-bearing kimberlites with old stable cratons, this tectonic setting is somewhat anomalous and, therefore, it is desirable to characterise the diamonds in terms of their infrared characteristics. A suite of diamonds of known paragenesis from the Venetia mine spans a large range of nitrogen concentrations from less than the detection limit to 1,355 ppm. Diamond nitrogen contents are, on average, higher in the eclogitic diamond population relative to the websteritic and peridotitic diamonds. Nitrogen aggregation states are variable, ranging from almost pure type IaA diamond (poorly aggregated nitrogen) to pure type IaB diamond (highly aggregated nitrogen). On a nitrogen aggregation diagram two distinct groups can be identified based on nitrogen content and nitrogen aggregation state. These are a minor population of diamonds with nitrogen contents generally higher than 500 ppm and nitrogen aggregation states of less than 40% IaB, and another, dominant population that is characterised by higher and more variable nitrogen aggregation. The unusually aggregated nature of the majority of the diamonds analysed is unique to Venetia relative to other intrusives on the Kaapvaal-Kalahari craton, but is similar to aggregation states observed for diamonds from other craton margin or adjacent mobile belt settings such as the Argyle lamproite and the George Creek kimberlite. This could be a consequence of diamond mantle residence at mantle temperatures higher than the norm for other kimberlites from the interior of cratons. Deformation of the mantle, associated with dynamic processes such as orogenesis or subduction, might also be responsible for accelerating the rate of nitrogen aggregation in these diamonds. Low numbers of diamonds with degradation of platelets at the Venetia kimberlite

  7. 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

  8. The Diamond Potential of the Tuwawi Kimberlite (Baffin Island, Nunavut).

    NASA Astrophysics Data System (ADS)

    Cross, J.; Kopylova, M.; Ritcey, D.; Kirkley, M.

    2009-05-01

    Baffin Island, underlain by Archean crust of the Rae craton with Paleoproterozoic reworking, is known to contain several kimberlites of possibly Cretaceous age. The most recent findings of kimberlite are located at the northwestern end of Baffin Island on the Brodeur Peninsula. The Tuwawi kimberlite, one in the cluster of 3 kimberlites, has an inverted cone shape. We studied drill core samples of kimberlite and mantle xenoliths from the Tuwawi kimberlite to constrain its diamond potential. Hypabyssal and volcaniclastic kimberlite types have been identified among available kimberlite core. Hypabyssal kimberlite is the predominant type in Tuwawi. The kimberlite consists of olivine macrocrysts set in a carbonate-serpentine groundmass with olivine microphenocrysts, phlogopite and spinel. Volcaniclastic kimberlite is characterized by the presence of 1) irregularly-shaped juvenile lapilli; 2) two semi-intermixed dark cryptocrystalline matrix materials; 3) olivine grains with a restricted size distribution and angular shapes. These features suggest mild sorting of the kimberlite, a possible incorporation of mud to the matrix, an epiclastic origin and formation in the crater facies. Peridotites and a garnet clinopyroxenite are found as xenoliths in the Tuwawi kimberlite. Peridotites include garnet lherzolite, garnet, spinel, and garnet-spinel harzburgites, and dunite. Both coarse and deformed (porphyroclastic and mosaic-porphyroclastic) textures are present within the peridotite xenoliths, and Cr- diopside from deformed xenoliths shows higher TiO2 (0.16 wt%) content than in coarse peridotites. Pyrope (Mg70-82) is present in all but one sample, whereas spinel occurs only in coarse peridotites and shows strong heterogeneity. It is controlled by random intra-grain compositional changes in FeO (from 13 to 16 wt%), MgO, Al2O3 and Cr2O3 (from 43 to 57 wt% ). Olivine and orthopyroxene in all xenoliths are very magnesian (Fo85-87 and En86-89), slightly more so in coarse

  9. 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

  10. Lattice preferred orientation of olivine found in diamond-bearing garnet peridotites in Finsch, South Africa and implications for seismic anisotropy

    NASA Astrophysics Data System (ADS)

    Lee, Jaeseok; Jung, Haemyeong

    2015-04-01

    Seismic anisotropy in the upper mantle provides important constraints on mantle dynamics, continental evolution and global tectonics and is believed to be produced by the flow-induced lattice-preferred orientation (LPO) of olivine. Recent experimental studies at high pressure and temperature have suggested that the LPO of olivine is affected by pressure in addition to water and stress. However, there has been no report yet for the pressure-induced LPO of natural olivine because samples from the deep upper mantle are rare and often unsuitable for study due to ambiguous foliation and lineation. Here we show evidence of the pressure-induced LPO of natural olivine in diamond-bearing garnet peridotites from Finsch, South Africa. We found that the [010] axes of olivine are aligned subnormal to foliation and that the [001] axes are aligned subparallel to lineation, which is known as B-type LPO of olivine. The equilibrium pressure of the samples, as estimated using geobarometer, was greater than 4 GPa, indicating that the samples originated from a depth greater than ~120 km. In addition, FTIR spectroscopy of the olivine showed that the samples are dry, with a water content of less than 90±20 ppm H/Si (5.5±1.2 ppm wt. H2O). These data suggest that the samples are the first natural examples of olivine displaying B-type LPOs produced due to high pressure under dry condition. Our data indicate that the trench-parallel seismic anisotropy observed in many subduction zones in and below subducting slabs at depths greater than ~90 km under dry condition may be attributed to the pressure-induced olivine fabrics (B-type LPO) and may be interpreted as the entrainment of the sub-lithospheric mantle in the direction of subduction rather than anomalous trench-parallel flow.

  11. Lattice-preferred orientation of olivine found in diamond-bearing garnet peridotites in Finsch, South Africa and implications for seismic anisotropy

    NASA Astrophysics Data System (ADS)

    Lee, Jaeseok; Jung, Haemyeong

    2015-01-01

    Seismic anisotropy in the upper mantle provides important constraints on mantle dynamics, continental evolution and global tectonics and is believed to be produced by the flow-induced lattice-preferred orientation (LPO) of olivine. Recent experimental studies at high pressure and temperature have suggested that the LPO of olivine is affected by pressure in addition to water and stress. However, there has been no report yet for the pressure-induced LPO of natural olivine because samples from the deep upper mantle are rare and often unsuitable for study due to ambiguous foliation and lineation. Here we show evidence of the pressure-induced LPO of natural olivine in diamond-bearing garnet peridotites from Finsch, South Africa. We found that the [010] axes of olivine are aligned subnormal to foliation and that the [001] axes are aligned subparallel to lineation, which is known as B-type LPO of olivine. The equilibrium pressure of the samples, as estimated using geobarometer, was greater than 4 GPa, indicating that the samples originated from a depth greater than ˜120 km. In addition, FTIR spectroscopy of the olivine showed that the samples are dry, with a water content of less than 90 ± 20 ppm H/Si (5.5 ± 1.2 ppm wt. H2O). These data suggest that the samples are the first natural examples of olivine displaying B-type LPOs produced due to high pressure under dry condition. Our data indicate that the trench-parallel seismic anisotropy observed in many subduction zones in and below subducting slabs at depths greater than ˜90 km under dry condition may be attributed to the pressure-induced olivine fabrics (B-type LPO) and may be interpreted as the entrainment of the sub-lithospheric mantle in the direction of subduction rather than anomalous trench-parallel flow.

  12. 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.

  13. H 2O and CO 2 in kimberlitic fluid as recorded by diamonds and olivines in several Ekati Diamond Mine kimberlites, Northwest Territories, Canada

    NASA Astrophysics Data System (ADS)

    Fedortchouk, Yana; Matveev, Sergei; Carlson, Jon A.

    2010-01-01

    Surface dissolution features on diamonds and Fourier Transform Infra Red spectroscopy (FTIR) of phenocrystal and xenocrystal olivines from kimberlites contain a record of magmatic fluid in kimberlite magmas. We investigated composition and behavior of kimberlitic fluid and the effect of volatiles on the eruption style and geology of kimberlites using microdiamonds and olivine concentrates from six kimberlite pipes with different lithologies and the character of diamond resorption (Ekati Diamond Mine, Northwest Territories, Canada). The study showed a clear correlation between the resorption style of diamond population of the kimberlites and the type of infrared (IR) spectra of their olivines. Four kimberlites have high quality diamonds with smooth regular surface features and high H 2O content of the olivines indicating the presence of H 2O-rich fluid during the emplacement. Fast ascent rates of fluid-rich magma can explain explosive eruption and filling the pipes with volcaniclastic kimberlite facies. Conversely, Grizzly and Leslie kimberlites have diamonds with complex sharp features diminishing diamond quality and indicating loss of the fluid. The slower ascent rates and less explosive eruption of the fluid-free magmas produced kimberlite pipes filled with magmatic facies kimberlite. Distinctive peaks in olivine IR spectra at 3356 and 3327 cm - 1 were found to correlate with the presence of hydrous magmatic fluid. Character of diamond morphology suggests that during the whole ascent of all six kimberlites, the magmatic fluid when present had a high H 2O:CO 2 ratio.

  14. 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.

  15. Asthenospheric kimberlites: Volatile contents and bulk compositions at 7 GPa

    NASA Astrophysics Data System (ADS)

    Stamm, Natalia; Schmidt, Max W.

    2017-09-01

    During ascent, kimberlites react with the lithospheric mantle, entrain and assimilate xenolithic material, loose volatiles and suffer from syn- and post-magmatic alteration. Consequently, kimberlite rocks deviate heavily from their primary melt. Experiments at 7 GPa, 1300-1480 °C, 10-30 wt% CO2 and 0.46 wt% H2O on a proposed primitive composition from the Jericho kimberlite show that saturation with a lherzolitic mineral assemblage occurs only at 1300-1350 °C for a carbonatitic melt with <8 wt% SiO2 and >35 wt% CO2. At asthenospheric temperatures of >1400 °C, where the Jericho melt stays kimberlitic, this composition saturates only in low-Ca pyroxene, garnet and partly olivine. We hence forced the primitive Jericho kimberlite into multiple saturation with a lherzolitic assemblage by adding a compound peridotite. Saturation in olivine, low- and high-Ca pyroxene and garnet was obtained at 1400-1650 °C (7 GPa), melts are kimberlitic with 18-29 wt% SiO2 + Al2O3, 22.1-24.6 wt% MgO, 15-27 wt% CO2 and 0.4-7.1 wt% H2O; with a trade-off of H2O vs. CO2 and temperature. Melts in equilibrium with high-Ca pyroxene with typical mantle compositions have ≥2.5 wt% Na2O, much higher than the commonly proposed 0.1-0.2 wt%. The experiments allow for a model of kimberlite origin in the convective upper mantle, which only requires mantle upwelling that causes melting at the depth where elemental carbon (in metal, diamond or carbide) converts to CO2 (at ∼250 km). If primary melts leading to kimberlites contain a few wt% H2O, then adiabatic temperatures of 1400-1500 °C would yield asthenospheric mantle melts that are kimberlitic (>18 wt% SiO2 + Al2O3) but not carbonatitic (<10 wt% SiO2 + Al2O3) in composition, carbonatites only forming 100-200 °C below the adiabat. These kimberlites represent small melt fractions concentrating CO2 and H2O and then acquire part of their chemical signature by assimilation/fractionation during ascent in the subcratonic lithosphere.

  16. The Morphology and Composition of Groundmass Spinel in Kimberlite

    NASA Astrophysics Data System (ADS)

    Roeder, P. L.

    2003-12-01

    Chromite and chromian spinel are a common, but very minor (<1%), early phase found in the groundmass of both basalt and kimberlite. The spinel is often zoned from a chromite core to a magnetite rim depending on the original melt composition, cooling rate and the nucleation of surrounding minerals. The primary silicate minerals in many kimberlites are often destroyed by late-stage alteration leaving spinel as one of the few minerals that retains morphological and chemical evidence of the progression from an early magmatic stage to a late-stage subsolidus alteration. The composition of basaltic and kimberlitic spinel can be compared by plotting FE2#(Fe2/(Fe2+Mg)) versus FE3#(Fe3/(Fe3+Al+Cr)). The primary chromite in both basalt and kimberlite usually have FE3# <0.15 and FE2# = 0.2-0.6 whereas the late stage magnetite for both rock types is high in FE3# and FE2#. Most spinel in basalts show a consistent trend of increasing FE2# with increasing FE3# whereas kimberlitic spinel can show a variety of different trends of increasing FE3# at a constant, or even decreasing, FE2#. Evidence will be presented that suggests that trends of FE3# vs. FE2# of spinel in various kimberlites may reflect variations in cooling rate of the kimberlite. It is useful to consider the morphology and compositional variation of spinel in kimberlite in terms of four stages: 1) High Cr2O3, TiO2<1 wt.%, FE2# < 0.5, FE3# <0.1. 2) High Cr2O3, TiO2 1-3 wt.%, FE2# <0.5, FE3# <0.1. 3) Cr2O3 1-50wt.%, TiO2 3-20 wt.%, FE2# 0.4-0.9, FE3# 0.1-0.9. 4) Cr2O3 <1 wt.%, TiO2 <2 wt.%, FE2# >0.8, FE3#>0.9 . The chromite of stage 1 reflects the bulk melt composition well before intrusion of the kimberlite whereas stage 2 is thought to reflect the relatively rapid and local change in melt composition during intrusion . Stage 3 reflects a very large change in composition due to very local crystallization of the groundmass minerals and diffusion-controlled crystallization that gives rise to atoll spinels. Stage 4

  17. 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

  18. Fractional ultrabasic-basic evolution of upper-mantle magmatism: Evidence from xenoliths in kimberlites, inclusions in diamonds and experiments

    NASA Astrophysics Data System (ADS)

    Litvin, Yuriy; Kuzyura, Anastasia

    2017-04-01

    Ultrabasic peridotites and pyroxenites together with basic eclogites are the upper-mantle in situ rocks among xenoliths in kimberlites. Occasionally their diamond-bearing varieties have revealed within the xenoliths. Therewith the compositions of rock-forming minerals demonstrate features characteristic for primary diamond-included minerals of peridotite and eclogite parageneses (the elevated contents of Cr-component in peridotitic garnets and Na-jadeitic component in eclogitic clinopyroxenes). High-pressure experimental study of melting equilibria on the multicomponent peridotie-pyroxenite system olivine Ol - orthopyroxene Opx - clinopyroxene Cpx - garnet Grt showed that Opx disappeared in the peritectic reaction Opx+L→Cpx (Litvin, 1991). As a result, the invariant peritectic equilibrium Ol+Opx+Cpx+Grt+L of the ultrabasic system was found to transform into the univariant cotectic assemblage Ol+Cpx+Grt+L. Further experimental investigation showed that olivine reacts with jadeitic component (Jd) with formation of garnet at higher 4.5 GPa (Gasparik, Litvin, 1997). Study of melting relations in the multicomponent system Ol - Cpx - Jd permits to discover the peritectic point Ol+Omph+Grt+L (where Omph - omphacitic clinopyroxene) at concentration 3-4 wt.% Jd-component in the system. The reactionary loss of Opx and Ol makes it possible to transform the 4-phase garnet lherzolite ultrabasic association into the bimineral eclogite assemblage. The regime of fractional Ol, Cpx and Grt crystallization must be accompanied by increasing content of jadeitic component in residual melts that causes the complete "garnetization of olivine". In the subsequent evolution, the melts would have to fractionate for basic SiO2-saturated compositions responsible for petrogenesis of eclogite varieties marked with accessory corundum Crn, kyanite Ky and coesite Coe. Both the peritectic mechanisms occur in regime of fractional crystallization. The sequence of the upper-mantle fractional

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

    PubMed

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

    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, δO(18) is predicted only to change slightly from mantle values. The model supports alteration of kimberlites predominantly by interactions with external non-magmatic fluids.

  20. Mineral inclusions in diamonds from the Kelsey Lake Mine, Colorado, USA: Depleted Archean mantle beneath the Proterozoic Yavapai province

    NASA Astrophysics Data System (ADS)

    Schulze, Daniel J.; Coopersmith, Howard G.; Harte, Ben; Pizzolato, Lori-Ann

    2008-03-01

    Thirty-four silicate and oxide inclusions large enough for in situ WDS electron microprobe analysis were exposed by grinding/polishing of 19 diamonds from the Kelsey Lake Mine in the Colorado-Wyoming State Line Kimberlite district. Eighteen olivines, seven Cr-pyropes, four Mg-chromites, and one orthopyroxene in 15 stones belong to the peridotite (P) suite and three garnets and one omphacite in three stones belong to the eclogite (E) suite. The fact that this suite is dominated by the peridotite population is in stark contrast to the other diamond suites studied in the State Line district (Sloan, George Creek), which are overwhelmingly eclogitic. Kelsey Lake olivine inclusions are magnesian (17 of 18 grains in 9 stones are in the range Fo 92.7-93.1), typical of harzburgitic P-suite stones worldwide, but unlike the more Fe-rich (lherzolitic) Sloan olivine suite. Mg-chromites (wt% MgO = 12.8-13.8; wt% Cr 2O 3 = 61.4-66.6) are in the lower MgO range of diamond inclusion chromites worldwide. Seven harzburgitic Cr-pyropes in five stones have moderately low calcium contents (wt% CaO = 3.3-4.3) but are very Cr-rich (wt% Cr 2O 3 = 9.7-16.7). A few stones have been analyzed by SIMS for carbon isotope composition and nitrogen abundance. One peridotitic stone is apparently homogeneous in carbon isotope composition (δ 13C PDB = -6.2‰) but with variable nitrogen abundance (1296-2550 ppm). Carbon isotopes in eclogitic stones range from "normal" for the upper mantle (δ 13C PDB = -5.5‰) to somewhat low (δ 13C PDB = -10.2‰), with little internal variation in individual stones (maximum difference is 3.6‰). Nitrogen contents (2-779 ppm) are lower than in the peridotitic stone, and are lower in cores than in rims. As, worldwide, harzburgite-suite diamonds have been shown to have formed in Archean time, we suggest that the Kelsey Lake diamond population was derived from a block of Archean lithosphere that, at the time of kimberlite eruption, existed beneath the Proterozoic

  1. Basaltic Diatreme To Root Zone Volcanic Processes In Tuzo Kimberlite Pipe (Gahcho Kué Kimberlite Field, NWT, Canada)

    NASA Astrophysics Data System (ADS)

    Seghedi, I.; Kurszlaukis, S.; Maicher, D.

    2009-05-01

    Tuzo pipe is infilled by a series of coherent and fragmental kimberlite facies types typical for a diatreme to root zone transition level. Coherent or transitional coherent kimberlite facies dominate at depth, but also occur at shallow levels, either as dikes or as individual or agglutinated coherent kimberlite clasts (CKC). Several fragmental kimberlite varieties fill the central and shallow portions of the pipe. The definition, geometry and extent of the geological units are complex and are controlled by vertical elements. Specific for Tuzo is: (1) high abundance of locally derived xenoliths (granitoids and minor diabase) between and within the kimberlite phases, varying in size from sub-millimeter to several tens of meters, frequent in a belt-like domain between 120-200 m depth in the pipe; (2) the general presence of CKC, represented by round-subround, irregular to amoeboid-shaped clasts with a macrocrystic or aphanitic texture, mainly derived from fragmentation of erupting magma and less commonly from previously solidified kimberlite, as well as recycled pyroclasts. In addition, some CKC are interpreted to be intersections of a complex dike network. This diversity attests formation by various volcanic processes, extending from intrusive to explosive; (3) the presence of bedded polymict wall- rock and kimberlite breccia occurring mostly in deep levels of the pipe below 345 m depth. The gradational contact relationships of these deposits with the surrounding kimberlite rocks and their location suggest that they formed in situ. The emplacement of Tuzo pipe involved repetitive volcanic explosions alternating with periods of relative quiescence causing at least partial consolidation of some facies. The volume deficit in the diatreme-root zone after each eruption was compensated by gravitational collapse of overlying diatreme tephra and pre-fragmented wall-rock xenoliths. Highly explosive phases were alternating with weak explosions or intrusive phases, suggesting

  2. Origin of salts and alkali carbonates in the Udachnaya East kimberlite: Insights from petrography of kimberlite phases and their carbonate and evaporite xenoliths

    NASA Astrophysics Data System (ADS)

    Kopylova, M. G.; Gaudet, M.; Kostrovitsky, S. I.; Polozov, A. G.; Yakovlev, D. A.

    2016-11-01

    The Udachnaya East kimberlite is characterized by the presence of chlorides, sulfates and alkali carbonates. This highly atypical mineralogy underpinned a model for an anhydrous alkali-rich primary kimberlite melt, despite the absence of petrographic studies providing textural context to the exotic minerals. The present work documents the petrography of the Udachnaya East kimberlite in order to address this problem. The pipe comprises two varieties of Fort-a-la-Corne type pyroclastic kimberlite, olivine-rich and magmaclast-rich, and coherent kimberlite. These kimberlites entrain xenoliths of limestones, altered shales and siltstones, halite-dominated rocks, dolomites, and coarse calcite rocks. The distinct varieties of the Udachnaya East kimberlite carry different populations of crustal xenoliths, which partially control the mineralogy of the host kimberlite. In magmaclast-rich pyroclastic kimberlite, where halite is absent from the crustal xenoliths, it is not observed in the interclast matrix, or within the magmaclasts. Halite occurs in the interclast matrix of olivine-rich pyroclastic kimberlite, where halite xenoliths are common. Large, 30 cm halite xenoliths are uniquely restricted to the coherent kimberlite and show a strong reaction with it. The halite xenoliths are sourced from depths of - 1500 to - 630 m, where carbonate beds host multiple karst cavities filled with halite and gypsum and occasional sedimentary evaporites. The style of secondary mineralization at Udachnaya depends on whether the kimberlite is coherent or pyroclastic. Shortite, pirssonite and other alkali carbonates replacing calcite and possibly serpentine are abundant only in porous pyroclastic kimberlites of both types and in their shale/siltstone xenoliths. The lower porosity of the coherent kimberlite prevented the interaction of kimberlite with Na brines. Serpentinization localized around halite xenoliths started at temperatures above 500 °C, as indicated by its association with high

  3. 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

  4. 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.

  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.

  6. An integrated model of kimberlite ascent and eruption.

    PubMed

    Wilson, Lionel; Head Iii, James W

    2007-05-03

    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.

  7. Kimberlites and aillikites as probes of the continental lithospheric mantle

    NASA Astrophysics Data System (ADS)

    Francis, Don; Patterson, Michael

    2009-04-01

    Although the mantle xenoliths carried by kimberlites are the source of much of our information about the composition of the mantle beneath the continents, the compositions of kimberlites themselves have received little attention for the information they carry about the nature of the lithospheric mantle. This neglect in part reflects their common fragmental, contaminated, and hybrid nature, but also the pervasive view that Group-I kimberlites are sourced in the underlying asthenosphere. Insight into the nature of kimberlites and their relationship to the other alkaline ultramafic rocks, such as aillikites, olivine lamproites, and meimechites, can be obtained by comparing their major element compositions in a way that treats their carbonate content as a primary magmatic phase. Group-I kimberlites and aillikites contain significant magmatic carbonate and their compositions fall to the Si-poor side of the composition of olivine. Group-I kimberlite can be distinguished from aillikite on the basis of Fe content, but there appears to be a gradation between these two end-members. In contrast, olivine lamproites and meimechites contain relatively little primary magmatic carbonate and have compositions that are more Si-rich than olivine. Pearce element ratio analysis assuming P as a conserved element indicates that much of the major element variation in hypabyssal kimberlites can be explained by variable amounts of olivine and orthopyroxene in proportions (˜ 70/30) similar to that of cratonic mantle xenoliths. Much of the olivine is present as xenocrysts, but the orthopyroxene is occult and has presumably been assimilated. The fact that individual fields of alkaline ultramafic rocks are characterized by uniform Fe and Ti contents that can be mapped on a regional scale suggests that the major element composition of these unusual rocks, and Group-I kimberlites in particular, is a reflection of the continental lithospheric mantle with which they have interacted. The

  8. 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.

  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. 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

  11. Kimberlite metasomatism at Murowa and Sese pipes, Zimbabwe

    NASA Astrophysics Data System (ADS)

    Smith, C. B.; Sims, K.; Chimuka, L.; Duffin, A.; Beard, A. D.; Townend, R.

    2004-09-01

    Metasomatism accompanying kimberlite emplacement is a worldwide phenomenon, although infrequently described or recognised. At the Cambrian-aged Murowa and Sese kimberlite clusters located within the Archean Zimbabwe Craton just north of the boundary with the Limpopo Mobile Zone in southern central Zimbabwe, the metasomatism is intense and well exposed and the processes can be readily studied. Dykes, sills and the root zones of pipes are exposed at the current erosion level. Kimberlite lithologies present are hypabyssal macrocrystic kimberlite (;HMK;), HMK breccia, and tuffisitic kimberlite breccia (;TKB;) including minor lithic tuffisitic kimberlite breccia (;LTKB;). Country rocks are 2.6 Ga Chibi and Zimbabwe granite batholiths emplaced into 2.6-2.9 Ga or earlier Archean tonalitic gneiss and greenstones. During initial metasomatism, the granites become spotted with green chlorite, needles of alkaline amphiboles (winchite, riebeckite, arfvedsonite) and pyroxenes (aegirine-augite) with minor carbonate and felts of talc. Oligoclase feldspar becomes converted to albite, extensively altered, dusted and reddened with hematite, whereas K-feldspar remains unaffected. The granites become converted to syenite through removal of quartz. More intense metasomatism at Murowa and Sese results in veins of green metasomatite which cut and disrupt the granite. Progressive disruption entrains granite blocks, breaking down the granite still further, spalling off needle-like granite slivers, and so giving rise to LTKB. This process of disruption and entrainment appears to be the manner of initial development of the pipe structure. The chemistry of the metasomatite is intermediate between granite and kimberlite. Compared to granite country rock it has markedly higher Mg, Cr, Ni, CO2 and H2O+, higher Ca, Mn, Nb, Sr, P, Fe3+/Fe2+ ratio, U, Co, and Cu, approximately equal TiO2, K2O, Na2O, La, Ta, Rb, Zr, Zn and resultant lower SiO2, Al2O3, Ga and Y. The metasomatite Na2O/K2O ratio is

  12. Experimental Study of Surface Dissolution Features on Kimberlite Indicator Minerals

    NASA Astrophysics Data System (ADS)

    McIsaac, E.; Fedortchouk, Y.

    2009-05-01

    During the ascent to the Earth's surface kimberlite magmas entrain mantle minerals - chromites, ilmenites, garnets and the most desirable - diamonds. Kimberlite magma partially dissolves these minerals during the ascent, producing different types of surface features on the minerals. Experiments showed that surface features on diamonds can be used to constrain composition of magmatic fluid. However, examining mantle minerals with more complex chemical compositions, such as chromites and ilmenites, could provide more detailed information about the composition and evolution of fluid system in the magmas, as determination of the depth of their entrainment is possible. This study experimentally investigates dissolution of chromites and ilmenites in melts with C-O-H fluid. The surface features produced at these conditions are then compared to the surface features on minerals recovered from kimberlites. The experiments were done in a piston-cylinder apparatus at 1350 - 1400°C and 1 GPa. Rounded natural mineral grains were placed in a synthetic mixture of Ca-Mg-Si-C-H-O composition with 0, 5, 13, 15, and 31 wt% H2O and 0, 5, and 27 wt% CO2. The experimental results investigated using Field-Emission Scanning Electron Microscope showed that angular step-like dissolution surfaces, which are common for natural kimberlitic chromites, develop only in the presence of H2O-rich fluid phase. The reaction of chromite with H2O dissolved in the melt and with dry melt caused smoothing of chromite surfaces. Chromite dissolution in CO2-rich melts produced rounded and polyhedral relief features. Both the smooth and polyhedral types of features are not typical for natural kimberlite-hosted chromite grains. Ilmenite underwent rapid dissolution at our experimental conditions. In H2O-rich fluid ilmenite produced "pyramidal" type of surface features previously described as the most common for natural kimberlitic ilmenites. The experimental results were compared to the natural minerals

  13. 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

  14. Synchroneity of cratonic burial phases and gaps in the kimberlite record: Episodic magmatism or preservational bias?

    NASA Astrophysics Data System (ADS)

    Ault, Alexis K.; Flowers, Rebecca M.; Bowring, Samuel A.

    2015-01-01

    A variety of models are used to explain an apparent episodicity in kimberlite emplacement. Implicit in these models is the assumption that the preserved kimberlite record is largely complete. However, some cratons now mostly devoid of Phanerozoic cover underwent substantial Phanerozoic burial and erosion episodes that should be considered when evaluating models for global kimberlite distributions. Here we show a broad temporal coincidence between regional burial phases inferred from thermochronology and gaps in the kimberlite record in the Slave craton, Superior craton, and cratonic western Australia. A similar pattern exists in the Kaapvaal craton, although its magmatic, deposition, and erosion history differs in key ways from the other localities. One explanation for these observations is that there is a common cause of cratonic subsidence and suppression of kimberlite magmatism. Another possibility is that some apparent gaps in kimberlite magmatism are preservational artifacts. Even if kimberlites occurred during cratonic burial phases, the largest uppermost portions of the pipes would have been subsequently eroded along with the sedimentary rocks into which they were emplaced. In this model, kimberlite magmatism was more continuous than the preserved record suggests, implying that evidence for episodicity in kimberlite genesis should be carefully evaluated in light of potential preservational bias effects. Either way, the correlation between burial and kimberlite gaps suggests that cratonic surface histories are important for understanding global kimberlite patterns.

  15. 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

  16. A Re Os isotope and PGE study of kimberlite-derived peridotite xenoliths from Somerset Island and a comparison to the Slave and Kaapvaal cratons

    NASA Astrophysics Data System (ADS)

    Irvine, Gordon J.; Pearson, D. Graham; Kjarsgaard, B. A.; Carlson, R. W.; Kopylova, M. G.; Dreibus, G.

    2003-12-01

    The concentrations of platinum-group elements (PGE; Os, Ir, Ru, Pd and Pt) and Re, and the Os isotopic compositions were determined for 33 lithospheric mantle peridotite xenoliths from the Somerset Island kimberlite field. The Os isotopic compositions are exclusively less radiogenic than estimates of bulk-earth ( 187Os/ 188Os as low as 0.1084) and require a long-term evolution in a low Re-Os environment. Re depletion model ages ( TRD) indicate that the cratonic lithosphere of Somerset Island stabilised by at least 2.8 Ga, i.e. in the Neoarchean and survived into the Mesozoic to be sampled by Cretaceous kimberlite magmatism. An Archean origin also is supported by thermobarometry (Archean lithospheric keels are characterised by >150 km thick lithosphere), modal mineralogy and mineral chemistry observations. The oldest ages recorded in the lithospheric mantle beneath Somerset Island are younger than the Mesoarchean (>3 Ga) ages recorded in the Slave craton lithospheric mantle to the southwest [Irvine, G.J., et al., 1999. Age of the lithospheric mantle beneath and around the Slave craton: a Rhenium-Osmium isotopic study of peridotite xenoliths from the Jericho and Somerset Island kimberlites. Ninth Annual V.M. Goldschmidt Conf., LPI Cont., 971: 134-135; Irvine, G.J., et al., 2001. The age of two cratons: a PGE and Os-Isotopic study of peridotite xenoliths from the Jericho kimberlite (Slave craton) and the Somerset Island kimberlite field (Churchill Province). The Slave-Kaapvaal Workshop, Merrickville, Ontario, Canada]. Younger, Paleoproterozoic, TRD model ages for Somerset Island samples are generally interpreted as the result of open system behaviour during metasomatic and/or magmatic processes, with possibly the addition of new lithospheric material during tectono-thermal events related to the Taltson-Thelon orogen. PGE patterns highly depleted in Pt and Pd generally correspond to older Archean TRD model ages indicating closed system behaviour since the time of

  17. 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.

