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Sample records for kimberlites

  1. Kimberlite Volcanism

    NASA Astrophysics Data System (ADS)

    Sparks, R. S. J.

    2013-05-01

    Kimberlite magmas are volatile-rich, silica-poor ultrabasic magmas originating as small-degree mantle melts at depths of 150 km or greater. Alteration and entrained xenoliths obscure their original magma chemistry and properties. Kimberlite magmas decrease temperature by a few hundred degrees during ascent. Changes of melt composition can result as a function of assimilation. Stalling of kimberlite can result in fractional crystallization, loss of xenocrysts, and loss of volatiles. Multiple pulses of kimberlite magmas form several distinct geological units in the same pipe or intrusion. Kimberlite pipes form by explosive disruption and deformation of country rocks. Confinement in a pipe introduces new processes such as fluidization, dynamic sintering, and intense mixing between volcanic jets and concentrated trapped mixtures. Occurrences of extravent and crater-fill lithofacies indicate that kimberlite eruptions generate eruptive products that are similar to those produced by common magma types. Alteration is largely attributed to hydrothermal systems, diagenesis, and weathering involving external water.

  2. 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

  3. Kimberlite ascent and eruption.

    PubMed

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

    2007-12-13

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

  4. 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

  5. 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.

  6. 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.

  7. 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.

  8. The Pimenta Bueno kimberlite field, Rondônia, Brazil: Tuffisitic kimberlite and transitional textures

    NASA Astrophysics Data System (ADS)

    Masun, K. M.; Scott Smith, B. H.

    2008-06-01

    The Pimenta Bueno kimberlite field of Rondônia State, Brazil is located near the edge of the Amazon Craton and contains more than thirty bodies. A late to mid-Triassic emplacement age has been obtained for two kimberlites. The Pimenta Bueno kimberlite field includes pipes of up to 15 ha and a few apparently flat-lying hypabyssal kimberlite sills. In the three pipes and one sill examined, three textural end member types of kimberlite are recognized: hypabyssal kimberlite (HK), tuffisitic kimberlite (TK), and resedimented volcaniclastic kimberlite (RVK). There are gradations in textures between the three types. The overall gradation with depth is VKB, VK, TK, TKt, HKt to HK (where "t" denotes transitional varieties), and the textural gradations are apparent at all scales, megascopically to microscopically. Importantly, these gradations appear to occur within single phases of kimberlite. TK and the associated transitional varieties observed in the Pimenta Bueno field have striking similarities to those of many kimberlites of southern Africa and Canada. The Pimenta Bueno kimberlite field thus represents a newly recognized example of southern-African-style kimberlite pipes occurring on a third continent. The TK-bearing pipes of Pimenta Bueno, southern Africa, and Canada are markedly different from those found in the Canadian Prairies and Lac de Gras, Northwest Territories, which indicates that pipes containing TK and related textures are repeated in space and time but only form in certain circumstances. There is an apparent correlation between kimberlite texture and pipe size. In the larger pipes (> 5 ha), the TK is overlain by VKB. In the small pipes (< 3 ha) there is a textural gradation with depth from TK to HK and VKB is not present. One TKt-HKt transition appears to be ~ 60 m thick. The nature of the TK-TKt-HKt-HK textural transition suggests that TK developed by the textural modification of magmatic kimberlite within the pipe. The textures may reflect increased

  9. Duration and periodicity of kimberlite volcanic activity in the Lac de Gras kimberlite field, Canada and some recommendations for kimberlite geochronology

    NASA Astrophysics Data System (ADS)

    Sarkar, Chiranjeeb; Heaman, Larry M.; Pearson, D. G.

    2015-03-01

    Establishing the emplacement ages and distribution pattern of Central Slave kimberlites has played a key role in diamond exploration within the Lac de Gras kimberlite field. Nonetheless, emplacement age information is lacking for approximately 80% of the known kimberlites in this field making the assessment of emplacement age patterns difficult. This study expands the number and geographic coverage of kimberlite emplacement ages within the Lac de Gras field to re-assess the absolute timing, duration, and possible number of pulses of kimberlite eruption. U-Pb perovskite ages for eight previously undated kimberlites and an additional thirteen kimberlites, which were previously dated by either the Rb-Sr or U-Pb methods, fall within the age range of 75-45 Ma, as previously suggested for kimberlite magmatism in this area. We report the first Carboniferous age kimberlite in the Central Lac de Gras field - the Eddie kimberlite - with a U-Pb perovskite age of 321.0 ± 3.0 Ma. A compilation of 57 kimberlite emplacement ages from the central Lac de Gras field was evaluated using probability density and mixture modeling methods. Five short-duration (4-5 Ma) periods of kimberlite magmatism are recognized at 48, 54, 61, 66 and 72 Ma; the three younger pulses have been previously recognized and remain relatively unchanged. The 54 Ma pulse represents the major kimberlite eruption event containing ~ 40% of the currently dated kimberlites in Lac de Gras field. A detailed evaluation of the temporal-spatial evolution of Lac de Gras kimberlites reveals that the oldest diamond-poor kimberlites (75-60 Ma) were emplaced in the northern and eastern parts of the field whereas the younger (55-48 Ma) economic kimberlites are concentrated in the center of the field.

  10. 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.

  11. 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

  12. 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

  13. Kimberlite ascent by assimilation-fuelled buoyancy.

    PubMed

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

    2012-01-19

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

  14. 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

  15. Kimberlite emplacement temperatures from conodont geothermometry

    NASA Astrophysics Data System (ADS)

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

    2015-02-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2015-11-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2011-03-01

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

  18. Distinct kimberlite pipe classes with contrasting eruption processes

    NASA Astrophysics Data System (ADS)

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

    2004-09-01

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

  19. 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.

  20. 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.

  1. 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.

  2. Geochemical Characteristics of Garnets from Tanzanian Kimberlites (Mwadui, Singida, Nyangwale, and Galamba kimberlites)

    NASA Astrophysics Data System (ADS)

    Mandl, Magdalena; Hauzenberger, Christoph; Konzett, Juergen; Jumanne, Richard; Gobba, John; Nguyen, Hoang

    2013-04-01

    More than 350 kimberlite pipes and clusters have been found in Tanzania up to date. Many of the occurrences are found in and around Shinyanga, northern Tanzania. They are characterised by the presence of crater deposits, suggesting that minimal erosion has taken place in this region since Neogene times (~50 Ma) when the kimberlites were emplaced. The kimberlites are typically found in Archean granitic basement and meta-sediments. The most prominent kimberlite pipe, the Mwadui kimberlite, which is mined for diamonds, is one of the worlds largest and measures ~146 ha at surface. Since only weathered crater deposits are exposed, no mantle xenoliths were found in the four visited / sampled kimberlite pipes: 1) Mwadui kimberlite, 2) Singida kimberlite, 3) Nyangwale kimberlite, 4) Galamba kimberlite. However, garnets could be sampled either in heavy mineral separates or as garnet megacrysts. These garnets have been studied using major, trace and rare earth element compositions in order to obtain information on the underlying upper mantle. Garnets from the Williamson diamond mine are cm sized megacrysts. All of these mantle garnet megacrysts are low Cr megacrysts in composition (group G1). The Cr2O3 values are low with 0.6-1.9 wt.%, TiO2 values are high with 0.5-1.2 wt.% and CaO ~4.7 wt.%. Only one megacryst from the Williamson Mine is harzburgitic (G10D diamond facies). The Cr2O3 values are also very low with 1.5 wt. % , TiO2 <0.04 wt.% and CaO ~2.4 wt.%. Garnets are depleted in LREE and enriched in HREE relative to a primitive mantle. Ni in garnet geothermometry for the harzburgitic megacryst shows a temperature of 1015°C. From the Singida kimberlite garnet grains of only a few mm could be recovered. Most of these garnet grains are eclogitic (G3) and low Cr megacrysts (G1), but there are also pyroxenitic (G4) and lherzolitic (G9) grains. The Cr2O3 content: G3= <0.1 wt.%, G1= 0.07-3 wt.%, G4= ~0.5 wt.% and G9= ~3.5 wt.%. TiO2: G3=<0.1 wt.%; G1= ~0.7 wt.%, G4= 0

  3. 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

  4. Detection of diamonds in kimberlite by the tagged neutron method

    NASA Astrophysics Data System (ADS)

    Alexakhin, V. Yu.; Bystritsky, V. M.; Zamyatin, N. I.; Zubarev, E. V.; Krasnoperov, A. V.; Rapatsky, V. L.; Rogov, Yu. N.; Sadovsky, A. B.; Salamatin, A. V.; Salmin, R. A.; Sapozhnikov, M. G.; Slepnev, V. M.; Khabarov, S. V.; Razinkov, E. A.; Tarasov, O. G.; Nikitin, G. M.

    2015-06-01

    A new technology for diamond detection in kimberlite based on the tagged neutron method is proposed. The results of experimental researches on irradiation of kimberlite samples with 14.1-MeV tagged neutrons are discussed. The source of the tagged neutron flux is a portable neutron generator with a built-in 64-pixel silicon alpha-detector with double-sided stripped readout. Characteristic gamma rays resulting from inelastic neutron scattering on nuclei of elements included in the composition of kimberlite are registered by six gamma-detectors based on BGO crystals. The criterion for diamond presence in kimberlite is an increased carbon concentration within a certain volume of the kimberlite sample.

  5. Oxygen isotope ratios in eclogites from kimberlites.

    PubMed

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

    1971-06-01

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

  6. 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.

  7. 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

  8. 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.

  9. Eclogite xenoliths from Wajrakarur kimberlites, southern India

    NASA Astrophysics Data System (ADS)

    Patel, S. C.; Ravi, S.; Thakur, S. S.; Rao, T. K.; Subbarao, K. V.

    2006-09-01

    Mineralogical characteristics of eclogite xenoliths from three kimberlite pipes (KL2, P2 and P10) of the Proterozoic Wajrakarur kimberlite field of southern India have been studied. In a rare sample of enstatite eclogite from the KL2 pipe garnet contains microscopic triangular arrays of needles or blebs of omphacite, enstatite and rutile consistent with an origin by exsolution parallel to the isometric form {111}. Discrete omphacite grains in the sample contain exsolved needles or blebs of enstatite and garnet. Kyanite eclogites are abundant in the KL2 pipe which occasionally show a secondary ring of pure celsian around kyanite grains. Omphacite Na2O contents in the eclogites of the KL2 and P2 pipes are typically between 3 and 6 wt%, and garnet has widely variable composition with end member ranges of Prp22-81Grs0-47Alm10-30Sps0-1Adr0-5Uv0-3. Eclogites of the P10 pipe comprise chromian omphacite and garnet. Phase relations in the ACF projection exhibit systematic increase of the Ca-Tschermak’s component in omphacite from enstatite eclogite through biminerallic eclogite to kyanite eclogite. Garnet-clinopyroxene Fe-Mg geothermometry yields temperatures mostly in the range of 900-1100 °C. A formerly supersilicic nature of garnet in enstatite eclogite as inferred from exsolution mineralogy indicates minimum peak pressure of 5 GPa.

  10. 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

  11. 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

  12. 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.

  13. 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

  14. Deep seated inclusions in kimberlites from Kharamai field and some kimberlite fields of Prianabarie.

    NASA Astrophysics Data System (ADS)

    Ashchepkov, I. V.; Kuligin, S. S.; Afanasiev, V. P.; Vladykin, N. V.; Kostrovitsky, S. I.; Lelyukh, M. I.; Vavilov, M. A.; Nigmatulina, E. N.; Palessky, S. V.

    2012-04-01

    The problem of the thickness of the lithospheric keel in the northern part of Yakutia is critical for the diamond grade. Reported delamination of the SCLM (Griffin et al., 2005)which is not supported by the geophysical methods (Koulakov et al ., 2011) was checked in number of localities in circum Anabar region. Pyropes, chrome-diopside, omphacites, enstatite, chromites and ilmenites from concentrate three kimberlitic pipes of the Kharamai field revealed compositional variations typical for thick SCLM: pyropes to 13% Of cr2O3 in the association with chromites (to 60% Of cr2O3.) low- Al - Cr diopsides and enstatite, omphacites, Cr-pargasite and K-Na richterite and picroilmenites (to 20% Of MgO). The pyropes belong to lherzolite filed (Sobolev et al., 1973) as those published only in the low-chromium part relate to Ca- Fe to pyroxenite magmatic trend (Tychkov et al., 2008) which is typical for the majority of Mesozoic (especially Jurassic) kimberlitic pipes in Prianabarie and other northern parts of Yakutia. Thermo-barometric reconstructions using four methods of monomineral thermobarometry reveal the thickness of lithospheric keel not less than 200 km which coinsides with the determined thickness of the SCLM beneath nearest Ary- Mastakh filed (Khardakh pipe). Straya Rechka and Kuranakh fields (Universitetskaya, Tudovaya , Los kimberlites etc). But the pipes in Evenkiyskaya kimberlite group (including Malysh and Tuzic pipes) carry material dominantly from the upper part of the SCLM, sub-calcium garnets are rare. Relatively high oxidation state, determined according to chromite, pyroxenes and pyropes are in accord with the rather low diamond grades. However some other Jurassic pipes in northern part of the Siberian craton reveal sufficiently deep mantle roots evidencing about lack of the delamination of mantle keel after Siberian PT superplume. The trace elements determined for the pyropes show variations and belong to the melt metasomatized groups I the middle and

  15. 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

  16. Garnets from kimberlites of northeast Angola and the relation of their composition to diamond content

    NASA Astrophysics Data System (ADS)

    Sobolev, N. V.; Mankenda, A.; Kaminskii, F. V.; Sobolev, V. N.

    High-chromium subcalcic pyropes were found in kimberlites from northeast Angola. A positive correlation between the content of these pyropes in concentrates with the diamond content of kimberlites is established, and it is thus concluded that such pyropes can serve as indicators in estimating the diamond content of kimberlites.

  17. 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

  18. 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.

  19. The Catoca kimberlite pipe, Republic of Angola: A paleovolcanological model

    NASA Astrophysics Data System (ADS)

    Pervov, V. A.; Somov, S. V.; Korshunov, A. V.; Dulapchii, E. V.; Félix, J. T.

    2011-08-01

    The Catoca kimberlite pipe is among the world's largest primary diamond deposits. The Catoca volcanic edifice is only slightly eroded. Kimberlitic rocks of various facies compose a crater of about 1 km in diameter and a diatreme. The structure of the pipe and mining conditions of the deposit are complicated by intense intrapipe tectonic processes related to large-amplitude subsidence. Based on geological data, we propose a structural model of the deposit and a paleovolcanological model of the Catoca pipe formed during a full cycle beginning with a stage of active volcanism and completed by stages of gradually waning volcanic activity and sedimentation. It is suggested that the banded tuffisitic kimberlite of the crater zone was deposited at the stage of active volcanic eruption from specific pyroclastic suspension as a low-viscosity mixture of crystals and aqueous sol rich in serpentine.

  20. 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.

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

    PubMed

    Basu, A R; Tatsumoto, M

    1979-07-27

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

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

    USGS Publications Warehouse

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

    1981-01-01

    Between 1876 and 1913, diamonds were found in at least seven localities in southern and central Wisconsin. All were found in Pleistocene glacial deposits or Holocene river gravel. The bedrock kimberlite source for the diamonds is unknown but has been presumed to be in northern Canada, the only area north of Wisconsin previously known to contain kimberlites. Recently, a kimberlite pipe, here named the Lake Ellen kimberlite, has been found in Iron County, Michigan. That find suggests the possibility that drift diamonds in Wisconsin have come from a more local source--kimberlites in northern Michigan and Wisconsin. The Lake Ellen kimberlite is very poorly exposed, but a strong positive magnetic anomaly indicates that it is roughly circular in plan and about 200 m in diameter. Although the kimberlite is entirely surrounded by Precambrian rocks, it contains abundant inclusions of fossiliferous dolomite, probably from the Ordovician Black River Group that overlay the area when the kimberlite was intruded. The post-Ordovician age of the kimberlite leads us to suspect that other possible cryptovolcanic structures in Paleozoic rocks in the region were formed over kimberlite pipes that are not yet exposed by erosion. Such structures include Limestone Mountain and Sherman Hill, in Houghton and Baraga Counties, Michigan; Glover Bluff, in Marquette County, Wisconsin; and possibly an area along the Brule River south of Iron River, Michigan. No diamonds are known in the Lake Ellen kimberlite, but it has not been adequately sampled. The cryptovolcanic structures could not be the source of the drift diamonds in Wisconsin because even if the structures are caused by kimberlites, those kimberlites have not yet been exposed by erosion. Elsewhere in the world, kimberlite is seldom found as a single isolated body; clusters of bodies are more common, and the presence of one kimberlite implies that others may exist nearby. The discovery of additional kimberlites may be very difficult

  3. 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, CO 2 and H 2O+, higher Ca, Mn, Nb, Sr, P, Fe 3+/Fe 2+ ratio, U, Co, and Cu, approximately equal TiO 2, K 2O, Na 2O, La, Ta, Rb, Zr, Zn and resultant lower SiO 2, Al 2O 3, Ga and Y. The metasomatite Na 2O/K 2O

  4. 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

  5. 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

  6. Reconstruction of a multi-vent kimberlite eruption from deposit and host rock characteristics: Jericho kimberlite, Nunavut, Canada

    NASA Astrophysics Data System (ADS)

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

    2011-03-01

    The Jericho kimberlite (173.1 ± 1.3 Ma) is a small (~ 130 × 70 m), multi-vent system that preserves products from deep (> 1 km?) portions of kimberlite vents. Pit mapping, drill core examination, petrographic study, image analysis of olivine crystals (grain size distributions and shape studies), and compositional and mineralogical studies, are used to reconstruct processes from near-surface magma ascent to kimberlite emplacement and alteration. The Jericho kimberlite formed by multiple eruptions through an Archean granodiorite batholith that was overlain by mid-Devonian limestones ~ 1 km in thickness. Kimberlite magma ascended through granodiorite basement by dyke propagation but ascended through limestone, at least in part, by locally brecciating the host rocks. After the first explosive breakthrough to surface, vent deepening and widening occurred by the erosive forces of the waxing phase of the eruption, by gravitationally induced failures as portions of the vent margins slid into the vent and, in the deeper portions of the vent (> 1 km), by scaling, as thin slabs burst from the walls into the vent. At currently exposed levels, coherent kimberlite (CK) dykes (< 40 cm thick) are found to the north and south of the vent complex and represent the earliest preserved in-situ products of Jericho magmatism. Timing of CK emplacement on the eastern side of the vent complex is unclear; some thick CK (15-20 m) may have been emplaced after the central vent was formed. Explosive eruptive products are preserved in four partially overlapping vents that are roughly aligned along strike with the coherent kimberlite dyke. The volcaniclastic kimberlite (VK) facies are massive and poorly sorted, with matrix- to clast-supported textures. The VK facies fragmented by dry, volatile-driven processes and were emplaced by eruption column collapse back into the volcanic vents. The first explosive products, poorly preserved because of partial destruction by later eruptions, are found in

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

    NASA Astrophysics Data System (ADS)

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

    2009-11-01

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

  8. 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

  9. Geochemistry of mantle metasomatism related to formation of kimberlites in the northern East European Platform

    NASA Astrophysics Data System (ADS)

    Kargin, A. V.

