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1.
Steven Creighton Thomas Stachel Sergei Matveev Heidi Höfer Catherine McCammon Robert W. Luth 《Contributions to Mineralogy and Petrology》2009,157(4):491-504
The oxidation state, reflected in the oxygen fugacity (fO2), of the subcratonic lithospheric mantle is laterally and vertically heterogeneous. In the garnet stability field, the Kaapvaal
lithospheric mantle becomes progressively more reducing with increasing depth from Δlog fO2 FMQ-2 at 110 km to FMQ-4 at 210 km. Oxidation accompanying metasomatism has obscured this crystal-chemical controlled depth-fO2 trend in the mantle beneath Kimberley, South Africa. Chondrite normalized REE patterns for garnets, preserve evidence of
a range in metasomatic enrichment from mild metasomatism in harzburgites to extensive metasomatism by LREE-enriched fluids
and melts with fairly unfractionated LREE/HREE ratios in phlogopite-bearing lherzolites. The metasomatized xenoliths record
redox conditions extending up to Δlog fO2 = FMQ, sufficiently oxidized that magnesite would be the stable host of carbon in the most metasomatized samples. The most
oxidized lherzolites, those in or near the carbonate stability field, have the greatest modal abundance of phlogopite and
clinopyroxene. Clinopyroxene is modally less abundant or absent in the most reduced peridotite samples. The infiltration of
metasomatic fluids/melts into diamondiferous lithospheric mantle beneath the Kaapvaal craton converted reduced, anhydrous
harzburgite into variably oxidized phlogopite-bearing lherzolite. Locally, portions of the lithospheric mantle were metasomatized
and oxidized to an extent that conversion of diamond into carbonate should have occurred.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
2.
I.?Leost T.?Stachel G.?P.?BreyEmail author J.?W.?Harris I.?D.?Ryabchikov 《Contributions to Mineralogy and Petrology》2003,145(1):15-24
Mineral inclusions in diamonds from Namibia document a range of mantle sources, including eclogitic, websteritic and peridotitic parageneses. Based on unusual textural features a group of inclusions showing websteritic, peridotitic and transitional chemical features is assigned to an 'undetermined suite' (12% of the studied diamonds). The mutual characteristic of this group is the occurrence of lamellar intergrowths of clinopyroxene and orthopyroxene. In addition, the 'undetermined suite' is associated with a number of uncommon phases: in one diamond MgCO3 is enclosed by clinopyroxene. Other minerals that form touching inclusions with the pyroxene lamellae are (1) a SiO2 phase observed in three diamonds, together with CaCO3 in one of them, (2) phlogopite and a Cr-rich 'titanate' (probably lindsleyite). The inclusions document a metamorphic path of decreasing pressures and temperatures after entrapment in diamond. First, homogeneous low-Ca clinopyroxenes were entrapped at high temperatures. They subsequently exsolved orthopyroxene and probably also SiO2 (coesite) on cooling along a P,T trajectory that did not allow garnet to be exsolved as well. Phlogopite, carbonates and LIMA phases are the result of overprint of a peridotitic source rock by a carbon-rich agent. The resulting unusual, olivine-free mineral association and the host diamonds are interpreted as products of extensive carbonation of the peridotite. 相似文献
3.
T. Stachel J. Harris S. Aulbach P. Deines 《Contributions to Mineralogy and Petrology》2002,142(4):465-475
Diamonds from the Kankan area in Guinea formed over a large depth profile beginning within the cratonic mantle lithosphere and extending through the asthenosphere and transition zone into the lower mantle. The carbon isotopic composition, the concentration of nitrogen impurities and the nitrogen aggregation level of diamonds representing this entire depth range have been determined. Peridotitic and eclogitic diamonds of lithospheric origin from Kankan have carbon isotopic compositions ('13C: peridotitic -5.4 to -2.2; eclogitic -19.7 to -0.7) and nitrogen characteristics (N: peridotitic 17-648 atomic ppm; eclogitic 0-1,313 atomic ppm; aggregation from IaA to IaB) which are generally typical for diamonds of these two suites worldwide. Geothermobarometry of peridotitic and eclogitic inclusion parageneses (worldwide sources) indicates that both suites formed under very similar conditions within the cratonic lithosphere, which is not consistent with a derivation of diamonds with light carbon isotopic composition from subducted organic matter within subducting oceanic slabs. Diamonds containing majorite garnet inclusions fall to the isotopically heavy side ('13C: -3.1 to +0.9) of the worldwide diamond population. Nitrogen contents are low (0-126 atomic ppm) and one of the two nitrogen-bearing diamonds shows such a low level of nitrogen aggregation (30% B-centre) that it cannot have been exposed to ambient temperatures of the transition zone (̿,400 °C) for more than 0.2 Ma. This suggests rapid upward transport and formation of some Kankan diamonds pene-contemporaneous to Cretaceous kimberlite activity. Similar to these diamonds from the asthenosphere and the transition zone, lower mantle diamonds show a small shift towards isotopic heavy compositions (-6.6 to -0.5, mode at -3.5). As already observed for other mines, the nitrogen contents of lower mantle diamonds were below detection (using FTIRS). The mutual shift of sublithospheric diamonds towards isotopic heavier compositions suggests a common carbon source, which may have inherited an isotopic heavy composition from a component consisting of subducted carbonates. 相似文献
4.