  18. 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

  19. The age of unusual xenogenic zircons from Yakutian kimberlites

    NASA Astrophysics Data System (ADS)

    Vladykin, N. V.; Lepekhina, E. A.

    2009-12-01

    Several spindle-shaped grains of zircon, which have a small size (<0.25 mm) and a distinct purplish pink coloration were found in the crushed samples of kimberlites from the Aykhal, Komsomolskaya-Magnitnaya, Botuobinskaya (Siberian platform), and Nyurbinskaya (Yakutia) pipes and olivine lamproites of the Khani massif (West Aldan). U-Pb SHRIMP II zircon dating performed at the VSEGEI Center for Isotopic Research yielded the ages of 1870-1890 Ma for the pipes of the Western province (Aykhal and Komsomolskaya) and 2200-2750 Ma for the pipes of the eastern province (Nyurbinskaya and Botuobinskaya), which allowed us to consider these zircons to be xenogenic to kimberlites. Although these zircons resemble in their age and color those from the granulite xenoliths in the Udachnaya pipe [2], no other granulite minerals are found there. Thus, major geological events in the mantle and lower crust, which led to the formation of zircon-bearing rocks, happened at 1800-1900 Ma in the northern part of the kimberlite province, whereas in the Eastern part of the province (Nakyn field) these events were much older (2220-2700 Ma). It is known that the period of 1800-1900 Ma in the Earth’s history was accompanied by intense tectonic movements and widespread alkaline-carbonatite magmatism. This magmatism was related to plume activity responsible for overheating the large portions of the mantle to the temperatures at which some diamonds in mantle rocks would burn (northern part of the kimberlite province). In the Nakyn area, the mantle underwent few or no geological processes at that time, and perhaps for this reason this area hosts more diamondiferous kimberlites. The age of olivine lamproites from the Khani massif is 2672-2732 Ma. Thus, these are some of the world’s oldest known K-alkaline rocks.

  20. The Nd and Sr isotopic evolution of Proterozoic seawater

    NASA Astrophysics Data System (ADS)

    Derry, Louis A.; Jacobsen, Stein B.

    1988-04-01

    Nd isotope measurements on banded iron formations and phosphorites, and Sr isotope measurements on carbonates, indicate that during both the Early and Late Proterozoic, hydrothermal input to the oceans was a significant fraction of the total input to ocean chemistry. Isotopic data from Early Proterozoic clastic sediments show systematic differences from coeval chemical sediments. These differences become less marked toward the end of the Proterozoic. This implies a higher hydrothermal water to river water flux ratio during the Early Proterozoic. The significant changes in seawater isotopic composition during the Proterozoic reflect the transition from mantle dominated Archean oceans to a typically modern system.

  1. 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

  2. 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.

  3. 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.

  4. 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.

  5. Diamondiferous kimberlites in central India synchronous with Deccan flood basalts

    NASA Astrophysics Data System (ADS)

    Lehmann, Bernd; Burgess, Ray; Frei, Dirk; Belyatsky, Boris; Mainkar, Datta; Rao, Nittala V. Chalapathi; Heaman, Larry M.

    2010-02-01

    Recently discovered diamondiferous kimberlite (Group-II) pipes in central India have surprisingly young 40Ar/ 39Ar whole rock and U-Pb perovskite ages around 65 million years. These ages overlap with the main phase of the Deccan flood basalt magmatism, and suggest a common tectonomagmatic control for both flood basalts and kimberlites. The occurrence of macrodiamonds in the pipes implies the presence of a thick subcratonic lithosphere at the Cretaceous/Tertiary boundary, significantly different from the present-day thickness of the Indian lithosphere. About one third of the Indian lithosphere was lost during or after the Deccan flood basalt event. The superfast northward motion of the Indian plate prior to the collision with Eurasia cannot be related to lithospheric thinning during the Gondwana break-up at 130 Ma, as previously thought.

  6. 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.

  7. 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.

  8. 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.

  9. 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

  10. 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.

  11. Kimberlite emplacement models — The implications for mining projects

    NASA Astrophysics Data System (ADS)

    Jakubec, Jaroslav

    2008-06-01

    The significance of the emplacement model for kimberlite pipes, or sheets, is commonly recognized in resource geology. However, its importance is not always appreciated in the mine design process. The fact is that knowledge of the orebody geometry, character of the contact zones, internal structures, rock mass competency and distribution of inclusions could directly influence the selection of the underground mining method, pit wall stability, dilution, treatability, and the dewatering strategy. The problems are exacerbated in smaller pipes and narrower sheets, and in more irregular shapes; they are more apparent in underground mining as opposed to open cast. Various kimberlite emplacement processes have a major impact on the nature of the kimberlite orebody and host rocks that will influence the mine design and mining strategy. Failure to understand these processes can adversely affect the economic outcome for developing a mine. It is therefore important to investigate those processes in order to better characterize the mining constraints and risks, and more accurately predict the mine's economic viability.

  12. [Proterozoic history and present state of cyanobacteria].

    PubMed

    Sergeev, V N; Gerasimenko, L M; Zavarzin, G A

    2002-01-01

    The paper delves into the main regularities of the distribution of fossil microorganisms in Precambrian rocks, beginning from the Archean Eon about 3.5 billion years ago and ending in the Cambrian Period about 0.5 billion years ago. The paper analyzes facial peculiarities in the lateral differentiation of microfossils in Proterozoic basins and the main stages of temporal changes in fossil cyanobacterial communities, which are based on the irreversible succession of physicochemical conditions on the Earth and the evolution of eukaryotic microorganisms and their incorporation into prokaryotic ecosystems. To gain insight into Proterozoic fossil records, modern stratified cyanobacterial mats built up from layers of prokaryotes are considered. The analysis of phosphatization, carbonatization, and silification processes in modern algal-bacterial communities suggests that analogous processes took place in Proterozoic microbiotas. A comparison of modern and Precambrian living forms confirms the inference that cyanobacterial communities are very conservative and have changed insignificantly both morphologically and physiologically during the past two billion years.

  13. In-vent column collapse as an alternative model for massive volcaniclastic kimberlite emplacement: An example from the Fox kimberlite, Ekati Diamond Mine, NWT, Canada

    NASA Astrophysics Data System (ADS)

    Porritt, L. A.; Cas, R. A. F.; Crawford, B. B.

    2008-06-01

    The origins of massive, poorly sorted fragmental kimberlite and kimberlite breccias in the diatremes of kimberlite volcanic pipes is currently poorly understood. Studies of the textural features of the major infill of the Fox kimberlite, Ekati Diamond Mine, NWT, Canada, show that it may have formed as a result of the collapse of the explosive eruption column above the vent into the deep open pipe during the climactic stage of the eruption. As the eruption intensity increased the column became critically overloaded with dense particles and rapidly collapsed onto itself. Unlike deposits typically preserved from column collapse events, such as ignimbrites, some portion of the collapsing mass was trapped in the deep pipe, forming a distinct type of pyroclastic deposit, "in-vent/vent-fill column collapse lapilli-tuff". This deposit has previously been tentatively identified as tuffisitic kimberlite (TK) or massive volcaniclastic kimberlite (MVK), based on broadly similar textural characteristics with these lithologies. Though every individual deposit will have formed under slightly different circumstances, mass emplacement from column collapse is proposed as a viable model for massive volcaniclastic kimberlite emplacement, based on extrapolation of modern volcanological processes.

  14. Growth of bultfonteinite and hydrogarnet in metasomatized basalt xenoliths in the B/K9 kimberlite, Damtshaa, Botswana: insights into hydrothermal metamorphism in kimberlite pipes

    NASA Astrophysics Data System (ADS)

    Buse, Ben; Schumacher, John C.; Sparks, R. Stephen J.; Field, Matthew

    2010-10-01

    Metamorphic assemblages within Karoo basalt xenoliths, found within volcaniclastic kimberlite of the B/K9 pipe, Damtshaa, Botswana, constrain conditions of kimberlite alteration. Bultfonteinite and chlorite partially replace the original augite-plagioclase assemblage, driven by the serpentinisation of the kimberlite creating strong chemical potential gradients for Si and Mg. Hydrogarnet and serpentine replace these earlier metamorphic assemblages as the deposits cool. The bultfonteinite (ideally Ca2SiO2[OH,F]4) and hydrogarnet assemblages require a water-rich fluid containing F-, and imply hydrothermal alteration dominated by external fluids rather than autometamorphism from deuteric fluids. Bultfonteinite and hydrogarnet are estimated to form at temperatures of ca. 350-250°C, which are similar to those for serpentinisation. Alteration within the B/K9 kimberlite predominantly occurs between 250 and 400°C. We attribute these conditions to increased efficiency of mass transfer and chemical reactions below the critical point of water and a consequence of volume-increasing serpentinisation and metasomatic reactions that take place over this temperature range. A comparison of the B/K9 kimberlite with kimberlites from Venetia, South Africa suggests that the composition and mineralogy of included xenoliths affects the alteration assemblages within kimberlite deposits.

  15. Application of Magnetic Mapping to Proterozoic Continental Reconstruction

    NASA Astrophysics Data System (ADS)

    Fry, B. N.; Morgan, P.

    2002-12-01

    Models of the Neoproterozoic (1.0-0.8 Ga) assembly of the supercontinent Rodinia include the SWEAT hypothesis, which places Proterozoic eastern Australia adjacent to the Proterozoic western margin of Canada, and the AUSWUS hypothesis, which juxtaposes the Proterozoic southwestern U.S. and Proterozoic eastern Australia. The disparity between the two models is largely due to an inability to correlate different basement mapping techniques used for the two continents. A thick weathering regolith in Australia has required the use of magnetic signature maps to define the Precambrian basement. Because of abundant basement exposure in the western U.S., isotopic methods have been used to define Precambrian terrane boundaries and little magnetic mapping has been done. The eastern margin of Proterozoic Australia is defined by magnetic mapping as the Tasman line. SWEAT places the eastern Proterozoic margin of Australia, west of the Tasman line against the northwestern margin of Proterozoic Canada. This model is mainly based on geologic piercing points and stratigraphic/isotopic correlations. The AUSWUS model connects the Tasman line to the western margin of the Proterozoic U.S., adjacent to the 87Sr/86Sr line. The 87Sr/86Sr line has been suggested to divide Proterozoic basement to the east and younger accreted terranes to the west. Magnetic mapping of the Proterozoic margin of the southwestern U.S. through the reduction of aeromagnetic data allows comparison with the magnetically defined margin of Proterozoic Australia. Preliminary evaluation of magnetic styles (amplitude and wavelength) and linear trends of the southwestern U.S. suggest that both Proterozoic and Phanerozoic igneous rocks greatly influence the total magnetic signature. Isolation of the Proterozoic component from the total aeromagnetic signal requires understanding the character, distribution, and magnitude of the magnetism in the Proterozoic rocks. The magnetic signatures of two suites of granites in the

  16. 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.

  17. 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

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

    NASA Astrophysics Data System (ADS)

    Afanasyev, Andrey; Melnik, Oleg; Porritt, Lucy; Schumacher, John; Sparks, Steve

    2015-04-01

    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. In order to simulate cooling of a kimberlite body by external water influx, we have used a modified version of the filtration code MUFITS (www.mufits.imec.msu.ru). The code is developed for simulation of multiphase multicomponent flows in porous media in a wide range of pressures and temperatures, including sub-critical and supercritical conditions. It solves mass conservation laws for individual components (water and a proxy component, not participating in serpentinisation) together with energy equation for the system as a whole including the solid rock matrix, and Darcy transport equations for different phases. Two modifications of the code were implemented: Serpentinisation of the olivine leads to a decrease in the density of the rock matrix and filling pore spaces resulting in significant decrease in porosity and permeability; latent heat of serpentinisation is accounted for in the energy equation. The simulation results indicate that 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

  19. Kimberlite Trends at the Surface and at Depth

    NASA Astrophysics Data System (ADS)

    Snyder, D. B.; Lockhart, G.

    2004-05-01

    Although the focus of much study as the host rock for diamonds, the emplacement mechanisms and structure of kimberlite deposits remains only poorly perceived. Recent application of geochronological and seismic techniques to the Lac De Gras kimberlite field that is home to the world's newest diamond mines in NW Canada revealed unexpected correlation in structural trends. The best fitting patterns for the variation in SKS splitting delay times for the Lac de Gras teleseismic stations are consistently those of two horizontal layers. The fast axis of each lower layer trends 045-050° and delay times are 0.9-1.0 seconds. Upper layers show greater variability; beneath the Ekati Diamond Mine, the fast axis trends 012° with a delay time of 0.45 seconds. At depths of about 120 km, the radial component of receiver functions from this same station has maximum amplitude at 285-290° and the transverse component at about 320\\deg; together these indicate a horizontal symmetry axis for hexagonal anisotropy at 108/288° and an associated fast axis at 018/198° . The 120 km depth is the top or bottom of the mantle layer containing this anisotropy; the large maximum amplitudes observed suggest it is here the bottom of the upper layer. Precise ages of over 40 kimberlites in the Lac de Gras field were constrained using standard Rb/Sr and U/Pb isotopic dating techniques correlated with a local geomagnetic polarity timescale; they indicate that one kimberlite group erupted from 75 to 64 Ma along a generally east-west (100-110° ) trend. Another 58.9±1.2 Ma group has a similar trend, whereas younger 55.4±0.5, 53.2±0.3 and 47.5±0.5 Ma clusters show tighter grouping along northeast (37-45° ) trends. The inferred age and direction of trends at both the surface and at >120 km depth suggests that kimberlites erupt along fractures controlled by continental stress fields related to global plate motions. This provides important clues about where to search for additional diamond deposits

  20. 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.

  1. 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.

  2. Petrology and geochemistry of diamondiferous Mesoproterozoic kimberlites from Wajrakarur kimberlite field, Eastern Dharwar craton, southern India: genesis and constraints on mantle source regions

    NASA Astrophysics Data System (ADS)

    Chalapathi Rao, N. V.; Srivastava, Rajesh K.

    2009-02-01

    The petrology and geochemistry of some new occurrences of Mesoproterozoic diamondiferous hypabyssal-facies kimberlites from the Chigicherla, Wajrakarur-Lattavaram and Kalyandurg clusters of the Wajrakarur kimberlite field (WKF), Eastern Dharwar craton (EDC), southern India, are reported. The kimberlites contain two generations of olivine, and multiple groundmass phases including phlogopite, spinel, calcite, dolomite, apatite, perovskite, apatite and rare titanite, and xenocrysts of eclogitic garnet and picro-ilmenite. Since many of the silicate minerals in these kimberlites have been subjected to carbonisation and alteration, the compositions of the groundmass oxide minerals play a crucial role in their characterisation and in understanding melt compositions. While there is no evidence for significant crustal contamination in these kimberlites, some limited effects of ilmenite entrainment are evident in samples from the Kalyandurg cluster. Geochemical studies reveal that the WKF kimberlites are less differentiated and more primitive than those from the Narayanpet kimberlite field (NKF), Eastern Dharwar craton. Highly fractionated (La/Yb = 108-145) chondrite-normalised distribution patterns with La abundances of 500-1,000 × chondrite and low heavy rare earth elements (HREE) abundances of 5-10 × chondrite are characteristic of these rocks. Metasomatism by percolating melts from the convecting mantle, rather than by subduction-related processes, is inferred to have occurred in their source regions based on incompatible element signatures. While the majority of the Eastern Dharwar craton kimberlites are similar to the Group I kimberlites of southern Africa in terms of petrology, geochemistry and Sr-Nd isotope systematics, others show the geochemical traits of Group II kimberlites or an overlap between Group I and II kimberlites. Rare earth element (REE)-based semi-quantitative forward modelling of batch melting of southern African Group I and II kimberlite source

  3. Geology of the Mwadui kimberlite, Shinyanga district, Tanzania

    NASA Astrophysics Data System (ADS)

    Stiefenhofer, J.; Farrow, D. J.

    2004-09-01

    The Mwadui pipe represents the largest diamondiferous kimberlite ever mined and is an almost perfectly preserved example of a kimberlitic crater in-fill, albeit without the tuff ring. The geology of Mwadui can be subdivided into five geological units, viz. the primary pyroclastic kimberlite (PK), re-sedimented volcaniclastic kimberlite deposits (RVK), granite breccias (subdivided into two units), the turbidite deposits, and the yellow shales listed in approximate order of formation. The PK can be further subdivided into two units—lithic-rich ash and lapilli tuffs which dominate the succession, and lithic-poor juvenile-rich ash and lapilli tuffs. The lower crater is well bedded down to at least 684 m from present surface (extent of current drill data). The bedding is defined by the presence of juvenile-rich lapilli tuffs vs. lithic-rich lapilli tuffs, and the systematic variation in granite content and clast size within much of the lithic-rich lapilli tuffs. Four distinct types of bedding have been identified in the pyroclastic deposits. Diffuse zones characterised by increased granite abundance and size, and upward-fining units, represent the dominant types throughout the deposit. Lateral heterogeneity was observed, in addition to the vertical changes, suggesting that the eruption was quite heterogeneous, or that more than one vent may have been present. The continuous nature of the bedding in the pyroclastic material and the lack of ash-partings suggest deposition from a high concentration (ejecta), sustained eruption column at times, e.g. the massive, very diffusely stratified deposits. The paucity of tractional bed forms suggest near vertical particle trajectories, i.e. a clear air-fall component, but the poorly sorted, matrix-supported nature of the deposits suggest that pyroclastic flow and/or surge processes may also have been active during the eruption. Available diamond sampling data were examined and correlated with the geology. Data derive from the old

  4. Dating kimberlite emplacement with zircon and perovskite (U-Th)/He geochronology

    NASA Astrophysics Data System (ADS)

    Stanley, Jessica R.; Flowers, Rebecca M.

    2016-11-01

    Kimberlites provide rich information about the composition and evolution of cratonic lithosphere. Accurate geochronology of these eruptions is key for discerning spatiotemporal trends in lithospheric evolution, but kimberlites can sometimes be difficult to date with available methods. We explored whether (U-Th)/He dating of zircon and perovskite can serve as reliable techniques for determining kimberlite emplacement ages. We obtained zircon and/or perovskite (U-Th)/He (ZHe, PHe) dates from 16 southern African kimberlites. Most samples with abundant zircon yielded reproducible ZHe dates (≤15% dispersion) that are in good agreement with published eruption ages. The majority of dated zircons were xenocrystic. Zircons with reproducible dates were fully reset during eruption or resided at temperatures above the ZHe closure temperature prior to entrainment in the kimberlite magma. Not dating hazy and radiation damaged grains can help avoid anomalous results for more shallowly sourced zircons that underwent incomplete damage annealing and/or partial He loss during the eruptive process. All seven kimberlites dated with PHe yielded reproducible (≤15% dispersion) and reasonable results. We conducted two preliminary perovskite 4He diffusion experiments, which suggest a PHe closure temperature of >300°C. Perovskite in kimberlites is unlikely to be xenocrystic and its relatively high temperature sensitivity suggests that PHe dates will typically record emplacement rather than postemplacement processes. ZHe and PHe geochronology can effectively date kimberlite emplacement and provide useful complements to existing techniques.

  5. 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

  6. (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

  7. High-Mg carbonatitic melts in diamonds, kimberlites and the sub-continental lithosphere

    NASA Astrophysics Data System (ADS)

    Weiss, Y.; Griffin, W. L.; Bell, D. R.; Navon, O.

    2011-09-01

    The trace elements of high-Mg carbonatitic high-density fluids (HDFs) trapped in six fibrous diamonds from Siberia exhibit patterns that are highly similar to those of Group I kimberlites, but are slightly more fractionated. The patterns of both are similar to the average pattern of post-Archaean xenoliths from the sub-continental lithospheric mantle (SCLM). The Siberian high-Mg carbonatitic HDFs are highly enriched in incompatible elements and have compositions comparable to those of high-Mg HDFs from Kankan, Guinea. However, in detail the latter show depletion of K, Rb, Cs, Nb and Ta and enrichment in Ba, Th, U and LREE relative to the Siberian HDFs. These differences correspond closely to those between the patterns of Group II and Group I kimberlites, respectively. Mixing, fractionation and melting were explored as possible scenarios to explain these similarities and to constrain the possible genetic relationships between HDFs, kimberlites and the SCLM. Addition of 2.5% of Group I kimberlitic magma or 0.5% of the Udachnaya high-Mg HDFs to a depleted peridotite closely reproduces the post-Archaean SCLM pattern. The formation of high-Mg HDFs through fractionation of kimberlitic magma calls for 80% crystallization of olivine, clinopyroxene, garnet, carbonate and ilmenite. However, mismatches in K, Rb, Y and Ho abundances, and absence of the postulated fractionating minerals as inclusions suggest other petrogenetic scenarios are more likely. High-Mg HDFs and kimberlites can be produced by melting of a common source. The pattern of the calculated source for Siberian HDF and Group I kimberlites resembles that of average post-Archean, rather than Archean, SCLM. Batch melting of such a source can produce high-Mg HDFs at 0.5% partial melting and Group I kimberlites at ~ 2%. Kankan HDFs and Group II kimberlites can be produced by 0.1 and 0.8% melting of average Archaean SCLM that carries phlogopite ± Fe-Ti oxides. The close correspondence between the trace

  8. 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.

  9. 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.

  10. Magnetic properties of xenoliths from Yakut kimberlite pipes

    NASA Astrophysics Data System (ADS)

    Tselebrovskiy, Alexey; Maksimochkin, Valeriy

    2014-05-01

    Lower continental crust is poorly known due to its limited availability. One source of information about the formation of the lower crust is the study of xenoliths found in kimberlites, mainly peridotites, eclogites and other rocks made by the kimberlite magma to the surface from great depths. Magnetic methods can solve problems related on the one hand, the definition of the phase composition of natural ferrimagnetics responsible for the magnetic properties of rocks, and on the other - with the establishment of the thermodynamic conditions in which they were formed - their genesis. For example, in [1, 2], there were differences in the magnetic properties of kimberlites taken from tubes with different diamond productivity. In this work, studies have been conducted of the magnetic properties and mineralogy of xenoliths from 10 Yakut kimberlit pipes, courtesy of Doctor of Geological and Mineralogical Sciences V. K. Garanin. Found that the natural remanent magnetization (NRM) and magnetic susceptibility (k0) of the investigated samples varies widely: NRM = (0.002-12.59) A/m, k0 = (0.23-59.9)*10-3 SI. Magnetic properties vary by species: average NRM peridotites (0.002-0.32) A/m order of magnitude smaller eclogitic rocks (0.58-12.59) A/m. Thermomagnetic analysis (TMA) of the test samples showed the presence of xenoliths of the ferromagnetic phase with a Curie point close to Tc magnetite. Because of the high correlation between the values of NRM, k0 and ferrimagnetic saturation magnetization (SM) can be inferred that the magnetic properties of the rocks studied at temperatures above ambient is basically determined by the concentration of magnetite in them. Besides magnetite TMA were also identified ferrimagnetic phase with Curie temperatures from -50°C to -125°C. Mineralogical analysis performed on three samples of peridotite tubes Udachnaya, Yubileynaya and Mir and two samples of eclogite tubes Udachnaya and Komsomolskaya, showed that at temperatures below room

  11. 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

  12. 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.

  13. Middle Proterozoic piggyback basin in the subsurface of northwestern Canada

    SciTech Connect

    Cook, F.A.; Clark, E.A. )

    1990-07-01

    Seismic reflection profiles from the interior platform in the northwestern Canadian Arctic show outstanding evidence for sedimentary basins formed and carried piggyback on thrust sheets during Middle Proterozoic compressional deformation. The seismic stratigraphic observations are the first documentation of a Proterozoic piggyback basin and thus also provide independent confirmation of proposed layer-parallel thrusting.

  14. Megacrysts from the Grib kimberlite pipe (Arkhangelsk Province, Russia)

    NASA Astrophysics Data System (ADS)

    Kostrovitsky, S. I.; Malkovets, V. G.; Verichev, E. M.; Garanin, V. K.; Suvorova, L. V.

    2004-09-01

    The megacryst suite of the Grib kimberlite pipe (Arkhangelsk province, Russia) comprises garnet, clinopyroxene, magnesian ilmenite, phlogopite and garnet-clinopyroxene intergrowths. Crystalline inclusions, mainly of clinopyroxene and picroilmenite, occur in garnet megacrysts. Ilmenite is characterized by a wide range in the contents of MgO (10.6-15.5 wt.%) and Cr 2O 3 (0.7-8.3 wt.%). Megacryst garnets show wide variations in Cr 2O 3 (1.3-9.6 wt.%) and CaO (3.6-11.0 wt.%) but relatively constant MgO (15.4-22.3 wt.%) and FeO (5.2-9.9 wt.%). The pyroxenes also show wide variations in such oxides as Cr 2O 3, Al 2O 3 and Na 2O (0.56-2.95; 0.86-3.25; 1.3-3.0 wt.%, respectively). The high magnesium and chromium content of all these minerals puts them together in one paragenetic group. This conclusion was confirmed by studies of the crystalline inclusions in megacrysts, which demonstrate similar variations in composition. Low concentration of hematite in ilmenite suggests reducing conditions during crystallization. P- T estimates based on the clinopyroxene geothermobarometer (Contrib. Mineral. Petrol. 139 (2000) 541) show wide variations (624-1208 °C and 28.8-68.0 kbars), corresponding to a 40-45 mW/m 2 conductive geotherm. The majority of Gar-Cpx intergrowths differ from the corresponding monomineralic megacrysts in having higher Mg contents and relatively low TiO 2. The minerals from the megacryst association, as a rule, differ from the minerals of mantle xenoliths, but garnets in ilmenite-bearing peridotite xenoliths are compositionally similar to garnet megacrysts. The common features of trace element composition of megacryst minerals and kimberlite (they are poor in Zr group elements) suggest a genetic relationship. The origin of the megacrysts is proposed to be genetically connected with kimberlite magma-chamber evolution on the one hand and with associated mantle metasomatism on the other. We suggest that, depending on the primary melt composition, different

  15. 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

  16. Terminal Proterozoic reorganization of biogeochemical cycles.

    PubMed

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

    1995-07-06

    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.

  17. 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.

  18. 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.

  19. Mineral inclusions in sublithospheric diamonds from Collier 4 kimberlite pipe, Juina, Brazil: subducted protoliths, carbonated melts and primary kimberlite magmatism

    NASA Astrophysics Data System (ADS)

    Bulanova, Galina P.; Walter, Michael J.; Smith, Chris B.; Kohn, Simon C.; Armstrong, Lora S.; Blundy, Jon; Gobbo, Luiz

    2010-10-01

    We report on a suite of diamonds from the Cretaceous Collier 4 kimberlite pipe, Juina, Brazil, that are predominantly nitrogen-free type II crystals showing complex internal growth structures. Syngenetic mineral inclusions comprise calcium- and titanium-rich phases with perovskite stoichiometry, Ca-rich majoritic-garnet, clinopyroxene, olivine, TAPP phase, minerals with stoichiometries of CAS and K-hollandite phases, SiO2, FeO, native iron, low-Ni sulfides, and Ca-Mg-carbonate. We divide the diamonds into three groups on the basis of the carbon isotope compositions (δ13C) of diamond core zones. Group 1 diamonds have heavy, mantle-like δ13C (-5 to -10‰) with mineral inclusions indicating a transition zone origin from mafic protoliths. Group 2 diamonds have intermediate δ13C (-12 to -15‰), with inclusion compositions indicating crystallization from near-primary and differentiated carbonated melts derived from oceanic crust in the deep upper mantle or transition zone. A 206Pb/238U age of 101 ± 7 Ma on a CaTiSi-perovskite inclusion (Group 2) is close to the kimberlite emplacement time (93.1 ± 1.5 Ma). Group 3 diamonds have extremely light δ13C (-25‰), and host inclusions have compositions akin to high-pressure-temperature phases expected to be stable in pelagic sediments subducted to transition zone depths. Collectively, the Collier 4 diamonds and their inclusions indicate multi-stage, polybaric growth histories in dynamically changing chemical environments. The young inclusion age, the ubiquitous chemical and isotopic characteristics indicative of subducted materials, and the regional tectonic history, suggest a model in which generation of sublithospheric diamonds and their inclusions, and the proto-kimberlite magmas, are related genetically, temporally and geographically to the interaction of subducted lithosphere and a Cretaceous plume.

  20. 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

  1. Picroilmenites in Yakutian kimberlites: variations and genetic models

    NASA Astrophysics Data System (ADS)

    Ashchepkov, I. V.; Alymova, N. V.; Logvinova, A. M.; Vladykin, N. V.; Kuligin, S. S.; Mityukhin, S. I.; Downes, H.; Stegnitsky, Yu. B.; Prokopiev, S. A.; Salikhov, R. F.; Palessky, V. S.; Khmel'nikova, O. S.

    2014-09-01

    Major and trace element variations in picroilmenites from Late Devonian kimberlite pipes in Siberia reveal similarities within the region in general, but show individual features for ilmenites from different fields and pipes. Empirical ilmenite thermobarometry (Ashchepkov et al., 2010), as well as common methods of mantle thermobarometry and trace element geochemical modeling, shows long compositional trends for the ilmenites. These are a result of complex processes of polybaric fractionation of protokimberlite melts, accompanied by the interaction with mantle wall rocks and dissolution of previous wall rock and metasomatic associations. Evolution of the parental magmas for the picroilmenites was determined for the three distinct phases of kimberlite activity from Yubileynaya and nearby Aprelskaya pipes, showing heating and an increase of Fe# (Fe# = Fe / (Fe + Mg) a.u.) of mantle peridotite minerals from stage to stage and splitting of the magmatic system in the final stages. High-pressure (5.5-7.0 GPa) Cr-bearing Mg-rich ilmenites (group 1) reflect the conditions of high-temperature metasomatic rocks at the base of the mantle lithosphere. Trace element patterns are enriched to 0.1-10/relative to primitive mantle (PM) and have flattened, spoon-like or S- or W-shaped rare earth element (REE) patterns with Pb > 1. These result from melting and crystallization in melt-feeding channels in the base of the lithosphere, where high-temperature dunites, harzburgites and pyroxenites were formed. Cr-poor ilmenite megacrysts (group 2) trace the high-temperature path of protokimberlites developed as result of fractional crystallization and wall rock assimilation during the creation of the feeder systems prior to the main kimberlite eruption. Inflections in ilmenite compositional trends probably reflect the mantle layering and pulsing melt intrusion during melt migration within the channels. Group 2 ilmenites have inclined REE enriched patterns (10-100)/PM with La / Ybn ~ 10

  2. Picroilmenites in Yakutian kimberlites: variations and genetic models

    NASA Astrophysics Data System (ADS)

    Ashchepkov, I. V.; Alymova, N. V.; Logvinova, A. M.; Vladykin, N. V.; Kuligin, S. S.; Mityukhin, S. I.; Stegnitsky, Y. B.; Prokopyev, S. A.; Salikhov, R. F.; Palessky, V. S.; Khmel'nikova, O. S.