    2014-11-01

    The geochemistry of mantle metasomatism related to the formation of kimberlites and allied rocks in the northern East European Platform (EEP) is considered with allowance for chemical systematics and geodynamic position. The Paleoproterozoic kimberlites of Kimozero, Mesoproterozoic orangeites of Karelia, Neoproterozoic kimberlites of Finland, and Devonian kimberlites of the Arkhangel'sk diamond province (ADP) are the objects of this research. Kimberlites from the EEP are characterized by wide variations in contents of rock-forming oxides and trace elements caused by secondary alteration, belonging to different lithofacies and specific sources. The diversity of kimberlites from the EEP is explained by the interaction of asthenospheric protokimberlitic melt with depleted or metasomatically enriched lithospheric mantle. A (Zr/Sm) n -Cr/Ni diagram has been proposed to identify specific attributes of mantle metasomatism. Comparison of the geodynamic setting of kimberlite formation in the EEP with the revealed geochemical features of mantle metasomatism shows that kimberlites having sources with the participation of MARID-type meta-somatic assemblages were formed under conditions of changing supercontinental cycles, when the breakdown of large supercontinent coincided in time with the initial stage of assembly of a new supercontinent. This is characteristic of the Kimozero kimberlites, orangeites of Karelia, and kimberlites from the ADP. Kimberlites of Finland were formed under geodynamic conditions characterized by supercontinent breakdown coeval with abundant within-plate mafic magmatism. These kimberlites bear geochemical features inherent to an asthenospheric source. The economic Devonian diamond-bearing kimberlites from the ADP display a subordinate role of the metasomatic lithospheric as a component of their source against the background of a significant interaction of melted asthenospheric material with the depleted lithospheric mantle.

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

    PubMed

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

    2013-01-01

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

  11. 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.

  12. The origin of pelletal lapilli in explosive kimberlite eruptions

    NASA Astrophysics Data System (ADS)

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

    2012-05-01

    Kimberlites are volatile-rich magmas from mantle depths of >=150km 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.

  13. 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.

  14. Linear stability analysis for hydrothermal alteration of kimberlitic rocks

    NASA Astrophysics Data System (ADS)

    Afanasyev, Andrey; Belyaeva, Ekaterina

    2016-04-01

    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 serpentinisation in slightly warmer regions of the kimberlite deposit. We conduct linear stability analysis of serpentinisation 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 destabilise the uniform distribution of parameters and the heat conduction tending to stabilise 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. Keywords: Hydrothermal systems, volcaniclastic deposits, phase transitions, instability analysis, numerical solutions

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

    PubMed

    Correia, Eugénio A; Laiginhas, Fernando A T P

    2006-06-01

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

  16. An integrated model of kimberlite ascent and eruption.

    PubMed

    Wilson, Lionel; Head Iii, James W

    2007-05-01

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

  17. 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.

  18. 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.

  19. 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.

  20. 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

  1. 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

  2. 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

  3. 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

  4. 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

  5. Geology of the Orion South kimberlite, Fort à la Corne, Canada

    NASA Astrophysics Data System (ADS)

    Kjarsgaard, B. A.; Harvey, S.; McClintock, M.; Zonneveld, J. P.; Du Plessis, P.; McNeil, D.; Heaman, L.

    2009-11-01

    A wide variety of primary pyroclastic, volcaniclastic and re-sedimented volcaniclastic deposits derived from multiple kimberlite eruptions are exceptionally well preserved at the Orion South kimberlite body in the Fort à la Corne field in central Saskatchewan. Construction of this kimberlite complex involved episodic kimberlite volcanic events punctuated by periods of volcanic quiescence, erosion and sedimentary deposition within the Western Canadian Sedimentary Basin. The Orion South kimberlite complex erupted into deltaic coastal plain/transitional estuarine (Mannville Group: Cantuar and Pense formations) and shallow marine (Lower Colorado Group: Joli Fou, Viking and Westgate formations) environments. At Orion South, eight distinct kimberlite phases erupted over 6 to 7 million years from ca. 106 Ma (Cantuar Formation equivalent) to ca. 99 Ma (Viking Formation equivalent). Detailed core logging, geochemistry, chronostratigraphy, petrography and geophysics were undertaken to define distinct eruptive phases and reconstruct the depositional environment, volcanic styles and 3-D architecture of the complex. The eight main kimberlite deposits record eruptive styles ranging from magmatic, to 'wet' and 'dry' phreatomagmatic, to submarine eruptions, a consequence of the varying interaction between kimberlite magma and seawater and/or groundwater.

  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. 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.

  8. 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.

  9. 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

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

    USGS Publications Warehouse

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

    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.

  11. 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

  12. 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

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

    PubMed

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

    2012-01-01

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

  14. Nature and origin of eclogite xenoliths from kimberlites

    NASA Astrophysics Data System (ADS)

    Jacob, D. E.

    2004-09-01

    Eclogites from the Earth's mantle found in kimberlites provide important information on craton formation and ancient geodynamic processes because such eclogites are mostly Archean in age. They have equilibrated over a range of temperatures and pressures throughout the subcratonic mantle and some are diamond-bearing. Most mantle eclogites are bimineralic (omphacite and garnet) rarely with accessory rutiles. Contrary to their overall mineralogical simplicity, their broadly basaltic-picritic bulk compositions cover a large range and overlap with (but are not identical to) much younger lower grade eclogites from orogenic massifs. The majority of mantle eclogites have trace element geochemical features that require an origin from plagioclase-bearing protoliths and oxygen isotopic characteristics consistent with seawater alteration of oceanic crust. Therefore, most suites of eclogite xenoliths from kimberlites can be satisfactorily explained as samples of subducted oceanic crust. In contrast, eclogite xenoliths from Kuruman, South Africa and Koidu, Sierra Leone stem from protoliths that were picritic cumulates from intermediate pressures (1-2 Ga) and were subsequently transposed to higher pressures within the subcratonic mantle, consistent with craton growth via island arc collisions. None of the eclogite suites can be satisfactorily explained by an origin as high pressure cumulates from primary melts from garnet peridotite.

  15. 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

  16. 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

  17. 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

  18. 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

  19. 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

  20. 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

  1. 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.

  2. 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

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

    NASA Astrophysics Data System (ADS)

    Chemia, Zurab; Artemieva, Irina; Thybo, Hans

    2015-04-01

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

  4. 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

  5. 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.

  6. 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.

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

    PubMed

    McCallum, M E; Eggler, D H

    1976-04-16

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

  8. 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.

  9. 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.

  10. Diamonds from the V. Grib pipe, Arkhangelsk kimberlite province, Russia

    NASA Astrophysics Data System (ADS)

    Rubanova, E. V.; Palazhchenko, O. V.; Garanin, V. K.

    2009-11-01

    A large collection (717 samples) of diamonds from the V. Grib deposit, discovered in 1996 in the Verhotinskoe field of the Arkhangelsk kimberlite province, was studied. The diamond crystals are characterized by high transparency and preservation. The collection consists of complete crystals (71%), chipped and damaged crystals (preservation > 50%; 14%), and fragments (preservation < 50%; 15%). Resorption is generally moderate resulting in a dominance of octahedral and mixed octahedral-dodecahedral shapes. Moderate resorption points to rapid ascent of the transporting kimberlite magma. A characteristic feature of V. Grib is a large number of green diamonds. This may relate to the close proximity of radioactive deposits. Microscopic green surface spots have no cathodoluminescence. The internal diamond morphology was studied by UV- and cathodoluminescence. The main typomorphic feature of diamond crystals from the V. Grib pipe is a high percentage of crystals without UV-luminescence. Presence of sectorial growth was also identified by luminescence. Analysis of mineral inclusions, carbon isotopic composition, nitrogen content and nitrogen aggregation state provided important genetic information. Peridotitic inclusions (olivine, chromian spinel and pyrope) predominate, sulfides are almost completely absent. The carbon isotopic composition of the host diamonds is typical for peridotitic diamonds worldwide. IR-spectroscopy suggests the presence of two diamond populations with low and high nitrogen concentrations. Three sub-populations may be identified based on a combination of morphology, nitrogen and hydrogen defects. Residence temperatures ( TNitrogen), based on a mantle residence time of 3 Ga, fall between 1050 and 1170 °C. Diamond crystallization in V. Grib occurred in multiple stages. This is documented through luminescence patterns, data on nitrogen concentration and aggregation state, and the presence of "diamond-in-diamond" inclusions.

  11. 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.

  12. 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.

  13. 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.

  14. 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

  15. Geophysical signatures of some recently discovered large (> 40 ha) kimberlite pipes on the Alto Cuilo concession in northeastern Angola

    NASA Astrophysics Data System (ADS)

    Pettit, Wayne

    2009-11-01

    This paper presents a comparison of geophysical responses from several large kimberlite complexes discovered and delineated on the Alto Cuilo concession in the diamond fields of northeastern Angola in the years 2005 to 2008. Several geophysical methods were used in combination with geochemical and mineralogical prioritization techniques to guide exploratory, delineation and bulk sample drilling, in order to rapidly identify and evaluate the kimberlite bodies. The kimberlites were emplaced through Karoo Supergroup sandstones and shales, have eruption ages contemporaneous with the sand-dominated Cretaceous-age Calonda Formation, and are covered by sand-dominated poorly consolidated sediments of the Kalahari Group. Given that sand-dominated non-kimberlite lithologies are magnetically transparent, a low level, high resolution helicopter-borne magnetic gradiometer survey proved to be exceptionally effective in discriminating kimberlite targets, even for low-amplitude anomalies (e.g. 1-2 nT). The helicopter magnetic data outlined approximately 244 probable kimberlite targets and drilling of 103 targets confirmed 80 new kimberlites greater than 5 ha in area. Most kimberlites take the form of well-preserved crater edifices containing a full range of crater-related kimberlite lithologies. Ground gravity and electromagnetic surveys were conducted over all kimberlites prioritized for follow-up investigation. Geophysical responses were ground-truthed against magnetic susceptibility and density measurements, which were routinely collected on all drill cores. The geophysical signatures resolved by the three independent geophysical methods were surprisingly variable and are inferred to be sourced primarily in the crater facies materials, which demonstrate characteristically variable lithologies. Geophysical interpretations guided the drill targeting at all stages of the program at Alto Cuilo, from exploration to evaluation. Combined with geochemical and mineralogical

  16. 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

  17. Are Monogenetic Basaltic Explosive Volcanoes Good Analogues for Explosive Kimberlite Volcanoes?

    NASA Astrophysics Data System (ADS)

    Cas, R. A.

    2009-05-01

    There is uncertainty as to the nature of kimberlite volcanoes because the edifices of ancient kimberlite volcanoes have been largely eroded. Monogenetic basaltic explosive volcanoes, such as scoria or cinder cones and maars, are often proposed as good analogues for kimberlite volcanoes. Basaltic scoria or cinder cones form from explosive eruptions driven by exsolving magmatic gases. The sparsity of country rock xenoliths in the deposits of scoria cones indicates the explosive fragmentation surface lay at or above the pre- eruption ground surface in the vent and conduit system, and therefore a sub-surface diatreme or pipe-like conduit cannot be formed. Basaltic scoria cones are therefore not good analogues for most kimberlite pipes and volcanoes. By contrast, the volcaniclastic kimberlite pipe fill of most kimberlite pipes contains a significant proportion of country rock xenolith and xenocryst debris (10 to 50 modal percent or more), indicating that the fragmentation surface lay below the ground surface, and this was responsible for explosively excavating the subsurface diatreme or pipe like conduit. Some authors propose that this is consistent with kimberlite pipes being the remains of maar volcanoes. The deposits of maar volcanoes usually contain substantial country rock debris suggesting excavation of a significant subsurface conduit has occurred. Maar forming explosive eruptions are phreatomagmatic, driven in large part by explosive superheating of ground water as magma passes upwards through a shallow crustal aquifer. The deposits however commonly show two key characteristics, base surge deposits and accretionary lapilli. Although these occur in some kimberlite deposits, attesting to the occurrence of phreatomagmatic explosive activity in those kimberlite volcanoes, in the absence of such features occurring universally in kimberlite pipes, it is difficult to support a maar volcanic model for kimberlite pipes. If the volatile content in a rising magma is high

  18. 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

  19. 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

  20. 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

  1. 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

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

    PubMed

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

    2012-01-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2015-09-01

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

  4. 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.

  5. Eruption processes and facies architecture of the Orion Central kimberlite volcanic complex, Fort à la Corne, Saskatchewan; kimberlite mass flow deposits in a sedimentary basin

    NASA Astrophysics Data System (ADS)

    Pittari, A.; Cas, R. A. F.; Lefebvre, N.; Robey, J.; Kurszlaukis, S.; Webb, K.

    2008-06-01

    The Fort à la Corne diamondiferous kimberlite field consists of at least 70 bodies of volcaniclastic kimberlite, hosted within a contemporaneous non-volcanic sedimentary succession. This study of the three-dimensional stratigraphy and facies architecture across the Orion Central kimberlite volcanic complex highlights the variations in upper and extra-vent processes. The sedimentary succession consists of continental to marginal marine quartz sandstones and mudstones, overlain by marginal to deep marine dark mudstone and muddy sandstones and siltstones. Relatively thin conformable volcaniclastic kimberlite packages are interbedded throughout the host rock stratigraphy. Extremely thick (up to at least 211 m thick) discordant to concordant, volcaniclastic packages/series, infill at least three elongate northwest-trending craters (145A, 145B and 219 craters), and contain laterally equivalent conformable extra-crater deposits bound by marine mudstones, indicative of a prevailing dominantly marine environment. The volcaniclastic deposits within the 145A and 145B craters, respectively, are separated by a considerable hiatus, whereas the deposit infilling the 219 crater was formed around the same time as 145B crater deposit. Multiple depositional units of massive to stratified, olivine-rich sand- to pebble-sized volcaniclastic facies infill craters and were emplaced by megaturbidite pulses fed by crystal-rich eruption fountains, which interacted with the crater relief. Stacked, normally graded, thick to very thick bedded matrix-supported olivine-rich facies characterized by brief depositional breaks between some beds represent syn- to post-eruptive turbidite pulses associated with the early eruptive event in the 145A crater. Thin layers of light grey kimberlitic mudstone underlie, or occur near the base of and above the main volcaniclastic packages associated with the 145B and 219 eruptions. Crater-infilling volcaniclastic deposits were later reworked by storm induced

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

    PubMed

    Kelley; Wartho

    2000-07-28

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

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

    NASA Astrophysics Data System (ADS)

    Stamm, N.; Schmidt, M. W.

    2015-12-01

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

  8. 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

  9. Geology of the Gahcho Kué kimberlite pipes, NWT, Canada: root to diatreme magmatic transition zones

    NASA Astrophysics Data System (ADS)

    Hetman, C. M.; Scott Smith, B. H.; Paul, J. L.; Winter, F.

    2004-09-01

    The Cambrian Gahcho Kué kimberlite cluster includes four main pipes that have been emplaced into the Archaean basement granitoids of the Slave Craton. Each of the steep-sided pipes were formed by the intrusion of several distinct phases of kimberlite in which the textures vary from hypabyssal kimberlite (HK) to diatreme-facies tuffisitic kimberlite breccia (TKB). The TKB displays many diagnostic features including abundant unaltered country rock xenoliths, pelletal lapilli, serpentinised olivines and a matrix composed of microlitic phlogopite and serpentine without carbonate. The HK contains common fresh olivine set in a groundmass composed of monticellite, phlogopite, perovskite, serpentine and carbonate. A number of separate phases of kimberlite display a magmatic textural gradation from TKB to HK, which is characterised by a decrease in the proportion of pelletal lapilli and country rock xenoliths and an increase in groundmass crystallinity, proportion of fresh olivine and the degree of xenolith digestion. The pipe shapes and infills of the Gahcho Kué kimberlites are similar to those of the classic South African pipes, particularly those of the Kimberley area. Similar intrusive magmatic emplacement processes are proposed in which the diatreme-zone results from the degassing, after breakthrough, of the intruding magma column. The transition zones represent 'frozen' degassing fronts. The style of emplacement of the Gahcho Kué kimberlites is very different from that of many other pipes in Canada such as at Lac de Gras, Fort à la Corne or Attawapiskat.

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

    NASA Astrophysics Data System (ADS)

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

    2015-11-01

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

  11. Mineral chemistry of a zircon-bearing, composite, veined and metasomatised upper-mantle peridotite xenolith from kimberlite

    NASA Astrophysics Data System (ADS)

    Dawson, J. B.; Hill, P. G.; Kinny, P. D.

    2001-02-01

    Zircon-bearing veins in a harzburgite xenolith from kimberlite have imposed Ca-metasomatism on the harzburgite wall rock, in addition to adding K, Fe, Ti and OH. The zircon, previously dated to have an age similar to that of the xenolith-hosting kimberlite, shows higher Y, Nb, Ba, REE, Th and U contents than other mantle-derived zircons. Peripheral alteration of the zircon to baddeleyite and zirconolite, and alteration of vein ilmenite to perovskite suggest reaction with an evolving carbonatitic kimberlite melt. The high Cr2O3 content (0.77 wt%) of the zirconolite extends the compositional range of terrestrial zirconolite.

  12. 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.

  13. 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

  14. 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.

  15. 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

  16. 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

  17. 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.

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

    NASA Astrophysics Data System (ADS)

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

    2009-05-01

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

  19. 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

  20. 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.