Inclusions of ferropericlase and former (Mg,Fe)(Si,Al)O3 perovskite in diamonds from Kankan, Guinea believed to originate in the lower mantle were studied using Mössbauer spectroscopy to determine Fe3+/ΣFe. Fe3+ concentration in the (Mg,Fe)(Si,Al)O3 inclusion is consistent with empirical relations relating Fe3+/ΣFe to Al concentration, supporting the inference that it crystallised in the perovskite structure at lower mantle conditions. In ferropericlase there is a nearly linear variation of trivalent cation abundance with monovalent cation abundance, suggesting a substitution of the form Na0.5M0.53+O (M=Fe3+, Cr3+, Al3+). Excess positive charge is likely balanced by cation vacancies, where their abundance is observed to increase with increasing iron concentration, consistent with high-pressure experiments. The abundance of cation vacancies is related to oxygen fugacity, where ferropericlase inclusions from Kankan and São Luiz (Brazil) are inferred to have formed at conditions more oxidising than Fe-(Mg,Fe)O equilibrium, but more reducing than Re-ReO2 equilibrium. Fe2+/Mg partition coefficients between (Mg,Fe)(Si,Al)O3 and ferropericlase were calculated for inclusions co-existing in the same diamond using Mössbauer data and empirical relations based on high-pressure experimental work. Most values are consistent with high-pressure experiments, suggesting that these inclusions equilibrated at lower mantle conditions. The measured ferropericlase Fe3+ concentrations are consistent with diamond formation in a region of redox gradients, possibly arising from the subduction of oxidised material into reduced lower mantle. Reduction of carbonate to form ferropericlase and diamond is consistent with a slight shift of Kankan δ13C values to isotopically heavy compositions compared to the worldwide dataset, and could supply the oxygen necessary to satisfy the high Fe3+ concentration in (Mg,Fe)(Si,Al)O3 perovskite, as well as account for the high proportion of ferropericlase in the lower mantle paragenesis. The heterogeneity of lower mantle diamond sources indicates that the composition of lower mantle diamonds do not necessarily reflect those of the bulk mantle. 相似文献
5.
Oxidation state of the lithospheric mantle beneath Diavik diamond mine, central Slave craton, NWT, Canada 总被引：1，自引：0，他引：1
Steven Creighton Thomas Stachel Dave Eichenberg Robert W. Luth 《Contributions to Mineralogy and Petrology》2010,159(5):645-657
Oxygen fugacity (fO2) conditions were determined for 29 peridotite xenoliths from the A154-North and A154-South kimberlites of the Diavik diamond
mine using the newly developed flank method modified specifically for measuring Fe3+ in mantle-derived pyropic garnets. The results indicate that the garnet-bearing lithospheric mantle beneath the central Slave
craton is vertically layered with respect to oxidation state. The shallow (<140 km), “ultra-depleted” layer is the most oxidized
section of garnet-bearing subcratonic mantle thus far measured, up to one log unit more oxidizing relative to the FMQ buffer
[Δlog fO2 (FMQ) + 1]. The lower, more fertile layer has fO2 conditions that extend down to Δlog fO2 (FMQ) − 3.8, consistent with xenolith suites from other localities worldwide. Based on trace element concentrations in garnets,
two distinct metasomatic events affected the mantle lithosphere at Diavik. An oxidized fluid imparted sinusoidal chondrite-normalized
REE patterns on garnets throughout the entire depth range sampled. In contrast, a reducing melt metasomatic event affected
only the lower portion of the lithospheric mantle. The fO2 state of the Diavik mantle sample suggests that diamond formation occurred by reduction of carbonate by fluids arising from
beneath the lithosphere. 相似文献
6.