    2013-08-01

    Major and trace element variations in picroilmenites from Late Devonian kimberlite pipes in Siberia reveal similarities within the region in general, but show individual features for ilmenites from different fields and pipes. Empirical ilmenite thermobarometry (Ashchepkov et al., 2010), as well as common methods of mantle thermobarometry and trace element geochemical modelling shows that long compositional trends for the ilmenites are a result of complex processes of polybaric fractionation of protokimberlite melts, accompanied by the interaction with mantle wall rocks and dissolution of previous wall rock and metasomatic associations. Evolution of picroilmenite's parental magmas was estimated for the three distinct phases of kimberlite activity from Yubileynaya and closely located Aprelskaya pipes showing heating and increase of Fe of mantle peridotites minerals from stage to stage and splitting of the magmatic system in the final stages. High pressure (5.5-7.0 GPa) Cr-bearing Mg-rich ilmenites (Group 1) reflect the conditions of high temperature metasomatic rocks at the base of the mantle lithosphere. Trace element patterns are enriched to 0.1-10/C1 and have flattened, spoon-like or S- or W-shaped REE patterns with Pb > 1. These result from melting and crystallization in melt - feeding channels in the base of the lithosphere, where high temperature dunite - harzburgites and pyroxenites were formed. Cr-poor ilmenite megacrysts (group2) trace the high temperature path of protokimberlites developed as result of fractional crystallization and wall rock assimilation during the creation of the feeder systems prior to the main kimberlite eruption. Inflections in ilmenite compositional trends probably reflect the mantle layering and pulsing melt intrusion during the melt migration within the channels. Group 2 ilmenites reveal inclined REE enriched patterns (10-100)/C1 with La/Ybn 10-25 similar to those derived from kimberlites, and HFSE peaks (typical megacrysts). A

  3. Thermodynamic Modelling of Volatiles in Kimberlite Ascent and Eruption

    NASA Astrophysics Data System (ADS)

    Russell, J. K.; Gordon, T. M.

    2009-04-01

    The unique aspect of kimberlite magmas is their potential for having high dissolved contents of primary volatiles (e.g., H2O + CO2 > 15 wt. %) coupled to a high ascent rate. The high ascent rates help couple the exsolved fluid to the magma as it rises to the point of eruption. During ascent the system evolves from a system featuring 30-40% suspended solids in a silicate melt to a system that is volumetrically dominated by the exsolved fluids (due to exsolution and expansion). The physical-chemical properties of kimberlite melt govern the transport and eruption behaviour of kimberlite magmas. For example, exsolution of a CO2-H2O fluid phase provides a logical and efficient means of reducing magma density and promoting the buoyancy critical for rapid ascent and eruption. The composition of the exsolved fluid depends on the total dissolved fluid content of the melt as well as the T-P ascent path. Under conditions of equilibrium degassing (e.g., closed system), the original dissolved fluid content limits the range of fluid compositions produced during ascent. Under perfect fractional degassing (open system), increments of equilibrium fluid are released and "fractionated". Such situations arise when 2-phase flow (melt and gas) develops and the gas phase decouples from the host magma. Separated two-phase flow is likely to develop in kimberlite and allows for highly transient fluid compositions beginning with fluids extremely enriched in CO2, and ending with H2O-dominated fluid. The physical properties and behaviour of the fluids during ascent are, thus, constantly changing in response to the evolving fluid composition. Here we use computational models calibrated on experimental data for multicomponent melts (e.g., MELTS; Ghiorso & Sack 1995) saturated with a CO2-H2O fluid (e.g., Papale et al. 2006) to explore the physical-chemical properties of volatile-saturated kimberlite during ascent and eruption. The exsolved magmatic fluid is modelled as mixtures of CO2 and H2O. No

  4. Role of Volatiles in Kimberlite Ascent and Eruption

    NASA Astrophysics Data System (ADS)

    Russell, J. K.; Gordon, T. M.

    2009-05-01

    The unique aspect of kimberlite magmas is their potential for having high dissolved contents of primary volatiles (e.g., H2O and CO2) coupled to a high ascent rate. The high ascent rates help couple the exsolved fluid to the magma as it rises to the point of eruption. During ascent the system evolves from a system featuring 30-40% suspended solids in a silicate melt to a system that is volumetrically dominated by the exsolved fluids (due to exsolution and expansion). The physical-chemical properties of kimberlite melt govern the transport and eruption behaviour of kimberlite magmas. For example, exsolution of a CO2-H2O fluid phase provides a logical and efficient means of reducing magma density and promoting the buoyancy critical for rapid ascent and eruption. The composition of the exsolved fluid depends on the total dissolved fluid content of the melt as well as the T-P ascent path (e.g., Holloway & Blank 1994). Under conditions of equilibrium degassing (e.g., closed system), the original dissolved fluid content limits the range of fluid compositions produced during ascent. Under perfect fractional degassing (open system), increments of equilibrium fluid are released and "fractionated". Such situations arise when 2-phase flow (melt and gas) develops and the gas phase decouples from the host magma. Separated two-phase flow is likely to develop in kimberlite and allows for highly transient fluid compositions beginning with fluids extremely enriched in CO2, and ending with H2O-dominated fluid. The physical properties and behaviour of the fluids during ascent are, thus, constantly changing in response to the evolving fluid composition. Here we use computational models calibrated on experimental data for multicomponent melts (e.g., MELTS; Ghiorso & Sack 1995) saturated with a CO2-H2O fluid (e.g., Papale et al. 2006) to explore the physical-chemical properties of volatile-saturated kimberlite during ascent and eruption. The exsolved magmatic fluid is modelled as

  5. 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

  6. Eruption of kimberlite magmas: physical volcanology, geomorphology and age of the youngest kimberlitic volcanoes known on earth (the Upper Pleistocene/Holocene Igwisi Hills volcanoes, Tanzania)

    NASA Astrophysics Data System (ADS)

    Brown, Richard J.; Manya, S.; Buisman, I.; Fontana, G.; Field, M.; Niocaill, C. Mac; Sparks, R. S. J.; Stuart, F. M.

    2012-09-01

    The Igwisi Hills volcanoes (IHV), Tanzania, are unique and important in preserving extra-crater lavas and pyroclastic edifices. They provide critical insights into the eruptive behaviour of kimberlite magmas that are not available at other known kimberlite volcanoes. Cosmogenic 3He dating of olivine crystals from IHV lavas and palaeomagnetic analyses indicates that they are Upper Pleistocene to Holocene in age. This makes them the youngest known kimberlite bodies on Earth by >30 Ma and may indicate a new phase of kimberlite volcanism on the Tanzania craton. Geological mapping, Global Positioning System surveying and field investigations reveal that each volcano comprises partially eroded pyroclastic edifices, craters and lavas. The volcanoes stand <40 m above the surrounding ground and are comparable in size to small monogenetic basaltic volcanoes. Pyroclastic cones consist of diffusely layered pyroclastic fall deposits comprising scoriaceous, pelletal and dense juvenile pyroclasts. Pyroclasts are similar to those documented in many ancient kimberlite pipes, indicating overlap in magma fragmentation dynamics between the Igwisi eruptions and other kimberlite eruptions. Characteristics of the pyroclastic cone deposits, including an absence of ballistic clasts and dominantly poorly vesicular scoria lapillistones and lapilli tuffs, indicate relatively weak explosive activity. Lava flow features indicate unexpectedly high viscosities (estimated at >102 to 106 Pa s) for kimberlite, attributed to degassing and in-vent cooling. Each volcano is inferred to be the result of a small-volume, short-lived (days to weeks) monogenetic eruption. The eruptive processes of each Igwisi volcano were broadly similar and developed through three phases: (1) fallout of lithic-bearing pyroclastic rocks during explosive excavation of craters and conduits; (2) fallout of juvenile lapilli from unsteady eruption columns and the construction of pyroclastic edifices around the vent; and (3

  7. Proterozoic Geomagnetic Field: Intensity, Morphology and Stability

    NASA Astrophysics Data System (ADS)

    Smirnov, A. V.

    2007-12-01

    Information about the geometry, stability, and intensity of Earth's magnetic field during the Proterozoic is of great importance for understanding the early geodynamo and in the interpretation of paleomagnetic data with regards to paleogeographic reconstructions, true polar wander, global glaciations and other problems. What are the space-time characteristics of the Proterozoic field, such as the relative significance of the dipole and non-dipole components, and are these characteristics significantly different from their Phanerozoic counterparts? In the absence of strict theoretical constraints, paleomagnetic data are the principal source of information about the Precambrian field. Field geometry can be estimated by combining paleomagnetic data with independent latitudinal indicators such as evaporites and glacial deposits. A hotter Precambrian Earth coupled with absence of the inner core could have resulted in higher field variation, including more frequent reversals. The current magnetostratigraphic database is insufficient to test whether the early geodynamo reversed its polarity with the same range of frequencies as in the Mesozoic-Cenozoic interval. However, Precambrian rocks may preserve directional information useful for constraining paleosecular variation (PSV). The estimates of PSV based on the Precambrian paleomagnetic database will be discussed. Paleointensity data provide information on the energy state of geodynamo and may prove crucial for constraining models of the thermal evolution of the Earth (e.g., the timing of nucleation and growth of the solid inner core). However, many Precambrian rock sequences have been affected by alteration, which hinders the measurement of paleointensity using bulk rock samples. In addition, many paleointensity determinations for that time period come from intrusive rocks in which the paleointensity signal may be complicated by thermochemical remanent magnetization and other factors. Therefore, a caution should be

  8. 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

  9. Sulfides in diamonds and in xenoliths from kimberlite pipes of Yakutiia

    NASA Astrophysics Data System (ADS)

    Bulanova, Galina P.; Spetsius, Zdislav V.; Leskova, Nelli V.

    The characteristics of sulfides from diamonds and xenoliths are compared using literature data on the mineralogy of sulfides in diamonds and in deep-seated xenoliths from kimberlite pipes. Results are presented on the Fe-Ni-Cu-Co-S mineral systems of mantle associations, sulfide inclusions in diamonds and megacrystals of kimberlite rocks, and minerals of the Fe-Ni-Cu-Co-S system in mantle xenoliths from kimberlite pipes. Particular consideration is given to the nature of sulfide mineralization in mantle xenoliths and diamonds.

  10. 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.

  11. 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

  12. 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.

  13. Uncertainty-based grade modelling of kimberlite: A case study of the Jay kimberlite pipe, EKATI Diamond Mine, Canada

    NASA Astrophysics Data System (ADS)

    Harrison, Sara; Leuangthong, Oy; Crawford, Barbara; Oshust, Peter

    2009-11-01

    Understanding uncertainty in resource models is a significant requirement of mineral resource evaluation. Geostatistical simulation is one method that can be used to quantify uncertainty and Sequential Gaussian Simulation (SGS) is one of the easiest techniques to understand and implement. Using SGS provides both a spatial model of a given variable and the ranges around that variable at any number of scales. The Jay kimberlite pipe is located in the southeastern quadrant of the EKATI property. Drilling to date has identified three kimberlitic domains characterized by varying lithological properties. These domains are not separated by hard contacts, but rather by boundaries that are transitional. Within these domains, vertical trends exist; in particular, diamond grade increases with depth. For these reasons, Jay required an in-depth investigation of the best uncertainty-based grade modelling method to use. Grade was modelled by organic SGS and by using the stepwise conditional transform (SCT) to incorporate a trend into the simulation routine. Although the SGS results were valid, they did not fully reproduce the trend and therefore, the results did not fully match the geological interpretation of the deposit. The SCT results reproduced the trend, however, did not correspond to the variability of the data and therefore under-represented the actual uncertainty in the model. This was confirmed through detailed uncertainty calculation and probabilistic resource classification. Therefore, the SGS model was chosen as the preferred uncertainty-based grade model for the Jay pipe.

  14. 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.

  15. 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

  16. 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.

  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.

  18. 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.

  19. Dating kimberlite emplacement with zircon and perovskite (U-Th)/He geochronology

    NASA Astrophysics Data System (ADS)

    Stanley, Jessica; Flowers, Rebecca

    2017-04-01

    Kimberlites provide rich information about the composition and evolution of cratonic lithosphere. They can entrain xenoliths and xenocrysts from the entire lithospheric column as they transit rapidly to the surface, providing information on the state of the deep lithosphere as well as any sedimentary units covering the craton at the time of eruption. Accurate geochronology of these eruptions is key for interpreting this information and discerning spatiotemporal trends in lithospheric evolution, but kimberlites can sometimes be difficult to date with available methods. Here we explore whether (U-Th)/He dating of zircon and perovskite can serve as reliable techniques for determining kimberlite emplacement ages by dating a suite of sixteen southern African kimberlites by zircon and/or perovskite (U-Th)/He (ZHe, PHe). Most samples with abundant zircon yielded ZHe dates reproducible to ≤15% dispersion that are in good agreement with published eruption ages, though there were several samples that were more scattered. Since the majority of dated zircon were xenocrystic, zircon with reproducible dates were fully reset during eruption or resided at temperatures above the ZHe closure temperature ( 180 °C) prior to entrainment in the kimberlite magma. We attribute scattered ZHe dates to shallowly sourced zircon that underwent incomplete damage annealing and/or partial He loss during the eruptive process. All seven kimberlites dated with PHe yielded dates reproducible to ≤15% dispersion and reasonable results. As perovskite has not previously been used as a (U-Th)/He chronometer, we conducted two preliminary perovskite 4He diffusion experiments to obtain initial estimates of its temperature sensitivity. These experiments suggest a PHe closure temperature of >300 °C. Perovskite in kimberlites is unlikely to be xenocrystic and its relatively high temperature sensitivity suggests that PHe dates will typically record emplacement rather than post-emplacement processes. ZHe

  20. 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

  1. Tuffisitic kimberlites from the Wesselton Mine, South Africa: Mineralogical characteristics relevant to their formation

    NASA Astrophysics Data System (ADS)

    Mitchell, Roger H.; Skinner, E. Michael W.; Scott Smith, Barbara H.

    2009-11-01

    Tuffisitic kimberlites from the Wesselton Mine consist, in order of formation, of the following primary components: chloritized olivine macrocrysts and phenocrysts; magmaclasts; cryptocrystalline diopside-phlogopite-rich mantles, and a smectite-chlorite interclast matrix. Magmaclasts consist of one to several crystals of chloritized olivine set in a microcrystalline groundmass of diopside, apatite, perovskite, spinel and chloritized and fresh phlogopite, the latter commonly rimming chloritized olivines. Magmaclasts have some similarities to holocrystalline hypabyssal kimberlite but lack monticellite, carbonate, carbonate-serpentine segregations and atoll spinels. Spinels in the magmaclasts show only a limited compositional evolution relative to spinels in spatially-associated hypabyssal kimberlite. Pre-existing solids, including discrete olivine grains, magmaclasts and most xenoliths, are mantled by acicular diopside and phlogopite. The interclast matrix is now represented by mixed layer phyllosilicates (chlorite-smectites) that are poorer in alumina and iron than chlorite pseudomorphing olivine and microlitic phlogopite and diopside. The interclast chlorite-smectite is considered to represent former phlogopite which has undergone late-stage deuteric hydrothermal-like modification. The interclast matrix crystallized from the volatile-rich remnants of the magma. None of the constituents of tuffisitic kimberlites, including the chlorite or chlorite-smectites, represent material formed from externally-derived fluids. These primary textures are unique to kimberlites and only form in certain circumstances. Tuffisitic kimberlites formed by progressive crystallization and volatile exsolution-induced segregation and/or disruption within a kimberlite magma (magmaclasts, mantles, interclast matrix) during a continuum of rapidly changing conditions in a subsurface (subvolcanic) magmatic system. This continuum represents a transition from a degassing magmatic system to a

  2. Mineral inclusions in diamonds from the River Ranch kimberlite, Zimbabwe

    NASA Astrophysics Data System (ADS)

    Kopylova, Maya G.; Gurney, John J.; Daniels, Leon R. M.

    More than 99% of mineral inclusions in diamonds from the River Ranch pipe in the Late Archean Limpopo Mobile Belt (Zimbabwe), are phases of harzburgitic paragenesis, namely olivine (Fo92-93), orthopyroxene (Mg#=93), G10 garnets and chromites. The diamond inclusion (DI) chemistry demonstrates a limited overlap with River Ranch kimberlite macrocrysts: the DI garnets are more Ca-undersaturated, and DI spinel and garnet are more Mg-rich. Most River Ranch diamond inclusions were equilibrated at T=1080-1320°C, P=47-61kbar, and fO2 between IW and WM buffers. The P/T profile beneath the Limpopo Mobile Belt (LMB) is consistent with a paleo-heat flow of 41-42mW/m2, similar to calculations for Roberts Victor, but hotter than for the Finsch, Kimberley, Koffiefontein and Premier Mines. This is ascribed to the younger tectonothermal age of the LMB and its proximity to Late Archean oceans. Like diamond inclusions from all other kimberlites studied, the River Ranch DI have a lithospheric affinity and therefore indicate that an ancient, chemically depleted, thick (at least 200km) mantle root existed beneath the Limpopo Mobile Belt 530-540Ma ago. The mantle root might have developed beneath the continental Central Zone of the LMB as early as the Archean, and could be alien to the overthrust allochthonous sheet of the Limpopo Belt. Oxygen fugacity estimates for diamond inclusions at River Ranch are similar to other diamondiferous harzburgites beneath the Kaapvaal craton, indicating that the Kaapvaal mantle as a whole was well buffered and homogeneous with respect to fO2 at the time of peridotitic diamond crystallization.

  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. 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.

  5. Isotope compositions of C and O of magmatic calcites from the Udachnaya-East pipe kimberlite, Yakutia

    NASA Astrophysics Data System (ADS)

    Tomilenko, A. A.; Dublyansky, Yu. V.; Kuzmin, D. V.; Sobolev, N. V.

    2017-07-01

    It has been demonstrated for the first time that the isotopic compositions of carbon (δ13C) in magmatic calcites from the Udachnaya-East pipe kimberlite groundmass varies from-2.5 to-1.0‰ (V-PDB), while those of oxygen (δ18O) range from 15.0 to 18.2‰ (V-SMOW). The obtained results imply that during the terminal late magmatic and postmagmatic stages of the kimberlite pipe formation, the carbonates in the kimberlite groundmass became successively heavier isotopically, which indicates the hybrid nature of the carbonate component of the kimberlite: it was formed with contributions from mantle and sedimentary marine sources.

  6. 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.

  7. 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.

  8. 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.

  9. Geochemical Dissection of a Kimberlite: What Makes up a Whole Rock Analysis?

    NASA Astrophysics Data System (ADS)

    Malarkey, J.; Pearson, D. G.; Davidson, J. P.; Nowell, G. M.; Kjarsgaard, B.; Ottley, C. J.

    2009-05-01

    The elemental and isotopic composition of "primary" kimberlite magma and its relationship to basaltic magmas has been the subject of considerable debate for decades. We present a trace element and isotopic study of multiple mineral phases from a kimberlite and compare these to an olivine melilitite with significantly less visible crustal input. Our objective is to determine whether different mineral phases clearly record different stages of crustal contamination in the evolving kimberlite and to assess which phases are most likely to give the best information about potential source regions. We have analysed phlogopite, perovskite, olivine and calcite from a Group I kimberlite from Jos, Somerset Island. These results are compared with olivine, melilite, phlogopite, perovskite and apatite hand picked from an olivine melilitite from Saltpetre Kop (SPK), S. Africa. Melilitites are less obviously affected by crustal contamination and are generally less altered by low-T processes than kimberlites, yet they contain several minerals in common with kimberlites and therefore offer a good, simpler analogue system. Preliminary Sr isotope data from the melilitite confirm that melilite, olivine and perovskite have similar initial 87Sr/86Sr ratios to the whole rock; while the later crytsallising phases, apatite and phlogopite, show increasingly more radiogenic values. Together they define a reasonable Rb-Sr isochron age of ~77±3.4Ma close to the published emplacement age (72.5-76.8Ma, [1]), suggesting that crustal assimilation is relatively minor. In contrast, preliminary results from the Jos kimberlite are more complex and not consistent with closed system behavior. Variations in initial 87Sr/86Sr both within and between the different mineral phases so far analysed can be explained by a combination of factors that include progressive crustal contamination and source heterogeneity. [1] Duncan et al (1978), Geological Magazine, 115, 317-396

  10. 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

  11. Construction of mantle sequence beneath Yubileynaya kimberlite pipe.

    NASA Astrophysics Data System (ADS)

    Ashchepkov, I. V.; Vladykin, N. V.; Logvinova, A. M.; Zinchuk, N. N.; Khmelnikova, O. S.; Palessky, S. V.; Nikolaeva, I. V.

    2003-04-01

    Concentrate from Yubileinaya kimberlite pipe (>1000 analyses), Alakite field, Russia, was used for thermobarometry (Nimis, Taylor, 2000; Ryan et al,1997; Ashchepkov, 2002). Cpx geotherm reveal divide at 35 kbars. Three lherzolitic units below this boundary correspond to 40mv/m2 geotherm at the upper part and are close to 35 mv/m2 at the lower part. Upper geotherm part is more dispersed irregularly deviating to LT conditions. The inflection from harzburgite layer low in Cpx to hot ‘asthenospheric’ branch at 1350oC 65 kbar was determined Sequence upper 35 kbars is represented by Ca-Ti- enriched peridotites- pyroxenites judging from Garnet and CPx compositions. Lower unites are represented by NaCr(3-4% each)- rich CPx and Ca-Fe enriched garnets. Garnet reveal also presence of harzburgites in each three units and relic unmetasomitised lherzolite fragments rare occurring in CPx concentrate. Amphiboles in upper part (< 35 kbar belong to) are Ca-rich while in lower part belong to richterite-kataforite groups. Spinels divided into 8 groups are TiO2 -enriched in the lower part of section and in upper part. Different scheme of isomorphism for- Cr-rich chromites in deep seated units suggest difference in oxygen fugacity and reactied agents. Ilmenite trend consisting from two units: >49% TiO2 Ilm slightly enriched in Al2O3 (~ 0.7 %) while more fractionated Fe- rich ilmenites are Al-poor. Seven ilmenite groups coinciding with amount of spinel groups are likely corresponding to mantle columns layers. Ilmenite megacrystalline suit trace feeding vein system of fractionating pre -eruption kimberlite- carbonatite melts. Accompanied Cr enrichment and Ti rise in spinels and other peridotite minerals in lower units is caused by interaction of melts evolving in chambers and rising in pre-eruption vein system with wall rock peridotites. Division into two parts at 35-40kbar boundary where slab’s PT path crosses dehydration peridotite line (van Keken et al, 2002) probably appeared

  12. 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

  13. 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

  14. 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

  15. 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

  16. Geologic Interpretation of Gravity Anomalies

    DTIC Science & Technology

    1990-04-19

    encountered together with the diamonds in the kimberlitic volcanic pipes. Diamond itself (o-3.5 g/cm’) presents the polymorphic modification of carbon, which...and wide distribution to the depth, some types of intrusive bodies, for example diamond-bearing kimberlitic tubes, the ore bodies of column-like form...steep contacts, which slope within the basement, diamond-bearing kimberlitic volcanic pipes in the sedimentary rocks, ore shoots of tube-shaped form, etc

  17. Morphometric convergence between Proterozoic and post-vegetation rivers

    PubMed Central

    Ielpi, Alessandro; Rainbird, Robert H.; Ventra, Dario; Ghinassi, Massimiliano

    2017-01-01

    Proterozoic rivers flowed through barren landscapes, and lacked interactions with macroscopic organisms. It is widely held that, in the absence of vegetation, fluvial systems featured barely entrenched channels that promptly widened over floodplains during floods. This hypothesis has never been tested because of an enduring lack of Precambrian fluvial-channel morphometric data. Here we show, through remote sensing and outcrop sedimentology, that deep rivers were developed in the Proterozoic, and that morphometric parameters for large fluvial channels might have remained within a narrow range over almost 2 billion years. Our data set comprises fluvial-channel forms deposited a few tens to thousands of kilometres from their headwaters, likely the record of basin- to craton-scale systems. Large Proterozoic channel forms present width:thickness ranges matching those of Phanerozoic counterparts, suggesting closer parallels between their fluvial dynamics. This outcome may better inform analyses of extraterrestrial planetary surfaces and related comparisons with pre-vegetation Earth landscapes. PMID:28548109

  18. Morphometric convergence between Proterozoic and post-vegetation rivers

    NASA Astrophysics Data System (ADS)

    Ielpi, Alessandro; Rainbird, Robert H.; Ventra, Dario; Ghinassi, Massimiliano

    2017-05-01

    Proterozoic rivers flowed through barren landscapes, and lacked interactions with macroscopic organisms. It is widely held that, in the absence of vegetation, fluvial systems featured barely entrenched channels that promptly widened over floodplains during floods. This hypothesis has never been tested because of an enduring lack of Precambrian fluvial-channel morphometric data. Here we show, through remote sensing and outcrop sedimentology, that deep rivers were developed in the Proterozoic, and that morphometric parameters for large fluvial channels might have remained within a narrow range over almost 2 billion years. Our data set comprises fluvial-channel forms deposited a few tens to thousands of kilometres from their headwaters, likely the record of basin- to craton-scale systems. Large Proterozoic channel forms present width:thickness ranges matching those of Phanerozoic counterparts, suggesting closer parallels between their fluvial dynamics. This outcome may better inform analyses of extraterrestrial planetary surfaces and related comparisons with pre-vegetation Earth landscapes.

  19. Morphometric convergence between Proterozoic and post-vegetation rivers.

    PubMed

    Ielpi, Alessandro; Rainbird, Robert H; Ventra, Dario; Ghinassi, Massimiliano

    2017-05-26

    Proterozoic rivers flowed through barren landscapes, and lacked interactions with macroscopic organisms. It is widely held that, in the absence of vegetation, fluvial systems featured barely entrenched channels that promptly widened over floodplains during floods. This hypothesis has never been tested because of an enduring lack of Precambrian fluvial-channel morphometric data. Here we show, through remote sensing and outcrop sedimentology, that deep rivers were developed in the Proterozoic, and that morphometric parameters for large fluvial channels might have remained within a narrow range over almost 2 billion years. Our data set comprises fluvial-channel forms deposited a few tens to thousands of kilometres from their headwaters, likely the record of basin- to craton-scale systems. Large Proterozoic channel forms present width:thickness ranges matching those of Phanerozoic counterparts, suggesting closer parallels between their fluvial dynamics. This outcome may better inform analyses of extraterrestrial planetary surfaces and related comparisons with pre-vegetation Earth landscapes.

  20. 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.

  1. Sr and Nd isotope composition of deformed peridotite xenoliths from Udachnaya kimberlite pipe

    NASA Astrophysics Data System (ADS)

    Surgutanova, E. A.; Agashev, A. M.; Demonterova, E. I.; Golovin, A. V.; Pokhilenko, N. P.

    2016-11-01

    New results of Rb-Sr and Sm-Nd isotope analyses have been obtained on samples of deformed peridotite xenoliths collected from the Udachnaya kimberlite pipe (Yakutia). The data obtained imply two main stages of metasomatic alteration of the lithospheric mantle base matter in the central part of the Siberian Craton. Elevated ratios of Sr isotopes may be considered as evidence of an ancient stage of metasomatic enrichment by a carbonatite melt. The acquired Nd isotope composition together with the geochemistry of the deformed peridotite xenoliths suggests that the second stage of metasomatic alteration took place shortly before formation of the kimberlite melt. The metasomatic agent of this stage had a silicate character and arrived from an asthenosphere source, common for the normal OIB type (PREMA) and the Group-I kimberlite.

  2. 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

  3. Various growth environments of cloudy diamonds from the Malobotuobia kimberlite field (Siberian craton)

    NASA Astrophysics Data System (ADS)

    Skuzovatov, Sergei; Zedgenizov, Dmitry; Howell, Daniel; Griffin, William L.

    2016-11-01

    Microinclusions of high-density fluids (HDF's) occur in cloudy diamonds from the Mir and Internatsionalnaya kimberlite pipes (Malobotuobia kimberlite field, Siberian platform). These HDFs are of typical high-Mg carbonatitic composition; a few diamonds contain microinclusions that define a low-Mg carbonatitic to silicic trend. The observed variations are interpreted as resulted from mixing of two contrasting fluids derived from the partial melting mainly of carbonated peridotite (the high-Mg carbonatitic HDFs) and eclogite (silica-rich HDFs and HDFs with high Ca/(Ca + Mg + Fe)). Immiscibility of carbonatitic and silica-rich fluids provides a possible mechanism for the co-existence of the observed HDFs but needs further proof. The uniform carbon isotope composition of cloudy diamonds with high-Mg carbonatitic microinclusions from both kimberlite pipes implies a single peridotitic source.

  4. Chlorine from the mantle: Magmatic halides in the Udachnaya-East kimberlite, Siberia

    NASA Astrophysics Data System (ADS)

    Kamenetsky, Vadim S.; Maas, Roland; Kamenetsky, Maya B.; Paton, Chad; Phillips, David; Golovin, Alexander V.; Gornova, Marina A.

    2009-07-01

    The Devonian Udachnaya-East pipe (Siberia) presents a rare example of exceptionally fresh kimberlite rocks, containing a rich assemblage of alkali carbonate, chloride and sulphate minerals in its groundmass. Accordingly, bulk groundmass compositions have unusually high concentrations of sodium and chlorine (up to 6 wt.% each), while water contents are very low. High Na 2O and low H 2O are inconsistent with currently accepted model kimberlite magma compositions; and high Na and Cl contents in the Udachnaya-East pipe have been ignored by the geoscience community, because of possibility of contamination from sedimentary crustal rocks. New textural evidence suggests that the chloride and alkali carbonate minerals in this kimberlite are co-magmatic with perovskite. Radiogenic isotope ratios for the perovskite ( 87Sr/ 86Sr ~ 0.7031, ɛNd + 5, ɛHf + 5.3, obtained by solution-mode and laser-ablation methods) are very primitive and provide no support for a crustal contaminant. The new perovskite data confirm previously published Sr-Nd-Pb isotopic evidence for the chlorides and alkali carbonates themselves, but are considered more reliable because of possible open system behaviour and analytical problems with unstable halide and carbonate minerals. Similar isotope ratios ( 87Sr/ 86Sr = 0.70292, ɛNd = + 5, ɛHf = + 6.5) are recorded in a clinopyroxene macrocryst, which contains chlorides and alkali carbonates within melt inclusions, implying a similarly primitive isotopic signature for the host kimberlite. We infer that assimilation of evaporitic sediments is unlikely and that abundant chlorine and sodium in the kimberlite originated in the upper mantle. Our results and published experimental data on mantle chloride melts and presence of a chloride component in diamond-hosted fluid inclusions suggest a previously unrecognised role for mantle-derived halide melts in the genesis, composition and rheological properties of kimberlite primary melts.

  5. 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.

  6. 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.

  7. 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.

  8. Tuffisitic Kimberlites and Their Emplacement Processes: A Review of Some Current Hypotheses

    NASA Astrophysics Data System (ADS)

    Mitchell, R. H.

