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1.
On a global scale, peridotitic garnet inclusions in diamonds from the subcratonic lithosphere indicate an evolution from strongly sinusoidal REE N, typical for harzburgitic garnets, to mildly sinusoidal or “normal” patterns (positive slope from LREE N to MREE N, fairly flat MREE N–HREE N), typical for lherzolitic garnets. Using the Cr-number of garnet as a proxy for the bulk rock major element composition it becomes apparent that strong LREE enrichment in garnet is restricted to highly depleted lithologies, whereas flat or positive LREE–MREE slopes are limited to less depleted rocks. For lherzolitic garnet inclusions, there is a positive relation between equilibration temperature, enrichment in MREE, HREE and other HFSE (Ti, Zr, Y), and decreasing depletion in major elements. For harzburgitic garnets, relations are not linear, but it appears that lherzolite style enrichment in MREE–HREE only occurs at temperatures above 1150–1200 °C, whereas strong enrichment in Sr is absent at these high temperatures. These observations suggest a transition from melt metasomatism (typical for the lherzolitic sources) characterized by fairly unfractionated trace and major element compositions to metasomatism by CHO fluids carrying primarily incompatible trace elements. Melt and fluid metasomatism are viewed as a compositional continuum, with residual CHO fluids resulting from primary silicate or carbonate melts in the course of fractional crystallization and equilibration with lithospheric host rocks. Eclogitic garnet inclusions show “normal” REEN patterns, with LREE at about 1× and HREE at about 30× chondritic abundance. Clinopyroxenes approximately mirror the garnet patterns, being enriched in LREE and having chondritic HREE abundances. Positive and negative Eu anomalies are observed for both garnet and clinopyroxene inclusions. Such anomalies are strong evidence for crustal precursors for the eclogitic diamond sources. The trace element composition of an “average eclogitic diamond source” based on garnet and clinopyroxene inclusions is consistent with derivation from former oceanic crust that lost about 10% of a partial melt in the garnet stability field and that subsequently experienced only minor reenrichment in the most incompatible trace elements. Based on individual diamonds, this simplistic picture becomes more complex, with evidence for both strong enrichment and depletion in LREE. Trace element data for sublithospheric inclusions in diamonds are less abundant. REE in majoritic garnets indicate source compositions that range from being similar to lithospheric eclogitic sources to strongly LREE enriched. Lower mantle sources, assessed based on CaSi–perovskite as the principal host for REE, are not primitive in composition but show moderate to strong LREE enrichment. The bulk rock LREEN–HREEN slope cannot be determined from CaSi–perovskites alone, as garnet may be present in these shallow lower mantle sources and then would act as an important host for HREE. Positive and negative Eu anomalies are widespread in CaSi–perovskites and negative anomalies have also been observed for a majoritic garnet and a coexisting clinopyroxene inclusion. This suggests that sublithospheric diamond sources may be linked to old oceanic slabs, possibly because only former crustal rocks can provide the redox gradients necessary for diamond precipitation in an otherwise reduced sublithospheric mantle. 相似文献
2.
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-bearing veins and disseminated phlogopite. Phlogopite-bearing veins may represent kimberlite-related addition and/or earlier K-metasomatism. Xenolith thermobarometry using published two-pyroxene and Al-in-opx methods suggest that garnet–spinel peridotites are derived from 1180 to 1390 °C and 3.6 to 4.7 GPa, close to the diamond–graphite boundary and above a 38 mW/m2 shield geotherm. Low-Ca garnet–spinel harzburgites with G10 garnets fall in about the same T and P range. Most spinel peridotites with assumed 2.0 GPa pressure are in the same T range, possibly indicating heating of the shallow mantle. Four of 79 Cr diopside xenocrysts have P–T estimates in the diamond stability field using published single-pyroxene P–T calculation methods. 相似文献
3.