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

    PubMed

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

    2013-09-01

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

  2. Hydroxyl in mantle olivine xenocrysts from the Udachnaya kimberlite pipe

    NASA Astrophysics Data System (ADS)

    Koch-Müller, Monika; Matsyuk, Stanislav S.; Rhede, Dieter; Wirth, Richard; Khisina, Natasha

    2006-06-01

    The incorporation of hydrogen in mantle olivine xenocrysts from the Udachnaya kimberlite pipe was investigated by Fourier-transform infrared spectroscopy and secondary ion mass spectrometry (SIMS). IR spectra were collected in the OH stretching region on oriented single crystals using a conventional IR source at ambient conditions and in situ at temperatures down to -180°C as well as with IR synchrotron radiation. The IR spectra of the samples are complex containing more than 20 strongly polarized OH bands in the range 3,730 3,330 cm-1. Bands at high energies (3,730 3,670 cm-1) were assigned to inclusions of serpentine, talc and the 10 Å phase. All other bands are believed to be intrinsic to olivine. The corresponding point defects are (a) associated with vacant Si sites (3,607 cm-1 E || a, 3,597 E || a, 3,571 cm-1 E || c, 3,567 E || c, and 3,556 E || b), and (b) with vacant M1 sites (most of the bands polarized parallel to a). From the pleochroic behavior and position of the OH bands associated with the vacant M1 sites, we propose two types of hydrogen—one bonded to O1 and another to O2, so that both OH vectors are strongly aligned parallel to a. The O2 H groups may be responsible for the OH bands at higher wavenumbers than those for the O1 H groups. The multiplicity of the corresponding OH bands in the spectra can be explained by different chemical environments and by slightly different distortions of the M1 sites in these high-pressure olivines. Four samples were investigated by SIMS. The calculated integral molar absorption coefficient using the IR and SIMS results of 37,500±5,000 L mol H2O cm-2 is within the uncertainties slightly higher than the value determined by Bell et al. (J Geophys Res 108(B2):2105 2113, 2003) (28,450±1,830 L mol H2O cm-2). The reason for the difference is the different distributions of the absorption intensity of the spectra of both studies (mean wavenumber 3,548 vs. 3,570 cm-1). Olivine samples with a mean wavenumber of about 3

  3. 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

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

    PubMed

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

    2011-10-01

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

  5. 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

  6. 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.

  7. 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

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

    NASA Astrophysics Data System (ADS)

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

    2016-02-01

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

  9. 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).

  10. 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.

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

    NASA Astrophysics Data System (ADS)

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

    2015-10-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2009-11-01

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

  13. Kimberlite wall-rock fragmentation processes: Venetia K08 pipe development

    NASA Astrophysics Data System (ADS)

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

    2011-10-01

    Current kimberlite pipe development models strongly advocate a downward growth process with the pipe cutting down onto its feeder dyke by means of volcanic explosions. Evidence is presented from the K08 kimberlite pipe in Venetia Mine, South Africa, which suggests that some pipes or sub-components of pipes develop upwards. The K08 pipe in pit exposure comprises >90 vol.% chaotic mega-breccia of country rock clasts (gneiss and schist) and <10 vol.% coherent kimberlite. Sub-horizontal breccia layers, tens of metres thick, are defined by lithic clast size variations and contain zones of shearing and secondary fragmentation. Textural studies of the breccias and fractal statistics on clast size distributions are used to characterize sheared and non-sheared breccia zones and to deduce a fragmentation mechanism. Breccia statistics are compared directly with the statistics of fragmented rock produced from mining processes in order to support interpretations. Results are consistent with an initial stage of brecciation formed by upward-moving collapse of an explosively pre-conditioned hanging wall into a sub-terranean volcanic excavation. Our analysis suggests that the pre-conditioning is most likely to have been caused by explosions, either phreatic or phreatomagmatic in nature, with a total energy output of 2.7 × 109 kJ (656 t of TNT). A second stage of fragmentation is interpreted as shearing of the breccia caused by multiple late kimberlite intrusions and possible bulk movement of material in the pipe conduit related to adjacent volcanism in the K02 pipe.

  14. The geology and emplacement of the volcaniclastic infill at the Voorspoed Group II kimberlite (orangeite) pipe, Kroonstad Cluster, South Africa

    NASA Astrophysics Data System (ADS)

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

    2012-06-01

    The Voorspoed Group II kimberlite pipe is atypical in terms of South African kimberlite pipes. Located in the central region of the Kaapvaal Craton (South Africa), the Voorspoed pipe is one of six pipes comprising the Kroonstad Kimberlite Cluster. Reconstruction of the palaeo-stratigraphy at the time of emplacement, of the Kroonstad kimberlites, indicates that significant post-emplacement erosion has occurred and the pipes are currently exposed at ~ 1600 m depth from the original land surface. The volcaniclastic kimberlite (VK) infilling the pipe is distinctly different from typical tuffisitic kimberlite infilling other South African type kimberlite pipes. Three textural volcaniclastic kimberlite varieties are observed: massive volcaniclastic kimberlite (MVK), bedded volcaniclastic kimberlite (BVK) and fine-grained volcaniclastic kimberlite (fgVK) layers. The BVK and fgVK are volumetrically insignificant and the bulk of the pipe is infilled with MVK. The MVK can be further sub-divided into seven varieties. The two dominant varieties are described in detail, which include: main MVK (mMVK) and the juvenile-rich MVK (jrMVK). The MVKs are typically massive, clastic, poorly sorted and supported by an interclast material consisting of juvenile and xenocrystic crystals in a mud-rich matrix. Magmaclasts within the MVKs are spherical, crystalline with a typical coherent hypabyssal texture and different mineralogical varieties are typically juxtaposed. BVK units are composed of several normally graded beds of volcaniclastic material. fgVK layers are well sorted relative to the MVKs and all constituents are typically < 2 mm in size. The BVK and fgVK can be deposited only within an open vent in order to account for the sorting of the components. A distinct basalt and sandstone rich unit (bsBreccia), which is essentially devoid of kimberlite material, is also observed. Three volcaniclastic zones are identified based on consistent vertical variation in the internal stratigraphy

  15. 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

  16. Petrology of two diamondiferous eclogite xenoliths from the Lahtojoki kimberlite pipe, eastern Finland

    NASA Astrophysics Data System (ADS)

    Peltonen, P.; Kinnunen, K. A.; Huhma, H.

    2002-08-01

    Diamondiferous Group A eclogites constitute a minor portion of the mantle-derived xenoliths in the eastern Finland kimberlites. They have been derived from the depth interval 150-230 km where they are inferred to occur as thin layers or small pods within coarse-grained garnet peridotites. The chemical and isotopic composition of minerals suggest that they represent (Proterozoic?) mantle-derived melts or cumulates rather than subducted oceanic lithosphere. During magma ascent and emplacement of the kimberlites, the eclogite xenoliths were mechanically and chemically rounded judging from the types of surface markings. In addition, those octahedral crystal faces of diamonds that were partially exposed from the rounded eclogite xenolith became covered by trigons and overlain by microlamination due to their reaction with the kimberlite magma. The diamonds bear evidence of pervasive plastic deformation which is not, however, evident in the eclogite host. This suggests that annealing at ambient lithospheric temperatures has effectively recrystallised the silicates while the diamond has retained its lattice imperfections and thus still has the potential to yield information about ancient mantle deformation. One of our samples is estimated to contain approximately 90,000 ct/ton diamond implying that some diamonds occur within very high-grade pods or thin seams in the lithospheric mantle. To our knowledge, this is one of the most diamondiferous samples described.

  17. Eclogite xenoliths in west African kimberlites as residues from Archaean granitoid crust formation

    NASA Astrophysics Data System (ADS)

    Rollinson, Hugh

    1997-09-01

    Eclogites are a comparatively rare but petrologically important member of kimberlite xenolith suites. Their broadly basaltic chemistry has led many authors to propose that they represent ancient, subducted ocean crust. Recent studies, however, have suggested an alternative origin and propose that kimberlitic eclogites are residues from the process of Archaean granitoid crust formation. Geochemical arguments in support of this new model were previously based on the trace-element chemistry of eclogitic minerals. Here I report that the major-element chemistry of eclogite xenoliths also supports a crustal residue model. I examine eclogite xenoliths from kimberlite pipes at Koidu, Sierra Leone, which sample the lithospheric mantle underlying the Archaean (2.8Gyr) granitoid crust of the West African craton. Geochemical plots of major elements measured in unaltered, whole-rock samples of low-silica eclogite demonstrate that they are complementary to the granitoids of the West African craton and have compositions which indicate that both were derived from a common basaltic parent rock.

  18. U-Pb geochronology and Sr/Nd isotope compositions of groundmass perovskite from the newly discovered Jurassic Chidliak kimberlite field, Baffin Island, Canada

    NASA Astrophysics Data System (ADS)

    Heaman, Larry M.; Pell, Jennifer; Grütter, Herman S.; Creaser, Robert A.

    2015-04-01

    We report the U-Pb age and Sr/Nd isotope composition for perovskite isolated from forty six kimberlite samples located in the newly discovered Chidliak field on Baffin Island, Canada. The minimum duration of kimberlite magmatism in this field was 17.9 m.y. from 157.0 to 139.1 Ma and represents a new Jurassic kimberlite field in NE Canada. The most prolific period of kimberlite magmatism occurred between 152 and 142 Ma (80% of dated kimberlites). Kimberlitic perovskite from these intrusions display a range in 87Sr/86Sr (0.7043 to 0.7030) and εNdT values (+3.9 to -0.4), overlapping the isotopic field previously defined for southern African Group I kimberlites. The ages and isotopic compositions obtained for Chidliak magmatism are identical to a number of Jurassic kimberlite fields in eastern North America and SW Greenland. Some of this Jurassic kimberlite magmatism has a link to one or more mantle plume hotspot tracks but the Chidliak kimberlites have an origin in the deep subcontinental lithospheric mantle and are part of a Jurassic magmatic province that erupted along both margins of Davis Strait; linked to upwelling asthenosphere, continental rifting, and Mesozoic-Cenozoic development of oceanic crust in the Labrador Sea basin. In contrast, the location of other eastern North American Jurassic kimberlites when plotted on a Jurassic continental reconstruction aligns closely to the northernmost projection of contours 2-4 of the African large low shearwave velocity province, consistent with a link to mantle plumes derived from the African mantle plume generating zone.

  19. 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.

  20. 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

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

    NASA Astrophysics Data System (ADS)

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

    2015-12-01

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

  2. Use of RADARSAT-1 satellite imagery and geophysical data for oil and kimberlite exploration

    NASA Astrophysics Data System (ADS)

    Paganelli, Flora

    The synergy of RADARSAT-1 and seismic imagery interpretation has been applied in the Blackstone area of the Central Alberta Foothills in the Canadian Cordillera thrust and fold belt to map the continuity of geological structures, which are of importance for oil and gas exploration. The reconstruction of the continuity of thrust-fold related major structures known in the area has been successful. Transverse faults and lineaments with ENE-WSW, NE-SW, and NNE-SSW trends have been delineated on the radar images. The ENE-WSW transverse faults have an extensional character, cut across the inner and outer Foothills and are persistent at the regional scale. The NE-SW and NNE-SSW transverse faults are wrench type faults, which are mainly localized in the inner Foothills. These structures have been identified for the first time in the area and are possibly a third generation fault-play type for oil and gas exploration. Principal Component Analysis (PCA) of RADARSAT-1 images was applied in the Buffalo Head Hills area, in the Western Canada Sedimentary Basin (WCSB), to provide an enhanced image base for structural mapping. North- and NNE-trending lineaments bounding the eastern edge of the Buffalo Head Hills along the Loon River valley, a conjugate set of NW- and NE-trending lineaments, and ENE-trending lineaments identifying the latest features in the area were outlined. The development of these structures has been related to Precambrian terrane assemblage in the WCSB during the Early Proterozoic, the development of the Peace River Arch, and the Laramide Orogeny. In the Buffalo Head Hills area a weights of evidence statistical approach was used to determine the spatial relationship of NNE-, NE-, -NW, and ENE-trending lineaments to known kimberlite locations. This method outlined different degrees of spatial correlation between kimberlites and lineaments, with higher correlations defined for the NNE, NE, and ENE lineament datasets. A weights of evidence model was then

  3. Contrasting eruption styles of the 147 Kimberlite, Fort à la Corne, Saskatchewan, Canada

    NASA Astrophysics Data System (ADS)

    Lefebvre, Nathalie; Kurszlaukis, Stephan

    2008-06-01

    The Cretaceous Fort à la Corne (FALC) kimberlite field was active over a time span of ~ 20 Ma with contemporaneous terrestrial (Mannville Group) to marine (Lower Colorado Group) background sedimentation. Steep-sided pipes, craters and positive landform volcanoes such as scoria or tuff cones are thought to have formed during that period. The 147 Kimberlite is located in the SE section of the field's main cluster and is part of the large (~ 377.5 ha) Orion North volcanic complex. Based on logging of 25 drill cores, the morphology of the country rock/kimberlite interface suggests excavation of a complex crater field down to the upper portion of the Mannville Group sedimentary deposits. At least two types of volcaniclastic deposits are identified: a main kimberlite unit that is typically characterized by crustal xenolith-poor (1-2%), normal graded beds possibly deposited as turbidites in a subaqueous environment, originating from the nearby 148 tephra cone and infilling the adjacent 147 crater, and a second unit, located on the NE margin of the 147 Kimberlite, that represents a thick (~ 60 m) sequence of large (up to 22 m) sedimentary country rock blocks located at least 60 m above their original stratigraphic position. We suggest the following time sequence of events: Crater excavation as a consequence of a shallow magma fragmentation level within the uppermost country rock sequences, together with several closely spaced eruptive centres initially formed the complex, intercalated crater field. Subsequently, ongoing eruptions with a fragmentation level above the country rock produced the lithic fragment poor main infill of the 148 Kimberlite. Resedimentation from the outer flanks of the 148 tephra cone resulted in the deposition of turbidites in the 147 area. A consolidation phase solidified the lowermost portion of the main infill in 147. A subsequent explosion(s) occurred within the Mannville Group in the 147 area, ejecting large blocks of sedimentary country rocks

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

    PubMed

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

    2006-03-01

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

  5. 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.

  6. Passive kimberlite intrusion into actively dilating dyke-fracture arrays: evidence from fibrous calcite veins and extensional fracture cleavage

    NASA Astrophysics Data System (ADS)

    Basson, I. J.; Viola, G.

    2004-09-01

    Calcite veins are invariably associated with en-echelon kimberlite dyke-fracture arrays. A detailed microstructural study of veining indicates four vein types. Type I stretched or ataxial veins are defined by high aspect ratio calcite fibers that are crystallographically continuous with calcite of the kimberlite matrix wall rock, by elongated phenocrystic phlogopite with sharp crystal terminations centered on contacts between adjacent calcite fibers and by phenocrystic phlogopite that grows or extends across these veins. Type I vein mineralogy indicates syn-dilational crystallization of vein minerals in local tensional areas within the kimberlite. Vein Types II (stretched to syntaxial elongate-blocky) and III (antitaxial) indicate late crystallization vein mineral growth during subsequent or repeated dilation. Calcite fibers in Type I to Type III veins are orthogonal to the contacts of their host dykes regardless of the orientation of vein margins. Type IV calcite veins, with blocky or mosaic/polycrystalline textures, are attributed to minor post-intrusion extension, which was potentially accompanied by repeated kimberlite intrusion within a given dyke array. Syn-crystallization/syn-intrusion Type I veins and an ubiquitous dyke-parallel fracture cleavage, in a zone up to 4 m on either side of dyke contacts, suggest that en-echelon kimberlite dyke-fracture arrays occupied the approximate center of zones of active dilation within the brittle carapace of the upper crust. Type II and III veins indicate that extension or dilation continued, independently of an occupying kimberlite fluid phase, after initial intrusion. Arrested mobile hydrofracturing, under low differential stress within the upper brittle or seismic carapace of the continental crust, followed by repeated dilation of the dyke-fracture system, is proposed as a mechanism for producing the features observed in this study. The conditions constrained in this study indicate passive dyke intrusion into dilating

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

    USGS Publications Warehouse

    Carter, Hearn B., Jr.

    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

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

    NASA Astrophysics Data System (ADS)

    Carter Hearn, B.

    2004-09-01

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

  9. 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.

  10. 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.

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

    NASA Astrophysics Data System (ADS)

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

    2009-11-01

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

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

    SciTech Connect

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

    1985-01-01

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

  13. 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.

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

    NASA Astrophysics Data System (ADS)

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

    2016-04-01

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

  15. 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

  16. Moissanite (SiC) from kimberlites: Polytypes, trace elements, inclusions and speculations on origin

    NASA Astrophysics Data System (ADS)

    Shiryaev, A. A.; Griffin, W. L.; Stoyanov, E.

    2011-03-01

    An extensive collection of moissanite (SiC) grains from the Mir, Aikhal and Udachnaya kimberlite pipes of Yakutia has been characterized in terms of structural perfection, defects and the major- and trace-element chemistry of SiC and its included phases. The natural grains are clearly distinct from synthetic SiC produced by various methods. Most of the natural SiC grains are 6H and 15R polytypes. Some of the grains (< 10%) show extremely complex Raman spectra indicating strongly disordered structures. Some grains also show zoning in impurities, C-isotope composition and cathodoluminescence brightness. Inclusions are heterogeneously distributed within the natural SiC; their size varies from a few nanometers to hundreds of microns. The most abundant inclusions in SiC are Si metal and iron silicide (FeSi 2); a Si-C-O phase with stoichiometry close to Si 4(C,O) 7 probably is related to the silicon oxycarbides. FeSi 2 commonly appears to have exsolved from Si metal; in some cases Ti metal then has exsolved from FeSi 2 to form symplectites. Trace elements are strongly concentrated in the inclusions of FeSi 2 and Si 4(C,O) 7. The trace-element patterns of these phases are generally similar in the different kimberlites, but there are some consistent minor differences between localities. The trace-element patterns of FeSi 2 and Si 4(C,O) 7 are strongly enriched in LREE/HREE and are broadly similar to the patterns of kimberlites, carbonatites and some diamond-forming fluids. However, extreme negative anomalies in Eu (and Sm) suggest highly reducing conditions. Yb also shows strong negative anomalies in FeSi 2 from all three localities, and in Si 4(C,O) 7 from Aikhal and Mir, but not in those from Udachnaya. Trace-element chemistry and the nature of the inclusions provide a reliable basis for distinguishing natural and synthetic SiC. Textural and chemical features and the presence of oxidation products (Si 4(C,O) 7 and SiO 2) suggest that moissanite grew at high temperatures

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

    USGS Publications Warehouse

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

    1984-01-01

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

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

    NASA Astrophysics Data System (ADS)

    Moussallam, Yves; Morizet, Yann; Gaillard, Fabrice

    2016-08-01

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

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

    PubMed

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

    2012-06-01

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

  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. PMID:12961906

  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., Jr.; 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. 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.

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

    NASA Astrophysics Data System (ADS)

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

    2012-12-01

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

  5. 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

  6. 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

  7. 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

  8. Rheological investigations of tailings of kimberlite ore dressing and numerical simulation of its behaviour in PLAXIS

    NASA Astrophysics Data System (ADS)

    Korshunov, A.; Nevzorov, A.

    2015-04-01

    The article presents the results of analysis of rheology properties of sandy-clay tailings (wastes) of kimberlite ore dressing of the diamond deposit (Russia, Arkhangelsk region). The coefficient of secondary compression as main parameter of soil's creep is defined by implementing standard one-dimensional consolidation test. The linear correlation between initial void ratio and coefficient of secondary compression of sandy-clay tailings were obtained. For numerical simulation of tailing's behaviour subject to its rheology properties in Soft Soil Creep (SSC) model (time independent behaviour) of PLAXIS software was used. According to laboratory tests calibration of SSC model was implemented. It allows predicting dam's safety and reliability for long-term outlook.