    2009-05-01

    Kimberlite diatremes are filled by the variety of kimberlite known variously as tuffisitic kimberlite (TK), volcaniclastic kimberlite (VK), subvolcanic magmaclastic kimberlite (SMK), or autolithic tuff breccia (AKB). The distinctive characteristics of these rock type are reviewed and compared with other volcaniclastic diatreme- zone rocks occurring in melilitite vents. It is shown that TKs are petrologically unique in consisting of massive unsorted matrix-to-clast supported rocks containing anhedral olivine macrocrysts and subhedral phenocrysts; a specific assemblage of sub-spherical to elliptical magmaclasts (formerly pelletal lapilli), xenolithic and autolithic clasts, thin cryptocrystalline mantles of microlitic diopside, phlogopite and apatite on the preceding constituents, and an interclast material consisting mainly of mixed layer phyllosilicates (chlorite-smectite) with lesser diopside. Magmaclasts are defined as solidified former melt-bearing clasts formed during, or prior to, emplacement by any process of magma disruption. Magmaclasts in TKs consist of chlorite-pseudomorphed olivine, fresh phlogopite, spinel, perovskite, apatite and chlorite. Petrologically they are similar to the groundmass of hypabyssal kimberlites but lack monticellite and carbonates. Microlitic diopside mantles decorating magmaclasts grade continuously into the interclast matrix and represent a continuum of crystallization. Clastic or carbonate matrices, accretionary and/or deformed welded or vesicular clasts are absent from TKs. Mechanisms proposed for the formation of TKs include fluidization, phreatomagmatism , embryonic pipe modification, and in-vent column collapse; however it very possible that diatremes are unlikely to form by any single process. TK formation has been considered to represent either downward or upwards migration of the foci of volcanic activity. Apart from their terminology, outstanding problems in TK genesis include: the formation of the magmaclasts and their

  9. 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

  10. Proterozoic granitoids of the Amazonian craton

    NASA Astrophysics Data System (ADS)

    dalĺAgnol, R.; Costi, H. T.; Lamarão, C. N.; Teixeira, N. P.; Bettencourt, J. S.; Fraga, L. M.

    2003-04-01

    Proterozoic granitoids are widespread in all provinces of the Amazonian craton. In the Maroni-Itacaiunas Province, granitoids associated with the Trans-Amazonian event include: subduction related, 2.16 to 2.14 Ga, calc-alkaline tonalites and trondhjemites; 2.10 to 2.08 Ga, syncolisional potassic granites; 2.05 Ga, charnockites. In the Tapajós Province, ˜2.01 Ga, tonalites are followed by ˜2.0 Ga volcanic sequences and ˜1.98 to 1.96 Ga calc-alkaline granitoids. A reappraisal of magmatic activity occurred at ˜1.88 Ga when calc-alkaline granitoids, as well as subalkaline, A-type granites, associated with felsic volcanic sequences were formed. A similar picture is observed in the northern Roraima region, where post-collisional 2.0 to 1.96 Ga calc-alkaline granitoids and associated volcanic sequences are followed by 1.92 Ga A-type granites. The remarkable 1.88 Ga magmatic event has a continental scale and is related to an extensional tectonism. It affected also the Archean Carajás Province, where, at this time, within-plate, shallow-level, A-type granites were emplaced. Coeval intermediate to felsic volcanic sequences are widespread in the Central Amazonian Province. In the Pitinga region, these sequences are intruded by ˜1.82 Ga, tin-mineralized granites. In the Central Guiana Belt and in the northwestern domains of the Guiana shield ˜1.55 Ga rapakivi complexes, locally with associated anorthosites and mangerites, are common. In the Rio Negro Province, 1.8 to 1.60 calc-alkaline (?) granitoids and gneisses are dominant. They are followed by 1.55 to 1.52 Ga, oxidized, titanite-bearing A-type granites and S-type, two-mica granites. The evolution of the southwestern part of the Amazonian craton is characterized by the occurrence of successive tectonic events extending from ˜1.75 Ga to ˜1.0 Ga. The oldest granitoids are dominantly calc-alkaline tonalites, trondhjemites and granodiorites. However, the Rondonia region is marked by the occurrence of 1.6 to 1.0 Ga old

  11. 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.

  12. Spatial distribution of kimberlite in the Slave craton, Canada: a geometrical approach

    NASA Astrophysics Data System (ADS)

    Stubley, M. P.

    2004-09-01

    Exploration within the Slave craton has revealed clusters of kimberlite intrusions, commonly with internally consistent geochemical and temporal characteristics. Translation diagrams ("Fry analysis") allow an unbiased geometrical examination of the distance and direction between each kimberlite occurrence and all others in the database. Recurrent patterns are visually accentuated due to the square function in data density. Circular histograms quantify the azimuthal density of kimberlite at various distances. For this study, the database comprises the geographic position of 212 kimberlite occurrences of which 70% are from the Lac de Gras field (LDG). Analyses are presented separately for the LDG data and for all non-LDG data in order to test for regional variations and to avoid overwhelming the craton-scale studies by the high density of LDG data. Empirical grouping of kimberlite locations results in delineation of five elliptical clusters that encompass all but four kimberlite occurrences. Clusters within the western part of the craton are elongate to the north-northeast and align within a narrow zone ("Western Corridor"). Elsewhere, the clusters are elongate to the northwest or west-northwest and appear to be arranged en echelon within a poorly defined north-northwest trending zone ("Central Corridor"). Geometrical spatial analyses of kimberlite locations highlight the craton-scale pattern of emplacement within the two main corridors. At regional and local scales, individual intrusions are preferentially located towards the west-northwest (ca. 280°) and north-northeast (ca. 015°) of other intrusions, and these orientations are interpreted to reflect upper mantle trends in magma generation. At local scales (10-25 km), kimberlite of the central and southern craton tends to be located to the northeast (ca. 045°), and possibly weakly to the east-northeast (ca. 070°), of other intrusions, and these orientations correspond to major crustal fractures systems. It is

  13. A link between geochemistry and geodynamics: carbonatites and kimberlites, Polar Siberia

    NASA Astrophysics Data System (ADS)

    Rass, Irene

    2010-05-01

    Geophysical evidence indicates that the Moho surface beneath the northern Siberian Platform composes crests (or ranges) up to 14 km high above deeper areas and 50-80 to 150 km wide (Chernyshov and Bokaya, 1983). These ranges at the Moho likely mark ancient rift zones with a thinner crust. More than 70% kimberlites in structures surrounding the Anabar Shield occur along these Moho crests (Kravchenko et al., 1997; Rosen and Kostyuchenko, 1998). Carbonate-rich rocks that compose pipes, along with kimberlites, in kimberlite fields, were recognized as an individual type of carbonatite rocks: kimberlitic carbonatites (Lapin and Marshintsev, 1984). They abound in kimberlite fields of both Paleozoic and Mesozoic age southeast and east of the Anabar Shield. The liquidus temperatures of related kimberlites, determined based on their major-component chemistries, are 1429-1441оC and 1349-1518оC, respectively (Perchuk and Vaganov, 1980). Compared to classic carbonatites in ring complexes, kimberlitic carbonatites are characterized by the lowest relative concentrations of P and Sr, slightly lower REE, and high contents of Cr, Ti, and Zr (Rass, 1998). Mesozoic kimberlitic carbonatites exhibit a dependence of their geochemistry, position relative to the axial zones of the Moho crests, and the temperatures of the associated kimberlites, from the Kuoika to the Lower Kuonamka field: from <42 km and 1518 оC to ~50 km and 1395 оC (Rass et al., 2000). Away from the maximum heights of the Moho crests, which mark ancient rifts in the northern part of the Siberian Platform and with a decrease in the liquidus temperatures of the associated kimberlites, the relative P and Nb concentrations in these rocks increase, and those of REE, Cr, and, to a lesser extent, Ni and Co decrease. The depths of the Moho surface beneath carbonatites in Mesozoic ring structures of the Odikhincha, Guli, Magan, and Yraas complexes in the Maymecha-Kotui alkaline-ultrabasic-carbonatite province west of the

  14. Primary melt and fluid inclusions in regenerated crystals and phenocrysts of olivine from kimberlites of the Udachnaya-East Pipe, Yakutia: The problem of the kimberlite melt

    NASA Astrophysics Data System (ADS)

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

    2017-08-01

    The primary melt and fluid inclusions in regenerated zonal crystals of olivine and homogeneous phenocrysts of olivine from kimberlites of the Udachnaya-East pipe, were first studied by means of microthermometry, optic and scanning electron microscopy, electron and ion microprobe analysis (SIMS), inductively coupled plasma mass-spectrometry (ICP MSC), and Raman spectroscopy. It was established that olivine crystals were regenerated from silicate-carbonate melts at a temperature of 1100°C.

  15. Limited role for methane in the mid-Proterozoic greenhouse.

    PubMed

    Olson, Stephanie L; Reinhard, Christopher T; Lyons, Timothy W

    2016-10-11

    Pervasive anoxia in the subsurface ocean during the Proterozoic may have allowed large fluxes of biogenic CH4 to the atmosphere, enhancing the climatic significance of CH4 early in Earth's history. Indeed, the assumption of elevated pCH4 during the Proterozoic underlies most models for both anomalous climatic stasis during the mid-Proterozoic and extreme climate perturbation during the Neoproterozoic; however, the geologic record cannot directly constrain atmospheric CH4 levels and attendant radiative forcing. Here, we revisit the role of CH4 in Earth's climate system during Proterozoic time. We use an Earth system model to quantify CH4 fluxes from the marine biosphere and to examine the capacity of biogenic CH4 to compensate for the faint young Sun during the "boring billion" years before the emergence of metazoan life. Our calculations demonstrate that anaerobic oxidation of CH4 coupled to SO4(2-) reduction is a highly effective obstacle to CH4 accumulation in the atmosphere, possibly limiting atmospheric pCH4 to less than 10 ppm by volume for the second half of Earth history regardless of atmospheric pO2 If recent pO2 constraints from Cr isotopes are correct, we predict that reduced UV shielding by O3 should further limit pCH4 to very low levels similar to those seen today. Thus, our model results likely limit the potential climate warming by CH4 for the majority of Earth history-possibly reviving the faint young Sun paradox during Proterozoic time and challenging existing models for the initiation of low-latitude glaciation that depend on the oxidative collapse of a steady-state CH4 greenhouse.

  16. Limited role for methane in the mid-Proterozoic greenhouse

    PubMed Central

    Olson, Stephanie L.; Reinhard, Christopher T.; Lyons, Timothy W.

    2016-01-01

    Pervasive anoxia in the subsurface ocean during the Proterozoic may have allowed large fluxes of biogenic CH4 to the atmosphere, enhancing the climatic significance of CH4 early in Earth’s history. Indeed, the assumption of elevated pCH4 during the Proterozoic underlies most models for both anomalous climatic stasis during the mid-Proterozoic and extreme climate perturbation during the Neoproterozoic; however, the geologic record cannot directly constrain atmospheric CH4 levels and attendant radiative forcing. Here, we revisit the role of CH4 in Earth’s climate system during Proterozoic time. We use an Earth system model to quantify CH4 fluxes from the marine biosphere and to examine the capacity of biogenic CH4 to compensate for the faint young Sun during the “boring billion” years before the emergence of metazoan life. Our calculations demonstrate that anaerobic oxidation of CH4 coupled to SO42− reduction is a highly effective obstacle to CH4 accumulation in the atmosphere, possibly limiting atmospheric pCH4 to less than 10 ppm by volume for the second half of Earth history regardless of atmospheric pO2. If recent pO2 constraints from Cr isotopes are correct, we predict that reduced UV shielding by O3 should further limit pCH4 to very low levels similar to those seen today. Thus, our model results likely limit the potential climate warming by CH4 for the majority of Earth history—possibly reviving the faint young Sun paradox during Proterozoic time and challenging existing models for the initiation of low-latitude glaciation that depend on the oxidative collapse of a steady-state CH4 greenhouse. PMID:27671638

  17. Limited role for methane in the mid-Proterozoic greenhouse

    NASA Astrophysics Data System (ADS)

    Olson, Stephanie L.; Reinhard, Christopher T.; Lyons, Timothy W.

    2016-10-01

    Pervasive anoxia in the subsurface ocean during the Proterozoic may have allowed large fluxes of biogenic CH4 to the atmosphere, enhancing the climatic significance of CH4 early in Earth’s history. Indeed, the assumption of elevated pCH4 during the Proterozoic underlies most models for both anomalous climatic stasis during the mid-Proterozoic and extreme climate perturbation during the Neoproterozoic; however, the geologic record cannot directly constrain atmospheric CH4 levels and attendant radiative forcing. Here, we revisit the role of CH4 in Earth’s climate system during Proterozoic time. We use an Earth system model to quantify CH4 fluxes from the marine biosphere and to examine the capacity of biogenic CH4 to compensate for the faint young Sun during the “boring billion” years before the emergence of metazoan life. Our calculations demonstrate that anaerobic oxidation of CH4 coupled to SO42- reduction is a highly effective obstacle to CH4 accumulation in the atmosphere, possibly limiting atmospheric pCH4 to less than 10 ppm by volume for the second half of Earth history regardless of atmospheric pO2. If recent pO2 constraints from Cr isotopes are correct, we predict that reduced UV shielding by O3 should further limit pCH4 to very low levels similar to those seen today. Thus, our model results likely limit the potential climate warming by CH4 for the majority of Earth history—possibly reviving the faint young Sun paradox during Proterozoic time and challenging existing models for the initiation of low-latitude glaciation that depend on the oxidative collapse of a steady-state CH4 greenhouse.

  18. Microbial Response in Peat Overlying Kimberlite Pipes in The Attawapiskat Area, Northern Ontario

    NASA Astrophysics Data System (ADS)

    Donkervoort, L. J.; Southam, G.

    2009-05-01

    Exploration for ore deposits occurring under thick, post-mineralized cover requires innovative methods and instrumentation [1]. Buried kimberlite pipes 'produce' geochemical conditions such as increased pH and decreased Eh in overlying peat [2] that intuitively select for bacterial populations that are best able to grow and, which in turn affect the geochemistry producing a linked signal. A microbiological study of peat was conducted over the Zulu kimberlite in the Attawapiskat area of the James Bay Lowlands to determine if the type of underlying rock influences the diversity and populations of microorganisms living in the overlying peat. Peat was sampled along an 800 m transect across the Zulu kimberlite, including samples underlain by limestone. Microbial populations and carbon source utilization patterns of peat samples were compared between the two underlying rock types. Results demonstrate an inverse relationship of increased anaerobic populations and lower biodiversity directly above the kimberlite pipe. These results support a reduced 'column' consistent with the model presented by Hamilton [3]. The combination of traditional bacterial enumeration and community- level profiling represents a cost-effective and efficient exploration technique that can serve to compliment both geophysical and geochemical surveys. [1] Goldberg (1998) J. Geochem. Explor. 61, 191-202 [2] Hattori and Hamilton (2008) Appl. Geochem. 23, 3767-3782 [3] Hamilton (1998) J. Geochem. Explor. 63, 155-172

  19. 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.

  20. 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

  1. Emplacement temperatures of pyroclastic and volcaniclastic deposits in kimberlite pipes in southern Africa

    NASA Astrophysics Data System (ADS)

    Fontana, Giovanni; Mac Niocaill, Conall; Brown, Richard J.; Sparks, R. Stephen J.; Field, Matthew

    2011-10-01

    Palaeomagnetic techniques for estimating the emplacement temperatures of volcanic deposits have been applied to pyroclastic and volcaniclastic deposits in kimberlite pipes in southern Africa. Lithic clasts were sampled from a variety of lithofacies from three pipes for which the internal geology is well constrained (the Cretaceous A/K1 pipe, Orapa Mine, Botswana, and the Cambrian K1 and K2 pipes, Venetia Mine, South Africa). The sampled deposits included massive and layered vent-filling breccias with varying abundances of lithic inclusions, layered crater-filling pyroclastic deposits, talus breccias and volcaniclastic breccias. Basalt lithic clasts in the layered and massive vent-filling pyroclastic deposits in the A/K1 pipe at Orapa were emplaced at >570°C, in the pyroclastic crater-filling deposits at 200-440°C and in crater-filling talus breccias and volcaniclastic breccias at <180°C. The results from the K1 and K2 pipes at Venetia suggest emplacement temperatures for the vent-filling breccias of 260°C to >560°C, although the interpretation of these results is hampered by the presence of Mesozoic magnetic overprints. These temperatures are comparable to the estimated emplacement temperatures of other kimberlite deposits and fall within the proposed stability field for common interstitial matrix mineral assemblages within vent-filling volcaniclastic kimberlites. The temperatures are also comparable to those obtained for pyroclastic deposits in other, silicic, volcanic systems. Because the lithic content of the studied deposits is 10-30%, the initial bulk temperature of the pyroclastic mixture of cold lithic clasts and juvenile kimberlite magma could have been 300-400°C hotter than the palaeomagnetic estimates. Together with the discovery of welded and agglutinated juvenile pyroclasts in some pyroclastic kimberlites, the palaeomagnetic results indicate that there are examples of kimberlites where phreatomagmatism did not play a major role in the generation

  2. Diamondiferous kimberlites in Central India synchronous with Deccan flood basalt volcanism

    NASA Astrophysics Data System (ADS)

    Lehmann, B.; Burgess, R.; Frei, D.; Belyatsky, B. V.; Mainkar, D.; Chalapathi Rao, N. V.

    2009-12-01

    India is known for its historic diamonds from alluvial gravels. The source rocks for these diamonds are thought to be among the so far nearly 100 identified kimberlitic/lamproitic pipes and dikes which occur mostly in the Dharwar craton (Andhra Pradesh) and the Bundelkhand craton (Madhya Pradesh), and which all have Mesoproterozoic ages with a peak at 1.1 Ga. However, diamondiferous kimberlite pipes in the recently discovered Mainpur kimberlite field in central India have surprisingly young 40Ar/39Ar whole-rock ages of 66.5 ±2.0 and 62.4 ±2.9 million years (2σ), confirmed by more precise laser ablation ICP-MS 206Pb/238U perovskite data of 65.1 ±0.8 and 62.3 ±0.8 Ma (2 σ). These ages overlap with the main phase of the Deccan flood basalt magmatism at 65 million years, and suggest a common tectonomagmatic control for both flood basalts (including carbonatite-alkaline rock variants) and kimberlites. The kimberlites were studied in drill core and have textural, bulk and mineral chemical composition typical of orangeite (African kimberlite Group 2), confirmed by Sr and Nd isotope data. The Mainpur kimberlite field is in the Archean Bastar craton with felsic rocks as old as 3.6 Ga. The presence of macrodiamonds in the pipes implies that Central India had a cool and thick lithospheric mantle root at the Cretaceous/Tertiary boundary, significantly different from the modern Indian lithosphere of about 80-100 km thickness only. The loss of India's cratonic roots must have taken place in the Tertiary, i.e. after much of the superfast northward motion of the Indian plate from Gondwana break-up at about 130 million years until the collision with Eurasia at about 50 million years ago. India's unique plate-tectonic behaviour in the Cretaceous cannot be related to a plume-eroded lithosphere. About one third of the Indian lithosphere was lost during or after the Deccan flood basalt event.

  3. 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

  4. 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.

  5. 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

  6. 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.

  7. [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.

  8. 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.

  9. Liberia’s Post-War Recovery: Key Issues and Developments

    DTIC Science & Technology

    2007-08-30

    Announces the Discovery of Kimberlite in Liberia and Progress with Gold Exploration,” January 9, 2006. Several other firms are involved in diamond...strong indications of a kimberlite , or pipes of igneous, volcanic, often diamond-bearing material, in Grand Cape County.83 If the kimberlite is verified

  10. Depth-related carbon isotope and nitrogen concentration variability in the mantle below the Orapa kimberlite, Botswana, Africa

    NASA Astrophysics Data System (ADS)

    Deines, Peter; Harris, J. W.; Gurney, J. J.

    1993-06-01

    Data on cubic diamonds with a very restricted delta C-13 range, a relatively high and constant nitrogen content, and low nitrogen aggregation state, from the Orapa kimberlite are analyzed. Results confirm the conclusion of the earlier studies (Deines, 1980; Deines et al., 1984, 1987, 1989) that a multitude of diamond sources and formation processes must be considered for the formation of diamonds recovered from any kimberlite.

  11. 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).

  12. 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

  13. 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

  14. 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

  15. 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.

  16. Reply to the Discussion by Mitchell and Tappe on “Kimberlites and aillikites as probes of the continental lithospheric mantle”

    NASA Astrophysics Data System (ADS)

    Francis, D.; Patterson, M.

    2010-03-01

    We thank Mitchell and Tappe for giving us the opportunity to discuss the nature of kimberlite magmas. We agree with Mitchell and Tappe about the importance of mineralogy, but the mineralogy of interstitial phases in kimberlites can shed little light on the origin of kimberlite magmas. Our use of cation units enables the interpretation of kimberlite whole-rock compositions in terms of the stoichiometry of their constituent minerals. When viewed in this manner, the variation in the whole-rock compositions of kimberlites clearly indicates that they are mixtures of harzburgitic lithospheric mantle and a carbonate-rich liquid. Aillikites are systematically more Fe-rich than kimberlites, but both contain less silica than olivine, such that they would evolve to lower Si contents via olivine fractionation. The opposite is true of olivine lamproites and meimechites. Although kimberlite whole-rock compositions do not reliably predict diamond grade on a local scale, there is a clear correlation between diamond grade and kimberlite composition on a global scale, which likely has the same explanation as the correlation between G10 garnets and diamonds. Considering that there is still no officially accepted definition of kimberlite, refusal to even consider the implications of their whole-rock compositional variation verges on lack of scientific due diligence.

  17. 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.

  18. 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.

  19. 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).

  20. Inclusions in diamonds from Snap Lake kimberlites (Slave Craton, Canada): Geochemical features of crystallization

    NASA Astrophysics Data System (ADS)

    Ivanova, O. A.; Logvinova, A. M.; Pokhilenko, N. P.

    2017-05-01

    The results of integrated studies of inclusion-containing diamonds from kimberlites of the Snap Lake dike complex (Canada) are presented. Features of the morphology, defect-impurity composition, and internal structure of the diamonds were determined by optic and scanning microscopy. The chemical composition of crystalline inclusions (olivine, garnet, and pyroxene) in diamonds was studied using a microanalyzer with an electronic probe. The inclusions of ultramafic paragenesis in the diamond (87%) are predominant. Carbonates, sulfide and hydrated silicate phases were found only in multiphase microinclusions. The large phlogopite inclusion studied was similar in composition to earlier studied nanosize inclusions of high-silica mica in diamonds from Snap Lake kimberlites. Revealed features of studied diamonds and presence of high-silica mica suggest that diamonds from Snap Lake have formed as the result of interaction between enriched in volatile and titanium high-potassium carbonate-silicate melts and peridotitic substrate at the base of thick lithospheric mantle.

  1. 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

  2. Geology and diamond distribution of the 140/141 kimberlite, Fort à la Corne, central Saskatchewan, Canada

    NASA Astrophysics Data System (ADS)

    Berryman, Adrian K.; Scott Smith, Barbara H.; Jellicoe, Brent C.

    2004-09-01

    The Cretaceous age Fort à la Corne (FALC) kimberlite province comprises at least 70 bodies, which were emplaced near the edge of the Western Canadian Interior Seaway during cycles of marine transgression and regression. Many of the bodies were formed during a marine regression by a two-stage process, firstly the excavation of shallow, but wide, craters and then subsequent infilling by xenolith-poor, crater-facies, subaerial, primary pyroclastic kimberlite. The bodies range in size up to 2000 m in diameter but are mainly less than 200 m thick and thus comprise relatively thin, but high volume, pyroclastic kimberlite deposits. Each body is composed of contrasting types of kimberlite reflecting different volcanic histories and, therefore, are considered separately. The 140/141 kimberlite is the largest delineated body in the province, estimated to have an areal extent below glacial Quaternary sediments in excess of 200 ha. The infilling of the 140/141 crater is complex, resulting from multiple phases of kimberlite. The central part of the infill is dominated by several contrasting phases of kimberlite. One of these phases is a primary pyroclastic airfall mega-graded bed up to 130 m in thickness. The constituents grade in size from very fine to coarse macrocrystic kimberlite, through to a basal breccia. The mega-graded bed is a widespread feature within parts of the body examined to date and at this current stage of evaluation appears to explain a variable diamond distribution within a tested portion of the pipe. A second different phase of kimberlite is interpreted as representing a younger nested crater within the mega-graded bed. Centrally located thicker intersections (>450 m) of this younger kimberlite may indicate a vent for the kimberlite crater. The thickness of the mega-graded bed increases with proximity to the younger kimberlite in the study area. Macrodiamond minibulk sample grades from the mega-graded bed have been obtained from nine large diameter drill

  3. 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.

  4. 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.

  5. Early Proterozoic transcontinental orogenic belts in the United States

    SciTech Connect

    Van Schmus, W.R. . Dept. of Geology); Bickford, M.E. . Dept. of Geology); Condie, K.C. . Dept. Geoscience)

    1993-02-01

    It has been recognized for many years that Early Proterozoic orogenic rocks in the western US range from 1.8 to 1.6 Ga, with a general distribution such that 1.8 to 1.7 Ga rocks underlie Colorado, northern Arizona, and northern New Mexico and 1.7 to 1.6 Ga rocks underlie southern Arizona and southern New Mexico. Recent U-Pb geochronologic and Sm-Nd isotopic studies by a variety of research groups have refined crustal history in the western region and have extended knowledge eastward into the buried midcontinent basement. As a result, the authors propose that 1.8 Ga to 1.6 Ga crust of the US by divided into two distinct, but overlapping, orogenic belts: a 1.8 to 1.7 Ga Inner Accretionary Belt and a 1.7 to 1.6 Ga Outer Tectonic Belt. The Inner Accretionary Belt (IAB) comprises rock suites with compositions and isotopic signatures compatible with origin as juvenile crustal terranes formed as oceanic or off-shore and related terranes that were accreted to southern Laurentia between 1.8 and 1.6 Ga. The IAB includes the Yavapai Province of Arizona, Early Proterozoic basement of Colorado and southern Wyoming, and the basement of Nebraska. The Mojave Province of California may be part of this belt, although it also includes components derived from older Proterozoic or Archean crust. Extension of the IAB eastward from Nebraska is uncertain at present, although coeval rocks that may be eastern manifestations of this 1.8 to 1.7 Ga orogenesis occur in Wisconsin (1.76 Ga granite-rhyolite suite), Ontario (Killarney granite), Labrador (Makkovic Province) and southern Greenland (Ketilidian orogen). The Outer Tectonic Belt (OTB) comprises rock suites which have compositions, structures, and isotopic signature compatible with origin in continental margin tectonic settings between 1.7 and 1.6 Ga.

  6. 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.

  7. The Evolution Of Steroids And Eukaryotes In The Proterozoic

    NASA Astrophysics Data System (ADS)

    Brocks, J. J.

    2005-12-01

    The ecology and diversity of eukaryotic organisms in the mid-Proterozoic ~1,800 to 800 Ma were fundamentally different than later in Earth history. The first fossils with eukaryotic affinity occur in the geological record about 2 billion years ago, but eukaryotic diversity and abundance remain low well into the Neoproterozoic (1,000-542 Ma). The first organism recognized as a member of a known eukaryotic kingdom, the red alga Bangiomorpha, only appears 1,200 Ma ago. In contrast to many younger assemblages, eukaryotes in the mid-Proterozoic predominantly inhabited agitated and well-oxygenated shoreline facies. In sedimentary rocks deposited in deep-water environments, fossils resembling eukaryotes usually remain rare, simple and small. The distinct ecology and apparently low diversity of early eukaryotes should also have left a distinct biomarker record of eukaryotic membrane lipids. Biomarkers are molecular fossils of natural products. They often retain the diagnostic carbon skeleton of their biological precursors and may endure billions of years enclosed in sedimentary rocks. Eukaryotic membranes contain a suite of distinct sterols with 26 to 30 carbon atoms. In the fossil record, these sterols are often preserved in the form of (C26) to (C30) hydrocarbon steranes. Their relative abundance is often characteristic of specific environments and geological periods in the Phanerozoic. We may expect particularly distinct sterane assemblages in the early history of eukaryote evolution. However, in conflicting contrast to the ecology and evolutionary status of earlier eukaryotes, the distribution of different sterane pseudo-homologues in the mid-Proterozoic is virtually indistinguishable from the Phanerozoic. Although the missing disparity between old and young biomarkers may, in principle, have a biological explanation, it is at least possible that many Proterozoic biomarker assemblages have experienced overprint with petroleum and petroleum products of younger origin

  8. A Superior Swath: Proterozoic Geology of the North American Midcontinent

    NASA Astrophysics Data System (ADS)

    Schneider, D. A.; Holm, D. K.; Chandler, V.

    2006-12-01

    Over fifty years of geologic investigations in the Upper Great Lakes region of southern Laurentia reveal a dynamic tectonic history of recurring Paleoproterozoic rifting and juvenile-crust accretionary events, and a final transformation to stable, cratonic lithosphere. A modern and revised understanding of the geologic evolution of this important Proterozoic margin is reported in a Precambrian Research special volume (in press), including a new regional tectonic province map that is based primarily on the integration of aeromagnetic and geochronologic data. Contrary to earlier models, ca. 1870-1830 Ma Penokean-interval crust is primarily constrained to northern Wisconsin, in a ~2100 Ma tectonic embayment of the rift-margin of the Archean Superior craton. The newly recognized Spirit Lake tectonic zone, a sharp magnetic discontinuity that marks the southern limit of Archean and Penokean-interval rocks, likely represents the eastward continuation of the Cheyenne belt suture zone in southern Wyoming. South of this boundary, 1800-1760 Ma Yavapai-interval rocks form the basement upon which 1750 Ma rhyolite and quartzite sequences were deposited. Yavapai- interval suturing was simultaneous with exhumation of a corridor of Archean-cored gneiss domes and significant magmatism and metamorphism along the main Penokean suture to the north. Much of the newly accreted Proterozoic terrane was strongly deformed during the 1650-1630 Ma Mazatzal orogeny, the dominant effect of which is represented by south-verging folds in 1700 Ma red quartzites; metamorphism is overall less intense and magmatism is absent. The northern boundary of Mazatzal terrane is obscured by abundant 1470- 1430 Ma "anorogenic" plutons that stitched the suture. These data reveal a successive accretion of juvenile arc terranes along the southern margin of Laurentia from ca. 1900-1600 Ma and an overall waning of Proterozoic metamorphism in this region. The final significant crustal modification represented in the

  9. Isotopes, ice ages, and terminal Proterozoic earth history.

    PubMed

    Kaufman, A J; Knoll, A H; Narbonne, G M

    1997-06-24

    Detailed correlations of ancient glacial deposits, based on temporal records of carbon and strontium isotopes in seawater, indicate four (and perhaps five) discrete ice ages in the terminal Proterozoic Eon. The close and repeated stratigraphic relationship between C-isotopic excursions and glaciogenic rocks suggests that unusually high rates of organic carbon burial facilitated glaciation by reducing atmospheric greenhouse capacity. The emerging framework of time and environmental change contributes to the improved resolution of stratigraphic and evolutionary pattern in the early fossil record of animals.

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

    PubMed

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

    2004-05-07

    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.

