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1.
Pyrope–almandine garnets (Mg# = 28.3–44.9, Ca# = 15.5–21.3) from a heavy mineral concentrate of diamondiferous kimberlites of the largest diamond deposit, the Yubileinaya pipe, along with kimberlite- like rocks and diamondiferous volcano–sediments of the Laptev Sea coast, have been found to contain polymineral, predominantly acicular inclusions, composed of aggregates of shrilankite (Ti2ZrO6), rutile, ilmenite, clinopyroxene, and apatite. The presence of shrilankite as an inclusion in garnets from assumed garnet–pyroxene rocks of the lower crust, lifted up by diamond-bearing kimberlite, allows it to be considered as an indicator mineral of kimberlite, which expands the possibilities when searching for kimberlite in the Arctic.  相似文献   

2.
《Geochimica et cosmochimica acta》1999,63(11-12):1825-1836
Oxygen isotope data have been obtained for silicate inclusions in diamonds, and similar associated minerals in peridotitic and eclogitic xenoliths from the Finsch kimberlite by laser-fluorination. Oxygen isotope analyses of syngenetic inclusions weighing 20–400 μg have been obtained by laser heating in the presence of ClF3. 18O/16O ratios are determined on oxygen converted to CO2 over hot graphite and, for samples weighing less than 750 μg (producing <12 μmoles O2) enhanced CO production in the graphite reactor causes a systematic shift in both δ13C and δ18O that varies as a function of sample weight. A “pressure effect” correction procedure, based on the magnitude of δ13C (CO2) depletion relative to δ13C (graphite), is used to obtain corrected δ18O values for inclusions with an accuracy estimated to be ±0.3‰ for samples weighing 40 μg.Syngenetic inclusions in host diamonds with similar δ13C values (−8.4‰ to −2.7‰) have oxygen isotope compositions that vary significantly, with a clear distinction between inclusions of peridotitic (+4.6‰ to +5.6‰) and eclogitic paragenesis (+5.7‰ to +8.0‰). The mean δ18O composition of olivine inclusions is indistinguishable from that of typical peridotitic mantle (5.25 ± 0.22‰) whereas syngenetic purple garnet inclusions possess relatively low δ18O values (5.00 ± 0.33‰). Reversed oxygen isotope fractionation between olivine and garnet in both diamond inclusions and diamondiferous peridotite xenoliths suggests that garnet preserves subtle isotopic disequilibrium related to genesis of Cr-rich garnet and/or exchange with the diamond-forming fluid. Garnet in eclogite xenoliths in kimberlite show a range of δ18O values from +2.3‰ to +7.3‰ but garnets in diamondiferous eclogites and as inclusions in diamond all have values >4.7‰.  相似文献   

3.
Two diamond bearing xenoliths found at Finsch Mine are coarse garnet lherzolites, texturally and chemically similar to the dominant mantle xenoliths in that kimberlite. A total of 46 diamonds weighing 0.053 carats have been recovered from one and 53 diamonds weighing 0.332 carats from the other. The diamonds are less corroded than diamonds recovered from the kimberlite. Geothermobarometric calculations indicate that the xenoliths equilibrated at 1,130° C and pressures 50 kb which is within the diamond stability field; this corresponds to depths of 160 km and would place the rocks on a shield geotherm at slightly greater depths than most coarse garnet lherzolites from kimberlite. The primary minerals in the two rocks are very similar to each other but distinctly different to the majority of mineral inclusions in Finsch diamonds. This suggests a different origin for the diamonds in the kimberlite and the diamonds in the xenoliths although the equilibration conditions for both suites are approximately coincident and close to the wet peridotite solidus.  相似文献   