D. Howell C. J. O’Neill K. J. Grant W. L. Griffin S. Y. O’Reilly N. J. Pearson R. A. Stern T. Stachel 《Contributions to Mineralogy and Petrology》2012,164(6):1011-1025
Fourier transform infrared mapping of diamonds can reveal detailed information on impurities, with a spatial context. We apply this technique, combined with in situ isotopic analysis of carbon, to the study of cuboid diamond growth in a sample that exhibits some mixed-habit growth. While there has been some uncertainty in the literature regarding sectoral differences in nitrogen aggregation and subsequent platelet development, the data from this study appear far more conclusive. We show that despite nitrogen being concentrated in octahedral sectors, there is no detectable difference in the concentration-adjusted rate of nitrogen aggregation within octahedral and cuboid sectors. However, the resultant platelet development is significantly reduced in cuboid sectors compared to contemporaneously formed octahedral sectors. This finding has significant implications for the classification of diamonds using the relationship between their platelet intensity and the absorption caused by B centres. It means that cuboid diamonds naturally fall below the linear relationship that has been termed regular, which would lead to them being incorrectly interpreted as having experienced heating or deformation. The data also support earlier suggestions that large hydrogen concentrations in the diamond-forming fluid may be required for cuboid growth. We further suggest that high nitrogen and hydrogen concentrations are required for mixed-habit diamond growth, which might be the product of specific fluid chemistries that occur in reducing mantle environments. 相似文献
7.
Steven Creighton Thomas Stachel Hayley McLean Karlis Muehlenbachs Antonio Simonetti Dave Eichenberg Robert Luth 《Contributions to Mineralogy and Petrology》2008,155(5):541-554
Twenty-five diamonds recovered from 21 diamondiferous peridotitic micro-xenoliths from the A154 South and North kimberlite
pipes at Diavik (Slave Craton) match the general peridotitic diamond production at this mine with respect to colour, carbon
isotopic composition, and nitrogen concentrations and aggregation states. Based on garnet compositions, the majority of the
diamondiferous microxenoliths is lherzolitic (G9) in paragenesis, in stark contrast to a predominantly harzburgitic (G10)
inclusion paragenesis for the general diamond production. For garnet inclusions in diamonds from A154 South, the lherzolitic
paragenesis, compared to the harzburgitic paragenesis, is distinctly lower in Cr content. For microxenolith garnets, however,
Cr contents for garnets of both the parageneses are similar and match those of the harzburgitic inclusion garnets. Assuming
that the microxenolith diamonds reflect a sample of the general diamond population, the abundant Cr-rich lherzolitic garnets
formed via metasomatic overprinting of original harzburgitic diamond sources subsequent to diamond formation, conversion of
original harzburgitic diamond sources occurred in the course of metasomatic overprint re-fertilization. Metasomatic overprinting
after diamond formation is supported by the finding of a highly magnesian olivine inclusion (Fo95) in a microxenolith diamond that clearly formed in a much more depleted environment than indicated by the composition of
its microxenolith host. Chondrite normalized REE patterns of microxenolith garnets are predominantly sinusoidal, similar to
observations for inclusion garnets. Sinusoidal REEN patterns are interpreted to indicate a relatively mild metasomatic overprint through a highly fractionated (very high LREE/HREE)
fluid. The predominance of such patterns may explain why the proposed metasomatic conversion of harzburgite to lherzolite
appears to have had no destructive effect on diamond content.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
8.