  11. 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

  12. Geochemical trends in kimberlites of the Ekati property, Northwest Territories, Canada: Insights on volcanic and resedimentation processes

    NASA Astrophysics Data System (ADS)

    Nowicki, Tom; Porritt, Lucy; Crawford, Barbara; Kjarsgaard, Bruce

    2008-06-01

    The Ekati property, Northwest Territories, Canada, hosts in excess of 150 volcanic kimberlite pipes occupied by a wide variety of rock types including coherent, magmatic material and a range of fragmental, volcaniclastic deposits of both pyroclastic and resedimented origin. Geochemical analysis of a suite of samples from several of these bodies provides valuable insight into the nature of their components and the processes by which they form. Observed variations in the bulk composition of coherent kimberlites correlate with their mode of emplacement and petrographic characteristics. High-volume bodies of coherent kimberlite, occurring within and in several cases dominating volcanic pipes (pipe-fill CK), are depleted in CO 2 and key incompatible elements (e.g. Ti and Nb) but enriched in Ni and SiO 2 relative to samples from narrow kimberlite dyke intrusions. The composition and certain textural features of pipe-fill CK can be interpreted to reflect formation by pyroclastic processes that involved a loss of volatiles and fines, and a concentration of olivine crystals relative to intrusive magmatic kimberlite. If this is the case, then the apparent coherent character of these rocks suggests that they represent coalescence of hot, fluidal pyroclasts (i.e. spatter) generated by fire-fountain style eruptions. Samples of massive pyroclastic kimberlite (PK) are geochemically similar to pipe-fill CK but generally show a greater degree of depletion of incompatible elements, suggestive of more explosive eruption processes and a greater degree of physical fractionation. The geochemical data support petrographic observations that Ekati PK contains very little xenolithic material. Resedimented volcaniclastic kimberlite (RVK) shows variable degrees of enrichment in Al 2O 3 relative to PK. The extent of Al-enrichment correlates with the proportion of dark, fine-grained clastic matrix material in the RVK samples and their composition can be explained as a mixture of a very olivine

  13. Some major problems with existing models and terminology associated with kimberlite pipes from a volcanological perspective, and some suggestions

    NASA Astrophysics Data System (ADS)

    Cas, R. A. F.; Hayman, P.; Pittari, A.; Porritt, L.

    2008-06-01

    Five significant problems hinder advances in understanding of the volcanology of kimberlites: (1) kimberlite geology is very model driven; (2) a highly genetic terminology drives deposit or facies interpretation; (3) the effects of alteration on preserved depositional textures have been grossly underestimated; (4) the level of understanding of the physical process significance of preserved textures is limited; and, (5) some inferred processes and deposits are not based on actual, modern volcanological processes. These issues need to be addressed in order to advance understanding of kimberlite volcanological pipe forming processes and deposits. The traditional, steep-sided southern African pipe model (Class I) consists of a steep tapering pipe with a deep root zone, a middle diatreme zone and an upper crater zone (if preserved). Each zone is thought to be dominated by distinctive facies, respectively: hypabyssal kimberlite (HK, descriptively called here massive coherent porphyritic kimberlite), tuffisitic kimberlite breccia (TKB, descriptively here called massive, poorly sorted lapilli tuff) and crater zone facies, which include variably bedded pyroclastic kimberlite and resedimented and reworked volcaniclastic kimberlite (RVK). Porphyritic coherent kimberlite may, however, also be emplaced at different levels in the pipe, as later stage intrusions, as well as dykes in the surrounding country rock. The relationship between HK and TKB is not always clear. Sub-terranean fluidisation as an emplacement process is a largely unsubstantiated hypothesis; modern in-vent volcanological processes should initially be considered to explain observed deposits. Crater zone volcaniclastic deposits can occur within the diatreme zone of some pipes, indicating that the pipe was largely empty at the end of the eruption, and subsequently began to fill-in largely through resedimentation and sourcing of pyroclastic deposits from nearby vents. Classes II and III Canadian kimberlite models

  14. Paleomagnetism of Middle Proterozoic mafic intrusions and Upper Proterozoic (Nankoweap) red beds from the Lower Grand Canyon Supergroup, Arizona

    NASA Astrophysics Data System (ADS)

    Weil, Arlo B.; Geissman, John W.; Heizler, Matt; Van der Voo, Rob

    2003-11-01

    Paleomagnetic data from lavas and dikes of the Unkar igneous suite (16 sites) and sedimentary rocks of the Nankoweap Formation (7 sites), Grand Canyon Supergroup (GCSG), Arizona, provide two primary paleomagnetic poles for Laurentia for the latest Middle Proterozoic (ca. 1090 Ma) at 32°N, 185°E (dp=6.8°, dm=9.3°) and early Late Proterozoic (ca. 850-900 Ma) at 10°S, 163°E (dp=3.5°, dm=7.0°). A new 40Ar/ 39Ar age determination from an Unkar dike gives an interpreted intrusion age of about 1090 Ma, similar to previously reported geochronologic data for the Cardenas Basalts and associated intrusions. The paleomagnetic data show no evidence of any younger, middle Late Proterozoic tectonothermal event such as has been revealed in previous geochronologic studies of the Unkar igneous suite. The pole position for the Unkar Group Cardenas Basalts and related intrusions is in good agreement with other ca. 1100 Ma paleomagnetic poles from the Keweenawan midcontinent rift deposits and other SW Laurentia diabase intrusions. The close agreement in age and position of the Unkar intrusion (UI) pole with poles derived from rift related rocks from elsewhere in Laurentia indicates that mafic magmatism was essentially synchronous and widespread throughout Laurentia at ca. 1100 Ma, suggesting a large-scale continental magmatic event. The pole position for the Nankoweap Formation, which plots south of the Unkar mafic rocks, is consistent with a younger age of deposition, at about 900 to 850 Ma, than had previously been proposed. Consequently, the inferred ˜200 Ma difference in age between the Cardenas Basalts and overlying Nankoweap Formation provides evidence for a third major unconformity within the Grand Canyon sequence.

  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. 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.

  17. 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.

  18. 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

    2016-01-07

    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.

  19. 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

  20. Caledonian and Proterozoic terrane accretion in the southwest Baltic Sea

    NASA Astrophysics Data System (ADS)

    Meissner, R.; Krawczyk, C. H.

    1999-12-01

    A marine seismic reflection survey was carried out in 1996 by DEKORP, Potsdam, and BGR, Hannover in the SW Baltic Sea. Several tectonic lineaments were crossed nearly perpendicularly, for example, the Caledonian Deformation Front which is assumed to mark the northern boundary of the terrane Avalonia which accreted to Baltica ca. 400 Ma. Here, a bivergent collision is clearly observed, confirming early ideas from the BABEL survey. The NE-dipping reflections in the uppermost mantle are interpreted as signs of the subducted Tornquist Ocean. A similar tectonic style of compression and indentation is observed in the Proterozoic crust northeast of Bornholm, where in addition a remarkable crustal thickening and a strong increase of reflection power is observed. The three DEKORP lines in this area provide a certain three-dimensional control and allows extension of similar observations from the BABEL line A southward. This Proterozoic terrane accretion seems to be connected to major tectonic lineaments in southern Sweden, either to the Gothian Thrust or the Sveconorwegian Front.

  1. 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.

  2. 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.

  3. 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

  4. High-resolution Seismic Reflection Imaging of Thin, Diamondiferous Kimberlite Dykes and Sills

    NASA Astrophysics Data System (ADS)

    Hammer, P. T.; Clowes, R. M.; Ramachandran, K.

    2003-12-01

    A unique seismic reflection experiment has successfully imaged a thin, diamondiferous kimberlite dyke intruded into granitic host rock. Although the typical dyke thickness is only 1-3 m, it is mapped from the near-surface to 1500 m depth. Such an application of seismic techniques to the diamond exploration and mining industry is unusual because the primary exploration targets are near-vertical kimberlite pipes that often are detected using magnetic and electromagnetic techniques. Subhorizontal dykes and sills do exist but they are poor potential field targets and have not been discovered by these techniques. However, one sub-horizontal structure, the Snap Lake dyke, was discovered in 1997 in the Archean Slave geological province of the Northwest Territories, Canada by tracking indicator minerals in the glacial till overburden. The Snap Lake dyke is a thin, dipping sheet that extends over at least 25 square km and plunges at approximately 15 degrees. The intrusion is richly diamondiferous and currently in the permitting stage for development of an underground mine. Its discovery heightened industry interest in dykes and sills, both in terms of their potential economic value and the information they yield regarding kimberlite emplacement. Since seismic reflection methods are especially well suited for mapping subhorizontal structures, dykes and sills have the potential to be excellent seismic targets. As a result, the Snap Lake seismic program was carried out to evaluate the seismic reflection method as a tool for exploration and deposit characterization of subhorizontal kimberlite intrusions. Snap Lake provides a superb test site for such a study because the dyke's gross geometry and composition have been determined through a substantial drilling program. Prior to the seismic field experiment, drill-core samples from the kimberlite and host rocks were used to measure P velocities and densities. These data were used to generate finite-difference and reflectivity

  5. 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.

  6. 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.

  7. 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.

  8. Formation of primary kimberlite melts - Constraints from experiments at 6-12 GPa and variable CO2/H2O

    NASA Astrophysics Data System (ADS)

    Girnis, A. V.; Bulatov, V. K.; Brey, G. P.

    2011-12-01

    Liquidus phase relations were experimentally studied in model kimberlite melts at 6-12 GPa. Four starting materials were used with different CO2 and H2O contents but almost identical proportions of other components (~ 35 wt.% SiO2, 3 wt.% Al2O3, 10 wt.% FeO, 33 wt.% MgO, 15 wt.% CaO, 0.3 wt.% Na2O, and 1 wt.% K2O on a 100% volatile-free basis). The liquidus phase of the CO2 richest mixture (33 wt.% CO2 and no H2O) is coesite at P > 6-8 GPa and low-Ca pyroxene at lower pressures. In an H2O-free mixture with lower CO2 content (18 wt.%) low-Ca pyroxene is the main liquidus phase and coesite was never observed in the experimental products. The addition of water (mixtures with 10 wt.% H2O + 9 wt.% CO2 and 12 wt.% H2O + 5 wt.% CO2) depresses the liquidus temperature and expands the crystallization field of olivine. At high pressures (> 8 GPa), garnet crystallization dominates near-liquidus phase relations. Based on the experimental data, possible conditions of the simultaneous saturation of kimberlite melt with olivine + low-Ca pyroxene ± garnet were estimated. High-Ca pyroxene was never found in the near-liquidus assemblages despite the high-CaO compositions of the starting mixtures. It was shown that the supposed primary kimberlite melt can be in equilibrium with the garnet harzburgite assemblage at 6-8 GPa and variable volatile contents, from ~ 15 wt.% CO2 under anhydrous conditions to 20 wt.% H2O and low CO2. These conditions correspond to possible parameters of kimberlite magma generation by partial melting of carbonated garnet harzburgite in the lithospheric mantle. The primary CO2 content could be higher if the kimberlite magma was poorer in SiO2 (more ‘carbonatitic'). The maximum CO2 content is limited by the appearance of magnesite on the liquidus at 20-22 wt.%, which is much lower than CO2 solubility in kimberlite melt at 7-8 GPa (> 30 wt.%). The content of H2O in primary kimberlite magmas depends on the bulk H2O content in the source, because no hydrous

  9. 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.

  10. 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.

  11. 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

  12. Thermal state and composition of the lithospheric mantle beneath the Daldyn kimberlite field, Yakutia

    NASA Astrophysics Data System (ADS)

    Griffin, W. L.; Kaminsky, F. V.; Ryan, C. G.; O'Reilly, S. Y.; Win, T. T.; Ilupin, I. P.

    1996-09-01

    The proton microprobe has been used to study the distribution of trace elements in garnet and chromite concentrates from the Udachnaya kimberlite and three smaller, low-grade kimberlites from the Daldyn kimberlite field. Garnet thermobarometry and classical P-T estimates for megacrystalline peridotite xenoliths both suggest a Paleozoic geotherm beneath the Daldyn area that is close to a 35 mW/m 2 conductive model. Finer-grained xenoliths with T < 1000° C scatter above this geotherm; high-temperature sheared xenoliths lie near or above a 40 mW/m 2 model geotherm at 1150-1400°C. The higher- T results are interpreted as the result of short-term heating, caused by magmatic intrusion and perturbation of a relatively cool conductive geotherm, especially near the base of the lithosphere. The stratigraphic distribution [inferred from nickel temperature ( TNi)] of garnets with different major-element chemistry indicates that the lithosphere is strongly layered in terms of rock type; depleted lherzolites predominate to depths of ca. 150 km, harzburgites comprise up to 60% of the volume between 150 and 180 km, and these are underlain by a mixture of depleted and metasomatically enriched lherzolites. Zinc temperatures ( TZn) indicate that chromite-bearing peridotites are essentially absent at depths > 190 km. High- T lherzolite garnets carry a distinctive trace-element fingerprint showing enrichment in Zr, Ti, Y and Ga, interpreted as due to the infiltration of asthenosphere-derived melts. This melt-related metasomatic signature becomes the dominant one at ca. 220-230 km depth, and this is interpreted as the base of the lithosphere. This depth also corresponds approximately to the Lehman Discontinuity at the top of a pronounced low-velocity zone, observed in deep seismic sounding experiments across this part of the Siberian Platform. The techniques used here provide a means of mapping the lithosphere in terms of thermal structure, lithology and fluid-related processes; both

  13. 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.

  14. Crustal thickening during Proterozoic metamorphism and deformation in New Mexico

    NASA Astrophysics Data System (ADS)

    Grambling, Jeffrey A.

    1986-02-01

    Proterozoic rocks in northern and central New Mexico underwent simultaneous metamorphism and deformation, tentatively dated at 1410 Ma. Structural relationships record a minimum of 20% 30% shortening during the latter part of deformation, and chemical zoning in garnet and plagioclase indicates a 20% increase in depth of burial during the same interval. Locally, deformation thickened the upper continental crust by at least 20%. This crustal thickening was distributed over a broad area and caused rocks across 75 000 km2 to recrystallize at peak metamorphic conditions near 525 °C and 4 kbar. The metamorphic terrane cooled isobarically, at rates less than 5 °C/m.y. Such slow cooling may be normal at middle crustal depths. Preservation of the regionally uniform peak metamorphic conditions reflects an unusual tectonic history: heat from the metamorphic event outlasted deformation, and the terrane was not subjected to rapid uplift following its thermal peak.

  15. 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

  16. Clay Mineralogy and Organic Carbon Burial in Proterozoic Basins

    NASA Astrophysics Data System (ADS)

    Tosca, N. J.; Johnston, D. T.; Mushegian, A.; Rothman, D. H.; Knoll, A. H.

    2008-12-01

    Pedogenic, or soil-derived, clay minerals have long been implicated in the efficiency of organic matter (OM) burial and coincident accumulation of atmospheric oxygen. As diagenesis and metamorphism obscure pedogenic clays in many Precambrian rocks, clay mineralogy and its role in OM burial through much of geologic time remains incompletely understood. In this study we analyzed the mineralogy and total organic carbon (TOC) of a number of organic rich shales deposited in Late Archean to Early Cambrian sedimentary basins. Across all samples, diagenetic transformation of pre-existing smectite minerals has led to the predominance of glauconite and the diagenetic 1M and 1Md illite polytypes, which, collectively, can be thought of as "proto-smectite". The correlations between TOC and illite crystallinity suggest that OM burial and preservation in the Proterozoic proceeded by the physical aggregation of OM and pedogenic clays upon deposition. This association, in turn, led to the interference of OM with the illitization process, resulting in the ubiquitous relationship between high surface area (or, finely crystalline) material and high TOC. This interpretation is consistent with suggestions that the preservation of OM after burial proceeds by physical exclusion, with mineral surfaces effectively isolating OM from enzymatic breakdown. Together, it appears that the deposition of pedogenic clays has remained broadly constant over Proterozoic time and into the Early Cambrian, which is incompatible with the hypothesis that late Neoproterozoic oxygenation was influenced by increases in pedogenic clay production. As no clear temporal relationship exists between clays and OM, Precambrian oxygenation was likely controlled by other mechanisms.

  17. 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.

  18. Geochemistry of clinopyroxene megacrysts from the Grib kimberlite pipe, Arkhangelsk province, Russia: metasomatic origin and genetic relationship with clinopyroxene-phlogopite metasomatic xenoliths

    NASA Astrophysics Data System (ADS)

    Lebedeva, Natalia; Kargin, Alexey; Sazonova, Lyudmila; Nosova, Anna

    2017-04-01

    Kimberlite is a composite rock that contains juvenile magmatic material and xenoliths of crustal and mantle rocks, including metasomatically reworked rocks and megacrysts. In spite of nearly 40-50 years of continuous study of kimberlites and SCLM, some aspects of their origin remain controversial. In particular, it is unclear yet whether the megacrysts are magmatic or metasomatic in origin and how they are related to kimberlite magmas. In this contribution, we compare the major (EMPA) and trace element (SIMS, LA-ICP-MS) compositions of clinopyroxene megacrysts from the Grib kimberlite (Arkhangelsk province, Russia) with clinopyroxenes from metasomatic clinopyroxene-phlogopite xenoliths and garnet peridotite xenoliths. The Grib kimberlite (376±3 Ma, Larionova et al., 2016) is located in the central part of the Arkhangelsk province (the northern part of the East European craton) in the Chernoozero kimberlite field. The geochemical composition of the kimberlites is similar to widespread South Africa group I kimberlites . The Grib kimberlite is well known for hosting a variety of mantle xenoliths, e.g., garnet peridotite, sheared peridotite, eclogite, metasomatised mantle material, as well as megacrysts of clinopyroxene, garnet, olivine, phlogopite, and ilmenite. The clinopyroxene megacrysts occur as rounded or angular grains up to 2 cm in size. They are usually surrounded by ultrafine kimberlite rim. The xenoliths of the metasomatic clinopyroxene-phlogopite rocks reach up to 6 cm in size and have a granoblastic texture. They consist of clinopyroxene (55 vol. %), phlogopite (45 vol. %) and minor calcite, barite, perovskite. Some clinopyroxene grains contain inclusion of relict olivine that is similar in composition to olivine from mantle-derived peridotite xenoliths within the Grib kimberlite (Sazonova et al., 2015). This suggests that these xenoliths could be formed by metasomatic reworking of SCLM peridotites. The megacryst clinopyroxene is compositionally similar

  19. 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.

  20. [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.

  1. 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.

  2. 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.

  3. [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.

  4. 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.

  5. 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.

  6. 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

  7. 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.

  8. 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.

  9. 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.

  10. Geothermobarometry for ultramafic assemblages from the Emeishan Large Igneous Province, Southwest China and the Nikos and Zulu Kimberlites, Nunavut, Canada

    NASA Astrophysics Data System (ADS)

    Zhao, D.

    2009-05-01

    To understand and contrast the origins of ultramafic assemblages from basaltic and kimberlitic rocks and their associated deposits, such as V-Ti magnetite and Ni-Cu-(PGE) sulfide deposits and diamond, applicable thermobarometers were evaluated and applied to the ultramafic assemblages from the Emeishan Large Igneous Province (ELIP), Southwest China and from the Nikos and Zulu Kimberlites of Nunavut, Canada. The ELIP is located in the Yangtze Block, Southwest China and composed of Permian Emeishan Flood basalt (EFB) and associated layered mafic-ultramafic intrusions. Some of these intrusions host V-Ti magnetite deposits; while others contain Ni-Cu-(PGE) sulfide deposits. It is not clear why some intrusions host magnetite deposits and others contain sulfide deposits. The P-T conditions for the ultramafic assemblages from the mafic-ultramafic intrusions in the ELIP were calculated in order to understand the origins and the associated mineral deposits. The ultramafic assemblages are peridotite, olivine pyroxenite, pyroxenite in the layered intrusions and the common minerals include spinel, olivine, clinopyroxene, orthopyroxene, and minor magnetite and ilmenite. Using a two pyroxene thermometer and a Ca-Mg exchange barometer between olivine and clinopyroxene, a spinel-olivine-clinopyroxene-orthopyroxene assemblage from the Xinjie intrusion yields a T-P of 905°C and 17 kbar; and a similar assemblage from the Jinbaoshan intrusion yields a T-P of 1124°C and 31 kbar. The Nikos kimberlite, near Elwin Bay on Somerset Island, is located at the northeast end of the northeast-southwest kimberlite zone; and the Zulu kimberlite is located on the neighboring Brodeur Peninsula of Baffin Island, Nunavut. The ultramafic assemblages from the Canadian Kimberlites include garnet lherzolite, garnet-spinel lherzolite, spinel lherzolite, dunite, garnet websterite, spinel websterite and garnet clinopyroxenite. The calculated P-T conditions are in the range of 760 to 1180°C and 25 to 60

  11. Diamond-facies chrome spinel from the Tokapal kimberlite, Indrāvati basin, central India and its petrological significance

    NASA Astrophysics Data System (ADS)

    Chalapathi Rao, N. V.; Lehmann, B.; Mainkar, D.; Panwar, B. K.

    2012-07-01

    Widespread and abundant spinel is the only primary mineral of petrogenetic significance preserved in the hydrothermally altered, crater-facies, Neoproterozoic (≥620 Ma) Tokapal kimberlite pipe that intruded the Indrāvati basin, Bastar craton, Central India. Two distinct spinel populations occur: (i) finer-grained (<50 μm) microcrysts which are zoned from titaniferous magnesiochromite-chromite to magnetite; and (ii) larger macrocrysts (>400 μm) with cores having distinctly chromium-rich (Cr2O3 up to 63.67 wt%), and TiO2-poor (<0.68 wt%) compositions. Based on their morphology and chemical composition the macrocrysts are inferred to be disaggregated mantle xenocrysts and their compositional range extends well into the diamond stability field. However, the reported absence of diamond and other indicator minerals (such as pyrope garnet, chrome diopside and magnesian ilmenite) in the Tokapal pipe is intriguing since diamondiferous cratonic roots are indeed preserved in the Bastar craton, and also the kimberlite itself was derived from a similar source region(s) as that of the well-known diamondiferous Mesoproterozoic (ca. 1,100 Ma) kimberlites from Wajrakarur, Dharwar craton, southern India. Given the large areal extent (>550 ha) of kimberlite, there is a possibility that it contains diamonds but they were not recovered during the sampling. Alternately, highly oxidising conditions imparted by the metasomatic fluids/melts derived from (i) asthenosphere-lithosphere interaction, or (ii) the kimberlite itself might have played an important role in the destruction of diamond, and other indicator minerals.

  12. Climate, paleoecology and abrupt change during the Late Proterozoic: A consideration of causes and effects

    NASA Astrophysics Data System (ADS)

    McMenamin, Mark A. S.

    This chapter examines the influence of the biosphere on the initiation, and termination of, the glaciations of the late Proterozoic. Recent considerations suggest that the biosphere controlled the timing of the onset of glaciation and also controlled the timing of the end of glaciation. Massive carbonate accumulation and giant stromatolites of the Late Proterozoic, combined with major blooms of phytoplankton, led to significant drops in the carbon dioxide content of the atmosphere, and forced climate from greenhouse to icehouse conditions. Cryoconites and hyperscums, each with a distinctively adapted cryophilic microbiota, developed during the Proterozoic ice ages and may have been a factor in melting the ice. The Proterozoic Tindir Group, Alaska provides evidence for such a cryophilic microbiota. Only by invoking the activity of such organisms can we explain the rapidity of deglaciation. A propensity to accumulate massive carbonates was present before the glaciation as well as after the deposition of the cap carbonates. Substrate disturbance by burrowing metazoa after the ice ages disrupted the microbial mat component of Proterozoic carbonate sequestration. Stromatolites after the glaciation tend to have porous, clotted and thrombolitic textures instead of evenly laminated textures and would therefore be less effective at retaining carbon dioxide (as carbonate and organic matter) and keeping it out of marine circulation. Newly emergent, burrowing metazoa of the Late Proterozoic eventually halted the development of ice-age inducing conditions, and may have prevented even worse glaciations by releasing hydrocarbons sequestered in seafloor sediment.

  13. Experimental constraints on crustal contamination in Proterozoic anorthosite petrogenesis

    NASA Astrophysics Data System (ADS)

    Hill, C. M.; Bybee, G. M.; Parman, S. W.; Ashwal, L. D.

    2016-12-01

    Massif-type anorthosites are characterized by their large volume, plagioclase- dominated mineralogy (up to 90% An40-60) and restriction to the Proterozoic Eon (0.6 to 2.7 Ga). The composition of the parental magma is believed to be a high-Al basalt. Whether this magma originated from the mantle or the crust, is not clear. In many massifs, including the 18,000 km2 Kunene Anorthosite Complex (KAC) in Angola, bodies of orthopyroxene-bearing and olivine-bearing anorthosites can be distinguished. This mineral dichotomy has been attributed to varying degrees of crustal contamination of a mantle-derived magma. However, previous experiments and modeling have shown a thermal divide on the plagioclase-pyroxene liquidus, implying that a mantle-derived melt cannot produce the observed range of mineral assemblages in the anorthosites at 10-13 kbar, even with contamination by surrounding crust (Longhi et al., J. Petrol. 40, 1999; Longhi, Lithos, 83, 2005). These models propose that lower crustal anatexis and subsequent fractionation would produce a closer match to the observed mineral compositions. We have conducted piston cylinder experiments at 10 kbar using a high-alumina basaltic starting composition (the proposed parental magma of Proterozoic anorthosites), as well as a mixture of basalt contaminated with a coeval granite composition from the KAC, to model the effects of contamination on phase equilibria relevant to anorthosites. The experiments show a shift from olivine to orthopyroxene in the crystallizing assemblage following contamination. The results indicate that the olivine-orthopyroxene dichotomy in anorthosites can be produced by crustal contamination of mantle-derived, high-alumina basalts at depth, and suggest that the parental magmas could be derived from melting of the mantle. Fractionation and contamination modeling using MELTS (Smith and Asimow, Geochem, Geophys. Geosyst. 6, 2005; Ghiorso et al. Geochem, Geophys. Geosyst. 3, 2002) matches the experiments well

  14. 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

  15. 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

  16. 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

  17. 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.

  18. Proterozoic diabase sills of northern Ontario: Magnetic properties and history

    NASA Astrophysics Data System (ADS)

    Middleton, R. S.; Borradaile, G. J.; Baker, D.; Lucas, K.

    2004-02-01

    Magnetic fabrics show that a series of massive diabase sills ("Logan sills") possess flow fabrics away from the midcontinental Proterozoic rift system, near its triple junction on the north shore of Lake Superior. Their intense remanences are commonly antiparallel to the geomagnetic field, requiring some knowledge of natural remanent magnetization in order to interpret of ground magnetic surveys. Thermal demagnetization, low-temperature demagnetization, and the combined techniques yield differently oriented characteristic remanence vectors in some of these sills. Although the commonly reported "Keweenawan" reversed paleofield direction is verified by most A components, the orientation of vectors within sites and through the thickness of sills is large and may be attributed to reversals, excursions, and secular variation. B components are mostly down, but their orientation distribution is poorly concentrated. Low-temperature demagnetization, prior to thermal demagnetization, tends to improve the consistency of vectors defined in subsequent thermal demagnetization. Moreover, in some cases, three to five cycles of low-temperature demagnetization alone produces the same result as the more time-consuming thermal demagnetization. Some within-site scatter of the A components is large, and some anomalous specimen directions and alternate polarities may be associated with platinum group mineralization. Whereas the grand mean direction for all sites lies on the available apparent polar wander path between 1100 and 1300 Ma, the mean paleopole locations differ at the 95% level for different demagnetization treatments of specimens from the same sites.

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

    PubMed

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

    2009-10-06

    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.

  20. Stability of phlogopite in ultrapotassic kimberlite-like systems at 5.5-7.5 GPa

    NASA Astrophysics Data System (ADS)

    Sokol, Alexander G.; Kruk, Alexey N.; Palyanov, Yury N.; Sobolev, Nikolay V.

    2017-04-01

    Hydrous K-rich kimberlite-like systems are studied experimentally at 5.5-7.5 GPa and 1200-1450 °C in terms of phase relations and conditions for formation and stability of phlogopite. The starting samples are phlogopite-carbonatite-phlogopite sandwiches and harzburgite-carbonatite mixtures consisting of Ol + Grt + Cpx + L (±Opx), according to the previous experimental results obtained at the same P- T parameters but in water-free systems. Carbonatite is represented by a K- and Ca-rich composition that may form at the top of a slab. In the presence of carbonatitic melt, phlogopite can partly melt in a peritectic reaction at 5.5 GPa and 1200-1350 °C, as well as at 6.3-7.0 GPa and 1200 °C: 2Phl + CaCO3 (L)↔Cpx + Ol + Grt + K2CO3 (L) + 2H2O (L). Synthesis of phlogopite at 5.5 GPa and 1200-1350 °C, with an initial mixture of H2O-bearing harzburgite and carbonatite, demonstrates experimentally that equilibrium in this reaction can be shifted from right to left. Therefore, phlogopite can equilibrate with ultrapotassic carbonate-silicate melts in a ≥ 150 °C region between 1200 and 1350 °C at 5.5 GPa. On the other hand, it can exist but cannot nucleate spontaneously and crystallize in the presence of such melts in quite a large pressure range in experiments at 6.3-7.0 GPa and 1200 °C. Thus, phlogopite can result from metasomatism of peridotite at the base of continental lithospheric mantle (CLM) by ultrapotassic carbonatite agents at depths shallower than 180-195 km, which creates a mechanism of water retaining in CLM. Kimberlite formation can begin at 5.5 GPa and 1350 °C in a phlogopite-bearing peridotite source generating a hydrous carbonate-silicate melt with 10-15 wt% SiO2, Ca# from 45 to 60, and high K enrichment. Upon further heating to 1450 °C due to the effect of a mantle plume at the CLM base, phlogopite disappears and a kimberlite-like melt forms with SiO2 to 20 wt% and Ca# = 35-40.

  1. 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

  2. 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

  3. Liberia’s Post-War Recovery: Key Issues and Developments

    DTIC Science & Technology

    2006-01-30

    International (DFI) Ltd. announced that it had discovered strong indications of a kimberlite , or pipe of igneous, volcanic, often diamond-bearing material...in Grand Cape County. DFI is also involved in gold exploration in Liberia. See DFI, “Diamond Fields Announces the Discovery of Kimberlite in Liberia

  4. 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.

  5. Airborne IP: examples from the Mount Milligan deposit, Canada, and the Amakinskaya kimberlite pipe, Russia

    NASA Astrophysics Data System (ADS)

    Viezzoli, Andrea; Kaminski, Vlad

    2016-07-01

    There have been multiple occurrences in the literature in the past several years of what has been referred to as the induced polarisation (IP) effect in airborne time domain electromagnetic (TDEM) data. This phenomenon is known to be responsible for incorrect inversion modelling of electrical resistivity, lower interpreted depth of investigation (DOI) and lost information about chargeability of the subsurface and other valuable parameters. Historically, there have been many suggestions to account for the IP effect using the Cole-Cole model. It has been previously demonstrated that the Cole-Cole model can be effective in modelling synthetic TDEM transients. In the current paper we show the possibility of extracting IP information from airborne TDEM data using this same concept, including inverse modelling of chargeability from TDEM data collected by VTEM, with field examples from Canada (Mt Milligan deposit) and Russia (Amakinskaya kimberlite pipe).