4.
Daniel J. Schulze 《Lithos》2003,71(2-4):195-213
A new empirical method has been devised for classification of mantle-derived garnets in kimberlite. Simple chemical screens have been developed to distinguish between garnets from different parageneses, based on Mg, Fe, Ca, Cr, Ti and Na values of published analyses of garnets from >2000 ultramafic xenoliths in kimberlite. Although crustal garnets are typically uncommon as xenocrysts in kimberlite, the first step in the classification is to screen these from the mantle population, using data from >600 garnet-bearing crustal rocks. Such a screen may also prove useful in evaluating the source (crust vs. mantle) of garnet in kimberlite exploration samples. Subsequent steps divide mantle garnets into eclogite, peridotite and Cr-poor megacryst groupings, and sub-groups of the peridotite (lherzolite, harzburgite, wehrlite) and eclogite (Groups I and II and A, B, C and grospydite) populations. Important features of this classification include the fact that it is based on distinctions between groups of fundamental geological significance (e.g., peridotite vs. eclogite) and it is based on a large, well-documented and well-understood xenolith database. As it utilizes oxide values and molar ratios of major and minor elements, the rationale for the screens is readily understood and it is simple to use.  相似文献   

5.
B. Carter Hearn Jr.   《Lithos》2004,77(1-4):473-491
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   


6.
This study describes the petrography of peridotite xenoliths,and the major and trace element geochemistry of garnets in bothxenoliths and coarse concentrate from the Drybones Bay kimberlite.The temperature and depth of equilibration of clinopyroxeneand garnet show that the mantle lithosphere beneath the SW marginof the Slave Province was at least 160 km thick at the timeof kimberlite emplacement (  相似文献   

7.
Mineral inclusions in diamonds from the Sputnik kimberlite pipe, Yakutia   总被引：9，自引：0，他引：9  
The Sputnik kimberlite pipe is a small “satellite” of the larger Mir pipe in central Yakutia (Sakha), Russia. Study of 38 large diamonds (0.7-4.9 carats) showed that nine contain inclusions of the eclogitic paragenesis, while the remainder contain inclusions of the peridotitic paragenesis, or of uncertain paragenesis. The peridotitic inclusion suite comprises olivine, enstatite, Cr-diopside, chromite, Cr-pyrope garnet (both lherzolitic and harzburgitic), ilmenite, Ni-rich sulfide and a Ti-Cr-Fe-Mg-Sr-K phase of the lindsleyite-mathiasite (LIMA) series. The eclogitic inclusion suite comprises omphacite, garnet, Ni-poor sulfide, phlogopite and rutile. Peridotitic ilmenite inclusions have high Mg, Cr and Ni contents and high Nb/Zr ratios; they may be related to metasomatic ilmenites known from peridotite xenoliths in kimberlite. Eclogitic phlogopite is intergrown with omphacite, coexists with garnet, and has an unusually high TiO2 content. Comparison with inclusions in diamonds from Mir shows general similarities, but differences in details of trace-element patterns. Large compositional variations among inclusions of one phase (olivine, garnet, chromite) within single diamonds indicate that the chemical environment of diamond crystallisation changed rapidly relative to diamond growth rates in many cases. P-T conditions of formation were calculated from multiphase inclusions and from trace element geothermobarometry of single inclusions. The geotherm at the time of diamond formation was near a 35 mW/m2 conductive model; that is indistinguishable from the Paleozoic geotherm derived by studies of xenoliths and concentrate minerals from Mir. A range of Ni temperatures between garnet inclusions in single diamonds from both Mir and Sputnik suggests that many of the diamonds grew during thermal events affecting a relatively narrow depth range of the lithosphere, within the diamond stability field. The minor differences between inclusions in Mir and Sputnik may reflect lateral heterogeneity in the upper mantle.  相似文献   