Garnet–biotite and garnet–cordierite geothermometers have been consistently calibrated, using the results of Fe 2+–Mg cation exchange experiments and utilizing recently evaluated nonideal mixing properties of garnet. Nonideal mixing parameters of biotite (including Fe, Mg, Al VI, and Ti) and of cordierite (involving Fe and Mg) are evaluated in terms of iterative multiple least-square regressions of the experimental results. Assuming the presence of ferric Fe in biotite in relation to the coexisting Fe-oxide phases (Case A), and assuming the absence of ferric Fe in biotite (Case B), two formulae of garnet–biotite thermometer have been derived. The garnet–cordierite geothermometer was constructed using Margules parameters of garnet adopted in the garnet–biotite geothermometers. The newly calibrated garnet–biotite and garnet–cordierite thermometers clearly show improved conformity in the calculated temperatures. The thermometers give temperatures that are consistent with each other using natural garnet–biotite–cordierite assemblages within ±50 °C. The effects of ferric Fe in biotite on garnet–biotite thermometry have been evaluated comparing the two calibrations of the thermometer. The effects are significant; it is clarified that taking ferric Fe content in biotite into account leads to less dispersion of thermometric results. 相似文献
4.
湖南沅江是我国砂矿金刚石的重要产地,石榴子石和金刚石是砂矿中常见的重矿物,与金刚石相关的石榴子石特征研究,对揭示湖南砂矿金刚石的来源与形成条件有重要意义。本文随机选取湖南沅江辰溪地区金刚石砂矿中160粒碎屑石榴子石和5粒金刚石包裹体中的石榴子石,采用矿物学、地球化学并借鉴统计学方法对它们进行了分析比较。结果显示,碎屑石榴子石主要为铁铝-锰铝榴石系列,其中个别石榴子石含有金刚石包裹体。聚类分析、线性判别、逻辑回归分析计算显示,部分G3榴辉岩型石榴子石与金刚石可能具有成生联系。同时,两个采集地点的石榴子石类型、主微量元素具有一定的差异,其中一个地点的石榴子石样品DJZ-7-1具有与金刚石更强的亲缘性。基于本文碎屑石榴子石Si值大于3.02以及前人对湖南金刚石限定的温压条件进行分析,认为湖南金刚石可能形成于深度小于220 km的橄榄岩-榴辉岩混杂区,该区域系钾镁煌斑岩型金刚石来源的优势区域。据此,建议可在辰溪赤岩村河段上游区域进一步寻找幔源G3型石榴子石以及钾镁煌斑岩,以期发现原生金刚石矿床。 相似文献
5.
Cathodoluminescence (CL) imaging of polished sections of a diamond from the Guaniamo region of Venezuela suggests a history of the diamond involving two periods of growth separated by a period of resorption and possibly brittle deformation. In situ electron probe analysis of multiple eclogitic garnet inclusions reveals a correlation between garnet composition and location in the stone. An early-formed garnet in the diamond core has higher Ca/(Ca+Mg) and lower Mg/(Mg+Fe) values than later garnets associated with the second period of diamond growth. This variation conforms to an extensive trend of variation in the suite of eclogitic garnets extracted from Venezuelan diamonds. The diamond is zoned in carbon isotope composition (in situ secondary ion mass spectrometry, SIMS, data). The core compositions ( δ13C PDB), corresponding to the first stage of growth, average −17.7‰. The second period of growth is apparently in two sub-sets of CL zones with mean values of −13.0‰ and −7.9‰. Nitrogen contents of diamond are low (30–300 atomic ppm) and do not correlate with carbon isotope composition. Oxygen isotope ratios of the garnet inclusions are elevated substantially above those expected for “common mantle”; δ18O VSMOW of early garnet is approximately +10.5‰ and two late garnets average +8.8‰. The evolutionary trend of magnesium enrichment in garnet is unlikely to represent igneous fractionation. The stable isotope data are consistent with diamond formation in subducted meta-basic rocks that had interacted with sea water at low temperatures at or near the sea floor and contained a substantial biogenic carbon component. During or following subduction, diamonds continued to form in an evolving system that was progressively modified by interaction with mantle material. 相似文献
6.