  9. Crust and Upper Mantle Structure in the Sarfartoq Kimberlite Province, West Greenland: A Receiver Function Study

    NASA Astrophysics Data System (ADS)

    Dahl-Jensen, Trine; Voss, Peter H.; Møller Steensgaard, Bo; Pinna, Line G.

    2015-04-01

    A marked change in crustal thickness is seen at the deformation boundary between undisturbed Archean core in the south and reworked Archean gneiss in the foreland of the Nagssugtoqidian orogen in West Greenland. In addition, intra-crustal boundaries can be tentativly interpreted. Interpretations on upper mantle structures are less clear. This is the first information on crust and upper mantle structure in the area, which is known for kimberlite, carbonatite and ultramafic lamprophyre occurrences, and diamond exploration. The data consists of two summer seasons of passive seismological data recorded on 5 broad-band seismological stations placed on an almost 200 km long profile crossing the deformation boundary. The stations were installed in the remote area with solar panels and batteries, and recorded two summer seasons. Between 7 and 28 events on the stations were used for the Receiver Function analysis.

  10. Crust and Upper Mantle Structure in the Sarfartoq Kimberlite Province, West Greenland: A Receiver Function Study

    NASA Astrophysics Data System (ADS)

    Dahl-Jensen, T.; Voss, P.; Larsen, L. M.; Steensgaard, B. M.; Pinna, L. G. B.

    2014-12-01

    A marked change in crustal thickness is seen at the deformation boundary between undisturbed Archean core in the south and reworked Archean gneiss in the foreland of the Nagssugtoqidian orogen in West Greenland. In addition, intra-crustal boundaries can be tentativly interpreted. Interpretations on upper mantle structures are less clear. This is the first information on crust and upper mantle structure in the area, which is known for kimberlite, carbonatite and ultramafic lamprophyre occurrences, and diamond exploration. The data consists of two summer seasons of passive seismological data recorded on 5 broad-band seismological stations placed on an almost 200 km long profile crossing the deformation boundary. The stations were installed in the remote area with solar panels and batteries, and recorded two summer seasons. Between 7 and 28 events on the stations were used for the Receiver Function analysis.

  11. 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.

  12. 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.

  13. 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.

  14. 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.

  15. 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

  16. 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

  17. 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

  18. Comparative analysis of ore-bearing structures in Maiskoe, Markha, and Ozernoe kimberlite bodies at the Nakyn field, Yakutia

    NASA Astrophysics Data System (ADS)

    Ignatov, P. A.; Novikov, K. V.; Shmonov, A. M.; Razumov, A. N.; Kilizhikov, O. K.

    2015-03-01

    Quantitative characteristics of ore-bearing structures in kimberlite dikes and veins at the Nakyn field (Yakutia) were compared, and local tension and compression zones were identified. According to the results, dikes, veins and their swells are controlled by local strike-slip tension zones, while their morphology and dimensions are highly dependent on the tectonodynamic conditions the ore-bearing structures were formed under.

  19. 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.

  20. 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.

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

    NASA Astrophysics Data System (ADS)

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

    2016-04-01

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

  2. 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

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

    NASA Astrophysics Data System (ADS)

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

    2012-04-01

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

  4. 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

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

    NASA Astrophysics Data System (ADS)

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

    2014-10-01

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

  6. 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

  7. 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

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

    NASA Astrophysics Data System (ADS)

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

    2013-12-01

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

  9. Discussion of “Geology and diamond distribution of the 140/141 kimberlite, Fort à la Corne, central Saskatchewan, Canada”, by A. Berryman, B.H. Scott-Smith and B.C. Jellicoe (Lithos v. 76, p. 99 114)

    NASA Astrophysics Data System (ADS)

    Kjarsgaard, Bruce A.; Leckie, Dale A.; Zonneveld, John-Paul

    2007-09-01

    A wide variety of geological data and geological observations by numerous geoscientists do not support a two-stage crater excavation and in-fill model, or a champagne glass-shaped geometry for the 169 or 140/141 kimberlite bodies in the Fort à la Corne kimberlite field, Saskatchewan as described by Berryman, A., Scott Smith, B.H., Jellicoe, B., (2004). Rather, these kimberlite bodies are best described as polygenetic kimberlite tephra cones and tuff rings with associated feeder vents of variable geometry as shown by previous workers for the 169 kimberlite, the 140/141 kimberlite and the Star kimberlite. The domal tephra cone geometry is preserved due to burial by conformable Cretaceous marine mudstones and siltstones and is not an artifact of Quaternary glacial processes.

  10. 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.

  11. 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

  12. 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.

  13. 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.

  14. Tracking the Late Jurassic apparent (or true) polar shift in U-Pb-dated kimberlites from cratonic North America (Superior Province of Canada)

    NASA Astrophysics Data System (ADS)

    Kent, Dennis V.; Kjarsgaard, Bruce A.; Gee, Jeffrey S.; Muttoni, Giovanni; Heaman, Larry M.

    2015-04-01

    Different versions of a composite apparent polar wander (APW) path of variably selected global poles assembled and averaged in North American coordinates using plate reconstructions show either a smooth progression or a large (˜30°) gap in mean paleopoles in the Late Jurassic, between about 160 and 145 Ma. In an effort to further examine this issue, we sampled accessible outcrops/subcrops of kimberlites associated with high-precision U-Pb perovskite ages in the Timiskaming area of Ontario, Canada. The 154.9 ± 1.1 Ma Peddie kimberlite yields a stable normal polarity magnetization that is coaxial within less than 5° of the reverse polarity magnetization of the 157.5 ± 1.2 Ma Triple B kimberlite. The combined ˜156 Ma Triple B and Peddie pole (75.5°N, 189.5°E, A95 = 2.8°) lies about midway between igneous poles from North America nearest in age (169 Ma Moat volcanics and the 146 Ma Ithaca kimberlites), showing that the polar motion was at a relatively steady yet rapid (˜1.5°/Myr) pace. A similar large rapid polar swing has been recognized in the Middle to Late Jurassic APW path for Adria-Africa and Iran-Eurasia, suggesting a major mass redistribution. One possibility is that slab breakoff and subduction reversal along the western margin of the Americas triggered an episode of true polar wander.

  15. Discrimination of reworked pyroclastics from primary tephra-fall tuffs: a case study using kimberlites of Fort a la Corne, Saskatchewan, Canada

    NASA Astrophysics Data System (ADS)

    Leahy, Kevin

    Reworked volcaniclastics are traditionally discriminated from primary tephra-fall pyroclastics by an absence of features such as blanketing, juvenile lapilli, grain welding and poor sorting. Frequently, these features are difficult to identify, especially in small outcrops, ancient or altered successions, debris flows and surge deposits. Crystal-rich volcaniclastics, such as kimberlites, have a large proportion of coarse euhedral crystals, and abrasion leading to rounding can be recognised and classified with relative ease. A petrographic method of discriminating reworked material has been devised, based on the degree of grain roundness, and is illustrated using volcaniclastic kimberlite from Fort a la Corne, Saskatchewan, Canada. Petrographic thin sections of samples at regular intervals throughout the borehole core, and from a nearby crater-facies kimberlite, show that the percentages of rounded, subrounded and euhedral grains define two distinct groupings. The first group contains a higher percentage of euhedral grains and includes all the samples from the basal 4.8m of the 14.1-m-thick kimberlite section in borehole 004, and all of the crater facies tephra-fall tuffs. A second group contains more rounded, subrounded and fragmental grains and includes all the data from the upper 6.3m, which are interpreted as reworked strata. Thus, point counting concurs with hand-sample interpretation and may be used as a verification tool in discriminating reworked pyroclastic sands from primary tephra-fall tuffs.

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

    NASA Astrophysics Data System (ADS)

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

    2012-03-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2009-11-01

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

  18. 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

  19. 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

  20. 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

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

    NASA Astrophysics Data System (ADS)

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

    2014-05-01

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

  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

    Are kimberlites, lamproites, ultramafic lamprophyres and carbonatites genetically associated or not? There are strong opinions for and against any relationship. The 20 Ha Khaderpet pipe, discovered by Rio Tinto Exploration, is an unusual diamondiferous carbonatite-kimberlite clan rock (KCR) association in the Anumpalle Cluster of the Wajrakarur Kimberlite Field in the Dharwar Craton of Peninsular Indian. The Khaderpet pipe has a discrete sovite phase intrusive into KCR breccia, not noticed elsewhere in the Wajrakarur kimberlite field. Petrographically, the KCR is a clast-supported lithic breccia and crystal lithic tuff, with occasional pelletal lapilli. Clasts show a weak horizontal imbrication. The overall appearance of the tuffs and breccias is suggestive of terminal-blocked, vent accumulations that formed by under pressure, with spallation of country rock causing an abundance of granitoid debris. The sovite phase has up to 95% calcite, occasionally showing flow textured polycrystalline laths set in a minor saponite matrix. There are mineralogical gradations from an olivine-rich ultramafic to a calcite-dominant rock resembling pure carbonatite. Rare REE mineral phases in the carbonatite include allanite and other REE-rich unidentified mineral phases. Xenocrystic high pressure phases in both ultramafic and carbonatite include mantle-derived diamonds, lherzolitic-, eclogitic- and subcalcic-pyrope, Ti-poor andradite, chrome diopside, picrochromite and picroilmenite. Extensive metasomatism in the form of reddening of country rock feldspars by hematite, introduction of green chlorite, and saponitic alteration of breccia clasts and the ultramafic phase is common. The chemistry of the Khaderpet ultramafic component, suggests that the KCR is transitional between kimberlite and ultramafic lamprophyres, like certain other pipes in the Wajrakarur Kimberlite Field, with strong enrichment in LREE, CaO and CO2. However, low MgO (~ 13%) and high CaO (~ 10%) values are more

  3. The origin of coarse garnet peridotites in cratonic lithosphere: new data on xenoliths from the Udachnaya kimberlite, central Siberia

    NASA Astrophysics Data System (ADS)

    Doucet, Luc S.; Ionov, Dmitri A.; Golovin, Alexander V.

    2013-06-01

    We report new textural and chemical data for 10 garnet peridotite xenoliths from the Udachnaya kimberlite and examine them together with recent data on another 21 xenoliths from the 80-220 km depth range. The samples are very fresh (LOI near zero), modally homogeneous and large (>100 g). Some coarse-grained peridotites show incipient stages of deformation with <10 % neoblasts at grain boundaries of coarse olivine. Such microstructures can only be recognized in very fresh rocks, because fine-grained interstitial olivine is strongly affected by alteration, and may have been overlooked in previous studies of altered peridotite xenoliths in the Siberian and other cratons. Some of the garnet peridotites are similar in composition to low-opx Udachnaya spinel harzburgites (previously interpreted as pristine melt extraction residues), but the majority show post-melting enrichments in Fe and Ti. The least metasomatized coarse peridotites were formed by 30-38 % of polybaric fractional melting between 7 and 4 GPa and ≤1-3 GPa. Our data together with experimental results suggest that garnet in these rocks, as well as in some other cratonic peridotites elsewhere, may be a residual mineral, which has survived partial melting together with olivine and opx. Many coarse and all deformed garnet peridotites from Udachnaya underwent modal metasomatism through interaction of the melting residues with Fe-, Al-, Si-, Ti-, REE-rich melts, which precipitated cpx, less commonly additional garnet. The xenoliths define a complex geotherm probably affected by thermal perturbations shortly before the intrusion of the host kimberlite magmas. The deformation in the lower lithosphere may be linked to metasomatism.

  4. 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

  5. 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

  6. 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.

  7. Kimberlite and related rocks from Garnet Lake, West Greenland, including their mantle constituents, diamond occurrence, age and provenance

    NASA Astrophysics Data System (ADS)

    Hutchison, Mark T.; Frei, Dirk

    2009-11-01

    Observations of thickness, orientation and morphology and mineral chemistry of the principal diamondiferous intrusive sheet and associated bodies in the vicinity of Garnet Lake, Sarfartoq, West Greenland are reported. The principal body dips to the east on a NE/SW (true) trend and reaches a maximum thickness of 4.25 m. Multiple intrusive events are identified within the main sheet including sub-parallel bands occasionally exhibiting grain size sorting, cross-cutting layers and late-stage carbonate-rich emplacement, particularly at the contacts with country rock. Phenocrystic mineral assemblages and compositional measurements reveal two principal petrological types. The dominant type is an aillikite and the second rock type is a kimberlite. The kimberlite exhibits thin Ba-rich rims (towards kinoshitalite) on Al-rich phlogopite crysts, and an abundance of perovskite. Compositional zonation in groundmass spinels suggest a later transition towards an aillikite component. The aillikite is characterised by abundant phlogopite, heavily zoned with tetraferriphlogopite rims, transitional Type 1-Type 2 spinel compositions, rare Al,Ti-rich groundmass clinopyroxene and occasional exotic Sr-carbonate phases such as olekminskite. The Garnet Lake main sheet is characterised by mantle phases occurring as individual grains, most strikingly as garnet xenocrysts up to 5 mm and disaggregated mantle olivine crysts. Xenoliths occur rarely and are typically garnet dunites and garnet lherzolites. Heavy mineral separation reveals an abundance of G10D garnets and, whilst peridotitic garnets dominate, eclogitic G3D and G4D garnets also occur. Trace element compositions of garnet crysts reveal sinusoidal REE patterns in harzburgitic garnets however a component of flat and REE-enriched G11 garnets is apparent, reflecting significant mantle refertilisation. Thermorbarometric calculations on assemblages in Garnet Lake main sheet garnet lherzolites reveal equilibrium conditions clustering closely

  8. 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

    We studied heavy minerals extracted from a diamondiferous metaconglomerate that formed 2697-2701 Ma in a successor basin within the Michipicoten Greenstone Belt (MGB) of the Wawa-Abitibi Terrane (Southern Superior Craton). The conglomerate is metamorphosed in the greenschist facies and contains mainly locally derived igneous mafic to felsic detritus, but also very minor components of medium grade metamorphic minerals, diamonds and paragenetic diamond indicator minerals. Comparison of the size distribution, resorbtion and N aggregation of diamonds in nearby Wawa lamprophyres and the metaconglomerate diamonds confirms that the latter were not derived from the proximal lamprophyric source. The heavy minerals in the metaconglomerate include diopside, olivine, corundum, chromite, almandine, pyrope with kelyphitic rims, picroilmenite, amphibole and anorthite. Low abundances of the heavy minerals (several grains per 4-70 tons of the metaconglomerate) are, in part, explained by their complete or partial replacement by the greenschist mineral assemblage. Detrital almandine and amphibole are inferred to originate in amphibolite facies rocks. Cr-diopside, olivine, chromite and anorthite were sourced from mafic-ultramafic anorthosite- and chromitite-bearing layered complexes mapped in the MGB. The presence of pyrope with more than 6 wt.% Cr 2O 3 suggests derivation from a cratonic root. Picroilmenite has compositions typical of kimberlite and unlike that of ultramafic lamprophyres and other unconventional diamondiferous volcanics. The Wawa metaconglomerate, therefore, should be considered analogous to the Witwatersrand successor basin conglomerate in recording indirect evidence for Archean kimberlites. The tight localization of the diamondiferous conglomerate in time and space was controlled by a quick (~ 3 Ma) erosion of the source kimberlite body. The location of the kimberlite-bearing > 2.7 Ga Superior protocraton was inferred from the provenance of the metaconglomerate

  9. Garnet lherzolite xenoliths in the kimberlites of northern Lesotho: revised P-T equilibration conditions and upper mantle Palaeogeotherm

    NASA Astrophysics Data System (ADS)

    Carswell, D. A.; Gibb, F. G. F.

    1987-12-01

    Evidence is presented that the inflected palaeogeotherm for northern Lesotho, previously highlighted by Boyd (1973), Boyd and Nixon (1973, 1975), Finnerty and Boyd (1984, 1987), is essentially an artifact of the unsatisfactory, over-simplified barometer formulation (based on MacGregor 1974) employed. The absence of an inflection in the palaeogeotherm for Udachnaya, Siberia based on P-T estimates for garnet lherzolite xenoliths calculated with the same barometer, does not prove the reality of an inflected palaeogeotherm for northern Lesotho. Rather, it reflects, at least in part, chemical differences between the equivalent deformed, high- T xenoliths in these two areas — most importantly expressed in the respective contents of Jadeite relative to ureyite in the constituent orthopyroxenes. Accurate estimation of P-T equilibration conditions for garnet lherzolite xenoliths requires both complete and precise mineral analyses and adequate consideration of the influence of minor elements, such as Cr and Na, on the element exchange reaction thermometers and barometers employed. The barometer formulation of Nickel and Green (1985) is judged to be the best currently available. As no single thermometer is entirely satisfactory and dependable throughout the P-T range of interest, equilibration temperatures are currently best assessed as a mean value obtained from application of the most accurate formulations for both the two-pyroxene solvus thermometer (Bertrand and Mercier 1985) and Fe2+-Mg2+ exchange reactions between garnet-clinopyroxene (Powell 1985), garnet-orthopyroxene (Harley 1984a) and garnet-olivine (O'Neill and Wood 1979) mineral pairs. Such ‘best’ P-T estimates for xenoliths in the kimberlites of northern Lesotho indicate a somewhat elevated, non-inflected, upper mantle palaeogeotherm, compatible with a 120 145 km thick thermally conductive lithosphere above a convecting asthenosphere. The common coarse textured, chemically depleted, garnet lherzolite

  10. Melting experiments on the Udachnaya kimberlite at 6.3-7.5 GPa: Implications for the role of H2O in magma generation and formation of hydrous olivine

    NASA Astrophysics Data System (ADS)