8.
We present petrography and mineral chemistry for both phlogopite,from mantle-derived xenoliths(garnet peridotite,eclogite and clinopyroxene-phlogopite rocks)and for megacryst,macrocryst and groundmass flakes from the Grib kimberlite in the Arkhangelsk diamond province of Russia to provide new insights into multi-stage metasomatism in the subcratonic lithospheric mantle(SCLM)and the origin of phlogopite in kimberlite.Based on the analysed xenoliths,phlogopite is characterized by several generations.The first generation(Phil)occurs as coarse,discrete grains within garnet peridotite and eclogite xenoliths and as a rock-forming mineral within clinopyroxene-phlogopite xenoliths.The second phlogopite generation(Phl2)occurs as rims and outer zones that surround the Phil grains and as fine flakes within kimberlite-related veinlets filled with carbonate,serpentine,chlorite and spinel.In garnet peridotite xenoliths,phlogopite occurs as overgrowths surrounding garnet porphyroblasts,within which phlogopite is associated with Cr-spinel and minor carbonate.In eclogite xenoliths,phlogopite occasionally associates with carbonate bearing veinlet networks.Phlogopite,from the kimberlite,occurs as megacrysts,macrocrysts,microcrysts and fine flakes in the groundmass and matrix of kimberlitic pyroclasts.Most phlogopite grains within the kimberlite are characterised by signs of deformation and form partly fragmented grains,which indicates that they are the disintegrated fragments of previously larger grains.Phil,within the garnet peridotite and clinopyroxene-phlogopite xenoliths,is characterised by low Ti and Cr contents(TiO_21 wt.%,Cr_2 O_31 wt.% and Mg# = 100 × Mg/(Mg+ Fe)92)typical of primary peridotite phlogopite in mantle peridotite xenoliths from global kimberlite occurrences.They formed during SCLM metasomatism that led to a transformation from garnet peridotite to clinopyroxene-phlogopite rocks and the crystallisation of phlogopite and high-Cr clinopyroxene megacrysts before the generation of host-kimberlite magmas.One of the possible processes to generate low-Ti-Cr phlogopite is via the replacement of garnet during its interaction with a metasomatic agent enriched in K and H_2O.Rb-Sr isotopic data indicates that the metasomatic agent had a contribution of more radiogenic source than the host-kimberlite magma.Compared with peridotite xenoliths,eclogite xenoliths feature low-Ti phlogopites that are depleted in Cr_2O_3 despite a wider range of TiO_2 concentrations.The presence of phlogopite in eclogite xenoliths indicates that metasomatic processes affected peridotite as well as eclogite within the SCLM beneath the Grib kimberlite.Phl2 has high Ti and Cr concentrations(TiO_22 wt.%,Cr_2O_31 wt.% and Mg# = 100× Mg/(Mg + Fe)92)and compositionally overlaps with phlogopite from polymict brecc:ia xenoliths that occur in global kimberlite formations.These phlogopites are the product of kimberlitic magma and mantle rock interaction at mantle depths where Phl2 overgrew Phil grains or crystallized directly from stalled batches of kimberlitic magmas.Megacrysts,most macrocrysts and microcrysts are disintegrated phlogopite fragments from metasomatised peridotite and eclogite xenoliths.Fine phlogopite flakes within kimberlite groundmass represent mixing of high-Ti-Cr phlogopite antecrysts and high-Ti and low-Cr kimberlitic phlogopite with high Al and Ba contents that may have formed individual grains or overgrown antecrysts.Based on the results of this study,we propose a schematic model of SCLM metasomatism involving phlogopite crystallization,megacryst formation,and genesis of kimberlite magmas as recorded by the Grib pipe.  相似文献   

9.
This study presents mineralogical and thermobarometric data for equilibrium peridotite assemblages from the V. Grib kimberlite pipe of the Arkhangelsk diamond province. We provided the first constraints on the composition, structure, thermal state, and lower boundary of the lithospheric mantle beneath the V. Grib kimberlite pipe. It was found that phlogopite-free and phlogopite-bearing peridotite xenoliths can be distinguished by their mineral chemistry. The occurrence of phlogopite in peridotites may represent evidence for modal metasomatism responsible for variation in the mineral composition of phlogopite-pyrope and pyrope peridotites. On the basis of P-T estimates, we conclude that modal metasomatism may have affected the entire thickness of the lithospheric mantle beneath the V. Grib kimberlite pipe. Comparison of our results with the available data from the literature shows strong vertical and lateral mantle heterogeneity beneath kimberlite pipes of the Lomonosov deposit and the V. Grib pipe.  相似文献   