Mineral inclusions recovered from 100 diamonds from the A154 South kimberlite (Diavik Diamond Mines, Central Slave Craton, Canada) indicate largely peridotitic diamond sources (83%), with a minor (12%) eclogitic component. Inclusions of ferropericlase (4%) and diamond in diamond (1%) represent “undetermined” parageneses. Compared to inclusions in diamonds from the Kaapvaal Craton, overall higher CaO contents (2.6 to 6.0 wt.%) of harzburgitic garnets and lower Mg-numbers (90.6 to 93.6) of olivines indicate diamond formation in a chemically less depleted environment. Peridotitic diamonds at A154 South formed in an exceptionally Zn-rich environment, with olivine inclusions containing more than twice the value (of 52 ppm) established for normal mantle olivine. Harzburgitic garnet inclusions generally have sinusoidal rare earth element (REEN) patterns, enriched in LREE and depleted in HREE. A single analyzed lherzolitic garnet is re-enriched in middle to heavy REE resulting in a “normal” REEN pattern. Two of the harzburgitic garnets have “transitional” REEN patterns, broadly similar to that of the lherzolitic garnet. Eclogitic garnet inclusions have normal REEN patterns similar to eclogitic garnets worldwide but at lower REE concentrations. Carbon isotopic values (δ13C) range from − 10.5‰ to + 0.7‰, with 94% of diamonds falling between − 6.3‰ and − 4.0‰. Nitrogen concentrations range from below detection (< 10 ppm) to 3800 ppm and aggregation states cover the entire spectrum from poorly aggregated (Type IaA) to fully aggregated (Type IaB). Diamonds without evidence of previous plastic deformation (which may have accelerated nitrogen aggregation) typically have < 25% of their nitrogen in the fully aggregated B-centres. Assuming diamond formation beneath the Central Slave to have occurred in the Archean [Westerlund, K.J., Shirey, S.B., Richardson, S.H., Gurney, J.J., Harris, J.W., 2003b. Re–Os systematics of diamond inclusion sulfides from the Panda kimberlite, Slave craton. VIIIth International Kimberlite Conference, Victoria, Canada, Extended Abstracts, 5p.], such low aggregation states indicate mantle residence at fairly low temperatures (< 1100 °C). Geothermometry based on non-touching inclusion pairs, however, indicates diamond formation at temperatures around 1200 °C. To reconcile inclusion and nitrogen based temperature estimates, cooling by about 100–200 °C shortly after diamond formation is required. 相似文献
7.
Subcalcic, high-Cr (G10) garnets are found as inclusions within diamonds and in peridotitic xenoliths. The strong spatial associations between G10 garnets and diamond make them an important tool in the investigation of diamond genesis. We present an integrated study of the major and trace element composition and oxygen-Sr-Nd-Hf isotopic ratios of eight G10 garnets from the Ekati mine (NWT-Canada) and four from the Murowa mine (Zimbabwe) in an attempt to determine their petrogenetic evolution and to further examine a possible relationship between the metasomatic agents responsible for G10 garnet signatures and diamond forming fluids.All garnets display sinusoidal to mildly sinusoidal REE patterns and have negative Ti, Sr and positive U anomalies. They have variably radiogenic 87Sr/ 86Sr (0.703261-0.731191) and non-radiogenic εNd values (−8.1 to −27.1), except for one sample from Murowa that has a positive εNd of 2.5. One Ekati sample has an extremely low εHf value of −61.6. The Ekati garnets we have studied all appear to come from a single depth in the Slave lithospheric mantle. On the base of Cr-Ca relations they have crystallized at 4.9 GPa and display dunitic Ca intercept values. Their δ 18O values range between +5.23‰ and +5.42‰.The Ekati G10 garnets record a complex, multi-stage metasomatic history involving the interaction of several components during their genesis. One metasomatic agent was enriched in HFSE, LREE, Sr, and depleted in Nb. This agent had the least radiogenic Sr. Another metasomatic agent had highly radiogenic Sr, and was enriched in LREE, Sr, Nb, Th and U.The G10 garnets have very low εNd and εHf values combined with radiogenic Sr, thus, they require an early lithospheric mantle enrichment event at some stage during their genesis or during the evolution of any precursor material that they formed from. The