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

    2013-01-01

    Melting experiments on kimberlite from the Udachnaya pipe have been performed at 6.3-7.5 GPa and 1300-1600 °C using a split-sphere multianvil apparatus. The water content in kimberlite varied from 2.5 to 11.6 wt.% and the CO2/(CO2 + H2O) molar ratio was from 0.61 to 0.23. The samples were placed in a graphite container inside a Pt capsule. The oxygen fugacity (fO2) during the experiment was controlled by the equilibrium between graphite and water-bearing carbonate-silicate melt. At relatively low temperatures, fO2 was close to the EMOG/D equilibrium, at higher temperatures, it shifted for approximately 1 log unit to more reduced conditions. An olivine + garnet + clinopyroxene assemblage was present at ⩽100 °C below the liquidus of the Udachnaya kimberlite, with 6-8 wt.% H2O at the pressure 6.3 GPa and 6-10 wt.% H2O at 7.5 GPa. At 2.5 wt.% H2O the same assemblage appeared at ⩾150 °C below liquidus, both at 6.3 and 7.5 GPa. Orthopyroxene did not form at any temperature and pressure of the experiments. The presence of clinopyroxene near the liquidus was due to the calcic nature and a high degree of silica undersaturation in the Udachnaya kimberlite. At the supra-solidus conditions, garnet and clinopyroxene compositionally distinct from minerals of the megacryst/macrocryst suite crystallized in equilibrium with low-H2O carbonated melt. Near the liquidus of high-H2O kimberlite, the stable olivine composition was from Fo91 to Fo98. The garnet composition (CaO ˜8 wt.%, TiO2 <1 wt.% and Cr2O3 up to 2.2 wt.%) approached that of Ti-bearing garnet typical of Cr-poor garnet megacrysts while the clinopyroxene was an analog of clinopyroxene megacrysts in kimberlite. Infrared absorption measurements showed that crystallized olivines contained water in the form of Ti-clinohumite-like and OH-clinohumite-like defects. The H2O content of olivine was found to depend mainly on water content in kimberlite melt and pressure. Olivine with 120-170 ppm H2O crystallized at 6 GPa in

  11. Geochemical and oxygen isotope signatures of mantle corundum megacrysts from the Mbuji-Mayi kimberlite, Democratic Republic of Congo, and the Changle alkali basalt, China

    NASA Astrophysics Data System (ADS)

    Giuliani, Gaston; Pivin, Marjorie; Fallick, Anthony E.; Ohnenstetter, Daniel; Song, Yucai; Demaiffe, Daniel

    2015-01-01

    Oxygen isotope signatures of ruby and sapphire megacrysts, combined with trace-element analysis, from the Mbuji-Mayi kimberlite, Democratic Republic of Congo, and the Changle alkali basalt, China, provide clues to specify their origin in the deep Earth. At Mbuji-Mayi, pink sapphires have δ18O values in the range 4.3 to 5.4‰ (N = 10) with a mean of 4.9 ± 0.4‰, and rubies from 5.5 to 5.6‰ (N = 3). The Ga/Mg ratio of pink sapphires is between 1.9 and 3.9, and in rubies, between 0.6 and 2.6. The blue or yellow sapphires from Changle have δ18O values from 4.6 to 5.2 ‰, with a mean of 4.9 ± 0.2‰ (N = 9). The Ga/Mg ratio is between 5.7 and 11.3. The homogenous isotopic composition of ruby suggests a derivation from upper mantle xenoliths (garnet lherzolite, pyroxenite) or metagabbros and/or lower crustal garnet clinopyroxenite eclogite-type xenoliths included in kimberlites. Data from the pink sapphires from Mbuji-Mayi suggest a mantle origin, but different probable protoliths: either subducted oceanic protolith transformed into eclogite with δ18O values buffered to the mantle value, or clinopyroxenite protoliths in peridotite. The Changle sapphires have a mantle O-isotope signature. They probably formed in syenitic magmas produced by low degree partial melting of a spinel lherzolite source. The kimberlite and the alkali basalt acted as gem conveyors from the upper mantle up to the surface.

  12. 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

  13. The molecular structure of melts along the carbonatite-kimberlite-basalt compositional joint: CO2 and polymerisation

    NASA Astrophysics Data System (ADS)

    Moussallam, Yves; Florian, Pierre; Corradini, Dario; Morizet, Yann; Sator, Nicolas; Vuilleumier, Rodolphe; Guillot, Bertrand; Iacono-Marziano, Giada; Schmidt, Burkhard C.; Gaillard, Fabrice

    2016-01-01

    Transitional melts, intermediate in composition between silicate and carbonate melts, form by low degree partial melting of mantle peridotite and might be the most abundant type of melt in the asthenosphere. Their role in the transport of volatile elements and in metasomatic processes at the planetary scale might be significant yet they have remained largely unstudied. Their molecular structure has remained elusive in part because these melts are difficult to quench to glass. Here we use FTIR, Raman, 13C and 29Si NMR spectroscopy together with First Principle Molecular Dynamic (FPMD) simulations to investigate the molecular structure of transitional melts and in particular to assess the effect of CO2 on their structure. We found that carbon in these glasses forms free ionic carbonate groups attracting cations away from their usual 'depolymerising' role in breaking up the covalent silicate network. Solution of CO2 in these melts strongly modifies their structure resulting in a significant polymerisation of the aluminosilicate network with a decrease in NBO/Si of about 0.2 for every 5 mol% CO2 dissolved. This polymerisation effect is expected to influence the physical and transport properties of transitional melts. An increase in viscosity is expected with increasing CO2 content, potentially leading to melt ponding at certain levels in the mantle such as at the lithosphere-asthenosphere boundary. Conversely an ascending and degassing transitional melt such as a kimberlite would become increasingly fluid during ascent hence potentially accelerate. Carbon-rich transitional melts are effectively composed of two sub-networks: a carbonate and a silicate one leading to peculiar physical and transport properties.

  14. Volatile composition of microinclusions in diamonds from the Panda kimberlite, Canada: Implications for chemical and isotopic heterogeneity in the mantle

    NASA Astrophysics Data System (ADS)

    Burgess, Ray; Cartigny, Pierre; Harrison, Darrell; Hobson, Emily; Harris, Jeff

    2009-03-01

    In order to better investigate the compositions and the origins of fluids associated with diamond growth, we have carried-out combined noble gas (He and Ar), C and N isotope, K, Ca and halogen (Cl, Br, I) determinations on fragments of individual microinclusion-bearing diamonds from the Panda kimberlite, North West Territories, Canada. The fluid concentrations of halogens and noble gases in Panda diamonds are enriched by several orders of magnitude over typical upper mantle abundances. However, noble gas, C and N isotopic ratios ( 3He/ 4He = 4-6 Ra, 40Ar/ 36Ar = 20,000-30,000, δ 13C = -4.5‰ to -6.9‰ and δ 15N = -1.2‰ to -8.8‰) are within the worldwide range determined for fibrous diamonds and similar to the mid ocean ridge basalt (MORB) source value. The high 36Ar content of the diamonds (>1 × 10 -9 cm 3/g) is at least an order of magnitude higher than any previously reported mantle sample and enables the 36Ar content of the subcontinental lithospheric mantle to be estimated at ˜0.6 × 10 -12 cm 3/g, again similar to estimates for the MORB source. Three fluid types distinguished on the basis of Ca-K-Cl compositions are consistent with carbonatitic, silicic and saline end-members identified in previous studies of diamonds from worldwide sources. These fluid end-members also have distinct halogen ratios (Br/Cl and I/Cl). The role of subducted seawater-derived halogens, originally invoked to explain some of the halogen ratio variations in diamonds, is not considered an essential component in the formation of the fluids. In contrast, it is considered that large halogen fractionation of a primitive mantle ratio occurs during fluid-melt partitioning in forming silicic fluids, and during separation of an immiscible saline fluid.

  15. Origin of eclogite and pyroxenite xenoliths from the Victor kimberlite, Canada, and implications for Superior craton formation

    NASA Astrophysics Data System (ADS)

    Smit, K. V.; Stachel, T.; Creaser, R. A.; Ickert, R. B.; DuFrane, S. A.; Stern, R. A.; Seller, M.

    2014-01-01

    A suite of 30 eclogite and pyroxenite xenoliths recovered from the Jurassic Victor kimberlite in the western Superior Province are investigated to determine their formation and emplacement in the sub-continental lithospheric mantle (SCLM). The samples have a wide compositional range, including low-Mg and high-Mg varieties. The low-Mg eclogites have a shallow origin as plagioclase-bearing protoliths that were subsequently subducted and emplaced into the SCLM. This is supported by their generally flat MREE to HREE compositions, the presence of kyanite and a positive Eu anomaly in the kyanite-bearing sample, as well as δ18O in three low-Mg eclogites that are higher than the pristine mantle value. LREE depletion in the low-Mg eclogites, along with unradiogenic 87Sr/86Sr indicate that they were not affected by widespread metasomatism after emplacement in the SCLM. The high-Mg eclogites and pyroxenites have compositional characteristics that require a distinct origin to the low-Mg eclogites. Their bulk compositions, LREEN-enriched trace element patterns and in particular, occurrence of unradiogenic 187Os/188Os in pyroxenite, is consistent with formation by reaction of broadly siliceous melts (generated from the melting of low-Mg eclogites) with depleted peridotite. A subduction origin of the eclogites studied here is consistent with seismic and field-based studies that have reported terrane accretion by successive subduction of the west-east orientated terranes in the western Superior Province. Although the timing of eclogite and pyroxenite formation could not be constrained, radiogenic 187Os/188Os require long-term isolation from the convecting mantle and supports a Neorchaean age for their formation.

  16. Partitioning of H2O between olivine and carbonate-silicate melts at 6.3 GPa and 1400 °C: Implications for kimberlite formation

    NASA Astrophysics Data System (ADS)

    Sokol, Alexander G.; Kupriyanov, Igor N.; Palyanov, Yuri N.

    2013-12-01

    Partitioning of H2O between olivine and carbonate-silicate melts has been studied at 6.3 GPa and 1400 °C using a split-sphere multianvil apparatus. Olivine was synthesized in equilibrium with hydrous silicate and hydrous carbonate-silicate±chloride melts saturated with respect to one of Opx, Grt, Ms or a harzburgitic (Ol+Opx+Grt) residue and had CO2/(CO2+SiO2) molar ratios from 0 to 0.8. The concentration of H2O in olivine was determined using FTIR spectroscopy. We found that depending on the melt carbonation and saturation in equilibrium silicate phases the H2O content in olivine varied from 100 to 1500 ppm. The obtained results and data reported in Sokol et al. (2013) indicate that H2O content in olivine becomes approximately two times lower as CO2/(CO2+SiO2) molar ratios in the equilibrium melt increases from 0 to 0.4-0.8 and the crystallization media transform from hydrous silicate to hydrous carbonate-silicate (kimberlite like) melt. The estimated water partitioning between carbonate-silicate melt and nominally anhydrous mantle minerals indicates that carbonatitic melt can effectively extract water once it invades H2O-poore the peridotite. We suggest that extraction of H2O owing to the freezing point depression may provide the necessary melting degree of metasomatized peridotite source and formation of kimberlitic magma.

  17. The influence of volcanological and sedimentological processes on diamond grade distribution in kimberlites: examples from the EKATI Diamond Mine, NWT, Canada

    NASA Astrophysics Data System (ADS)

    Porritt, Lucy A.; Cas, R. A. F.; Ailleres, L.; Oshust, P.

    2011-10-01

    The distribution of diamonds within individual kimberlite pipes is poorly documented in the public domain due to the proprietary nature of the data. The study of the diamond distribution within two pipes, Fox and Koala, from the EKATI Diamond Mine, NWT, Canada, in conjunction with detailed facies models has shown several distinct relationships of deposit type and grade distribution. In both pipes, the lithological facies represent grade units which can be distinguished from each other in terms of relative size and abundance of diamonds. A positive relationship between olivine grain size and abundance with diamond size and abundance is observed, indicating that sorting of fragmental kimberlites influences diamond distribution. Though surface geological processes do not control the diamond potential of the erupting magma, they can be responsible for concentrating diamonds into economically significant proportions. A good understanding of the eruption, transport and depositional processes responsible for the individual lithological units and the diamond distribution within them is important for successful resource estimation. This may lead to recognition of areas suitable for selective mining, making a marginal deposit economic.

  18. Evolution of diamond resorption in a silicic aqueous fluid at 1-3 GPa: Application to kimberlite emplacement and mantle metasomatism

    NASA Astrophysics Data System (ADS)

    Zhang, Zhihai; Fedortchouk, Yana; Hanley, Jacob J.

    2015-06-01

    Natural diamonds grow and partially dissolve during mantle metasomatism and undergo further resorption during the ascent to the Earth's surface in kimberlite magmas. This study uses atomic force microscopy (AFM) for quantitative characterization of diamond resorption morphology in order to provide robust constraints of the composition of kimberlitic and mantle metasomatic fluids. We performed experiments in a piston-cylinder apparatus at pressures (P) of 1-3 GPa and temperatures (T) of 1150-1400 °C to examine the impact of P, T, and silica content of an aqueous fluid on diamond dissolution. Petrographic observation and microthermometry of synthetic fluid inclusions trapped in olivine at the run conditions provide constraints on the composition and density of the fluid reacting with the diamond. Our results confirm an inverse relationship between P and T on diamond dissolution kinetics. A P increase of 1 GPa suppresses diamond oxidation rates by the same value as a T decrease by 50 °C, while the transformation rate of diamond crystal morphology from octahedron to tetrahexahedron increases with both P and T. All dissolved diamonds develop glossy surfaces, ditrigonal {111} faces, sheaf striations, and negative trigons, while circular pits only occur in aqueous fluids with low silica content (≤ 4.2 mol/kg) at 1 GPa. We identify five distinct morphological groups of trigons: two types of point-bottomed (p/b) (trumpet- and V-shaped) and three types of flat-bottomed (f/b) (trumpet-shaped, trapezoid-shaped and rounded). AFM measurements of trigons from two successive runs showed three stages of their evolution. Etch pits nucleate at defects as trumpet p/b trigons with the vertical dissolution rate (Vd) faster than the dissolution rates at the surface free of defects; they further develop by growth of the bottoms in (111) plane to create trumpet-shaped f/b trigons accompanied by decrease in Vd; and finally form trapezoid-shaped f/b trigon with constant wall angles. The

  19. Comparison of the mantle modification of the mantle column between two phases of kimberlite intrusion in Dalnyaya pipe, Yakutia

    NASA Astrophysics Data System (ADS)

    Ashchepkov, Igor; Spetsius, Zdislav; Salikhov, Ravil; Khmelnikova, Olga

    2013-04-01

    Dalnaya pipe is one of the largest in Daldyn field, Yakutia is composed of autolite breccia (AKB) and porphyric kimberlite (PK). Minerals from concentrates of both phases were compared and with the peridotite xenoliths minerals. Cpx from Dalnyaya are showing common tendencies Fe--Ti rise and Cr, Al, Na decrease. Garnets belong to lherzolite field with more deviation to harzbuirgitic one for (PK) . The chromites show two subtrends for Cr Fe, Ni vs TiO2. In general the variations of the AKB minerals and dispersion are higher but amount of depleted varieties is higher in PK. We used >50 xenoliths and ~1200 concentrate minerals for PT reconstructions. Combine PTX diagram show deep SCLM root beneath Dalnyaya with the main heating ~7 GPa. The HT 45mwm-2 branch is traced by some xenoliths from base to 2GPa. Essential inflection and heating detected by PT for OPx ~3GPa referring to Ca- enriched pyroxenitic garnets . Small Fe enrichment for Cpx and Gar found near 6 GPa referring to heated porhyroclastic varieties. Continuous and irregular growth of Fe# for Gar and low Fe Cpx Fe# 6 to 12# suggest that primary mantle layering beneath this pipe was smoothed by the high scale interaction with melts. The refertilization trend with Fe#9-15% rising upward in two branches refer to the Ilm and Cpx parental melt evolutions produced the intergrowth sometimes with garnets. In the PFO2 diagrams garnets and Cpx show continuous reduction to the lithosphere base to 4QMF higher for Cpx. Ilm - garnet trend is rising upward between -2 -0 QMF. The PT diagram for the AKB minerals from Dalnyaya pipe is nearly the same with the high dispersion to Fe rich varieties and smaller amount o f Mg rich minerals. Since the diamond grade is often determined by the amount of depleted varieties it is higher for the PK. Trace elements determined for Gar and Cpx from 13 xenoliths from the middle part of mantle section reveal very similar patterns in general. Supported by RBRF grant 11-05-00060.

  20. Eclogites and the Metasomatism of Eclogites from the Jagersfontein Kimberlite: Punctuated Transport and Implications for Alkali Magmatism

    NASA Astrophysics Data System (ADS)

    Pyle, Joseph M.; Haggerty, Stephen E.

    1998-04-01

    The first detailed petrographic and electron microbeam study of eclogite xenoliths and associated metasomatism from the Jagersfontein Kimberlite are presented with the objectives of determining depths of origin and of establishing possible links to cratonic magmatism. Jagersfontein eclogites are primarily bimineralic garnet + clinopyroxene with wide variations in color, size, and modal abundances, and with initial equilibration in the diamond stability field; five of seventy-nine eclogites derive from P > 8 Gpa. Phlogopite, amphibole, and calcite are the dominant metasomatic minerals, with accessory spinel, feldspar, and serpentine. Garnets are py 39 to py 77 in composition, and zoned garnets are observed as overgrowth rims on early garnet, adjacent to phlogopite, and as relicts in spinel-bearing subassemblages. Phlogopite and amphibole are enriched in Ti, and serpentine contains noticeable Al and Fe. Pleonaste has limited magnetite and minor Cr; and a strong correlation exists between MgO and Cr 2O 3 contents in spinel and in coexisting garnet. Pyroxene is compositionally diverse, and three distinct varieties exist: (1) primary omphacite (jd 7-jd 55) with distinct high-Al [VI] and low-Al [VI] groups; (2) secondary symplectitic diopsidic omphacite (jd 2-jd 25) with slightly greater Al [IV] content than primary omphacite; and (3) spinel-associated pyroxene with little or no jadeite component (max 0.96 wt% Na 2O), variable Ca/(Ca + Mg + Fe) (0.03-0.47), and high Tschermak content (up to ˜0.3 cations Al [IV] pfu). Feldspar compositions are variable and controlled by associated minerals (garnet, pyroxene, phlogopite) and composition of reacting phases. Pressure-T estimates for metasomatism are <2 Gpa and <900 C, in the field of graphite stability. From mass balance considerations, the metasomatic melts contained K, Ti, Cr, H 2O, and CO 2, but also Na in amphibole and Nb in rutile. Garnet:pyroxene ratios control metasomatic end-product minerals. Hydrous metasomatism of

  1. Identification of /sup 13/C depleted mantle carbon in diamonds from the Roberts Victor Kimberlite, South Africa

    SciTech Connect

    Deines, P.