10.
The xenoliths of garnet–clinopyroxene–disthene rocks(grospydites and associated kyanite eclogites) from the Zagadochnayakimberlite pipe in Yakutia have been studied in detail. Contraryto previous data, the presence of a continuous range in thepyrope-grossular series of garnets is shown on the basis ofnumerous X-ray data and 17 chemical analyses of garnets. Thisconclusion is confirmed by the study of separate grains withkyanite intergrowths from the kimberlite heavy fraction, whichare present in the kimberlite as the result of destruction ofgrospydite xenoliths, and possible, of garnet-kyanite rocksalso. A close connection of the calcium content in the garnetwith the sodium content in the coexisting clinopyroxes is alsoshown. An increase in the chemical potential of sodium resultsin the stability of the pryoxene-kyanite assemblage insteadof a garnet of intermediate composition (50 percent of grossular).The interval of the miscibility gap between calcium-rich andcalcium-poor garnets is increased in this way. The data of chemicalanalyses of 14 pyroxenes from the xenoliths indicate that theydiffer in the high aluminium and sodium content from other pyroxenesof eclogite-facies rocks. Chromium-rich bands with a high chromiumcontent in coexisting garnet, pyroxene, and kyantic have beenoccasionally found in the xenoliths. A study has been made ofthe chrome-kyanite with 12.86 per Cr202. The presence of chromium-richminerals in the grospydite xenoliths confrms their connectionwith ultrabasic rocks.  相似文献   

11.
The chemical compositions of garnets from 58 eclogite, 72 peridotite and 4 pyroxenite xenoliths in kimberlites have been estimated from their unit cell edge length and refractive indices. The samples studied were obtained from 17 kimberlite occurrences and include all those of known source which remain in the famous Williams (1932) collection which is stored at the University of Cape Town. Every suitable sample available to the authors has been examined.A gap in the range of garnet volume percentages occurs in the samples studied between approximately 15 and 30%. Garnet peridotites characteristically have <15% garnet and eclogites >30% garnet. Very rare exceptions occur. Our collection contains no eclogites with olivine and only one with orthopyroxene. All but two of the peridotite-pyroxenite group contain orthopyroxene. The garnets from the peridotites and pyroxenites plot on a pyrope-almandine-uvarovite triangle in a narrow band with a remarkably constant almandine/uvarovite ratio. Garnets from the eclogites are plotted on a pyrope-almandine-grossularite triangle and have a wide spread of compositions. These fall into 4 groups viz. eclogite I, eclogite II, kyanite eclogite and corundum eclogite.The reasons for the differences in garnet chemistry are considered and a tentative evolutionary scheme suggested by partial melting of the garnet peridotite which is assumed to occur in the upper mantle. Recent models of upper mantle composition and the genesis of garnet-bearing xenoliths in kimberlite are briefly and critically examined.S.A. UMP Publication No. 9.  相似文献   

12.
Henry O.A. Meyer 《Earth》1977,13(3):251-281
The importance of ultramafic and eclogitic xenoliths in kimberlite as representing the rocks and minerals of the upper mantle has been widely perceived during the last decade. Studies of the petrology and mineral chemistry of these mantle fragments as well as of inclusions in diamond, have led to significant progress in our understanding of the mineralogy and chemistry of the upper mantle. For example, it is now known that textural differences in the ultramafic xenoliths (lherzolite, harzburgite, pyroxenite and websterite) are partially reflected in chemical differences. Thus xenoliths that display a ‘fluidal’ texture, indicative of intense deformation are less depleted in Ca, Al, Na, Fe and Ti than those xenoliths in which granular textures are predominant. It is believed this relative depletion may indicate the sheared (fluidal texture) xenoliths are representative of primary, undifferentiated mantle. This material on partial melting would produce ‘basaltic-type’ material, and leave a residuum whose chemistry and mineralogy is reflected by the granular xenoliths.Also present in kimberlite are large single phase xenoliths that may be either one single crystal (xenocryst, megacryst) or an aggregate of several crystals of the same mineral (discrete xenolith, or discrete nodule). These large single phase samples consist of similar minerals to those occurring in the ultramafic xenoliths but chemically they are distinct in being generally more Fe-rich. The relation between these xenocrysts to their counterparts in the ultramafic xenoliths is unknown. Also unknown, at the present time, is the exact relation between diamond and kimberlite. Evidence obtained from study of the mineral inclusions in diamond suggests that diamond forms in at least two chemically distinct environments in the mantle; one eclogitic, the other, ultramafic. Interestingly, this suggestion is true for diamonds from worldwide localities.The mineral-chemical results of studies on xenoliths and inclusions in diamond have been convincingly interpreted in the light of experimental studies. It is now possible based on several different geothermometers and barometers to determine relatively reasonable physical conditions for the final genesis of many of these mantle rocks. For the most part the final equilibration temperatures range between 1000 and 1400°C and pressure in the region 100–200 km. These conditions are consistent with an upper mantle origin. Future studies will undoubtedly attempt to more concisely, and accurately, define these conditions, as well as understand better the chemical and spatial relationship of the rock-types in the mantle.  相似文献   