  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. 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

  10. First Report of Majoritic-Garnet Diamond Inclusions From Yakutian Kimberlites

    NASA Astrophysics Data System (ADS)

    Sobolev, N. V.; Taylor, L. A.; Logvinova, A. M.; Seryotkin, Y. V.; Koptil, V. I.; Yefimova, E. S.

    2002-12-01

    The presence of a majoritic component in mantle garnets is significant in that it indicates a deeper-than-normal origin for their host diamonds. We have discovered the first majoritic garnets both of peridotitic (P-/U-type) and eclogitic (E-type) parageneses, included in microdiamonds (<1 mm) from three Yakutian kimberlite pipes: Yubileynaya, Komsomolskaya, and Krasnopresnenskaya, all located in the Alakit kimberlite field of Upper Devonian age. Up until now, a considerable number of majoritic garnets have been recovered from placers. The new finds of majoritic garnets reported here practically double the number of kimberlitic pipes worldwide where such garnets have been detected. Multiple inclusions of garnet and olivine occur in single P-type diamond from Yubileynaya. Here, a CrCa-rich majoritic garnet coexists with a CrCa-rich non-majoritic garnet and olivine, but the 3 grains are not in contact. Positive identification of the majoritic garnet was obtained by single-crystal X-ray diffraction: space group Ia3d; a = 11.775 (1) Å; V = 1632.6 (2) Å3. All garnets were analyzed extensively by electron microprobe. The specific features of the compositions of `coexisting' majoritic and non-majoritic garnets, resp., are: Si (pfu) = 3.22 and 3.02: Cr2O3 (wt %) = 10.2 and 13.7; CaO (wt %) = 20.8 and 12.7; Mg# 77.6 and 69.9. Coexisting olivine is Fo 91.5, which is consistent with the relatively low Mg# of the majoritic garnet. This Yubileynaya majoritic garnet diamond inclusion (DI) represents the first find of a garnet, containing solid solution pyroxene, from a wehrlitic paragenesis. Furthermore, its CaCr-component (uvarovite) content is unusually high (~50%). The chemical differences of the wehrlitic garnets in this one Yubileynaya diamond testifies directly to the complex history of this diamond, specifically to a large range of pressures. Majoritic garnet DIs from the Komsomolskaya and Krasnopresnenskaya pipes are both of E-type and are characterized by the following

  11. 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

  12. 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

  13. Sr-Nd-Pb isotope systematics of mantle xenoliths from Somerset Island kimberlites: Evidence for lithosphere stratification beneath Arctic Canada

    NASA Astrophysics Data System (ADS)

    Schmidberger, S. S.; Simonetti, A.; Francis, D.

    2001-11-01

    Sr, Nd, and Pb isotopic compositions were determined for a suite of Archean garnet peridotite and garnet pyroxenite xenoliths and their host Nikos kimberlite (100 Ma) from Somerset Island to constrain the isotopic character of the mantle root beneath the northern Canadian craton. The Nikos peridotites are enriched in highly incompatible trace elements (La/Sm N = 4-6), and show 143Nd/ 144Nd (t) (0.51249-0.51276) and a large range in 87Sr/ 86Sr (t) (0.7047-0.7085) and Pb ( 206Pb/ 204Pb (t) = 17.18 to 19.03) isotope ratios that are distinct from those estimated for "depleted mantle" compositions at the time of kimberlite emplacement. The Nd isotopic compositions of the peridotites overlap those of the Nikos kimberlite, suggesting that the xenoliths were contaminated with kimberlite or a kimberlite-related accessory phase (i.e., apatite). The highly variable Sr and Pb isotopic compositions of the peridotites, however, indicate that kimberlite contribution was restricted to very small amounts (˜1 wt % or less). The high-temperature peridotites (>1100°C) that sample the deep Somerset lithosphere trend toward more radiogenic 87Sr/ 86Sr (t) (0.7085) and unradiogenic 206Pb/ 204Pb (t) (17.18) isotopic ratios than those of the low-temperature peridotites (<1100°C). This is in agreement with Sr isotopic compositions of clinopyroxene from the low-temperature peridotites ( 87Sr/ 86Sr (t) = 0.7038-0.7046) that are significantly less radiogenic than those of clinopyroxene from the high-temperature peridotites ( 87Sr/ 86Sr (t) = 0.7052-0.7091). The depth correlation of Sr isotopes for clinopyroxene and Sr and Pb isotopic compositions for the Nikos whole-rocks indicate that the deep Somerset lithosphere (>160 km) is isotopically distinct from the shallow lithospheric mantle. The isotopic stratification with depth suggests that the lower lithosphere is probably younger and may have been added to the existing Archean shallow mantle in a Phanerozoic magmatic event. The radiogenic Sr

  14. Reconstructing Earth's Surface Oxidation Across The Archean- Proterozoic Transition

    NASA Astrophysics Data System (ADS)

    Kaufman, A. J.; Guo, Q.; Strauss, H.; Schröder, S.; Gutzmer, J.; Wing, B. A.; Baker, M.; Bekker, A.; Jin, Q.; Kim, S.; Farquhar, J.

    2010-12-01

    The Archean-Proterozoic transition is characterized by the widespread deposition of organic-rich shale, sedimentary iron formation, glacial diamictite, and marine carbonates recording profound carbon isotope anomalies, but notably lacks bedded evaporites. All deposits reflect environmental changes in oceanic and atmospheric redox states, in part associated with Earth’s earliest ice ages. Time-series data for multiple sulfur isotopes from carbonate associated sulfate as well as sulfides in the glaciogenic Duitschland Formation of the Transvaal Supergroup, South Africa, capture the concomitant buildup of sulfate in the ocean and the loss of mass independent sulfur isotope fractionation. This is arguably associated with the atmospheric rise of oxygen (as well as the protective ozone layer) coincident with profound changes in ocean chemistry and biology. The loss of the MIF signal within the Duitschland succession is in phase with the earliest recorded positive carbon isotope anomaly, convincingly linking these environmental perturbations to the Great Oxidation Event (ca. 2.3 Ga). The emergence of cyanobacteria and oxygenic photosynthesis may be associated with a geochemical “whiff of oxygen” recorded in 2.5 Ga sediments. If true, the delay in the GOE can then be understood in terms of a finite sink for molecular oxygen - ferrous iron, which was abundant in deep Neoarchean seawater and sequestered in a worldwide episode of iron formation deposition ending shortly before accumulation of the Duitschland Formation. Insofar as early Paleoproterozoic glaciation is associated with oxygenation of a methane-rich atmosphere, we conclude that Earth’s earliest ice age(s) and the onset of a modern and far more energetic carbon cycle are directly related to the global expansion of cyanobacteria that released oxygen to the environment, and of eukaryotes that respired it.

  15. 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.

  16. A global survey of Precambrian evaporites: Implications for Proterozoic paleoenvironments

    NASA Astrophysics Data System (ADS)

    Evans, D. A.

    2006-05-01

    Evaporites are sedimentary rocks comprising minerals that crystallized from supersaturation of surface waters due to solar-driven desiccation. They, or their metamorphic relics and pseudomorphs, are abundant in the geologic record and document changes in paleoclimate, sealevel, and marine chemistry. Phanerozoic evaporites have been well described and summarized, in no small part due to their role as hydrocarbon seals, as well as sources of salinity in hydrothermal fluids that concentrate metal deposits. Precambrian evaporites are abundant in discrete number of instances but are generally less voluminous; their long-term preservation is limited by subsurface mineral dissolution as well as tectonic crustal recycling. Unlike Precambrian glacial deposits, which have been globally catalogued several times during the past fifty years, Precambrian evaporites have been compiled only partially in a few rare studies. A new, global survey of Precambrian evaporites (mainly pseudomorphs after gypsum, anhydrite, and halite) documents over 100 examples, including ten of Archean age. About 20 deposits have total preserved or estimated salt volumes exceeding 1000 cubic km, and these are restricted to the Proterozoic Era. One of the most impressive episodes of evaporite deposition in the entire geologic record occurred at about 800 Ma, coincident with the onset of Rodinia supercontinental fragmentation. These evaporites are preserved primarily as calcium-sulfates, totalling about 350,000 cubic km in volume. The next major global peak in evaporite deposition occurred in late Ediacaran to Early Cambrian time, totalling more than 1.5 million cubic km of mixed sulfates and halites. These peaks rival the great salt records of the Late Devonian, Late Permian, and Late Jurassic, and the molar volumes of deposited salt are comparable to the current inventory of oceanic salinity. Questions for future consideration include: what does the removal of this much salinity from the oceans, in these

  17. 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).

  18. 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.

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

    PubMed

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

    2003-06-05

    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. Perovskite and baddeleyite from kimberlitic intrusions in the Tarim large igneous province signal the onset of an end-Carboniferous mantle plume

    NASA Astrophysics Data System (ADS)

    Zhang, Dongyang; Zhang, Zhaochong; Santosh, M.; Cheng, Zhiguo; He, Huang; Kang, Jianli

    2013-01-01

    Several tens of kimberlitic pipes and dykes are exposed in the Wajilitag area in the western Tarim large igneous province. Here we report for the first time secondary ion mass spectrometric U-Pb age data on perovskite and baddeleyite grains in a kimberlitic pipe and a kimberlitic dyke from the Tarim Craton. The perovskite yielded a well-defined intercept age of 299.8±4.3 Ma, which is consistent with its corresponding concordia and 206Pb/238U ages, corrected for the common Pb contribution, of about 300 Ma. The baddeleyite separated from two kimberlitic samples from a dyke display identical concordia U-Pb ages of 300.8±4.7 Ma and 300.5±4.4 Ma. Our age data show that the kimberlitic intrusions were emplaced at ca. 300 Ma, rather than in the late Permian as previously regarded. These new ages are slightly older than the eruption ages of Tarim flood basalts (291-273 Ma), offering a critical regional time marker for the onset of Permo-Carboniferous magmatism in the Tarim Craton. Detailed petrographic observations did not reveal any ultrahigh pressure mineral assemblage in the Wajilitag kimberlitic intrusions. Phlogopites from these intrusions show ɛNd(t) values of +3.7 to +4.2. The baddeleyites which are texturally primary and therefore inferred to have crystallized directly from the kimberlitic magma, yield a range of ɛHf(t) from +4.8 to +8.7. These results combined with previously reported geochemical data, suggest that the Wajilitag kimberlitic intrusions were most likely derived from a moderately refractory and depleted subcontinental lithosphere mantle, metasomatized by subduction components associated with an early-middle Paleozoic convergent regime. The kimberlitic magma was generated by small-degree partial melting of the lithospheric mantle in response to the impingement of the Tarim mantle plume. Thus, our new geochronological data suggest the arrival of the mantle plume beneath the Tarim lithosphere at least 10 million years before the onset of Tarim

  1. 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.

  2. 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.

  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

    2006-11-02

    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. Molybdenum isotope evidence for widespread anoxia in mid-Proterozoic oceans.

    PubMed

    Arnold, G L; Anbar, A D; Barling, J; Lyons, T W

    2004-04-02

    How much dissolved oxygen was present in the mid-Proterozoic oceans between 1.8 and 1.0 billion years ago is debated vigorously. One model argues for oxygenation of the oceans soon after the initial rise of atmospheric oxygen approximately 2.3 billion years ago. Recent evidence for H(2)S in some mid-Proterozoic marine basins suggests, however, that the deep ocean remained anoxic until much later. New molybdenum isotope data from modern and ancient sediments indicate expanded anoxia during the mid-Proterozoic compared to the present-day ocean. Consequently, oxygenation of the deep oceans may have lagged that of the atmosphere by over a billion years.

  6. A conceptual model for kimberlite emplacement by solitary interfacial mega-waves on the core mantle boundary

    NASA Astrophysics Data System (ADS)

    Sim, B. L.; Agterberg, F. P.

    2006-07-01

    If convection in the Earth's liquid outer core is disrupted, degrades to turbulence and begins to behave in a chaotic manner, it will destabilize the Earth's magnetic field and provide the seeds for kimberlite melts via turbulent jets of silicate rich core material which invade the lower mantle. These (proto-) melts may then be captured by extreme amplitude solitary nonlinear waves generated through interaction of the outer core surface with the base of the mantle. A pressure differential behind the wave front then provides a mechanism for the captured melt to ascend to the upper mantle and crust so quickly that emplacement may indirectly promote a type of impact fracture cone within the relatively brittle crust. These waves are very rare but of finite probability. The assumption of turbulence transmission between layers is justified using a simple three-layer liquid model. The core derived melts eventually become frozen in place as localised topographic highs in the Mohorovicic discontinuity (Moho), or as deep rooted intrusive events. The intrusion's final composition is a function of melt contamination by two separate sources: the core contaminated mantle base and subducted Archean crust. The mega-wave hypothesis offers a plausible vehicle for early stage emplacement of kimberlite pipes and explains the age association of diamondiferous kimberlites with magnetic reversals and tectonic plate rearrangements.

  7. 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

  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. Precisely dating Paleozoic kimberlites in the North China Craton and Hf isotopic constraints on the evolution of the subcontinental lithospheric mantle

    NASA Astrophysics Data System (ADS)

    Li, Qiu-Li; Wu, Fu-Yuan; Li, Xian-Hua; Qiu, Zhi-Li; Liu, Yu; Yang, Yue-Heng; Tang, Guo-Qiang

    2011-09-01

    Kimberlite, a deep-sourced ultramafic potassic rock, carries not only diamond, but also invaluable mantle xenoliths and/or xenocrysts, which are important for tracking the evolution of subcontinental lithospheric mantle (SCLM). However, it is challenging to accurately determine the emplacement age of kimberlite and its compositions of primary magma because of modifications by crustal and/or mantle contamination and post-emplacement alteration. This paper reports emplacement ages of diamondiferous kimberlites in Mengyin and Fuxian of the North China Craton (NCC) using three different dating methods. For Mengyin kimberlite, single-grain phlogopite Rb-Sr dating yields an isochron age of 485 ± 4 Ma, U-Th-Pb analyses on perovskite give a 238U- 206Pb age of 480.6 ± 2.9 Ma and a 232Th- 208Pb age of 478.9 ± 3.9 Ma, and baddeleyite yields a 207Pb- 206Pb age of 480.4 ± 3.9 Ma. For Fuxian kimberlite, baddeleyite gives a 207Pb- 206Pb age of 479.6 ± 3.9 Ma, indicating that the Paleozoic kimberlites in the NCC were emplaced at ~ 480 Ma. Numerous lines of evidence indicate that the studied baddeleyites are xenocrysts from the SCLM, and can be used to constrain Hf isotope compositions ( ɛHf(t) ~ - 6) of the SCLM when kimberlite erupted. Combined with data from Mesozoic-Cenozoic mantle-derived rocks and xenoliths, the Hf isotope evolution trend of the SCLM beneath NCC before craton destruction was tentatively constructed, which suggested that the Archean SLCM was enriched by metasomatism at ~ 1.3 Ga. Further Hf isotope investigations on additional SCLM-derived materials could be used to compare with the constructed Hf isotope evolution trend before craton destruction to determine when lithospheric thinning occurred.

  10. 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.

  11. Provenance of zircon xenocrysts in the Neoproterozoic Brauna Kimberlite Field, São Francisco Craton, Brazil: Evidence for a thick Palaeoproterozoic lithosphere beneath the Serrinha block

    NASA Astrophysics Data System (ADS)

    Donatti-Filho, José Paulo; Oliveira, Elson P.; McNaughton, Neal J.

    2013-08-01

    The 642 Ma-old Brauna Kimberlite Field is located on the northeastern sector of the São Francisco Craton (Serrinha block) and is one of the rare Neoproterozoic kimberlitic events in South America. Zircon xenocrysts from the volumetric most important kimberlite pipes Brauna 03, Brauna 07 and Brauna 04 were used as a tool to identify different components of the lithosphere beneath the northeast region of the São Francisco craton. A composite kimberlite sample of eight representative and different drill holes and three samples of the host rocks (Nordestina granodiorite) were sampled for SHRIMP geochronology. The results were compared with precise U-Pb age data for the regional rocks, i.e. the Archaean basement and the Palaeoproterozoic Rio Itapicuru greenstone belt. Samples from the Nordestina granodiorite gave three different ages: 2155 Ma in the western part of the batholith, 2139 Ma in its central part, and 2132 Ma in its eastern part. Zircon 207Pb/206Pb ages of the Brauna kimberlite zircon grains spread over the timespan 2107-2223 Ma and indicate four age groups at 2105 ± 3 Ma, 2138 ± 7 Ma, 2166 ± 5 Ma, and 2198 ± 4 Ma. Source rocks for the three former age groups can be found in the Rio Itapicuru greenstone belt, including zircon xenocrysts from the Nordestina granodiorite, whereas the latter age group has not yet been reported in the Serrinha block. The new zircon ages show that only rocks of the Palaeoproterozoic Rio Itapicuru greenstone belt and of a hidden 2.17-2.20 Ga crust were sampled by the kimberlite magma during its ascent through the lithosphere. It is proposed that there is none or a few Archaean crust beneath the kimberlite emplacement area, hence implying a thick Palaeoproterozoic lithosphere for this portion of the São Francisco craton.

  12. 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.

  13. Secular variation in carbon isotope ratios from Upper Proterozoic successions of Svalbard and East Greenland.

    PubMed

    Knoll, A H; Hayes, J M; Kaufman, A J; Swett, K; Lambert, I B

    1986-06-26

    Analyses of stratigraphically continuous suites of samples from Upper Proterozoic sedimentary successions of East Greenland, Spitsbergen and Nordaustlandet (Svalbard) provide an approximation to the secular variation in carbon isotope ratios during a geologically and biologically important period of change from around 900 million years ago to the beginning of the Cambrian period. Late Riphean carbonates and organic material show a stratigraphically useful pattern of enrichment in 13C relative to Phanerozoic or earlier Proterozoic samples. Isotopic compositions of isolated samples from other localities are consistent with a worldwide extended interval of enhanced organic burial and consequent net survival of oxidized material, probably O2, just before the initial radiation of metazoans.

  14. A sulfur isotope study of pyrite genesis: The mid-proterozoic Newland formation, belt supergroup, Montana

    NASA Astrophysics Data System (ADS)

    Strauss, Harald; Schieber, Jürgen

    1990-01-01

    Different generations of sedimentary pyrite from the Mid-Proterozoic Newland Formation, USA, have been analysed for their sulfur isotopic compositions. The results indicate bacterial sulfate reduction as the pyrite forming process. The δ 34S values for early diagenetic pyrite, around -14%., are in contrast to dominantly more positive values for many other Middle Proterozoic units. A progressive reduction of sulfate availability during diagenesis can be recognized by an increase in 34S content (Rayleigh Distillation) as well as through detailed petrographic observations. Contemporaneous seawater had a sulfur isotopic ratio between +14 and +18%. as measured from sedimentary barite within the unit.

  15. Latest Proterozoic to early Cambrian sedimentary-tectonic evolution of a passive margin sequence, northeastern Washington

    SciTech Connect

    Lindsey, K.A.; Gaylord, D.R.

    1987-08-01

    The late Proterozoic to Early Cambrian Three Sisters formation, Addy Quartzite, and Gypsy Quartzite lie near the base of the Cordilleran miogeocline in northeastern Washington. Detailed stratigraphic and sedimentary examination of these units extends understanding of the evolution of western North America. These units were deposited on a newly rifted passive margin and record the final stages of late Proterozoic rifting and the early stages of subsequent early Paleozoic subsidence and transgression. The three Sisters formation, Addy Quartzite, and Gypsy Quartzite are correlative with the Brigham Group in southeastern Idaho and Utah, the Gold Creek Quartzite in northern Idaho, and the Flathead Quartzite in Montana and Wyoming.

  16. Petrogenetic significance of minor elements in olivines from diamonds and peridotite xenoliths from kimberlites of Yakutia

    NASA Astrophysics Data System (ADS)

    Sobolev, N. V.; Logvinova, A. M.; Zedgenizov, D. A.; Pokhilenko, N. P.; Malygina, E. V.; Kuzmin, D. V.; Sobolev, A. V.

    2009-11-01

    Peridotite xenoliths and diamonds from kimberlites represent an important source of information about the composition of the continental lithosphere at depths exceeding 120-150 km. Ultramafic (or peridotitic) U(P)-type of geological environment is dominating at these depths compared to eclogitic (E-type). Olivine is the most typical mineral both of peridotite xenoliths and as diamond inclusions in most kimberlites worldwide. In spite of its simple chemical composition it contains a number of petrogenetically significant minor elements such as Ti, Al, Mn, Ca, Cr, Ni, Co in low concentrations, mostly below 0.1 wt.% of oxide except of NiO. More than 500 industrial quality diamonds of size range between 0.8 and 3 mm containing olivine inclusions sometimes associated with enstatite, pyrope, chrome diopside and chromite were selected from current diamond production of nine major Siberian diamond mines. This collection also includes revised olivine diamond inclusions from Arkhangelsk (Russia), Majhgawan (India) diamond mines and Urals (Russia) alluvial mines. More than 30% of studied samples were prepared for analysis of olivines on a single polished surface with diamond. More than 300 peridotite xenoliths were selected for olivine studies from a representative collection from unaltered kimberlites of Udachnaya diamond mine. These xenoliths include low-temperature coarse lherzolites, harzburgites and dunites which are represented by spinel, garnet-spinel and garnet varieties. More than 70 xenoliths of high-temperature porphyroclastic (sheared) lherzolites from Udachnaya are also included in the examined collection. Olivines were analyzed for major and minor elements with a JEOL JXA 8200 electron microprobe at the Max-Planck Institute of Chemistry, Mainz. Special efforts were made to obtain high precision and accuracy in electron microprobe analyses, especially, for Ti, Al, Ni, Co, Ca, Mn and Cr. These elements were analyzed by using long counting time and high beam

  17. CONTINUITY BETWEEN EASTERN AND WESTERN BUSHVELD COMPLEX, SOUTH AFRICA, CONFIRMED BY XENOLITHS FROM KIMBERLITE

    NASA Astrophysics Data System (ADS)

    Ashwal, L. D.; Webb, S. J.; Cawthorn, G.

    2009-12-01

    The eastern and western limbs of the Bushveld Complex, South Africa have been interpreted by some as dipping sheets of mafic cumulate rocks ever since the modeling of gravity and geoelectrical data by Meyer & de Beer (Nature 325, 610, 1987). However, re-interpretation of the regional gravity data by Cawthorn & Webb (Tectonophys. 330, 195, 2001), with consideration of isostatic crustal flexure and depressing of the Moho, allows Bushveld to be modeled as a single, connected body. This is consistent with the anomalously thick sub-Bushveld crust (to 48 km) as revealed by seismic data (Nguuri et al, Geophys. Res. Lett 28, 2501, 2001). Here we provide direct evidence from xenoliths in kimberlite for a regionally interconnected Bushveld Complex, implying its emplacement as a single sill-like body ~400 km across and ~8 km thick. The Cretaceous Palmietgat Group 1 kimberlites, located mid-way between the eastern and western lobes, about 70 km N of Pretoria, form a cluster of 6 pipes linked by dikes, over a distance of ~3 km. The K15W pipe is actively mined for diamonds. We recovered 5 small (~4 cm across) xenoliths of pyroxenitic rocks in the waste pile representing the high density portion of crushed material that was rejected for diamond recovery by a Sortex apparatus. The xenoliths are medium-grained orthopyroxene cumulates (80-90% opx) with interstitial zoned plagioclase (8-15%), clinopyroxene (<1-2%) and phlogopite. One sample contains ~2% of small, euhedral chromite grains. Orthopyroxenes have mg# = 75-80, with Al2O3 = 0.4-0.6 wt%, Cr2O3 = 0.4-0.5 wt% and TiO2 = 0.19-0.22 wt% and clinopyroxenes have mg# = 82-85. Disseminated chromites show a limited compositional range, with average Fe3+:Al:Cr = 0.55:0.25:1.03 and mg# = 12.6. These compositions match well with Bushveld Complex cumulate rocks, particularly with those from the Upper Critical Zone, which shows orthopyroxenes with mg# = 78-82, and similar Ti, Al and Cr concentrations to those measured here. Likewise, Upper

  18. The olivine macrocryst problem: New insights from minor and trace element compositions of olivine from Lac de Gras kimberlites, Canada

    NASA Astrophysics Data System (ADS)

    Bussweiler, Yannick; Foley, Stephen F.; Prelević, Dejan; Jacob, Dorrit E.

    2015-04-01

    This study presents detailed petrographical and geochemical investigations on remarkably fresh olivines in kimberlites from the EKATI Diamond Mine™ located in the Tertiary/Cretaceous Lac de Gras kimberlite field within the Slave craton of Canada. Olivine, constituting about 42 vol.% of the analyzed samples, can be divided into two textural groups: (i) macrocrystic olivines, > 100 μm sub-rounded crystals and (ii) groundmass olivines, < 100 μm subhedral crystals. Olivines from both populations define two distinct chemical trends; a "mantle trend" with angular cores, showing low Ca (< 0.1 wt.% CaO) and high Ni (0.3-0.4 wt.% NiO) at varying Mg# (0.86-0.93), contrasts with a "melt trend" typified by thin (< 100 μm) rims with increasing Ca (up to 1.0 wt.% CaO) and decreasing Ni (down to 0.1 wt.% NiO) contents at constant Mg# (~ 0.915). These findings are in agreement with recent studies suggesting that virtually all olivine is composed of xenocrystic (i.e. mantle-related) cores with phenocrystic (i.e. melt-related) overgrowths, thereby challenging the traditional view that the origin of kimberlitic olivine can be distinguished based on size and morphology. The two main trends can be further resolved into sub-groups refining the crystallization history of olivine; the mantle trend indicates a multi-source origin that samples the layered lithosphere below the Slave craton, whereas the melt trend represents multi-stage crystallization comprising a differentiation trend starting at mantle conditions and a second trend controlled by the crystallization of additional phases (e.g. chromite) and changing magma conditions (e.g. oxidation). These trends are also seen in the concentrations of trace elements not routinely measured in olivine (e.g. Na, P, Ti, Co, Sc, Zr). Trace element mapping with LA-ICP-MS reveals the distribution of these elements within olivine grains. The trace element distribution between the two trends appears to be consistent with phenocrystic olivine

  19. Syngenetic inclusions of yimengite in diamond from Sese kimberlite (Zimbabwe) — evidence for metasomatic conditions of growth

    NASA Astrophysics Data System (ADS)

    Bulanova, G. P.; Muchemwa, E.; Pearson, D. G.; Griffin, B. J.; Kelley, S. P.; Klemme, S.; Smith, C. B.

    2004-09-01

    Syngenetic inclusions of yimengite K (Cr, Ti, Mg, Fe, Al) 12O 19, a potassium member of the magnetoplumbite mineral group, have been recorded in an octahedral macrodiamond from the Sese kimberlite (50 km south of Masvingo, Zimbabwe). One yimengite inclusion carries lamellae of chromite suggesting peridotitic diamond paragenesis. The diamond and inclusions were studied in situ in a plate polished parallel to (011). Cathodoluminescence (CL) imaging has shown blue colour and octahedral zonation of the diamond, lack of cracks and the location of five yimengites in different growth zones. Nitrogen (N) contents (at. ppm) in the diamond determined by Fourier transform infrared spectroscopy (FTIR) steadily decrease from 576 (core) to 146 (rim). N aggregation (%1aB) is correspondingly 40% in the core and 30% in the rim. Hydrogen (H) content is high in the core, moderate in the intermediate and very high in the rim zones. Four yimengites were dated using the laser 40Ar/ 39Ar method. Three inclusions yielded total gas ages that agree with, or are younger than, or within error of, the Sese kimberlite eruption age (538±11 Ma) but may be compromised by gas loss. One inclusion, with the highest tapped interface gas yield, gave a total gas age of 892±21 Ma that is a likely minimum yimengite age. Time-T °C constraints from N aggregation systematics give a range of possible ages from kimberlite eruption date back to Archean and do not resolve the variable results of the 40Ar/ 39Ar dating. Compared with the published chemistry of yimengite from kimberlites, inclusions from the Sese diamond contain higher Al, Mg, and Sr and have lower concentration of Fe 3+. The chondrite-normalised REE pattern of the yimengite shows enrichment in LREE and depletion in HREE, but LREE/HREE fractionations are lower than for lindsleyite-mathiasite series mantle titanates and rather similar to the REE concentrations in kimberlite and lamproite rocks. It is suggested that Sese yimengite formed in the

  20. Paleoclimatology of the Early Paleogene using Kimberlite-Hosted Mummified Wood from the Canadian Arctic

    NASA Astrophysics Data System (ADS)

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

    2011-12-01

    Fifty-five million years ago, during the Paleocene-Eocene transition, average global temperatures were 4 - 7°C warmer than today and tropical forest ecosystems persisted in Arctic regions for millennia (ref. 1). Superimposed on this warmth were several hyperthermal periods of intense warming (10 - 12°C above modern-day average), such as the Paleocene-Eocene Thermal Maximum (PETM; 55.5 Ma), caused by an influx of isotopically-light carbon of unknown origin into the atmosphere (ref. 2). During this time, massive kimberlite eruptions buried forest fragments under pyroclastic debris. Evidence can be found in several diamondiferous kimberlite mines in the Northwest Territories of Canada, aged at 56.0 ±0.7 Ma (Diavik Mine) and 53.2 ±0.3 Ma (Ekati Mine) (ref. 3). Due to these unique burial conditions, the original woody material is intact (i.e. not petrified) allowing paleoclimatic analyses from wood growing before and after the PETM. Morelet Wavelet Analysis detected a significant 4 - 7 year periodicity in tree-ring width suggesting Cenozoic El Niño climate cycles were similar to those existing today, thus validating paleoclimatic models (ref. 4). In addition, evidence of wood-boring insect galleries within samples suggests the existence of a rich ecosystem above the Arctic Circle. Our multi-proxy study of δ13C, δ18O, and δD isotope ratios in α-cellulose at sub-annual scales will permit a better understanding of seasonal and yearly trends in Early Paleogene temperature and precipitation. It will also allow comparisons with studies of Eocene-aged wood (45 Ma) suggesting Arctic regions were 10 - 12°C warmer than modern-day temperatures, with relative humidity reaching 90 - 100% by the end of the growing season (refs. 5,6). 1. Zachos, J, M Pagani, L Sloan, E Thomas, and K Billups. 2001. Science 292(5517): 686 - 693. 2. Higgins, JA, and DP Schrag. 2006. Earth and Planetary Science Letters 245: 523-537. 3. Creaser, RA, H Grütter, J Carlson, and B Crawford. 2004

  1. Stress modelling of magma storage zones and its implications for rapid kimberlitic magma ascent

    NASA Astrophysics Data System (ADS)

    Baruah, A.; Mandal, N.