    1985-01-01

    The Roberts Victor Kimberlite is known for the abundance of eclogite xenoliths, some of which show an unusual depletion in /sup 18/O. The question whether the observed oxygen isotope variations can be related to carbon isotopic composition variations has been investigated. Peridotite-suite diamons (X = -5.4 per thousand vs. PDB, s = +/-0.9 per thousand, n = 65) and sulfide containing diamonds (X = -4.9, s = +/-0.9, n = 20) do not differ in their /sup 13/C content. For these samples, delta/sup 13/C is not related to diamond shape, color, minerals occluded, or the inclusion chemistry. Eclogite suite diamonds (11) can be subdivided into two groups, GI and GII, based on delta/sup 13/C : GI = (X = -15.4, s = +/-0.4, n = 8); GII = (X = -5.9, s = +/-0.4, n = 3). The composition of the gt and cpx inclusions of the two groups is distinct; e.g. cpx of GI is significantly depleted in SiO/sub 2/, MgO, and CaO, and significantly enriched in Al/sub 2/O/sub 3/, FeO and MnO, compared to cpx of GII. Comparison of the chemical composition of the inclusions in E-type diamonds with those of eclogite xenoliths showing /sup 18/O depletion suggests that /sup 13/C and /sup 18/O depletion are not likely to be related. Evaluation of compositional trends of gt and cpx in eclogite xenoliths indicates that GI and GII are not related by a single fractionation event, but represent products from different reservoirs. Equilibration conditions deduced from coexisting gt and cpx demonstrate that GI diamonds come from larger depths than eclogite xenoliths and by inference GII diamonds. The high FeO and MnO content of a gt inclusion in cpx of an eclogite xenolith is used to argue for the existence of two separate events responsible for the formation of GI and GII diamonds.

  2. Origin of sub-lithospheric diamonds from the Juina-5 kimberlite (Brazil): constraints from carbon isotopes and inclusion compositions

    NASA Astrophysics Data System (ADS)

    Thomson, A. R.; Kohn, S. C.; Bulanova, G. P.; Smith, C. B.; Araujo, D.; Walter, M. J.

    2014-12-01

    Forty-one diamonds sourced from the Juina-5 kimberlite pipe in Southern Brazil, which contain optically identifiable inclusions, have been studied using an integrated approach. The diamonds contain <20 ppm nitrogen (N) that is fully aggregated as B centres. Internal structures in several diamonds revealed using cathodoluminescence (CL) are unlike those normally observed in lithospheric samples. The majority of the diamonds are composed of isotopically light carbon, and the collection has a unimodal distribution heavily skewed towards δ13C ~ -25 ‰. Individual diamonds can display large carbon isotope heterogeneity of up to ~15 ‰ and predominantly have isotopically lighter cores displaying blue CL, and heavier rims with green CL. The light carbon isotopic compositions are interpreted as evidence of diamond growth from abiotic organic carbon added to the oceanic crust during hydrothermal alteration. The bulk isotopic composition of the oceanic crust, carbonates plus organics, is equal to the composition of mantle carbon (-5 ‰), and we suggest that recycling/mixing of subducted material will replenish this reservoir over geological time. Several exposed, syngenetic inclusions have bulk compositions consistent with former eclogitic magnesium silicate perovskite, calcium silicate perovskite and NAL or CF phases that have re-equilibrated during their exhumation to the surface. There are multiple occurrences of majoritic garnet with pyroxene exsolution, coesite with and without kyanite exsolution, clinopyroxene, Fe or Fe-carbide and sulphide minerals alongside single occurrences of olivine and ferropericlase. As a group, the inclusions have eclogitic affinity and provide evidence for diamond formation at pressures extending to Earth's deep transition zone and possibly the lower mantle. It is observed that the major element composition of inclusions and isotopic compositions of host Juina-5 diamonds are not correlated. The diamond and inclusion compositions are

  3. Infrared spectral and carbon isotopic characteristics of micro- and macro-diamonds from the Panda kimberlite (Central Slave Craton, Canada)

    NASA Astrophysics Data System (ADS)

    Melton, G. L.; Stachel, T.; Stern, R. A.; Carlson, J.; Harris, J. W.

    2013-09-01

    One hundred and twenty-one micro-diamonds (< 1 mm) and 90 macro-diamonds (2.5 mm to 3.4 mm) from the Panda kimberlite (Ekati mine, Central Slave Craton, Canada) were analyzed for nitrogen content, nitrogen aggregation state (%B) and platelet and hydrogen peak areas (cm- 2). Micro-diamond nitrogen concentrations range from < 10 at. ppm to 1696 at. ppm (median = 805 at. ppm) and the median aggregation state is 23%B. Macro-diamonds range from < 10 at. ppm to 1260 at. ppm (median = 187 at. ppm) nitrogen and have a median nitrogen aggregation of 26%B. Platelet and hydrogen peaks were observed in 37% and 79% of the micro-diamonds and 79% and 56% of the macro-diamonds, respectively. Nitrogen based time averaged residence temperatures indicate that micro- and macro-diamonds experienced similar thermal mantle residence histories, both populations displaying bimodal residence temperature distributions with a gap between 1130 °C and 1160 °C (at 3.5 Ga residence). In addition, SIMS carbon isotopic analyses for the micro-diamonds were obtained: δ13C compositions range from - 6.9‰ to + 1.8‰ (median = - 4.3‰). CL imaging reveals distinct growth layers that in some samples differ by > 2‰, but mostly vary by < 0.5‰. Comparison of only the “gem-quality” samples (n = 49 micro- and 90 macro-diamonds) between the two diamond sets, indicates a statistically significant shift of + 1.3‰ in average δ13C from macro- to micro-diamonds and this shift documents distinct diamond forming fluids, fractionation process or growth histories. A broad transition to heavier isotopic values is also observed in connection to decreasing mantle residence temperatures. The bimodal mantle residence temperature distribution may coincide with the transition from highly depleted shallow to more fertile deep lithospheric mantle observed beneath the Central Slave Craton. The increase in δ13C with decreasing residence temperature (proxy for decreasing depth) is interpreted to reflect diamond

  4. Evidence for the mantle metasomatism at the formation of polycrystalline diamond aggregates from Mir kimberlite, Yakutia, Russia

    NASA Astrophysics Data System (ADS)

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

    2013-12-01

    Polycrystalline diamond aggregates (boart, framesites, diamondites) have been widely studied but their origin is poorly understood. We report the results of a study of two polished fragments of fine-grained (40-400 μm size of individual diamond grains) dense polycrystalline diamond aggregates from the Mir pipe containing visible multiple interstitial garnet inclusions. They were analyzed for major and trace elements of inclusions and for δ13C and N abundance of host diamonds. These aggregates are classified as variety IX by Orlov (1977). No cavities were observed in these samples. Sixty two irregular garnet grains and one clinopyroxene inclusion were detected and analyzed in sample MR-832. Garnets are homogeneous within single grains but variable in mg# [100Mg/(Mg+Fe)] from 60 up to 87 and CaO contents (3.3-5.3 wt.%) among grains with a trend to negative correlation. Low Cr (550-640 ppm) confirms eclogitic (E-type) paragenesis. High Na2O contents (5.2 wt.%) of a single pyroxene inclusion is additional evidence of eclogitic nature of this sample. Wide variations in trace elements (ppm) are characteristic for garnet grains: Sr (2.7-25.6), Y (9.7-14.1), Zr (15.6-38.7) and positive Eu anomaly is present. The δ13C of diamonds within studied sample is relatively constant (-7.1÷-7.8 ‰ PDB) but N abundance is variable (620-1150 ppm). The second peridotitic (U/P-type) sample MR-838 contains eight inclusions of Mg-rich Cr-pyropes (mg# ~85, Cr2O3 3.2-3.4 wt.%) and magnesite inclusion with 4.35 wt.% FeO and 1.73 wt.% CaO. Trace elements content in pyropes is relatively uniform (ppm): Sr (0.4-1.6), Y (13.2-13.4) and Zr (13.0). The δ13C is between -5.6 and -9.8 ‰ PDB and N abundance is low and relatively constant (75-98 ppm). We conclude that heterogeneous distribution of the trace elements among garnet grains and magnesite presence are indicative of the effects of mantle metasomatism and rapid crystallization shortly before the eruption of the kimberlite (e.g. Shimizu

  5. Trace element chemistry of peridotitic garnets in diamonds from the Premier (Cullinan) and Finsch kimberlites, South Africa: Contrasting styles of mantle metasomatism

    NASA Astrophysics Data System (ADS)

    Viljoen, K. S.; Harris, J. W.; Ivanic, T.; Richardson, S. H.; Gray, K.

    2014-11-01

    The purpose of this paper is to provide, discuss, and interpret a comprehensive set of geochemical data (involving major elements as well as Ni, Ti, Sr, Y, Zr, Nb, Hf and the rare earth elements) for peridotitic garnets in diamonds from Premier and Finsch, with a view on the nature of the metasomatic processes operating up to the time of diamond crystallisation, and the location of these two diamondiferous kimberlites within and outside the region of low seismic velocity in the Kaapvaal lithosphere. Trace element data were acquired using an ion microprobe, and a new method for the analysis of Ni in garnet by ion microprobe is presented. Peridotitic garnets in diamonds from the Premier mine are characterised by a significantly higher proportion of the lherzolite paragenesis relative to diamonds from other South African mines, such as Finsch, Venetia and De Beers Pool. Based on Ni-in-garnet thermometry, inclusion encapsulation temperatures of 1055 °C to 1669 °C are calculated for peridotitic garnets from Premier, with an average temperature of 1215 °C. Calculated temperatures for garnets from Finsch range from 1036 °C to 1167 °C, and are generally lower than for Premier, with an average of 1098 °C. The garnets in the diamonds from Premier and Finsch reflect contrasting styles of metasomatism associated with diamond crystallisation, with a low temperature fluid-type metasomatism prevalent in the case of the Paleoarchean diamonds from Finsch, and a higher temperature melt-related metasomatism occurring in the case of the Paleoproterozoic diamonds from Premier. The metasomatic agent accompanying diamond crystallisation at Finsch is effective at introducing Sr, the light rare earth elements, and some Zr into the lithosphere, but is ineffective at transporting much Ca, Ti, Y and heavy rare earth elements. In the case of Premier the metasomatic agent is highly effective at element transport, introducing e.g. Ca, Fe, Ti, Zr, Y and the rare earth elements. The location

  6. Nd, Sr, Pb, Ar, and O isotopic systematics of Sturgeon Lake kimberlite, Saskatchewan, Canada: constraints on emplacement age, alteration, and source composition

    NASA Astrophysics Data System (ADS)

    Hegner, E.; Roddick, J. C.; Fortier, S. M.; Hulbert, L.

    1995-06-01

    Rb-Sr isotopic dating of phlogopite megacryst samples separated from Sturgeon Lake kimberlite, Saskatchewan, yields a crystallization age of 98±1 Ma (2 σ, MSWD=1.2; 87Sr/86Sr( t)=0.7059). The 40Ar/39Ar analyses of a phlogopite megacryst sample indicate the presence of large amounts of excess 40Ar and yield an excessively old age of ˜410 Ma. Assessment of the Ar data using isotope correlation plots indicates clustering of the data points about a mixing line between the radiogenic 40Ar component at 98 Ma and a trapped component with uniform 36Ar/40Ar and Cl/40Ar. Values of δ 18O as high as +20‰ (VSMOW) for calcite from the groundmass and a whole-rock sample indicate pervasive low-temperature alteration. The δ 13C of matrix carbonate is -11.3‰ (PDB), slightly lighter than typical values from the literature. The δ 18O values of about +5‰ (VSMOW) for brown phlogopite megacrysts may be primary, green phlogopites are interpreted to be an alteration product of the brown variety and are 2‰ heavier. Initial Nd-Sr-Pb isotopic ratios for a whole-rock sample ( ɛ Nd=+0.8; 87Sr/86Sr=0.7063, 206Pb/204Pb=18.67, 207Pb/204Pb=15.54, 208Pb/204Pb=38.97) suggest an affinity with group I kimberlites. Initial ɛ Nd values of +1.7 and +0.5 (87Sr/86Sr( t)=0.7053 and 0.7050) for eclogitic and lherzolitic garnet megacryst samples, and values of 0.0 for two phlogopite megacryst samples reflect an origin from an isotopically evolving melt due to assimilation of heterogeneous mantle. Lilac high-Cr lherzolitic garnet megacrysts give an unusually high ɛ Nd(98. Ma) of +28.6 (87Sr/86Sr=0.7046) indicating a xenocrystic origin probably from the lithospheric mantle. The very radiogenic 87Sr/86Sr and 206Pb/204Pb ratios of the kimberlite are consistent with melting of EM II (enriched) mantle components.

  7. Ti-rich Silicate Perovskite: A New Lower Mantle Phase and the Possible Source of Unradiogenic Hf in Kimberlites and Carbonatites

    NASA Astrophysics Data System (ADS)

    Collerson, K. D.; Terasaki, H.; Ohtani, E.; Suzuki, A.; Kondo, T.

    2005-12-01

    In an attempt to synthesize the pre-exsolution homogeneous phase proposed by [1] as the protolith of exsolution-textured cpx-ilm xenoliths in kimberlite, we conducted a subsolidus MA experiment at 25 GPa and 1800°C using a natural cpx-ilm xenolith from Monastery kimberlite containing 17% TiO2 as the starting composition. Phases identified (EPMA, Raman &XRD) were Ti-rich MgSi perovskite, Ti-rich CaSi perovskite and stishovite. TiO2 contents ranged from 16-18% in the CaTiSiPv to between 12.5 and 25% in the MgTiSiPv. This indicates that an extensive field of solid solution exists in the system MgSiPv - CaSiPv - CaTiPv at pressures greater than 24 GPa [cf. 2,3]. Furthermore, a multi-phase system was observed using XRD in a DAC experiment at 30 GPa and 1800°C. By contrast, the maximum TiO2 in majorite garnet in this composition is only 5-6% at 18 GPa [4]. Raman spectra for CaTiSiPv vary systematically with Ti content. This observation could have application for interpretation of spectra obtained in subsequent DA experiments on the stability of CaTiSiPv. The protolith of the cpx-ilm xenoliths does not exist as a single homogeneous Ti-rich silicate phase in the upper mantle. However, presence of a Ti-bearing phase in the lower mantle (LM) is inferred from crystals of CaSiPv and CaTiPv that occur in contact with each other in LM diamonds [5]. These were interpreted as reversion products, formed from CaSiPv and CaTiPv solid solution during ascent in kimberlite magma of at P < 9 GPa [2]. However, following [2] these phases must have existed as a single phase at higher pressure. Using the mean composition of CaTiSiPv produced in our experiments, we calculated that this solid solution is likely to involve 0.7 CaSiPv and 0.3 CaTiPv. Ti-rich SiPv in the LM phase may explain the "hidden" low Lu/Hf reservoir required by unradiogenic Hf isotopic compositions in kimberlites and carbonatites [6,7]. [1] Ringwood & Lovering (1970) EPSL,7, 371. [2] Kubo et al., (1997) PCM 24: 488

  8. Subduction-related origin of eclogite xenoliths from the Wajrakarur kimberlite field, Eastern Dharwar craton, Southern India: Constraints from petrology and geochemistry

    NASA Astrophysics Data System (ADS)

    Dongre, A. N.; Jacob, D. E.; Stern, R. A.

    2015-10-01

    Major and trace elements as well as the first oxygen isotopes are reported on eclogite xenoliths from the Mesoproterozoic KL2 and P3 kimberlite pipes of the Wajrakarur kimberlite field, Eastern Dharwar craton, Southern India. Garnets in kyanite-bearing samples are rich in grossular, whereas they are predominantly pyrope-almandines in the bimineralic (kyanite-free) samples. The kyanite eclogite from the P3 pipe is more Mg-rich than those from KL2 pipe. Equilibration temperatures indicate derivation from 4.5 to 5.3 GPa and 1060 to 1220 °C for the KL2 samples and 3.6 GPa, 918 °C for the P3 sample. Garnet rare earth element patterns show two characteristic types, one with relatively low and flat heavy rare earth element patterns: Wajrakarur Group 1 and a second with lower light to heavy rare earth element ratios: Wajrakarur Group 2. Most samples in Wajrakarur Group 1 show pronounced positive Eu anomalies in garnet and positive Eu and Sr anomalies in the reconstructed whole rock trace element patterns; these are among the strongest anomalies in eclogite xenoliths worldwide. In contrast, Wajrakarur Group 2 samples show only subtle positive Eu anomalies. Oxygen isotopic ratios of garnets range between +5.3‰ and +7.8‰ δ18O. This range extends significantly beyond the range for unchanged mantle. Similar to many other eclogite suites worldwide, the Wajrakarur Group 1 and Group 2 eclogite suites shows evidence for an origin as crustal gabbroic material, likely once part of the oceanic crust, which was subducted and imbricated under the Eastern Dharwar craton. Their surface origin therefore lends support to geodynamic models that favor amalgamation of the Dharwar craton by subduction.

  9. New Data From a new Craton : N- and C-Isotopes in Diamonds From the Panda Kimberlite (Canada).

    NASA Astrophysics Data System (ADS)

    Cartigny, P.; Harris, J. W.; Javoy, M.

    2002-05-01

    \\delta{13}C \\delta{15}N and N content analyses of 87 known-parageneses diamonds from the Panda mine have been carried out. Seventy-five, 8 and 4 samples belong to the peridotitic, eclogitic and lower-mantle paragenesis respectively. Peridotitic diamonds have a mean \\delta{13}C-value of -5.2 \\permil and range from -6.9 to -3.0 \\permil, with one single extreme value at -14.1 \\permil. Their associated \\delta{15}N values range from -17.0 to +8.5 \\permil for a mean value of -4.0 \\permil. Their N contents range from 0 to 1280 ppm. Eclogitic diamonds have \\delta{13}C-values ranging from -11.2 to -4.4 \\permil with one single extreme value at -19.4 \\permil. Their \\delta{15}N range from -2.1 to +7.9 \\permil and N contents from 0 to 3452 ppm. Lower mantle diamonds are all Type II (nitrogen-free) and contained within a narrow \\delta{13}C-interval of 1 per mil from -4.5 to -3.5 \\permil; these latter values being thus very similar to previous data from other localities. The present dataset shows very strong similarities with results obtained previously. For example, the mode of the \\delta{13}C distribution is strikingly similar to samples suites from yakutian or south-african kimberlites. The study of N-isotopes in peridotitic diamonds which remain limited geographically to the sino-korean and south-african cratons is now extended to a new craton. By showing a similar \\delta{15}N-range and mean value, the present results the {15}N depleted character of the sublithospheric mantle relative to external reservoirs (atmosphere, crust, sediments) of the Earth. Moreover, very evident \\delta{13}C, \\delta{15}N and N-content similarities between eclogitic and peridotitic diamonds further supports the idea that eclogitic and peridotitic diamonds derive from a similar source and that two distinct sources are unlikely. However, one eclogitic diamond displaying a low \\delta{13}C value and high N content cannot be explained by the "fractionation processes" suggested

  10. Eclogite xenoliths from the Lace kimberlite, Kaapvaal craton: From convecting mantle source to palaeo-ocean floor and back

    NASA Astrophysics Data System (ADS)

    Aulbach, S.; Viljoen, K. S.