13.

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

  相似文献   

14.
Trace element concentrations of peridotitic garnet inclusions in diamonds from two Chinese kimberlite pipes were determined using the ion microprobe. Garnet xenocrysts from the same two kimberlite pipes were also analyzed for comparison. In contrast to their extremely refractory major element compositions, all harzburgitic garnets showed enrichment in light rare earth elements (REE) relative to chondrite, resulting in sinuous REE patterns. Both normal and sinuous REE patterns were observed from the lherzolitic garnets. Concentrations of REE in garnets changed significantly from diamond to diamond and no specific correlations were observed with their major element compositions. Analyses of randomly selected two to three points within every grain of a large number of garnet inclusions by the ion microprobe demonstrated that there was no evident compositional heterogeneity, and multiple grains of one phase from a single diamond host also exhibit very similar compositions. This implies that the trace element heterogeneity within one grain or among multiple inclusions from the same diamond host, as reported from Siberian diamonds, is not a common feature for these Chinese diamonds. Concentrations of Na, Ti, and Zr tend to decrease when garnets become more refractory, but variations of Sr and Li are more complex. Compositions rich in light REE and relatively poor in high field strength elements (HFSE) of the harzburgitic garnet inclusions in diamonds are generally consistent with metasomatism by carbonatite melts. The trace element features observed from the garnet inclusions in Chinese diamonds may be caused by carbonatite melt infiltration and partial melt extraction. Spatial and temporal gradients in melt/rock ratio and temperature are the main reasons for the large variations of REE patterns and other trace element concentrations. Received: 27 April 1999 / Accepted: 1 March 2000  相似文献   

15.
Megacrysts from the Grib kimberlite pipe (Arkhangelsk Province, Russia)   总被引：3，自引：0，他引：3  
The megacryst suite of the Grib kimberlite pipe (Arkhangelsk province, Russia) comprises garnet, clinopyroxene, magnesian ilmenite, phlogopite and garnet-clinopyroxene intergrowths. Crystalline inclusions, mainly of clinopyroxene and picroilmenite, occur in garnet megacrysts. Ilmenite is characterized by a wide range in the contents of MgO (10.6–15.5 wt.%) and Cr2O3 (0.7–8.3 wt.%). Megacryst garnets show wide variations in Cr2O3 (1.3–9.6 wt.%) and CaO (3.6–11.0 wt.%) but relatively constant MgO (15.4–22.3 wt.%) and FeO (5.2–9.9 wt.%). The pyroxenes also show wide variations in such oxides as Cr2O3, Al2O3 and Na2O (0.56–2.95; 0.86–3.25; 1.3–3.0 wt.%, respectively). The high magnesium and chromium content of all these minerals puts them together in one paragenetic group. This conclusion was confirmed by studies of the crystalline inclusions in megacrysts, which demonstrate similar variations in composition. Low concentration of hematite in ilmenite suggests reducing conditions during crystallization. P–T estimates based on the clinopyroxene geothermobarometer (Contrib. Mineral. Petrol. 139 (2000) 541) show wide variations (624–1208 °C and 28.8–68.0 kbars), corresponding to a 40–45 mW/m2 conductive geotherm. The majority of Gar-Cpx intergrowths differ from the corresponding monomineralic megacrysts in having higher Mg contents and relatively low TiO2. The minerals from the megacryst association, as a rule, differ from the minerals of mantle xenoliths, but garnets in ilmenite-bearing peridotite xenoliths are compositionally similar to garnet megacrysts. The common features of trace element composition of megacryst minerals and kimberlite (they are poor in Zr group elements) suggest a genetic relationship. The origin of the megacrysts is proposed to be genetically connected with kimberlite magma-chamber evolution on the one hand and with associated mantle metasomatism on the other. We suggest that, depending on the primary melt composition, different paragenetic associations of macro/megacrysts can be crystallized in kimberlites. They include: (1) Fe–Ti (Mir, Udachnaya pipes); (2) high-Mg, Cr (Zagadochna, Kusova pipes); (3) high-Mg, Cr, Ti (Grib pipe).  相似文献   