only Hf isotope composition measurable from the Ekati suite is so unradiogenic ( εHf = −61) that it yields a Lu/Hf model age of 3521 Ma. This indicates that the lithospheric enrichment event seen by the Ekati garnets or their precursors may have occurred in the early stages of the craton stabilization, during the diamond forming event [Westerlund K., Shirey S., Richardson S., Carlson R., Gurney J. and Harris J. (2006) A subduction wedge origin for Paleoarchean peridotitic diamonds and harzburgites from the Panda kimberlite, Slave craton: evidence from Re-Os isotope systematics. Contrib. Mineral. Petrol.152(3), 275-294]. Although our data cannot unequivocally discriminate between a variety of models for the genesis of subcalcic garnets it is clear that the host peridotite originated via melting at shallow depths followed by subduction and that the observed geochemical fingerprint of the garnets is strongly influenced by diamond forming fluids. Diamond forming fluids sampled from fibrous diamonds, have steep REE patterns, negative Ti and Sr anomalies and very low Sm/Nd ratios that are very similar to G10 garnet characteristics. These diamond forming fluids have been recently shown to have extreme Sr and Nd isotopic compositions [Klein-BenDavid O., Pearson D. G., Nowell G. M. and Cantigny P. (2008) Origins of diamond forming fluids—constraints from a coupled Sr-Nd isotope and trace element approach. Extended abstracts to the 9th International Kimberlite Conference, Frankfurt, Germany, 9IKC-A-00118.] that are closely concordant with G10 garnets. The fluids are also rich in LREE, P, K and water, sharing these features with mica-rich metasomes. These similarities suggest that ancient lithospheric metasomes could either provide a source region for, or be a product of diamond forming fluids. Diamond forming fluids appear to be intimately involved in the evolution of G10 garnets in the lithospheric mantle, either acting as a metasomatic agent, or being integral to triggering or enhancing garnet growth in a Cr-rich protolith. Such a link explains the strong association between G10 garnets and diamonds. 相似文献
8.
辽宁瓦房店金刚石矿区金伯利岩中的石榴石一直被当作镁铝榴石。为了确定矿区颜色复杂的石榴石种类,本文对矿区的石榴石进行了系统的采样分析,测定了112件石榴石样品的晶胞参数、50件样品的微区化学成分和40件样品的红外光谱。利用石榴石晶胞参数、红外光谱、化学成分和化学分子式方法对矿区石榴石进行分类,结果显示:晶胞参数分类法误差大,容易得出错误结论;红外图谱分类法准确度不高,只能作为参考方法;化学成分分类法太过笼统,达不到详细划分石榴石种类的目的;化学分子式分类法可把矿区的石榴石详细划分6个矿种:镁钙铁-铝铬铁榴石、镁铁钙-铝铬铁榴石、镁钙铁-铝铬榴石、镁钙-铝铬铁镁榴石、镁铁钙-铝铬榴石、镁铁钙-铝铁铬榴石,每种石榴石都充分反映了A、B离子的种类及占位特征,是4种分类方法中最为科学的方法。研究认为瓦房店金刚石矿区金伯利岩中石榴石A端元成分以Mg 2+离子占位为主;B端元成分以Al 3+离子占位为主。由于阳离子替代普遍,A、B端元成分复杂,瓦房店金伯利岩中不存在单纯意义上的镁铝榴石。 相似文献
9.
Diamond exploration focuses on geochemical analysis of indicator minerals that are more abundant than diamond itself. Among such indicators, low-Cr (Cr2O3 < 1 wt%) garnets from mantle eclogites are problematic since they overlap compositionally with many lower-crust-derived garnets also transported by kimberlite. Misclassification of these garnets may create “false positive” mantle signatures and possible misdirection of exploration efforts. Statistical solutions using major elements in low-Cr garnet (Hardman et al. in J Geochem Explor 186:24–35, 2018) provide improved error rates for the discrimination of low-Cr crustal and mantle garnets recovered from kimberlite. In this study we analysed a large suite of garnets (n = 571) from both crustal and mantle settings, already characterised for major elements, for a wide range of trace elements by laser ablation inductively-coupled plasma mass spectrometry and use these new data along with literature data (n = 169) to evaluate the effectiveness of adding trace elements to garnet-based diamond exploration programs. A new garnet classification scheme, initially using a major-element based filter, uses garnet Sr contents and Eu anomalies to help identify low-Cr garnets that are misclassified using major element methods. Combined with existing methods, our new trace element classifiers offer improvement in classification error rates for low-Cr, crustal and mantle garnets to as low as 4.7% for calibration data. 相似文献
10.
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 (Fo 95) 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 REE N 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
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