    2012-12-01

    Rapid ascent of low viscous kimberlitic magmas is reflected from the presence of meta-stable diamond phenocrysts. Existing models suggest that high velocity magma ascent takes place as a mechanical coupling interaction between the CO2-rich volatile phase originating from the magma and the hydraulic fracture (Type-I). However, for such fracturing to occur at a depth of ~200 km, the system need to have a huge tensile stress to overcome the lithostatic pressure (~60 Kb) and the tensile strength of the rocks (0.4 - 0.5 Kb). The objective of the present work is to present a mechanical model and show the specific conditions in which the magma storage zone (MSZ) can build up such large tensile stresses to cause fracturing for magma ascent. Finite Element (FE) method was employed to map the stress field in the mantle rock around a magma chamber. MSZ was modeled as a semi-elliptical zone at bottom of the model of 150 km depth and 300 km width. Two types of FE modelling was performed considering two factors: (1) density contrast (Δρ) between magma and ambient mantle, and (2) shape (Ar: ratio of vertical and horizontal dimensions) of the MSZ. Figure 1 show the Δρ contrasts required for tensile fracturing to occur at the MSZ tip for different values of their Ar. Results reveal a distinct zone of maximum tensile stresses in the neighborhood of the MSZ, suggesting the potential locations of tensile fracturing. It shows that the tensile stress magnitude decreases exponentially away from the MSZ top vertically. The results illustrate a nonlinear relation of stress with increasing Δρ (Figure 1). We show that for models with Ar >1 there is a localization of tensile stress at the MSZ tip, and for the models with Ar << 1 it diffuse along the boundary (Figure 2). We also show that for a particular Δρ, tensile stress increases for increasing Ar. The results indicate that MSZ with large Ar are more potential for tensile fracturing to occur at their vertices. Considering the

  2. Proterozoic Stability of the Kaapvaal Craton from Titanite (U-Th)/He Thermochronology and Strong Influence of Radiation Damage on this Underutilized Thermochronometer

    NASA Astrophysics Data System (ADS)

    Baughman, J. S.; Flowers, R. M.; Dhansay, T.

    2014-12-01

    The Kaapvaal craton of southern Africa is an archetypal Archean craton that formed and initially stabilized between 3.7 and 2.7 Ga. Geochronology and isotopic studies have constrained periods of lithospheric growth and stabilization, and low temperature thermochronology has yielded information about the Mesozoic-Cenozoic history of burial and erosion across the craton. However, there is a substantial thermal history gap between these end-member events, because few thermochronometers provide access to temperatures of ~300-120°C. Such data are critical for evaluating Kaapvaal's response to Proterozoic lateral accretion and intracontinental magmatism. Our study assesses cratonic stability by applying a little-utilized but promising mid-temperature thermochronologic technique, titanite (U-Th)/He dating, to decipher cooling through 230-190°C (based on early diffusion studies). We obtained titanite (U-Th)/He data for Archean basement samples across an ~200,000 km2 area of the northern Kaapvaal craton. Multiple samples with titanite eU values < 70 ppm yield He dates as old as 1200-800 Ma. In contrast, titanites with eU of 70-700 ppm yield younger dates (350-20 Ma) that display a dramatic correlation between date and eU. This pattern clearly manifests the influence of radiation damage on titanite He retentivity that has been observed in other He thermochronometers, but never previously documented for titanite. There is strong future potential to exploit this effect to decipher more detailed thermal histories, as has been done for apatite and zircon He thermochronometry. In our dataset, the oldest titanite results postdate extensive ~1.4-1.2 Ga carbonatite and kimberlite magmatism across the Kaapvaal craton, and overlap with ~1.2-1.1 Ga Namaqua-Natal arc accretion and ~1.1 Ga Umkondo intraplate large igneous province activity. The volcanic character of many of the northern Kaapvaal alkaline and carbonatite complexes indicates that the basement was exhumed to the surface

  3. In situ U-Pb age determination and Nd isotopic analysis of perovskites from kimberlites in southern Africa and Somerset Island, Canada

    NASA Astrophysics Data System (ADS)

    Wu, Fu-Yuan; Yang, Yue-Heng; Mitchell, Roger H.; Li, Qiu-Li; Yang, Jin-Hui; Zhang, Yan-Bin

    2010-03-01

    Determination of the emplacement ages and initial isotopic composition of kimberlite by conventional isotopic methods using bulk rock samples is unreliable as these rocks usually contain diverse clasts of crustal- and mantle-derived materials and can be subject to post-intrusion sub-aerial alteration. In this study, 8 samples from 5 kimberlites in southern Africa and twelve samples from 7 kimberlites from Somerset Island, Canada have been selected for in situ perovskite U-Pb isotopic age determination and Nd isotopic analysis by laser ablation using thin sections and mineral separates. These fresh perovskites occur as primary groundmass minerals with grain-sizes of 10-100 μm. They were formed during the early stage of magmatic crystallization, and record data for the least contaminated or contamination-free kimberlitic magma. U-Pb isotopic data indicate that the majority of the southern Africa kimberlites investigated were emplaced during the Cretaceous with ages of 88 ± 3 to 97 ± 6 Ma, although one sample yielded an Early Paleozoic age of 515 ± 6 Ma. Twelve samples from Somerset Island yielded ages ranging from 93 ± 4 Ma to 108 ± 5 Ma and are contemporaneous with other Cretaceous kimberlite magmatism in central Canada (103-94 Ma). Although whole-rock compositions of the kimberlites from southern Africa have a large range of ɛNd( t) values (- 0.5 to + 5.1), the analysed perovskites show a more limited range of + 1.2 to + 3.1. Perovskites from Somerset Island have ɛNd( t) values of - 0.2 to + 1.4. These values are lower than that of depleted asthenospheric mantle, suggesting that kimberlites might be derived from the lower mantle. This study shows that in situ U-Pb and Nd isotopic analysis of perovskite by laser ablation is both rapid and economic, and serves as a powerful tool for the determination of the emplacement age and potential source of kimberlite magmas.

  4. Geochemistry of hypabyssal kimberlites from Lac de Gras, Canada: Comparisons to a global database and applications to the parent magma problem

    NASA Astrophysics Data System (ADS)

    Kjarsgaard, B. A.; Pearson, D. G.; Tappe, S.; Nowell, G. M.; Dowall, D. P.

    2009-11-01

    We present 104 whole-rock geochemical analyses of hypabyssal kimberlite from the Lac de Gras field. Screens using Yb versus Al 2O 3 and ln Si/Al versus ln Mg/Yb effectively discriminate crustally contaminated samples. The remaining "non-contaminated" kimberlites samples have variable (5 to 50%) entrainment of cratonic peridotite. It is problematic to effectively screen for small amounts (< 5%) of digested crust in samples with higher (> 20%) contents of peridotite contamination. We utilize the Lac de Gras data suite to calculate, by two different methods, parent magma compositions and identify two (and potentially three) geochemically distinct parent magma types. The Lac de Gras parent magma compositions are compared to those calculated from other localities in Canada, Greenland, South Africa and Russia. Together, these calculated parent magmas define a range, albeit limited, of viable, yet distinct, kimberlite parent magma compositions. Geochemically, kimberlite parent magmas have high volatile contents (H 2O and CO 2), high MgO, and low SiO 2, Al 2O 3 and alkalis, with K > Na and Na + K/Al < 1. It is difficult to reconcile differences between various calculated kimberlite parent magma compositions from different cratonic areas as merely due to the effects of craton specific lithospheric mantle contamination, indicating the intra- and inter-cratonic variation of parent magma compositions reflect differing source region characteristics and/or partial melting regimes.

  5. The origin of Triassic/Jurassic kimberlite magmatism, Canada: Two mantle sources revealed from the Sr-Nd isotopic composition of groundmass perovskite

    NASA Astrophysics Data System (ADS)

    Zurevinski, S. E.; Heaman, L. M.; Creaser, R. A.

    2011-09-01

    The crystallization ages and the Sr and Nd isotopic compositions of groundmass perovskite from a well-established, SE trending, Triassic-Jurassic corridor of kimberlite magmatism in central and eastern North America were determined to investigate the origin of this magmatism. The results obtained from kimberlite fields located along this corridor are interpreted to indicate that at least two distinct mantle sources contributed to this magmatism. The most primitive Rankin Inlet and Timiskaming kimberlites have a relatively unradiogenic strontium isotopic signature (0.7032-0.7036), interpreted to be derived from recycled and metasomatized oceanic lithosphere in the deep mantle. In contrast, the Attawapiskat and Kirkland Lake kimberlites have CHUR-like (Chondritic Uniform Reservoir) signatures (0.7040-0.7042) interpreted to have an origin in the asthenosphere. The progressive decrease in the age of magmatism from the Triassic Rankin Inlet kimberlites to the Miocene Great Meteor seamount, combined with the similarity in the isotopic composition of these diverse magmas along the proposed >3000 km long hot spot track, provides strong evidence in support of a common mantle plume origin for both the continental and oceanic components.

  6. 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

  7. 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.

  8. 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.

  9. 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.

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

    PubMed

    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.

  11. 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.

  12. Lemhi Arch, a late Proterozoic and early Paleozoic landmass, central Idaho

    SciTech Connect

    Ruppel, E.T.

    1985-05-01

    The northwest-trending Lemhi arch of central Idaho first formed in late middle Proterozoic time, and as much as 4500 m (14,760 ft) of middle Proterozoic clastic rocks were eroded in later proterozoic time. The west flank of the arch was partly covered in late Proterozoic(.) and Early Cambrian time by the Wilbert Formation. On the east flank, westward-thinning marine sedimentation began with deposition of the Middle Cambrian Flathead Formation, and continued through the Late Cambrian. During Ordovician and Silurian times, the east flank of the arch was dry. The west flank was submerged in the Ordovician, and the Summerhouse Formation, Kinnikinnic Quartzite, and Saturday Mountain formation were deposited. The west flank of the arch was briefly exposed after deposition of the Saturday Mountain Formation, but was partly submerged later in the Silurian, when the Laketwon Dolomite was deposited. During the Middle and Late Devonian, deposition was renewed on the west flank of the arch, where the Jefferson formation indicates eastward transgression. The east flank was exposed until the late Devonian, when a thin sequence of the Jefferson and Three Forks Formations was deposited across the top of the arch, and marine sedimentation was continuous from the miogeocline far onto the craton. The Lemhi arch continued to influence marine deposition even after it was submerged, separating shelf deposits in southwest Montana and eastcentral Idaho from miogeoclinal deposits in central Idaho. The arch was overridden by the Medicine Lodge thrust in late Early and Late Cretaceous times.

  13. Continent-scale linearity of kimberlite-carbonatite magmatism, mid-continent North America

    NASA Astrophysics Data System (ADS)

    Duke, Genet Ide; Carlson, Richard W.; Frost, Carol D.; Hearn, B. C.; Eby, G. Nelson

    2014-10-01

    Cretaceous-Tertiary kimberlite-carbonatite magmatism in mid-continent North America extends along a N40°W linear trend from Louisiana to Alberta, and occurs in at least four different pulses (∼109-85, 67-64, 55-52, and less than 50 Ma). The lack of spatial age progressions of magmatism consistent with motion of North America over a fixed hot spot, the presence of Sr-Nd-Hf-Pb isotopic and trace-element compositions that show a temporal evolution from lithospheric to asthenospheric melt-sources, and the orientation of the magmatic belt parallel to the western subduction margin of the North American plate, suggest that this linear zone is the surface expression of mantle melting related to the subduction system. We propose that fragmentation of Farallon and Kula plates opened slab windows perpendicular to their convergence direction. In this model, sheet-like mantle upwellings were induced along slab-window margins, and these upwellings underwent low-degree partial melting to produce highly alkalic magmas along the trend parallel to, but ∼2000 km east of, the convergent margin. The N40°W trend may reflect melting associated with penetration of the mantle transition-zone by the downgoing oceanic plate(s).

  14. Perspectives on Proterozoic surface ocean redox from iodine contents in ancient and recent carbonate

    NASA Astrophysics Data System (ADS)

    Hardisty, Dalton S.; Lu, Zunli; Bekker, Andrey; Diamond, Charles W.; Gill, Benjamin C.; Jiang, Ganqing; Kah, Linda C.; Knoll, Andrew H.; Loyd, Sean J.; Osburn, Magdalena R.; Planavsky, Noah J.; Wang, Chunjiang; Zhou, Xiaoli; Lyons, Timothy W.

    2017-04-01

    The Proterozoic Eon hosted the emergence and initial recorded diversification of eukaryotes. Oxygen levels in the shallow marine settings critical to these events were lower than today's, although how much lower is debated. Here, we use concentrations of iodate (the oxidized iodine species) in shallow-marine limestones and dolostones to generate the first comprehensive record of Proterozoic near-surface marine redox conditions. The iodine proxy is sensitive to both local oxygen availability and the relative proximity to anoxic waters. To assess the validity of our approach, Neogene-Quaternary carbonates are used to demonstrate that diagenesis most often decreases and is unlikely to increase carbonate-iodine contents. Despite the potential for diagenetic loss, maximum Proterozoic carbonate iodine levels are elevated relative to those of the Archean, particularly during the Lomagundi and Shuram carbon isotope excursions of the Paleo- and Neoproterozoic, respectively. For the Shuram anomaly, comparisons to Neogene-Quaternary carbonates suggest that diagenesis is not responsible for the observed iodine trends. The baseline low iodine levels in Proterozoic carbonates, relative to the Phanerozoic, are linked to a shallow oxic-anoxic interface. Oxygen concentrations in surface waters would have at least intermittently been above the threshold required to support eukaryotes. However, the diagnostically low iodine data from mid-Proterozoic shallow-water carbonates, relative to those of the bracketing time intervals, are consistent with a dynamic chemocline and anoxic waters that would have episodically mixed upward and laterally into the shallow oceans. This redox instability may have challenged early eukaryotic diversification and expansion, creating an evolutionary landscape unfavorable for the emergence of animals.

  15. 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

  16. 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

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

    NASA Astrophysics Data System (ADS)

    Heigold, Paul 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.

  18. 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.

  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. 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

  1. Nature of the mantle roots beneath the North American craton: mantle xenolith evidence from Somerset Island kimberlites

    NASA Astrophysics Data System (ADS)

    Schmidberger, S. S.; Francis, D.

    1999-09-01

    The recently discovered Nikos kimberlite on Somerset Island, in the Canadian Arctic, hosts an unusually well preserved suite of mantle xenoliths dominated by garnet-peridotite (lherzolite, harzburgite, dunite) showing coarse and porphyroclastic textures, with minor garnet-pyroxenite. The whole rock and mineral data for 54 Nikos xenoliths indicate a highly refractory underlying mantle with high olivine forsterite contents (ave. Fo=92.3) and moderate to high olivine abundances (ave. 80 wt.%). These characteristics are similar to those reported for peridotites from the Archean Kaapvaal and Siberian cratons (ave. olivine Fo=92.5), but are clearly distinct from the trend defined by oceanic peridotites and mantle xenoliths in alkaline basalts and kimberlites from post-Archean continental terranes (ave. olivine Fo=91.0). The Nikos xenoliths yield pressures and temperatures of last equilibration between 20 and 55 kb and 650 and 1300°C, and a number of the peridotite nodules appear to have equilibrated in the diamond stability field. The pressure and temperature data define a conductive paleogeotherm corresponding to a surface heat flow of 44 mW/m 2. Paleogeotherms based on xenolith data from the central Slave province of the Canadian craton require a lower surface heat flow (˜40 mW/m 2) indicating a cooler geothermal regime than that beneath the Canadian Arctic. A large number of kimberlite-hosted peridotites from the Kaapvaal craton in South Africa and parts of the Siberian craton are characterized by high orthopyroxene contents (ave. Kaapvaal 32 wt.%, Siberia 20 wt.%). The calculated modal mineral assemblages for the Nikos peridotites show moderate to low contents of orthopyroxene (ave. 12 wt.%), indicating that the orthopyroxene-rich mineralogy characteristic of the Kaapvaal and Siberian cratons is not a feature of the cratonic upper mantle beneath Somerset Island.

  2. Archean Lithosphere Beneath Arctic Canada: Lu-Hf Isotope Systematics for Kimberlite-Hosted Garnet-Peridotites From Somerset Island

    NASA Astrophysics Data System (ADS)

    Schmidberger, S. S.; Simonetti, A.; Francis, D.; Gariepy, C.

    2001-05-01

    Knowledge of the age of lithospheric mantle underlying the continents provides valuable constraints for the timing of formation and stabilization of Archean cratons. This study reports Lu-Hf isotopic data for garnet-peridotites, and their constituent garnets, from the Nikos kimberlite (100 Ma) on Somerset Island in the Canadian Arctic obtained using a Micromass IsoProbe multicollector inductively coupled plasma mass spectrometer (MC-ICP-MS) at GEOTOP-UQAM. The low temperature peridotites (<1100 C; 80-150 km) exhibit a significant range in 176Lu/177Hf ratios (0.03-0.05) and are characterized by radiogenic 176Hf/177Hf(0.1Ga) isotopic compositions (0.28294-0.28419) corresponding to \\epsilonHf(0.1Ga) values of +8 to +52. In contrast, 176Lu/177Hf ratios (0.004-0.04) are lower for the high temperature peridotites (>1100 C; 160-190 km) and their 176Hf/177Hf(0.1Ga) isotopic compositions (0.28265-0.28333; \\epsilonHf(0.1Ga)=-2 to +22) are less radiogenic than those of the shallow xenoliths. A Lu-Hf isochron for six peridotites yields a mid Archean age of 3.4\\pm0.3 Ga and an initial 176Hf/177Hf ratio of 0.28101\\pm24. The remaining peridotites (n=9), in contrast, are characterized by extremely high (+35) initial \\epsilonHf(3.4Ga) values, which correlate negatively with their 176Lu/177Hf ratios, suggesting addition of Hf as a result of metasomatic interaction with the host kimberlite. The garnets from the low temperature (3.4 Ga old) peridotites are characterized by high 176Lu/177Hf ratios and define an errorchron age of 1.4\\pm0.2 Ga, which may reflect re-equilibration of Hf during kimberlite magmatism.

  3. 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

  4. 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.

  5. 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

  6. 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.

  7. 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.

  8. A Methane-rich Proterozoic Atmosphere: Possible Link to the Neoproterozoic Snowball Earth Glaciations

    NASA Astrophysics Data System (ADS)

    Pavlov, A. A.; Kasting, J. F.; Hurtgen, M.; Arthur, M. A.

    2001-12-01

    An enhanced atmospheric greenhouse effect is required throughout Archean and Proterozoic to offset reduced solar luminosity. In the anoxic Archean atmosphere CH4 could have been an important greenhouse gas because of the decreased levels of the primary oxidants - OH, O and H2O2. However, after the major transition of the atmospheric oxidation state at 2.0-2.3 Gyr, the photochemical lifetimes of reduced atmospheric gases (like methane) should have been much shorter. Therefore, a common view of the Proterozoic climate suggests that CO2 was the major greenhouse gas (along with H2O) and that atmospheric CH4 concentrations were low. Here we argue that substantial methane levels could have been present in the Proterozoic atmosphere if O2 levels were somewhat lower than today. In agreement with earlier calculations, our 1-D photochemical model shows that the atmospheric methane mixing ratio is a highly nonlinear function of the surface methane flux. In our model, a factor of 10 increase in the methane flux results in a 60-fold increase of the surface methane concentration. 1-D climate calculations show that such a high methane abundance would keep the mean global surface temperature at ~296 K under reduced solar luminosity conditions ( ~17 % decreased solar luminosity at 2.3 Gyr ago), even if CO2 was present only at today's level. Here we propose several reasons why the net methane flux could have been indeed substantially higher in the Proterozoic, compared to the present day. In the modern ecosystem, 99.9 % of methane, produced by methanogens, is being consumed by methanotrophic bacteria. These bacteria would presumably consume much less methane if O2 levels were lower. Moreover, in the present day sulfate-rich ocean methanogens living in sediments are outcompeted by sulfate reducers and forced to live in the nutrient-poor environments. Methane is also consumed in marine sediments by anaerobic methanotrophs living in consortium with sulfate reducing bacteria. In an

  9. 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.

  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. High CO2 levels in the Proterozoic atmosphere estimated from analyses of individual microfossils.

    PubMed

    Kaufman, Alan J; Xiao, Shuhai

    2003-09-18

    Solar luminosity on the early Earth was significantly lower than today. Therefore, solar luminosity models suggest that, in the atmosphere of the early Earth, the concentration of greenhouse gases such as carbon dioxide and methane must have been much higher. However, empirical estimates of Proterozoic levels of atmospheric carbon dioxide concentrations have not hitherto been available. Here we present ion microprobe analyses of the carbon isotopes in individual organic-walled microfossils extracted from a Proterozoic ( approximately 1.4-gigayear-old) shale in North China. Calculated magnitudes of the carbon isotope fractionation in these large, morphologically complex microfossils suggest elevated levels of carbon dioxide in the ancient atmosphere--between 10 and 200 times the present atmospheric level. Our results indicate that carbon dioxide was an important greenhouse gas during periods of lower solar luminosity, probably dominating over methane after the atmosphere and hydrosphere became pervasively oxygenated between 2 and 2.2 gigayears ago.

  12. Isotopic evidence for early Proterozoic age of the Idono Complex, west-central Alaska

    USGS Publications Warehouse

    Miller, M.L.; Bradshaw, J.Y.; Kimbrough, D.L.; Stern, T.W.; Bundtzen, T.K.

    1991-01-01

    The Idono Complex is a fault-bounded, ~300 km2 fragment of Early Proterozoic continental crust surrounded by overlap assemblages and younger terranes accreted in Mesozoic time. It is composed of granitic to dioritic orthogneiss, amphibolite, and metasedimentary rocks. Trace element compositions of the granitoids and amphibolite suggest rock formation in a subduction-related volcanic arc terrain. Nine zircon fractions from three samples of granitoid orthogneiss define a U-Pb discordia line intersecting concordia at 2062 ?? 7 Ma and 182 ?? 8 Ma. The upper and lower intercepts are interpreted, respectively, as approximations of the time of granitoid crystallization and major episodic Pb-loss. Rocks of the Idono Complex may provide important insights into both Early Proterozoic evolution along the craton margin, and subsequent displacements. -from Authors

  13. Nature of type IaB diamonds from the Mir kimberlite pipe (Yakutia): evidence from spectroscopic observation

    NASA Astrophysics Data System (ADS)

    Yuryeva, Olga P.; Rakhmanova, Mariana I.; Zedgenizov, Dmitry A.

    2017-04-01

    In this study, the specific features of structural defects of type IaB diamonds from the Mir kimberlite pipe (Yakutian diamondiferous province) have been characterized using FTIR and photoluminescence spectroscopy. Mineral inclusions in these diamonds [olivine (Ol), orthopyroxene (OPx), chromite (Chr), sulphide (Sf)] correspond to associations of peridotite rocks at the base of the lithosphere. Nitrogen content in type IaB diamonds shows significant variations, suggesting different growth media and/or several growth stages. A specific feature of these diamonds is the absence or very small amount of platelets, which may be related to annealing during their long-term residence at the temperatures of the base of the lithosphere. All studied diamonds show the presence of hydrogen defects that are active in IR spectra with an intense line at 3107 cm-1, and additional weaker lines at 3085 and 3237 cm-1, which correlated with high nitrogen content. Type IaB diamonds are also characterized by the presence of nitrogen-nickel luminescence centres S2, S3 and 523.2 nm. This feature distinguishes them from superdeep diamonds with extreme nitrogen aggregation states, which clearly attest to different growth conditions and crystallization media of type IaB diamonds from the Mir kimberlite pipe.

  14. 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.

  15. Restoration of Laramide right-lateral strike slip in northern New Mexico by using Proterozoic piercing points: Tectonic implications from the Proterozoic to the Cenozoic

    NASA Astrophysics Data System (ADS)

    Karlstrom, K. E.; Daniel, C. G.

    1993-12-01

    Early Proterozoic rocks in northern New Mexico can be used to document 100-170 km of right-lateral slip along a network of north-striking Laramide (Late Cretaceous-early Tertiary) faults that formed the precursor of the Rio Grande rift. Piercing lines are defined by the intersection of a subborizontal 4 kbar isobaric surface with steeply dipping stratigraphic markers and regional structures. Restoration of slip provides new insight into the nature of Proterozoic orogenic and crustal province boundaries. The Yavapai crustal province (1.75-1.72 Ga juvenile volcanogenic rocks) is imbricated with overlying Hondo Group sedimentary cover in an originally east-trending ductile thrust belt that formed near the southern margin of the Yavapai province. A series of offset magnetic anomalies (Jemez lineament) is interpreted to mark the thrust-buried boundary between older (Yavapai province) and younger (Mazatzal province) crust. Our microplate tectonic model suggests that Laramide strike-slip movement (New Mexico) was kinematically linked with coeval extensional collapse (Arizona) and foreland shortening (Wyoming) during decoupling and northward movement of the rigid Colorado Plateau relative to the undeformed midcontinent. Crustal-scale strike-slip faults in New Mexico were reactivated during later Tertiary extension and continue to localize high heat flow in the Rio Grande rift and northward into Colorado.

  16. Sensitivity of biomarkers to changes in chemical emissions in the Earth’s Proterozoic atmosphere

    NASA Astrophysics Data System (ADS)

    Grenfell, J. L.; Gebauer, S.; von Paris, P.; Godolt, M.; Hedelt, P.; Patzer, A. B. C.; Stracke, B.; Rauer, H.

    2011-01-01

    The search for life beyond the Solar System is a major activity in exoplanet science. However, even if an Earth-like planet were to be found, it is unlikely to be at a similar stage of evolution as the modern Earth. It is therefore of interest to investigate the sensitivity of biomarker signals for life as we know it for an Earth-like planet but at earlier stages of evolution. Here, we assess biomarkers, i.e. species almost exclusively associated with life, in present-day and in 10% present atmospheric level oxygen atmospheres corresponding to the Earth's Proterozoic period. We investigate the impact of proposed enhanced microbial emissions of the biomarker nitrous oxide, which photolyses to form nitrogen oxides which can destroy the biomarker ozone. A major result of our work is regardless of the microbial activity producing nitrous oxide in the early anoxic ocean, a certain minimum ozone column can be expected to persist in Proterozoic-type atmospheres due to a stabilising feedback loop between ozone, nitrous oxide and the ultraviolet radiation field. Atmospheric nitrous oxide columns were enhanced by a factor of 51 for the Proterozoic "Canfield ocean" scenario with 100 times increased nitrous oxide surface emissions. In such a scenario nitrous oxide displays prominent spectral features, so may be more important as a biomarker than previously considered in such cases. The run with "Canfield ocean" nitrous oxide emissions enhanced by a factor of 100 also featured additional surface warming of 3.5 K. Our results suggest that the Proterozoic ozone layer mostly survives the changes in composition which implies that it is indeed a good atmospheric biomarker.

  17. 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.

  18. 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. © 2011 Blackwell Publishing Ltd.

  19. Tracking the Archean-Proterozoic suture zone in the northeastern Great Basin, Nevada and Utah

    USGS Publications Warehouse

    Rodriguez, B.D.; Williams, J.M.

    2008-01-01

    It is important to know whether major mining districts in north-central Nevada are underlain by crust of the Archean Wyoming craton, 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 these provinces is also important because it may influence subsequent patterns of sedimentation, deformation, magmatism, and hydrothermal activity. The suture zone is exposed in northeastern Utah and south-western Wyoming and exhibits a southwest strike. In the Great Basin, the suture zone strike is poorly constrained because it is largely concealed below a Neoproterozoic-Paleozoic miogeocline and Cenozoic basin fill. Two-dimensional resistivity modeling of three regional north-south magnetotelluric sounding profiles in western Utah, north-central Nevada, and northeastern Nevada, and one east-west profile in northeastern Nevada, reveals a deeply penetrating (>10 km depth), broad (tens of kilometers) conductor (1-20 ohm-meters) that may be the Archean-Proterozoic suture zone, which formed during Early Proterozoic rifting of the continent and subsequent Proterozoic accretion. This major crustal conductor changes strike direction from southwest in Utah to northwest in eastern Nevada, where it broadens to ???100 km width that correlates with early Paleozoic rifting of the continent. Our results suggest that the major gold belts may be over-isolated blocks of Archean crust, so Phanerozoic mineral deposits in this region may be produced, at least in part, from recycled Archean gold. Future mineral exploration to the east may yield large gold tonnages. ?? 2008 Geological Society of America.

  20. 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.

  1. 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.

  2. 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

  3. 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.

  4. 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.

  5. 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.

  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. 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.

  8. 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.

  9. 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.

  10. 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.

  11. 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.

  12. 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.

  13. Polycrystalline diamond aggregates from the Mir kimberlite pipe, Yakutia: Evidence for mantle metasomatism

    NASA Astrophysics Data System (ADS)

    Sobolev, N. V.; Shatsky, V. S.; Zedgenizov, D. A.; Ragozin, A. L.; Reutsky, V. N.

    2016-11-01

    Polycrystalline diamond aggregates (boart, framesites, diamondites) have been widely studied but their origin is poorly understood. We report the results of a study in situ 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 one of them - for δ13C and N abundance and isotopic composition 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 are 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 variable (- 6.4 ÷- 9.8 ‰) as well as N abundance (75-1150 ppm) and δ15N - 27, - 38, - 58 ‰. 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 element content in pyropes is relatively uniform (ppm): Sr (0.4-1.6), Y (13.2-13.4) and Zr (13.0). We conclude that heterogeneous distribution of the trace elements among garnet grains in Mr 832 and magnesite presence in Mr 838 are indicative of the effects of mantle metasomatism and rapid crystallization shortly before the eruption of the kimberlite.

  14. 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

  15. Petrography and mineral compositions of eclogites from the Koidu Kimberlite Complex, Sierra Leone

    NASA Astrophysics Data System (ADS)

    Fung, Agnes T.; Haggerty, Stephen E.