Bussweiler Yannick Pearson D Graham Stachel Thomas Kjarsgaard Bruce A. 《Mineralogy and Petrology》2018,112(2):583-596
Kimberlites from the Diavik and Ekati diamond mines in the Lac de Gras kimberlite field contain abundant large (>1 cm) clinopyroxene (Cr-diopside) and garnet (Cr-pyrope) crystals. We present the first extensive mineral chemical dataset for these megacrysts from Diavik and Ekati and compare their compositions to cratonic peridotites and megacrysts from the Slave and other cratons. The Diavik and Ekati Cr-diopside and Cr-pyrope megacrysts are interpreted to belong to the Cr-rich megacryst suite. Evidence for textural, compositional, and isotopic disequilibrium suggests that they constitute xenocrysts in their host kimberlites. Nevertheless, their formation may be linked to extensive kimberlite magmatism and accompanying mantle metasomatism preceding the eruption of their host kimberlites. It is proposed that the formation of megacrysts may be linked to failed kimberlites. In this scheme, the Cr-rich megacrysts are formed by progressive interaction of percolating melts with the surrounding depleted mantle (originally harzburgite). As these melts percolate outwards, they may contribute to the introduction of clinopyroxene and garnet into the depleted mantle, thereby forming lherzolite. This model hinges on the observation that lherzolitic clinopyroxenes and garnets at Lac de Gras have compositions that are strikingly similar to those of the Cr-rich megacrysts, in terms of major and trace elements, as well as Sr isotopes. As such, the Cr-rich megacrysts may have implications for the origin of clinopyroxene and garnet in cratonic lherzolites worldwide.
相似文献9.
Poitras St&#;phane P. Pearson D. Graham Hardman Matthew F. Stachel Thomas Nowell Geoff M. Cairns Scott 《Mineralogy and Petrology》2018,112(2):719-736
The Central Mackenzie Valley (CMV) area of Northwest Territories is underlain by Precambrian basement belonging to the North American Craton. The potential of this area to host kimberlitic diamond deposits is relatively high judging from the seismologically-defined lithospheric thickness, age of basement rocks (2.2–1.7 Ga) and presence of kimberlite indicator minerals (KIMs) in Quaternary sediments. This study presents data for a large collection of KIMs recovered from stream sediments and till samples from two study areas in the CMV, the Horn Plateau and Trout Lake. In the processed samples, peridotitic garnets dominate the KIM grain count for both regions (> 25% each) while eclogitic garnet is almost absent in both regions (< 1% each). KIM chemistry for the Horn Plateau indicates significant diamond potential, with a strong similarity to KIM systematics from the Central and Western Slave Craton. The most significant issue to resolve in assessing the local diamond potential is the degree to which KIM chemistry reflects local and/or distal kimberlite bodies. Radiogenic isotope analysis of detrital kimberlite-related CMV ilmenite and rutile grains requires at least two broad age groups for eroded source kimberlites. Statistical analysis of the data suggests that it is probable that some of these KIMs were derived from primary and/or secondary sources within the CMV area, while others may have been transported to the area from the east-northeast by Pleistocene glacial and/or glaciofluvial systems. At this stage, KIM chemistry does not allow the exact location of the kimberlitic source(s) to be constrained.
相似文献10.
Thomas Stachel Gerhard P. Brey Jeff W. Harris 《Contributions to Mineralogy and Petrology》2000,140(1):1-15
Peridotitic inclusions in alluvial diamonds from the Kankan region of Guinea in West Africa are mainly of lherzolitic paragenesis.
Nevertheless, extreme Cr2O3 contents (max. 17 wt%) in some of the exclusively lherzolitic garnets document that the diamond source experienced a previous
stage of melt extraction in the spinel stability field. This initial depletion was followed by at least two metasomatic stages:
(1) enrichment of LREE and Sr and (2) introduction mainly of MREE–HREE and other HFSE (Ti, Y, Zr, Hf). The Ti- and HFSE-poor
character of stage (1) points towards a CHO-rich fluid or carbonatitic melt, the high HFSE in stage (2) favour silicate melts
as enriching agent. Eclogitic inclusions are derived from a large depth interval ranging from the lithosphere through the
asthenosphere into the transition zone. The occurrence of negative Eu anomalies in garnet and clinopyroxene from both lithosphere
and transition zone suggests a possible relationship to subducted oceanic crust. Lithospheric eclogitic inclusions are derived
from heterogeneous sources, that may broadly be divided into a low-Ca group with LREE depleted trace element patterns and
a high-Ca group representing a source with negative LREE–HREE slope that is moderately enriched in incompatible elements relative
to primitive mantle. High-Ca inclusions of majoritic paragenesis are significantly more enriched in incompatible elements,
such as in Sr and LREE. Calculated whole rock compositions require metasomatic enrichment even if a derivation from MORB is
assumed.
Received: 26 January 2000 / Accepted: 18 May 2000 相似文献