    2015-12-01

    Major- and trace-element compositions of eclogite and pyroxenite xenoliths of ≥2.5 Ga age (in situ Pb-Pb data on clinopyroxene) from the Lace kimberlite on the Kaapvaal craton were investigated in order to constrain: (1) the nature and evolution of their protoliths; (2) the extent to which they preserve information on the state of the asthenospheric mantle source that gave rise to their low-pressure protoliths; and (3) the effect of their deep recycling on the radiogenic isotope evolution of the convecting mantle. Their elemental relationships are consistent with low-pressure fractionation of olivine ± plagioclase and clinopyroxene during oceanic crust formation, whereby the residual melt was enriched in rare-earth elements (REE), high field-strength elements and Y, producing inverse correlations of ΣREE with the size of Eu- and Sr-anomalies. LREE-depletion may indicate loss of on average 20% of a partial melt upon subduction and metamorphism (eclogitisation) of oceanic crust, which did not, however, contribute to juvenile growth of continental crust. The eclogites have median Sm/Nd (0.40) and Lu/Hf (0.27) similar to Depleted Mantle, and lower U/Pb (0.02) and Th/Pb (0.02). If deeply subducted, these rocks cannot explain unradiogenic Nd and Hf, and radiogenic Pb isotope compositions in the sources of some modern ocean island basalts. Low incompatible trace-element contents similar to picrites, and Yb concentrations at a given TiO2 content similar to modern MORB, indicate derivation of the protoliths by average melt fractions of ∼ 0.20- 0.25 that left a spinel peridotite residue at pressures ≤2.5 to 3.0 GPa. This shallow intersection of the peridotite solidus suggests moderate Archaean ambient mantle potential temperatures of ≤1420 to 1470 °C. Samples filtered for clinopyroxene fractionation and metasomatism have V/Sc (4.7 ± 1.2; n = 11) indicating lower fO2 (-1.9 relative to the fayalite-magnetite-quartz buffer = ΔFMQ) than modern MORB. This is in part

  11. Megacrysts and xenoliths in kimberlite, Elliott County, Kentucky: A mantle sample from beneath the Permian Appalachian Plateau

    NASA Astrophysics Data System (ADS)

    Garrison, James R.; Taylor, Lawrence A.

    1980-12-01

    Two kimberlite pipes in Elliott County contain rare ultramafic xenoliths and abundant megacrysts of olivine (Fo85 93), garnet (0.21 9.07% Cr2O3), picroilmenite, phlogopite, Cr-poor clinopyroxene (0.56 0.88% Cr2O3), and Cr-poor orthopyroxene (<0.03 0.34% Cr2O3) in a matrix of olivine (Fo88 92), picroilmenite, Cr-spinel, magnetite, perovskite, pyrrhotite, calcite, and hydrous silicates. Rare clinopyroxene-ilmenite intergrowths also occur. Garnets show correlation of mg (0.79 0.86) and CaO (4.54 7.10%) with Cr2O3 content; the more Mg-rich garnets have more uvarovite in solution. Clinopyroxene megacrysts show a general decrease in Cr2O3 and increase in TiO2 (0.38 0.56%) with decreasing mg (0.87 0.91). Clinopyroxene megacrysts are more Cr-rich than clinopyroxene in clinopyroxene-ilmenite intergrowths (0.06 0.38% Cr2O3) and less Cr-rich than peridotite clinopyroxenes (1.39 1.46% Cr2O3). Orthopyroxene megacrysts and orthopyroxene inclusions in olivine megacrysts form two populations: high-Ca, high-Al (1.09 1.16% CaO and 1.16 1.18% Al2O3) and low-Ca, low-Al (0.35 0.46% CaO and 0.67 0.74% Al2O3). Three orthopyroxenes belonging to a low-Ca subgroup of the high-Ca, high-Al group were also identified (0.86 0.98% CaO and 0.95 1.01% Al2O3). The high-Ca, high-Al group (Group I) has lower mg (0.88 0.90) than low-Ca, low-Al group (Group II) with mg=0.92 0.93; low mg orthopyroxenes (Group Ia) have lower Cr2O3 and higher TiO2 than high mg orthopyroxenes (Group II). The orthopyroxene megacrysts have lower Cr2O3 than peridotite orthopyroxenes (0.46 0.57% Cr2O3). Diopside solvus temperatures indicate equilibration of clinopyroxene megacrysts at 1,165° 1,390° C and 1,295° 1,335° C for clinopyroxene in clinopyroxene-ilmenite intergrowths. P-T estimates for orthopyroxene megacrysts are bimodal: high-Ca, high-Al (Group I) orthopyroxenes equilibrated at 1,165° 1,255° C and 51 53 kb (± 5kb) and the low-Ca, low-Al (Group II) orthopyroxenes equilibrated at 970° 1,020°C and 46 56 kb (

  12. In-situ Analysis of Diamonds and Their Mineral Inclusions From the Lynx Kimberlite Dyke Complex, Central Quebec

    NASA Astrophysics Data System (ADS)

    van Rythoven, A.; McCandless, T. E.; Schulze, D. J.; Bellis, A.; Taylor, L. A.; Liu, Y.

    2009-05-01

    Twenty diamonds from the 522 Ma Lynx kimberlite dyke complex were selected from 442 stones in the 1.47- 3.45mm (+3 to +11 DTC) sieve class on the basis of visible inclusions. The 442 diamonds are part of a larger population of 6598 stones produced from 34 t and 494 t bulk samples taken in 2005 and 2007, respectively. The twenty diamonds all have octahedral primary growth forms. Three macles occur, as does one example of two intergrown octahedra connected along their {111} faces. Two samples are coarse intergrowths of octahedra. Most of the diamonds display a significant degree of resorption and range from octahedra with rounded corners and edges to tetrahexahedroida. Shield and serrate laminae, and hillocks are the most common resorption-related surface features. Nineteen of the samples have light brown to brown colouration. After their external morphology was examined, the diamonds were cut and polished along a single plane to expose included mineral grains for compositional analysis and to image internal structure. Cathodoluminescence imaging reveals deformation lamellae in the majority of the diamonds. A subset of these stones show deformation lamellae truncated by growth/resorption zones and in some cases intersection of planes of different orientation. Oscillatory planar growth patterns are the most common. However, examples of simple homogeneous, complex planar, and complex undulating growth zones occur. Inclusions, particularly olivine, typically occur in core/early growth regions of the diamonds. Of the twenty diamonds, sixteen have primary inclusions. The inclusion suite is largely peridotitic. Seventeen forsteritic olivine inclusions occur in ten diamonds and have molar Mg/(Mg+Fe)= 0.916-0.933. Seven Cr-diopside inclusions occur in one diamond (2.2-2.3 wt. % Cr2O3). Four Cr-pyropes (Cr/(Cr+Al) = 0.28-0.41) occur in three diamonds. Two enstatite inclusions (Mg/(Mg+Fe) = 0.938-0.94) occur in two diamonds. One heterogeneous inclusion of monosulfide solid

  13. Post-Archean formation of the lithospheric mantle in the central Siberian craton: Re-Os and PGE study of peridotite xenoliths from the Udachnaya kimberlite

    NASA Astrophysics Data System (ADS)

    Ionov, Dmitri A.; Doucet, Luc S.; Carlson, Richard W.; Golovin, Alexander V.; Korsakov, Andrey V.

    2015-09-01

    The formation age of the Siberian cratonic mantle is not well established. Re-Os data on various mantle-derived materials brought up by kimberlite magmas have shown that it contains Archean components, but the reported ages range broadly (3.4 to <1 Ga). We report Re-Os isotope and PGE concentration data for a suite of 29 fresh, well-characterized xenoliths from the Udachnaya-East kimberlite representing all major peridotite rock types and a large part of the cratonic mantle profile. Several xenoliths with very low Os contents (<0.3 ppb) and/or high Re/Os ratios are not suitable for age estimates. The Os (and Ir) depletions are common in cpx-bearing spinel harzburgites and coarse garnet harzburgites, but are not found in deformed, high-T peridotites. Twenty refractory (Al2O3 0.1-1.6%) peridotites yield TRD ages from 0.9 to 2.2 Ga. TRD for a subset of six high-Mg# (0.92-0.93), low-T (⩽930 °C) spinel harzburgites and a single garnet harzburgite yield a narrow range from 2.0 to 2.2 Ga with an average of 2.1 ± 0.1 Ga, which we consider the best estimate for the age of the melting event that initially formed the lithospheric mantle beneath Udachnaya. The TRD estimates for less refractory (Mg# 0.907-0.919) deformed garnet peridotites show a greater range and are generally lower (0.9-2.0 Ga; average 1.54 ± 0.28 Ga) apparently due to the effects of melt metasomatism on the initial melting residues. The predominant part of the mantle in the central Siberian craton formed in the Paleoproterozoic and not in the Archean, unlike cratons in southern Africa and North America. Minor older components reported earlier from Udachnaya may be fragments of pre-existing lithosphere trapped during stacking of melting residues formed about 2 Ga ago. We argue that the formation of cratonic lithospheric mantle, with common high-Mg# (⩾0.92) and opx-enriched peridotites, was not limited to the Archean as previously thought, but continued in the Paleoproterozoic, i.e. that asthenospheric

  14. The age and history of the lithospheric mantle of the Siberian craton: Re-Os and PGE study of peridotite xenoliths from the Obnazhennaya kimberlite

    NASA Astrophysics Data System (ADS)

    Ionov, Dmitri A.; Carlson, Richard W.; Doucet, Luc S.; Golovin, Alexander V.; Oleinikov, Oleg B.

    2015-10-01

    The formation age of the lithospheric mantle of the Siberian craton (one of the largest on Earth) is not well established; nearly all published whole-rock Re-Os data are for mantle xenoliths from a single kimberlite in the center of the craton (Udachnaya). We report Re-Os isotope and PGE concentration data for 19 spinel and garnet peridotite xenoliths from the Obnazhennaya kimberlite in the northeastern portion of the craton. Most samples in this study, and many Obnazhennaya peridotites in general, show a combination of relatively low Al2O3 (0.1-2%) with high CaO (1.4-4%) concentrations. Only four dunites and harzburgites in our sample suite have low contents of both Al2O3 and CaO (0.1-0.8%), but their relatively low Mg# (0.888-0.919) and highly variable Os concentrations (0.6-35 ppb) suggest they may have formed in melt migration channels rather than as residues of partial melt extraction. A group of six Ca-rich (2.0-3.2% CaO) peridotites yields the highest Re-Os model ages (mean TRD = 2.8 Ga, mean TMA = 3.5 Ga). Eight peridotites with low to moderate Al2O3 (<2%) and Mg# ≥0.91, including three low-Ca harzburgites, yield lower Re-Os model ages (mean TRD = 1.9 Ga, mean TMA = 2.2 Ga). The remainder of the samples may not yield meaningful TRD ages because they are not refractory (Al2O3 >2.6% and/or Mg# ≤0.90). We interpret these results as evidence for a two-stage formation of the lithospheric mantle. The peridotites formed at the two stages show very similar chemical compositions. The enrichment in Ca, which we attribute to widespread post-melting metasomatism by carbonate-rich melts, may have taken place either at the end of the Archean melting event, when at least one Ca-Al-rich peridotite was formed, or later. The combined Re-Os age data on xenoliths from Obnazhennaya and Udachnaya suggest that the lithospheric mantle beneath the Siberian craton was not formed in a single event, but grew in at least two events, one in the late Archean and the other in the

  15. 40Ar/ 39Ar-ages of phlogopite in mantle xenoliths from South African kimberlites: Evidence for metasomatic mantle impregnation during the Kibaran orogenic cycle

    NASA Astrophysics Data System (ADS)

    Hopp, Jens; Trieloff, M.; Brey, G. P.; Woodland, A. B.; Simon, N. S. C.; Wijbrans, J. R.; Siebel, W.; Reitter, E.

    2008-12-01

    We applied the 40Ar/ 39Ar dating method to an extensive suite of phlogopites from kimberlite-hosted mantle xenoliths (dominantly garnet bearing) from the mines of Bultfontein (South Africa), Letseng-la-Terae and Liqhobong (Lesotho). Argon extraction was performed by conventional high resolution stepwise heating technique, laser incremental heating technique and laser spot analysis. All age spectra obtained by conventional analysis indicate various degrees of 40Ar loss during kimberlite emplacement, but never resulted in a total reset of the argon system. Most intriguingly, the sample-specific maximum apparent ages cluster between 1.0 and 1.22 Ga for the phlogopites with the least disturbed age spectra. A maximum apparent age of 1.02 Ga was observed during laser heating analysis. Individual grains tend to yield older ages in their cores, with successively younger ages at their rims. The range in age obtained via the laser fusion technique and with conventional stepwise heating technique agrees with each other, as well as with literature data. The often inferred presence of excess 40Ar in those phlogopites cannot explain the coherent age pattern in the large suite of samples. Hence, the age constraint of 1.0-1.25 Ga is regarded as geologically meaningful and assigned to metasomatism of the local cratonic mantle during the advent of Kibaran orogenesis (1.00-1.25 Ga). The major consequences of our findings are: (i) The argon system of phlogopite can remain closed for long time scales, even at ambient temperatures of 800-1200 °C within the mantle, most likely because the solid/solid partitioning behaviour of Ar between phlogopite and other major phases in the mantle strongly favours phlogopite, or because conventionally inferred diffusivity of argon in phlogopite is seriously overestimated. Thus, the 40Ar/ 39Ar phlogopite system appears to be a valuable tool for deciphering ancient metasomatic events affecting the lithospheric mantle. (ii) The cratonic lithospheric

  16. Magnetic signatures recorded in rocks and trees located inside the Tunguska blast 100 years ago, implications for Mirror Matter, Comet, and Kimberlitic Pipe explosion hypotheses

    NASA Astrophysics Data System (ADS)

    Kletetschka, G.

    2008-12-01

    Hundred years ago an unknown object impacted in Siberia, Tunguska region and created a seismic signature 100-1000 times stronger that the Hiroshima explosion. To this day, nothing has been found to suggest a foreign material (e.g. meteoritic) dispersion during this event. Various hypotheses were put forward, for example: Comet impact, Kimberlite Pipe explosion, and Mirror Matter interaction with the regular matter. We collected samples of black chert, conglomerate and wood from 5 different locations within 2 km from the epicenter. We used these samples for magnetic analysis and searched for any evidence of magnetic contamination that may date the Tunguska blast. All samples, wood, chert, and conglomerate showed sufficient content of magnetic material that should be capable of recording strong magnetic pulse. Our analysis shows no evidence of magnetic enhancement recorded in any of the three types (Chert, wood and conglomerate) of Tunguska samples. We will discuss this result in terms of three possible hypotheses of the created the Tunguska event.

  17. Relationships between oxygen fugacity and metasomatism in the Kaapvaal subcratonic mantle, represented by garnet peridotite xenoliths in the Wesselton kimberlite, South Africa

    NASA Astrophysics Data System (ADS)

    Hanger, Brendan J.; Yaxley, Gregory M.; Berry, Andrew J.; Kamenetsky, Vadim S.

    2015-01-01

    A suite of 12 peridotite xenoliths from the Wesselton kimberlite was studied and found to sample the subcratonic lithospheric mantle over a pressure range from 3.6 to 4.7 GPa and a temperature range of 880 to 1120 °C. Major, minor and trace element compositions indicate that both metasomatised and un-metasomatised samples are present over this pressure range. Fe3 +/∑ Fe in garnet from four xenoliths was determined using Fe K-edge XANES spectroscopy, enabling the redox state of the sampled subcratonic mantle to be determined for three garnet bearing samples. ΔlogfO2[FMQ] varied from 0 to - 3.3 over the sampled pressure interval, with the un-metasomatised samples falling within the global trend of decreasing ΔlogfO2[FMQ] with increasing depth. Superimposed on this was an oxidation trend, at higher pressures (≥ 4.5 GPa), with ΔlogfO2 increasing by 1.5 to 2 units in the metasomatically enriched samples, indicating a clear link between metasomatism and oxidation. One potential source of this oxidation is a carbonated silicate melt, which will increase in carbonate content as ΔlogfO2 increases. Mantle minerals affected by such a melt have the potential to shift from the field of diamond stability into that of carbonate, threatening the stability of diamond.

  18. A Lower Mantle Origin for Megacryst Suite Pyroxene-Ilmenite Xenoliths in Kimberlites: High-Pressure Experimental Constraints and Geodynamic Significance

    NASA Astrophysics Data System (ADS)

    Collerson, K. D.; Terasaki, H.; Ohtani, E.; Suzuki, A.; Kondo, T.

    2004-12-01

    Megacryst suite xenoliths (MSX's) in kimberlites, alnoites and alkali basalts are an important and poorly understood association. MSX's comprise medium- to coarse-grained monomineralic, or rare, multi-grain aggregates of low Cr, high Ti-Na pyrope, Mg ilmenite, sub-calcic pyroxene, Fo85±3 olivine, orthopyroxene and zircon. Some MSX's exceed 30 cm in diameter. They are interpreted to form by fractional crystallization from their host magma, near the base of the lithosphere [1-2]. However, majorite, and other high-pressure phases in some garnetite MSX's, indicates a mantle transition zone (TZ) origin [3]. A sub-lithospheric, deeply subducted slab source is also supported by Hf isotopic data [4]. A common member of the MSX suite, are graphic intergrowths of pyroxene and Mg-ilmenite interpreted to reflect cotectic, or non-equilibrium crystallization [5-6] from the kimberlite magma. However, Pb isotopic data for Monastery [7], and Namibian [8] megacrysts shows that MSX's and their host magmas are unrelated. Thus the mineralogy of the Ti-rich px-ilm MSX's needs to be determined at TZ and higher P. We have conducted multi-anvil (MA) and diamond anvil (DA) experiments on natural px-ilm xenoliths from Monastery and Malaita with different TiO2 contents (17% and 12%), in an attempt to synthesize the pre-exsolution phase. MA experiments were carried out on both starting compositions at 18 and 21 GPa, at 1800° C and 2100° C. None of the experiments yielded a single phase. Phases identified (EPMA, Raman & XRD) include: majorite, Si-rich ilmenite and Ca-Si-Ti Pv. At 21 GPa and 2100° C wadsleyite formed part of the assemblage, and melt was locally developed. Majorite is the most abundant phase in all experiments. Maximum majorite TiO2 occurs at 18 GPa (i.e., 5.4% - Malaita and 6.2% - Monastery). In the lower Ti Malaita composition, at 25 GPa and 1800° C, the assemblage is dominated by almost equal amounts of majorite (TiO2 1.3% to 2.1%) and Ca-Si-Ti Pv, with a small amount of

  19. Application of Fe K-edge XANES determinations of Fe3+/totalFe in garnet to peridotite xenoliths from the Udachnaya Kimberlite

    NASA Astrophysics Data System (ADS)

    Yaxley, G. M.; Berry, A. J.; Woodland, A. B.; Kamenetsky, V. S.; Paterson, D.; de Jonge, M. D.; Howard, D. L.