    1995-10-01

    The origins of eclogite (clinopyroxene + garnet) and the relative proportions of eclogite in the upper mantle are issues of considerable uncertainty and debate that bear upon the chemical and physical dynamics of petrogenesis, recycling, and remote sensing interpretation. Forty-one upper mantle eclogites from the Koidu Kimberlite Complex, Sierra Leone, were selected for detailed petrographic and chemical examination to bolster an earlier database with a view to the identification of protoliths and possible source regions of eclogite origin. On the basis of MgO contents in garnets, eclogites are divided into a high-MgO suite and a low-MgO suite. High-MgO eclogites contain pyrope (16.5-20.2 wt % MgO), with an average garnet composition of Pyr65Alm20Gross15. Almandine and grossular (5.3-13.2 wt % MgO) are present in low-MgO eclogites and vary from Pyr20Alm60Gross20 to Pyr35Alm30Gross35. Pyroxenes in high-MgO eclogites are diopsidic (Jd11Di89-Jd26Di74); those in low-MgO eclogites range from jadeitic-diopside to omphacite (Jd20Di80-Jd48Di52). Oriented apatite crystals in garnet and clinopyroxene are interpreted to be products of exsolution and, coupled with coexisting rutile, imply that garnet is a major repository for P, Cl, F, OH, and Ti in the upper mantle. Reconstructed bulk compositions of high-(15.0-18.9 wt %) and low-MgO (7.1-12.2 wt %) eclogites are distinct, and major elements in these xenoliths broadly resemble basalts, picrites, and komatiites. Most high-MgO eclogites equilibrated at 1080°C at 4.7 GPa to 1130°C at 5.2 GPa, whereas most low-MgO eclogites cluster at 880°C at 3.3 GPa to 930°C at 3.8 GPa. Estimated PT and depths of origin of the Koidu eclogites imply that high-MgO eclogites are asthenospheric, low-MgO eclogites are lithospheric, and both are likely products of plume activity. Diamondiferous eclogites, worldwide, have characteristic Na, K, Ti, and IVAl (Si) in garnet-clinopyroxene pairs that point to distinctive source regions and petrogenesis

  16. 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

  17. 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

  18. Probing Archean lithosphere using the Lu-Hf isotope systematics of peridotite xenoliths from Somerset Island kimberlites, Canada

    NASA Astrophysics Data System (ADS)

    Schmidberger, Stefanie S.; Simonetti, Antonio; Francis, Don; Gariépy, Clément

    2002-04-01

    A knowledge of the Hf isotopic composition of the subcontinental lithosphere beneath Archean cratons is essential to constrain the Hf isotope budget of the Earth's mantle. Hf isotopic measurements were obtained by MC-ICP-MS for a suite of refractory peridotite xenoliths and constituent garnets from the Nikos kimberlite (100 Ma) on Somerset Island in order to constrain the isotopic composition and age of the lithosphere beneath the northern Canadian craton. The low-temperature Nikos peridotites (<1100°C), which represent the shallow Somerset lithosphere, are characterized by higher 176Lu/ 177Hf ratios (0.03-0.05) and Hf isotopic values ( 176Hf/ 177Hf (0.1Ga)=0.28296-0.28419) than the deep-seated high-temperature peridotites (>1100°C; 0.004-0.03, 0.28265-0.28333, respectively). These differences in Hf isotope signatures suggest that shallow and deep subcontinental lithosphere beneath Somerset Island represent isotopically distinct domains and do not share a common petrogenetic history. The Lu-Hf isotope systematics of the shallow low-temperature peridotites define a positively sloped line that plot along a 2.8 Ga reference isochron. A number of these peridotites are characterized by highly radiogenic Hf isotopic compositions suggestive of long-term radiogenic ingrowth (billions of years). These findings are consistent with an interpretation that the shallow Somerset lithosphere (to depths of ˜150 km) stabilized in the Archean. The majority of the high-temperature peridotites plot closer to the composition of the host kimberlite. Although the observed isotopic variation may be attributed in part to kimberlite-related Hf addition, it is possible that these deep-seated xenoliths represent younger mantle. The superchondritic 176Lu/ 177Hf ratios observed for a number of the shallow low-temperature peridotites indicate strong fractionation of Lu and Hf, suggesting mantle root formation in the garnet stability field (depths >80 km). The Hf isotope compositions for the

  19. Composition of the lithospheric mantle in the Siberian craton : New constraints from fresh peridotites from the Udachnaya-East Kimberlite

    NASA Astrophysics Data System (ADS)

    Doucet, Luc-Serge; Ionov, Dmitri A.; Ashchepkov, Igor

    2010-05-01

    Peridotite xenoliths from the Udachnaya kimberlite pipe represent the major source of lithospheric mantle samples beneath central Siberian craton. An important problem with the availble data [1], however, is that the Udachnaya xenoliths, like many other kimberlite-hosted peridotite suites worldwide, are extensively altered due to interaction with host magma and post-eruption alteration. This alteration causes particular dificulties for whole-rock studies including microstructures, modal estimates and chemical compositions. We report petrographic data and major and trace element compositions for whole-rocks and minerals of some 30 unusually fresh peridotite xenolith from the Udachnaya-East kimberlite. Our study has two goals. The first is to present and discuss trace element data on rocks and minerals from Udachnaya, whose composition remains little known. The other one is to explore how the availability of the fresh peridotites improves our knowledge of petrology and geochemistry of cratonic mantle in relation to published data on altered samples [1]. The xenoliths are spinel, garnet-spinel and garnet facies peridotites including garnet- and cpx-rich lherzolites, garnet and spinel harzburgites and dunites. Thermobarometric estimates for garnet bearing rocks yield T = 800-1350°C and P = 20-70 kbar, low-T spinel facies rocks may originate from shallower levels. Thus, the suite represents a lithospheric profile from the sub-Moho mantle down to ~210 km. The deeper peridotites commonly have porphyroclastic microstructures with mainly neoblast olivine, opx porphyroclasts and cpx and garnet with broadly variable morphologies whereas rocks of shallow origin are commonly protogranular. Trace element compositions in bulk rocks appear to be affected by host magma contamination with enrichments in highly to moderately incompatible elements as well as in alkalis. Nevertheless, the kimberlite-related contamination cannot explain a combination of low Th and U and high Sr

  20. Silicate globules in kyanite from grospydites of the Zagadochnaya kimberlite pipe, Yakutia: The problem of the origin

    NASA Astrophysics Data System (ADS)

    Tomilenko, A. A.; Kovyazin, S. V.; Pokhilenko, L. N.; Sobolev, N. V.

    2011-01-01

    The results of complex study of silicate globules and α-quartz paramorphs after coesite in kyanite from grospydites from the Zagadochnaya kimberlite pipe, Yakutia, using optical and scanning electron spectroscopy, electron and ion microprobes, LA ICP MS and Raman spectroscopy, are presented. The existence of radial fractures diverging from silicate globules into the matrix (kyanite) attests to the fact that the content of the globules is extremely condensed. A zonal structure is usually typical for globules: a coat and a core, which can be explicitly distinguished under the electron microscope, can be differentiated in them. Compositionally, the coat of the globule corresponds to potassium feldspar (wt %: 66.4 SiO2; 16.9 Al2O3; 0.4 FeO; 0.1 CaO; 0.2 Na2O; 14.7 K2O). The globules were also detected in which along with K, a high content of Na and Ca was also ascertained in the silicate coat. The globule coat is considerably enriched with Ba, La, Ce, Nb, and a number of other noncompatible elements as compared with xenolith minerals. The water content in globules is ˜0.6 wt %. As compared with the host mineral (kyanite), the core part of the globules is also enriched with Co, Ni, Zn, and Cu; their content in kyanite is negligibly low. The entire data collection attests to the fact that the formation of silicate globules could have been caused by interaction of the conservated fluid and/or water-silicate melt with the host mineral and crystalline inclusions of clinopyroxene and garnet with decreasing pressure during the transportation of grospydite xenoliths by the kimberlite melt to the Earth's surface.

  1. Evolution of the mantle sections beneath the kimberlite pipes example of Yakutia.

    NASA Astrophysics Data System (ADS)

    Ashchepkov, Igor; Ntaflos, Ttheodoros; Logvinova, Alla; Vladykin, Nikolai; Ivanov, Alexandr; Spetsius, Zdislav; Stegnitsky, Yury; Kostrovitsky, Sergei; Salikhov, Ravil; Makovchuk, Igor; Shmarov, Gleb; Karpenko, Mikhail; Downes, Hilary; Madvedev, Nikolai

    2017-04-01

    of most Ilmenites but more regular for the Cpx and Garnets revealing the sub parallel patterns elevating LREE with the rising TRE. But commonly these are not continuous sequances because they developed in the pulsing moving systems like beneath Zarnitsa. The minerals from the feeders like dunites also show the inflected or S-type REE patterns. From the earlier to later phases the TRE compositions became more evolved reflecting the evolution of protokimberlites. The wall rocks also often show the interaction with the more evolved melts and sometimes "cut" spectrums due to the dissolution some phases and repeated melting events So we could suggest the joint evolution of the mantle column protokimberlites and megacrysts composition and type of kimberlites with the diamond grade. The mantle lithospheric base captured by the PK. The developing and rising protokimbelrites was followed by the crystallization of the diamonds in the gradient in FO2 zone in wall rocks due to reductions of C -bearing fluids and carbonatites (> 1 QMF) on peridotites ((< -2 -5 QMF). The most intensive reactions are near the graphite - diamond boundary where protokimberlites are breaking and where most framesites are forming. Grant RBRF 16-05-000860Grant RBRF 16-05-000860

  2. 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

  3. 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.

  4. Carbon isotopic composition and origin of SiC from kimberlites of Yakutia, Russia

    NASA Astrophysics Data System (ADS)

    Mathez, E. A.; Fogel, R. A.; Hutcheon, I. D.; Marshintsev, V. K.

    1995-02-01

    The stability of moissanite (SiC) has been computed for upper mantle conditions using the internally optimized thermodynamic dataset for the MgSiO compounds of Fei et al. (1990). The computations consider the effects of pressure and temperature on the elastic properties of phases involved in the reactions. The maximum stability of moissanite throughout the upper mantle is typically five to six orders of magnitude lower in oxygen fugacity ( fO2) than the Fe metal-wüstite oxygen buffer at equivalent temperature and pressure, in agreement with previous calculations. Under conditions of SiC stability, silicates will be Fe-free, Fe metal will contain substantial amounts of Si but little C in solution, and Mg-rich sulfides will be stable. Moissanite from the heavy mineral concentrate of the Mir and Aikhal kimberlite pipes, Yakutia, has been studied. Moissanite crystals are gemmy and vary in color from a characteristic blue-green to pale green to nearly colorless to blue-black. Most exhibit crystallographic faces and are in the size range 0.5 to 1 mm in long dimension. Their compositions include small quantities of Fe, which is ubiquitous, Al, Ca, V, Cr, and Mn, all of which may be present in concentrations > 100 ppmwt. Mineral inclusions are present in some crystals. Silicon metal is the most common; inclusions of ferrosilicite (Fe 3Si 7), FeTi silicides, REE silicate, and sinoite (Si 2N 2O) have also been observed. The carbon isotopic compositions of individual moissanite grains have been determined by ion microprobe. The nine analyzed crystals from Aikhal and fourteen from Mir are characterized by a narrow range in δ 13C values of -22 to -29‰; the majority of crystals fall within a more restricted range of -24 to -27‰. Two grains were analyzed for N and found to have a δ 15N of +9.7 ± 4.0 and +5.6 ± 2.0‰. Five mechanisms for the formation of moissanite are considered. Moissanite may be a relict of a reduced, primordial Earth and now present only as

  5. 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.

  6. Model photoautrophs isolated from a Proterozoic ocean analog - aerobic life under anoxic conditions

    NASA Astrophysics Data System (ADS)

    Hamilton, T. L.; de Beer, D.; Klatt, J.; Macalady, J.; Weber, M.; Lott, C.; Chennu, A.

    2016-12-01

    The 1-2 billion year delay before the final rise of oxygen at the end of the Proterozoic represents an important gap in our understanding of ancient biogeochemical cycling. Primary production fueled by sulfide-dependent anoxygenic photosynthesis, including the activity of metabolically versatile cyanobacteria, has been invoked as a mechanism for sustaining low atmospheric O2 throughout much of the Proterozoic. However, we understand very little about photoautotrophs that inhabit Proterozoic-like environments present on Earth today. Here we report on the isolation and characterization of a cyanobacterium and a green sulfur bacterium that are the dominant members of pinnacle mats in Little Salt Spring—a karst sinkhole in Florida with perennially low levels of dissolved oxygen and sulfide. The red pinnacle mats bloom in the anoxic basin of the sinkhole and receive light that is of very poor quality to support photosynthesis. Characterization of the isolates is consistent with observations of oxygenic and anoxygenic photosynthesis in situ—both organisms perform anoxygenic photosynthesis under conditions of very low light quality and quantity. Oxygenic photosynthesis by the cyanobacterium isolate is inhibited by the presence of sulfide and under optimal light conditions, rates of anoxygenic photosynthesis are nearly double that of oxygenic photosynthesis. The green sulfur bacterium is tolerant of oxygen and has a very low affinity for sulfide. In Little Salt Spring, oxygenic photosynthesis occurs for only four hours a day and the water column remains anoxic because of a continuous supply of sulfide. Isolation and characterization of these photoautotrophs combined with our high resolution microsensor data in situ highlight microbial biogeochemical cycling in this exceptional site where aerobic microorganisms persist in a largely anoxic ecosystem.

  7. Towards Definition of Early Proterozoic/Archean Paleomagnetic Dipole Moments from Karelia (Russia)

    NASA Astrophysics Data System (ADS)

    Smirnov, A. V.; Tarduno, J. A.

    2001-05-01

    The onset of the geomagnetic dynamo may be related to the formation of the Earth's inner core. The timing of this event during the Precambrian differs in a number of theoretical models and the paleointensity database needs to be significantly extended to test various model predictions. We determine paleointensity from single plagioclase crystals derived from border dikes of the Early Proterozoic/Archean ( ~2.45 Ga) Burakovka layered intrusion (North Karelia, Russia). The use of these single crystals can result in data less affected by alteration. Magnetic hysteresis properties of plagioclases suggest that they contain single-domain to pseudo-single-domain magnetic inclusions. A study of low-temperature magnetic properties of single plagioclase crystals shows a sharp Verwey transition at 120 K, indicating the presence of stoichiometric, non-stressed magnetite. A significant magnetic anisotropy was not observed in the crystals, implying that magnetite exsolution was minor, consistent with fast cooling of the border dikes. We investigated four dikes using a modified Thellier technique. Of the twenty-two samples studied thus far, eight paleointensity determinations meet our reliability criteria (success rate is 36%). Based on these eight determinations, the average paleointensity is 36.5 +/- 1.2 μ T which corresponds to a virtual dipole moment of 6.49 +/- 0.21 x 1022 Am2. The latter value is consistent with the existence of a fully developed geomagnetic dynamo by the Early Proterozoic. Our results suggest that border dikes of the Burakovka layered intrusion can be used to obtain additional Early Proterozoic/Archean paleointensity values. Because of their fast cooling rates relative to the interiors of layered intrusions, border dikes also offer the opportunity to obtain secular variation data which, together with paleointensity values, could be used to define paleomagnetic dipole moments.

  8. 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

  9. Energy metabolism among eukaryotic anaerobes in light of Proterozoic ocean chemistry.

    PubMed

    Mentel, Marek; Martin, William

    2008-08-27

    Recent years have witnessed major upheavals in views about early eukaryotic evolution. One very significant finding was that mitochondria, including hydrogenosomes and the newly discovered mitosomes, are just as ubiquitous and defining among eukaryotes as the nucleus itself. A second important advance concerns the readjustment, still in progress, about phylogenetic relationships among eukaryotic groups and the roughly six new eukaryotic supergroups that are currently at the focus of much attention. From the standpoint of energy metabolism (the biochemical means through which eukaryotes gain their ATP, thereby enabling any and all evolution of other traits), understanding of mitochondria among eukaryotic anaerobes has improved. The mainstream formulations of endosymbiotic theory did not predict the ubiquity of mitochondria among anaerobic eukaryotes, while an alternative hypothesis that specifically addressed the evolutionary origin of energy metabolism among eukaryotic anaerobes did. Those developments in biology have been paralleled by a similar upheaval in the Earth sciences regarding views about the prevalence of oxygen in the oceans during the Proterozoic (the time from ca 2.5 to 0.6 Ga ago). The new model of Proterozoic ocean chemistry indicates that the oceans were anoxic and sulphidic during most of the Proterozoic. Its proponents suggest the underlying geochemical mechanism to entail the weathering of continental sulphides by atmospheric oxygen to sulphate, which was carried into the oceans as sulphate, fueling marine sulphate reducers (anaerobic, hydrogen sulphide-producing prokaryotes) on a global scale. Taken together, these two mutually compatible developments in biology and geology underscore the evolutionary significance of oxygen-independent ATP-generating pathways in mitochondria, including those of various metazoan groups, as a watermark of the environments within which eukaryotes arose and diversified into their major lineages.

  10. 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.

  11. 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.

  12. 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.

  13. 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

  14. 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

  15. 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.

  16. Occurrence of natural fullerenes in low grade metamorphosed Proterozoic shungite from Karelia, Russia

    NASA Astrophysics Data System (ADS)

    Parthasarathy, G.; Srinivasan, R.; Vairamani, M.; Ravikumar, K.; Kunwar, A. C.

    1998-11-01

    We report on the occurrence of fullerenes in Proterozoic shungite (˜2 Ga) from the shungite mine, Kondopoga, Karelia, Russia (62.12°N 34.17°E). The presence of fullerenes has been confirmed by mass spectrometry, with peaks at 360 and 720 amu (atomic mass unit), powder X-ray diffraction showing ten diffraction peaks corresponding to the fullerite structure with a = 1.4201(5) nm, and 13C nuclear magnetic resonance (NMR) spectroscopic studies, showing a peak at 143.2 ppm. In the Kondopoga shungite mine, fullerenes occur in silty shales that have experienced greenshist facies metamorphism.

  17. 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.

  18. 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.

  19. 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.

  20. 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

  1. Proterozoic sequences and their implications for precamorian and cambrian geologic evolution of Western Kentucky: Evidence from seismic-reflection data

    USGS Publications Warehouse

    Drahovzal, J.A.

    1997-01-01

    Analyses of two seismic-reflection lines in western Kentucky indicate the presence of two Proterozoic, unconformity-bounded sequences. One is autochthonous and of probable Late Proterozoic age; the other is allochthonous and of probable Middle Proterozoic age. Reflector patterns and apparent relationships to similar sequences elsewhere in the region suggest that the two sequences are of continental-rift origin. The two Proterozoic sequences lie beneath and adjacent to rocks of the Cambrian rift sequence in the Rough Creek Graben. The oldest sequence, the pre-Grenville sequence, was apparently folded and thrust faulted by the Grenville compressional event, implying that it is older than ???0.975 Ga (Middle Proterozoic). Two seismic-reflection pattern types are present in the western Kentucky data that may relate to the Middle Run (lithic arenite) and volcanic sequences defined farther east near the Grenville Front. The presence of imbricate, thrust-belt geometries in the pre-Grenville sequence extends the known westward limit of Grenville compressional structures into western Kentucky. The younger, post-Grenville sequence is less deformed and was apparently formed after the Grenville compressional event; several lines of evidence indicate that it is Late Proterozoic (0.7 to 0.6 Ga) in age. This probable siliciclastic and volcanic-rift sequence is represented by only thin remnants in western Kentucky and has no equivalent near the Grenville Front in southwestern Ohio and central Kentucky. Rocks of the better documented Cambrian rifting event belong to the thick, pre-Knox sequence in the Rough Creek Graben of western Kentucky and lie unconformably above these earlier sequences. A previously undocumented, northward-thickening interval within the lower part of the Cambrian pre-Knox sequence is recognized north of the Rough Creek Graben.

  2. 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.

  3. 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

  4. 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.

  5. 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.

  6. 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.

  7. 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.

  8. 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.

  9. 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

  10. 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.

  11. 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.

  12. 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.

  13. A Proterozoic lithospheric source for Karoo magmatism: evidence from the Nuanetsi picrites

    NASA Astrophysics Data System (ADS)

    Ellam, R. M.; Cox, K. G.

    1989-03-01

    Highly magnesian Jurassic picrite basalts from the Nuanetsi area of southeastern Zimbabwe are close to primary mantle melts, not substantially modified by high level fractionation processes. The lavas display a wide variation in isotope ratios and incompatible trace element abundances, which may be attributed to a heterogeneous mantle source. Sm sbnd Nd isotope results, on samples carefully selected to be thoroughly representative of the chemical variation within the whole suite, reveal a late Proterozoic isochron. One possibility is that this represents a mantle isochron corresponding to the last episode of Sm sbnd Nd fractionation within a lithospheric source. Alternatively, binary mixing might explain the apparent isochron, but there are problems identifying a mantle reservoir with sufficiently high Sm/Nd ratios. In either case a significant role for an ancient sub-crustal lithospheric source is implied. Rb sbnd Sr data do not lie on a well-defined isochron, and probably reflect Rb sbnd Sr fractionation at, or close to, the time of eruption. Pb isotope data are broadly consistent with the late Proterozoic age. The apparent age of the mantle isochron is similar to that of a crust-forming event in the Natal-Namaqualand belt, which may indicate coupled evolution of the continental crust and lithospheric mantle.

  14. Laurentia, Australia, and Antarctica as a Late Proterozoic supercontinent. Constraints from isotopic mapping

    SciTech Connect

    Borg, S.G.; DePaolo, D.J. Lawrence Berkeley Lab., CA )

    1994-04-01

    The reconstruction of Laurentia, Australia, and Antarctica into a Proterozoic supercontinent is evaluated by analyzing the fit of Precambrian provinces defined by isotopic and geochronologic mapping. The analysis is complicated by allochthonous segments of the Antarctic and eastern Australian margins. Removal of the allochthonous provinces produces a closer fit of the continents; there is a match of Early Proterozoic basement between southwestern Laurentia and the only exposure of craton known from the paleo-Pacific margin of Antarctica. In addition, western Laurentia is brought closer to the Australian Gawler block, consistent with provenance interpretations of the Belt Supergroup. Removal of the allochthonous provinces by right-lateral translation relative to the Antarctic craton margin places them in a pre-750 Ma position where they could be southwestward extensions of the Yavapai-Mazatzal and Grenville provinces of southern Laurentia. This modified reconstruction leads to a prediction of extensive Archean basement in Antarctica between the South Pole and Victoria Land, a prediction partly borne out by Archean rocks in the Miller Range of the Transantarctic Mountains; it also predicts the presence of 1.4 Ga rapakivi granites in the Transantarctic Mountains basement. This configuration implies assembly of the Australia-Antarctica Gondwana margin by terrane accretion following, or accompanied by, left-lateral translation. 27 refs., 2 figs., 1 tab.

  15. Laurentia, Australia, and Antarctica as a Late Proterozoic supercontinent: Constraints from isotopic mapping

    NASA Astrophysics Data System (ADS)

    Borg, Scott G.; Depaolo, Donald J.

    1994-04-01

    The reconstruction of Laurentia, Australia, and Antarctica into a Proterozoic supercontinent is evaluated by analyzing the fit of Precambrian provinces defined by isotopic and geochronologic mapping. The analysis is complicated by allochthonous segments of the Antarctic and eastern Australian margins. Removal of the allochthonous provinces produces a closer fit of the continents; there is a match of Early Proterozoic basement between southwestern Laurentia and the only exposure of craton known from the paleo-Pacific margin of Antarctica. In addition, western Laurentia is brought closer to the Australian Gawler block, consistent with provenance interpretations of the Belt Supergroup. Removal of the allochthonous provinces by right-lateral translation relative to the Antarctic craton margin places them in a pre-750 Ma position where they could be southwestward extensions of the Yavapai-Mazatzal and Grenville provinces of southern Laurentia. This modified reconstruction leads to a prediction of extensive Archean basement in Antarctica between the South Pole and Victoria Land, a prediction partly borne out by Archean rocks in the Miller Range of the Transantarctic Mountains; it also predicts the presence of 1.4Ga rapakivi granites in the Transantarctic Mountains basement. This configuration implies assembly of the Australia-Antarctica Gondwana margin by terrane accretion following, or accompanied by, left-lateral translation. This is compatible with a tectonic regime of clockwise rotation of Laurentia relative to Australia and Antarctica after rifting. Thus, the proposed supercontinent, with some modifications, has potential for explaining several aspects of the pattern of Precambrian provinces in the three continents.

  16. 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.

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

    PubMed

    Robbins, E I; Porter, K G; Haberyan, K A

    1985-09-01

    Microfossils resembling fecal pellets occur in acid-resistant residues and thin sections of Middle Cambrian to Early Proterozoic shale. The cylindrical microfossils average 50 x 110 mum 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.

  18. Midcontinent U.S. fault and fold zones: A legacy of Proterozoic intracratonic extensional tectonism?

    NASA Astrophysics Data System (ADS)

    Marshak, Stephen; Paulsen, Timothy

    1996-02-01

    The U.S. continental interior (midcontinent) contains numerous fault and fold zones. Seismic and drilling data indicate that some of these zones first formed as Proterozoic-Eocambrian rift faults, but the origin of most remains enigmatic. We propose that the enigmatic fault and fold zones also began as Proterozoic-Eocambrian normal faults. We base our hypothesis on the following: (1) enigmatic zones parallel known rifts, (2) the structural style of enigmatic zones mirrors the structural style of known rifts, (3) the map pattern of some enigmatic zones (e.g., the La Salle deformation belt of Illinois) resembles the map pattern of contemporary rifts, and (4) it is easier to rupture an intact craton by normal faulting than by reverse or strike-slip faulting. These zones, along with known rifts, represent the legacy of widespread extensional tectonism that brittlely broke up the craton into fault-bounded blocks prior to deposition of Phanerozoic platform cover. Once formed, midcontinent fault and fold zones remained weak, allowing cratonic blocks to jostle relative to one another during the Phanerozoic, thereby inverting faults (and creating transpressional or transtensional structural assemblages), localizing seismicity, and channeling (or releasing) ore-generating fluids.

  19. 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).

  20. 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.

  1. 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.

  2. 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.

  3. 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.

  4. Mantle xenocrysts from the Arkhangelskaya kimberlite (Lomonosov mine, NW Russia): Constraints on the composition and thermal state of the diamondiferous lithospheric mantle

    NASA Astrophysics Data System (ADS)

    Lehtonen, M.; O'Brien, H.; Peltonen, P.; Kukkonen, I.; Ustinov, V.; Verzhak, V.

    2009-11-01

    The Arkhangelskaya kimberlite pipe belongs to the Zolotitsa kimberlite field in the Arkhangelsk region, NW Russia. It is the first pipe of the Lomonosov diamond mine to be put into production, with 2 million tons of ore already extracted. In this study major and trace element compositions of garnet, clinopyroxene (Cpx), Mg-ilmenite and chromite xenocrysts from the Arkhangelskaya pipe have been used to infer information about the compositional variability of the mantle underlying the Zolotitsa field. Single-grain thermobarometry of peridotitic Cpx xenocrysts yields a cool cratonic geotherm that follows a ca. 36 mW/m 2 conductive model. Equilibration temperatures of garnet and chromite grains based on Ni- and Zn-thermometry, respectively, indicate a sampling interval of ca. 70-230 km of the lithospheric mantle when projected onto the Cpx-derived geotherm. The major element chemistry of Mg-ilmenite xenocrysts suggests that almost optimal redox conditions for diamond preservation prevailed in the mantle during the time of emplacement of the host kimberlite magmas. Garnet major and trace element compositions combined with the Cpx-geotherm indicate that the peridotitic diamond window extends from 130 to 210 km under Zolotitsa and that the deeper parts of the lithosphere have been affected by metasomatic events. Arkhangelskaya seems to have sampled the bulk of its diamonds from the deepest portion of the diamond stability field, between 190 and 210 km. In comparison, the neighbouring Lomonosova and Pionerskaya pipes are known to have collected their diamonds from 130-160 km. The comparable grade of the three pipes suggests that diamondiferous material is generously distributed within the diamond stability field. The remarkable difference evidenced by garnet composition and thermobarometry between Arkhangelskaya and the two other Zolotitsa pipes probably derives from differences in rheology and eruption rates of the rising kimberlite magmas.

  5. Tectonic implications of Late Proterozoic-Early Palaeozoic igneous rock associations in western Tasmania

    NASA Astrophysics Data System (ADS)

    Crawford, Anthony J.; Berry, Ron F.

    1992-11-01

    In contrast to the mainland Lachlan Fold Belt, Late Proterozoic basement(?) and Cambrian rocks are exposed over large areas of western Tasmania. They offer the possibility of solving the paradox regarding the continental versus oceanic nature of the basement beneath southern Australia, and better understanding the Late Proterozoic and Cambrian tectonic history of this southern part of the Lachlan Fold Belt. Around 600 Ma, attenuation and eventual rifting of Proterozoic continental crust resulted in formation of a thinned passive continental margin transected by small rift basins in which rift tholeiites transitional to MORB accumulated. Eastward-directed intra-oceanic subduction commenced to the east of the passive margin at some time before the Middle Cambrian, forming an oceanic arc with boninites and low-Ti magnesian quartz tholeiites in the forearc region. Continued subduction of intervening oceanic crust between the arc and the passive margin resulted in an arc-continent collision within the Middle Cambrian. One or more extensive sheets of forearc crust, dominated by low-Ti basalts and boninites and their cumulate complements, were overthrust onto thinned continental crust. Continued compression at the suture initiated what was probably a short-lived episode of westward-directed subduction beneath the newly-emplaced allochthon and passive margin basement, and generated medium- to high-K andesites and more felsic rocks of the Central Volcanic Complex of the Mount Read Volcanics. Rebound backthrusting along the eastern side of the Central Volcanic Complex of the Mount Read Volcanics belt exhumed underthrust Proterozoic crystalline crust which now forms the Tyennan region, and a foreland basin half-graben, the Dundas Trough, formed along the western edge of this basement inlier. The upper Mount Read Volcanics are mainly post-collisional high-K to shoshonitic basalts and andesites generated by delayed partial melting of subduction-modified, underthrust passive

  6. 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

  7. Geochronological and lead-isotope evidences for rapid crust formation in middle-proterozoic time: The Labrador example

    NASA Technical Reports Server (NTRS)

    Schaerer, Urs

    1988-01-01

    Extensive U-Pb geochronological studies in the Grenville and Makkovik provinces have shown that eastern Labrador is underlain by two distinct crustal blocks. In order to substantiate the juvenile character of the middle-Proterozoic crustal block, the isotopic compositon of lead in leached k-feldspars from the same rocks were analyzed. The results of the analysis are briefly discussed.

  8. Geochronological and lead-isotope evidences for rapid crust formation in middle-proterozoic time: The Labrador example

    NASA Technical Reports Server (NTRS)

    Schaerer, Urs

    1988-01-01

    Extensive U-Pb geochronological studies in the Grenville and Makkovik provinces have shown that eastern Labrador is underlain by two distinct crustal blocks. In order to substantiate the juvenile character of the middle-Proterozoic crustal block, the isotopic compositon of lead in leached k-feldspars from the same rocks were analyzed. The results of the analysis are briefly discussed.

  9. Lead isotopic evidence for mixed sources of Proterozoic granites and pegmatites, Black Hills, South Dakota, USA

    NASA Astrophysics Data System (ADS)

    Krogstad, Eirik J.; Walker, Richard J.; Nabelek, Peter I.; Russ-Nabelek, Carol

    1993-10-01

    The lead isotopic compositions of K-feldspars separated from the ca. 1700 Ma Harney Peak Granite complex and spatially associated granitic pegmatites indicate that these rocks were derived from at least two sources. It has been reported previously that the core of the Harney Peak Granite complex is dominated by relatively lower/ gd18O (avg. 11.5 %.) granites, whereas higher / gd18O (avg. 13.2%.) granites occur around the periphery of the complex. The higher δ 18O granites and one simple pegmatite have low values of 207Pb /204Pb for their 206Pb /204Pb Thus, they likely were derived from a source with a short crustal residence time. This source may have been the pelitic schists into which the Harney Peak Granite complex and pegmatites were intruded. Feldspars from granites with lower / gd18O values have significantly higher 207Pb /204Pb for their 206Pb /204Pb . The data define a linear array with a slope equivalent to an age of ca. 2.6 Ga with t 2 defined to be 1.7 Ga. Such a slope could represent a mixing array or a secondary isochron for the source. These low δ18O granites could have been derived from a source with a high U/ Pb and with a crustal residence beginning before the Proterozoic. The source (s) of these granites may have been a sediment derived from late Archean continental crust. The highly evolved Tin Mountain pegmatite has lead isotopic systematics intermediate between those of the two granite groups, suggesting either a mixed source or contamination. Two late Archean granites, the Little Elk Granite and the Bear Mountain Granite, had precurso