    2012-04-01

    The garnet structure can accommodate both Fe2+ and Fe3+. Garnet Fe3+/∑Fe in kimberlite-bourne garnet peridotite xenoliths can be used to determine the oxygen fugacity (fO2) of the cratonic lithosphere. This is important as an indicator of diamond (versus carbonate) stability. In cratonic lithosphere the ƒO2 of peridotite is expected to broadly decrease with increasing depth, and is consistent with graphite or diamond stability. However metasomatic events may locally perturb this trend, possibly leading to oxidation that could result in diamond breakdown or resorption. Such events will usually be recorded by the coexisting garnet. Fe3+/∑Fe of garnets has traditionally been determined by Mössbauer Spectroscopy of powdered samples. This lacks spatial resolution and the data for each measurement take several days to acquire. X-ray Absorption Near Edge Structure (XANES) spectroscopy is now commonly being used to determine Fe3+/∑Fe in minerals, is capable of micron spatial resolution and spectra can be recorded in ~15 minutes. We have recently reported a new method for quantifying Fe3+/∑Fe from the XANES spectra of mantle garnets with an accuracy and precision comparable to Mössbauer Spectroscopy. We applied the XANES technique to investigate the ƒO2-depth variation in the Siberian Craton using a suite of fresh garnet lherzolites from the Udachnaya East kimberlite. Garnet Fe3+/∑Fe was detemined using XANES spectroscopy on the X-ray Fluorescence Microscopy beamline of the Australian Synchrotron. XANES spectra were recorded in fluorescence mode from garnets prepared as either polished thin sections or electron probe mounts. A calibration curve relating the spectra to Fe3+/∑Fe of mantle garnets previously analysed by Mössbauer spectroscopy allowed garnet unknowns to be quantified. Thermobarometry established that the samples range in pressure from 3.9-7.1 GPa and lie along a typical cratonic geotherm. Several samples exhibit elevated abundances of Ti, Zr

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

    NASA Astrophysics Data System (ADS)

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

    2015-09-01

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

  1. Structural analysis and implicit 3D modelling of high-grade host rocks to the Venetia kimberlite diatremes, Central Zone, Limpopo Belt, South Africa

    NASA Astrophysics Data System (ADS)

    Basson, I. J.; Creus, P. K.; Anthonissen, C. J.; Stoch, B.; Ekkerd, J.

    2016-05-01

    The Beit Bridge Complex of the Central Zone (CZ) of the Limpopo Belt hosts the 519 ± 6 Ma Venetia kimberlite diatremes. Deformed shelf- or platform-type supracrustal sequences include the Mount Dowe, Malala Drift and Gumbu Groups, comprising quartzofeldspathic units, biotite-bearing gneiss, quartzite, metapelite, metacalcsilicate and ortho- and para-amphibolite. Previous studies define tectonometamorphic events at 3.3-3.1 Ga, 2.7-2.5 Ga and 2.04 Ga. Detailed structural mapping over 10 years highlights four deformation events at Venetia. Rules-based implicit 3D modelling in Leapfrog Geo™ provides an unprecedented insight into CZ ductile deformation and sheath folding. D1 juxtaposed gneisses against metasediments. D2 produced a pervasive axial planar foliation (S2) to isoclinal F2 folds. Sheared lithological contacts and S2 were refolded into regional, open, predominantly southward-verging, E-W trending F3 folds. Intrusion of a hornblendite protolith occurred at high angles to incipient S2. Constrictional-prolate D4 shows moderately NE-plunging azimuths defined by elongated hornblendite lenses, andalusite crystals in metapelite, crenulations in fuchsitic quartzite and sheath folding. D4 overlaps with a: 1) 2.03-2.01 Ga regional M3 metamorphic overprint; b) transpressional deformation at 2.2-1.9 Ga and c) 2.03 Ga transpressional, dextral shearing and thrusting around the CZ and d) formation of the Avoca, Bellavue and Baklykraal sheath folds and parallel lineations.

  2. A subduction wedge origin for Paleoarchean peridotitic diamonds and harzburgites from the Panda kimberlite, Slave craton: evidence from Re-Os isotope systematics

    NASA Astrophysics Data System (ADS)

    Westerlund, K. J.; Shirey, S. B.; Richardson, S. H.; Carlson, R. W.; Gurney, J. J.; Harris, J. W.

    2006-09-01

    An extensive study of peridotitic sulfide inclusion bearing diamonds and their prospective harzburgitic host rocks from the 53 Ma Panda kimberlite pipe, Ekati Mine, NWT Canada, has been undertaken with the Re-Os system to establish their age and petrogenesis. Diamonds with peridotitic sulfide inclusions have poorly aggregated nitrogen (<30% N as B centers) at N contents of 200-800 ppm which differs from that of chromite and silicate bearing diamonds and indicates residence in the cooler portion of the Slave craton lithospheric mantle. For most of the sulfide inclusions, relatively low Re contents (average 0.457 ppm) and high Os contents (average 339 ppm) lead to extremely low 187Re/188Os, typically << 0.05. An age of 3.52 ± 0.17 Ga (MSWD = 0.46) and a precise initial 187Os/188Os of 0.1093 ± 0.0001 are given by a single regression of 11 inclusions from five diamonds that individually provide coincident internal isochrons. This initial Os isotopic composition is 6% enriched in 187Os over 3.5 Ga chondritic or primitive mantle. Sulfide inclusions with less radiogenic initial Os isotopic compositions reflect isotopic heterogeneity in diamond forming fluids. The harzburgites have even lower initial 187Os/188Os than the sulfide inclusions, some approaching the isotopic composition of 3.5 Ga chondritic mantle. In several cases isotopically distinct sulfides occur in different growth zones of the same diamond. This supports a model where C-O-H-S fluids carrying a radiogenic Os signature were introduced into depleted harzburgite and produced diamonds containing sulfides conforming to the 3.5 Ga isochron. Reaction of this fluid with harzburgite led to diamonds with less radiogenic inclusions while elevating the Os isotope ratios of some harzburgites. Subduction is a viable way of introducing such fluids. This implies a role for subduction in creating early continental nuclei at 3.5 Ga and generating peridotitic diamonds.

  3. Thermal state, oxygen fugacity and COH fluid speciation in cratonic lithospheric mantle: New data on peridotite xenoliths from the Udachnaya kimberlite, Siberia

    NASA Astrophysics Data System (ADS)

    Goncharov, A. G.; Ionov, D. A.; Doucet, L. S.; Pokhilenko, L. N.

    2012-12-01

    Oxygen fugacity (fO2) and temperature variations in a complete lithospheric mantle section (70-220 km) of the central Siberian craton are estimated based on 42 peridotite xenoliths in the Udachnaya kimberlite. Pressure and temperature (P-T) estimates for the 70-140 km depth range closely follow the 40 mW/m2 model conductive geotherm but show a bimodal distribution at greater depths. A subset of coarse garnet peridotites at 145-180 km plots near the "cold" 35 mW/m2 geotherm whereas the majority of coarse and sheared rocks at ≥145 km scatter between the 40 and 45 mW/m2 geotherms. This P-T profile may reflect a perturbation of an initially "cold" lithospheric mantle through a combination of (1) magmatic under-plating close to the crust-mantle boundary and (2) intrusion of melts/fluids in the lower lithosphere accompanied by shearing. fO2 values estimated from Fe3+/∑Fe in spinel and/or garnet obtained by Mössbauer spectroscopy decrease from +1 to -4 Δlog fO2 (FMQ) from the top to the bottom of the lithospheric mantle (˜0.25 log units per 10 km) due to pressure effects on Fe2+-Fe3+ equilibria in garnet. Garnet peridotites from Udachnaya appear to be more oxidized than those from the Kaapvaal craton but show fO2 distribution with depth similar to those in the Slave craton. Published fO2 estimates for Udachnaya xenoliths based on C-O-H fluid speciation in inclusions in minerals from gas chromatography are similar to our results at ≤120 km, but are 1-2 orders of magnitude higher for the deeper mantle, possibly due to uncertainties of fO2 estimates based on experimental calibrations at ≤3.5 GPa. Sheared peridotites containing garnets with u-shaped, sinusoidal and humped REE patterns are usually more oxidized than Yb, Lu-rich, melt-equilibrated garnets, which show a continuous decrease from heavy to light REE. This further indicates that mantle redox state may be related to sources and modes of metasomatism.

  4. A mantle origin for Paleoarchean peridotitic diamonds from the Panda kimberlite, Slave Craton: Evidence from 13C-, 15N- and 33,34S-stable isotope systematics

    NASA Astrophysics Data System (ADS)

    Cartigny, Pierre; Farquhar, James; Thomassot, Emilie; Harris, Jeffrey W.; Wing, Bozwell; Masterson, Andy; McKeegan, Kevin; Stachel, Thomas

    2009-11-01

    In order to address diamond formation and origin in the lithospheric mantle underlying the Central Slave Craton, we report N- and C-stable isotopic compositions and N-contents and aggregation states for 85 diamonds of known paragenesis (73 peridotitic, 8 eclogitic and 4 from lower mantle) from the Panda kimberlite (Ekati Mine, Lac de Gras Area, Canada). For 12 peridotitic and two eclogitic sulfide inclusion-bearing diamonds from this sample set, we also report multiple-sulfur isotope ratios. The 73 peridotitic diamonds have a mean δ13C-value of - 5.2‰ and range from - 6.9 to - 3.0‰, with one extreme value at - 14.1‰. The associated δ15N-values range from - 17.0 to + 8.5‰ with a mean value of - 4.0‰. N-contents range from 0 to 1280 ppm. The 8 eclogitic diamonds have δ13C-values ranging from - 11.2 to - 4.4‰ with one extreme value at - 19.4‰. Their δ15N ranges from - 2.1 to + 7.9‰ and N-contents fall between 0 and 3452 ppm. Four diamonds with an inferred lower mantle origin are all Type II (i.e. nitrogen-free) and have a narrow range of δ13C values, between - 4.5 and - 3.5‰. The δ34S of the 14 analyzed peridotitic and eclogitic sulfide inclusions ranges from - 3.5 to +5.7‰. None of them provide evidence for anomalous δ33S-values; observed variations in δ33S are from +0.19 to - 0.33‰, i.e. within the 2 sigma uncertainties of mantle sulfur ( δ33S = 0‰). At Panda, the N contents and the δ13C of sulfide-bearing peridotitic diamonds show narrower ranges than silicate-bearing peridotitic diamonds. This evidence supports the earlier suggestion established from eclogitic diamonds from the Kaapvaal that sulfide-(±silicate) bearing diamonds sample a more restricted portion of sublithospheric mantle than silicate-(no sulfide) bearing diamonds. Our findings at Panda suggest that sulfide-bearing diamonds should be considered as a specific diamond population on a global-scale. Based on our study of δ34S, Δ 33S, δ15N and δ13C, we find no

  5. Mantle Evolution Beneath The Colorado Palteau: Interpreta-tion of The Study of Mineral Concentrate From Kimberlite Pipe Kl-1 Colorado.

    NASA Astrophysics Data System (ADS)

    Ashchepkov, I.; Vladykin, N.; Mitchell, R.; Coopersmith, H.; Garanin, V.; Saprykin, A. I.; Khmelnikova, O. S.

    Mineral grains and their intergrowth from the concentrate form the KL1- kimberlite pipe Colorado plateau was analyzed by EPMA and LAM ICP MS in Analytic Center of UIGGM. Garnets reveal nearly continuous trend of the compositions divided into 5 intervals. 1 cumulates from the crust and Sp facie mantle; 2. Gar-Sp lherzolites; 3- Gar- wehrlites, lherzolites and harzburgites; 4- Gar lherzolites and harzburgites; 4. Pyroxenites and Il peri-dotites . They reveal three trend of Ti decrease with the ris- ing Cr content. Those in the inter-growth with the pyroxenes are less in Tio2 as well as the pyroxenes. Discrete large Cpx grains are richer in Na, Al, Cr. TP conditions determined for the clinopyroxenes with Nimis- Taylor, 2001 thermobarometer and barometer Ashchepkov, 2001 reveal the heating from 35 to 40-42 mv/m2 in 30-50kbar interval. The spinels show two compositional intervals 64-50% Cr2O3 and 47-30%. The branch with the essential enrichment to 8% TiO2with the Cr decrease what also suppose the peridotite alteration due to rising of evolving Ti-rich melts. Two descend- ing crystallization lines for the ilmenites suggest the (polybaric) differentiation in two magmatic chambers. The Cr-rich ilmenites and most Cr-rich subcalcis garnets were found in the serpentinized ilmenite harzburgites that probably surround the most deep mag-matic chamber. The Ilm -Q (coesite) intergrowth suggests the deep differenti- ation. Several ilmenites contain up to 11%MnO. Trace elements determined for the clinopyroxenes suppose small decree melting possibly under influence of subducted- related melts having definite U peak and Ta-Nb minimums. Their reaction with peri- dotites with garnet dissolution according to AFC model decrease La/Ybn ration as well as the Pb* and U peak. Two stages of the Ti-rich melt percolations suggested to be accompanied the plum- re-lated melts influence on the peridotite of Wyoming craton keel which was followed with fur-ther followed by submelting of the subducted

  6. Chemical and U-Pb dating investigation of zircons from alnöites on Malaita, Solomon Islands: evidence for prolonged kimberlite-type magmatic activity

    NASA Astrophysics Data System (ADS)

    Simonetti, A.; Neal, C. R.

    2009-12-01

    alkaline, silica-undersaturated melts (e.g. carbonatites, kimberlites). In-situ U-Pb dating conducted by LA-ICP-MS analysis of 18 zircon crystals (n= 94 total analyses) investigated here indicate a range in weighted mean (WM) 206Pb/238U ages between ~35 and ~52 Ma. The zircons from the Aluta region define a range of WM 206Pb/238U ages between 34.9 ± 1.7 Ma and 45.1 ± 2.7 Ma (2σ), and correlate negatively with Zr/Hf ratios and total REE content. In contrast, the zircons from Kwainale define a uniform WM 206Pb/238U age spectrum yielding an age of 36.9 ± 0.6 Ma. The zircons from Faufaumela yield a range of WM 206Pb/238U dates from 38.1 ± 1.5 Ma to 51.9 ± 2.6 Ma, with the latter containing the lowest total REE content. The trace element compositions of the zircon crystals investigated here clearly indicate and support their mantle-derived origin, and their correlations with the age determinations (i.e. Aluta, Faufaumela zircons) suggest their progressive formation within an evolving mantle source region over a ~20 Ma interval. It is concluded that alnöite magmatism within the Ontong Java Plateau occurred over a prolonged period rather than a single event as previously suggested.

  7. 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

  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. Sulfur isotope composition of metasomatised mantle xenoliths from the Bultfontein kimberlite (Kimberley, South Africa): Contribution from subducted sediments and the effect of sulfide alteration on S isotope systematics

    NASA Astrophysics Data System (ADS)

    Giuliani, Andrea; Fiorentini, Marco L.; Martin, Laure A. J.; Farquhar, James; Phillips, David; Griffin, William L.; LaFlamme, Crystal

    2016-07-01

    Sulfur isotopes are a powerful geochemical tracer in high-temperature processes, but have rarely been applied to the study of mantle metasomatism. In addition, there are very limited S isotope data on sub-continental lithospheric mantle (SCLM) material. For cratonic regions, these data are restricted to sulfide inclusions in diamonds. To provide new constraints on the S isotope composition of the SCLM and on the source(s) of mantle metasomatic fluids beneath the diamondiferous Kimberley region (South Africa), we investigated the S isotope systematics of five metasomatised mantle xenoliths from the Bultfontein kimberlite. Pentlandite and chalcopyrite in these xenoliths were analysed by in situ secondary-ion mass spectrometry (SIMS), with bulk-rock material measured by gas source isotope ratio mass spectrometry techniques. Based on previous studies, the xenoliths experienced different types of metasomatism to one another at distinct times (∼180 and ∼90-80 Ma). Contained pentlandite grains show variable alteration to heazlewoodite (i.e. Ni sulfide) + magnetite. The in situ S isotope analyses of pentlandite exhibit a relatively restricted range between -5.9 and - 1.4 ‰δ34 S (compared to VCDT), with no statistically meaningful differences between samples. Chalcopyrite only occurs in one sample and shows δ34 S values between -5.4 and - 1.0 ‰. The bulk-rock Ssulfide isotope analyses vary between -3.4 and + 0.8 ‰δ34 S. Importantly, the only sample hosting dominantly fresh sulfides shows a bulk-rock δ34 S value consistent with the mean value for the sulfides, whereas the other samples exhibit higher bulk 34S/32S ratios. The differences between bulk-rock and average in situδ34 S values are directly correlated with the degree of sulfide alteration. This evidence indicates that the elevated 34S/32S ratios in the bulk samples are not due to the introduction of heavy S (commonly as sulfates) and are best explained by isotopic fractionation coupled with the removal

  10. The Local Structural State of Aluminosilicate Garnet Solid Solutions: An Investigation of Grospydite Garnet from the Roberts Victor Kimberlite Using Paramagnetically Shifted 27Al and 29Si MAS NMR Resonances

    NASA Astrophysics Data System (ADS)

    Geiger, C. A.; Palke, A. C.; Stebbins, J. F.

    2014-12-01

    Most rock-forming silicates are substitutional solid solutions. Over the years extensive research has been done to determine their structural and crystal chemical properties. Here, the distribution of cations, or order-disorder behavior, is of central importance. In the case of aluminosilicate garnet solid solutions (X3Al2Si3O12 with X = Mg, Fe2+, Mn2+ and Ca) it has been shown that both synthetic and natural crystals have random long-range X-cation disorder in space group Ia-3d, as given by X-ray single-crystal diffraction measurements. However, the structural state of natural garnets at the local scale is not known. Garnet from a grospydite xenolith from the Roberts Victor kimberlite, South Africa, was studied by 27Al and 29Si MAS NMR spectroscopy. The research thrust was placed on measuring and analyzing paramagnetically shifted resonances to determine the local (short range) structural state of the X-cations in a grossular-rich ternary aluminosilicate garnet solid solution. The garnet crystals are compositionally homogeneous based on microprobe analysis, showing no measurable zoning, and have the formula Grs46.7Prp30.0Alm23.3. The garnet is cubic with the standard garnet space group Ia-3d. The 27Al MAS NMR spectrum shows a very broad asymmetric resonance located between about 100 and -50 ppm. It consists of a number of individual overlapping paramagnetically shifted resonances, which are difficult to analyze quantitatively. The 29Si MAS NMR spectrum, showing better resolution, has two observable resonances termed S0 and S4. S0 is located between about -60 ppm and -160 ppm and S4 is centered at roughly 95 ppm. Both S0 and S4 are composite resonances in nature containing many overlapping individual peaks. S0 contains information on local cation configurations whereby an isolated SiO4 group in the garnet structure does not have an edge-shared Fe2+-containing dodecahedron. S4 involves local configurations where there is one edge-shared dodecahedron containing Fe2