16.
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   

17.
We have performed dissections of two diamondiferous eclogites (UX-1 and U33/1) from the Udachnaya kimberlite, Yakutia in order to understand the nature of diamond formation and the relationship between the diamonds, their mineral inclusions, and host eclogite minerals. Diamonds were carefully recovered from each xenolith, based upon high-resolution X-ray tomography images and three-dimensional models. The nature and physical properties of minerals, in direct contact with diamonds, were investigated at the time of diamond extraction. Polished sections of the eclogites were made, containing the mould areas of the diamonds, to further investigate the chemical compositions of the host minerals and the phases that were in contact with diamonds. Major- and minor-element compositions of silicate and sulfide mineral inclusions in diamonds show variations among each other, and from those in the host eclogites. Oxygen isotope compositions of one garnet and five clinopyroxene inclusions in diamonds from another Udachnaya eclogite (U51) span the entire range recorded for eclogite xenoliths from Udachnaya. In addition, the reported compositions of almost all clinopyroxene inclusions in U51 diamonds exhibit positive Eu anomaly. This feature, together with the oxygen isotopic characteristics, is consistent with the well-established hypothesis of subduction origin for Udachnaya eclogite xenoliths. It is intuitive to expect that all eclogite xenoliths in a particular kimberlite should have common heritage, at least with respect to their included diamonds. However, the variation in the composition of multiple inclusions within diamonds, and among diamonds, from the same eclogite indicates the involvement of complex processes in diamond genesis, at least in the eclogite xenoliths from Yakutia that we have studied.  相似文献   

18.
The diamond population from the Jagersfontein kimberlite is characterized by a high abundance of eclogitic, besides peridotitic and a small group of websteritic diamonds. The majority of inclusions indicate that the diamonds are formed in the subcratonic lithospheric mantle. Inclusions of the eclogitic paragenesis, which generally have a wide compositional range, include two groups of eclogitic garnets (high and low Ca) which are also distinct in their rare earth element composition. Within the eclogitic and websteritic suite, diamonds with inclusions of majoritic garnets were found, which provide evidence for their formation within the asthenosphere and transition zone. Unlike the lithospheric garnets all majoritic garnet inclusions show negative Eu-anomalies. A narrow range of isotopically light carbon compositions (δ13C −17 to −24 ‰) of the host diamonds suggests that diamond formation in the sublithospheric mantle is principally different to that in the lithosphere. Direct conversion from graphite in a subducting slab appears to be the main mechanism responsible for diamond formation in this part of the Earth’s mantle beneath the Kaapvaal Craton. The peridotitic inclusion suite at Jagersfontein is similar to other diamond deposits on the Kaapvaal Craton and characterized by harzburgitic to low-Ca harzburgitic compositions.  相似文献   

19.
Djerfisherite, a Cl-bearing potassium sulfide (K6Na(Fe,Ni,Cu)24S26Cl), is a widespread accessory mineral in kimberlite-hosted mantle xenoliths. Nevertheless, the origin of this sulfide in nodules remains disputable. It is usually attributed to the replacement of primary Fe–Ni–Cu sulfides when xenoliths interact with a K-and Cl-enriched hypothetical melt/fluid. The paper is devoted to a detailed study of the composition and morphology of djerfisherite from a representative collection (22 samples) of the deepest mantle xenoliths—sheared garnet peridotite, taken from the Udachnaya-East kimberlite pipe (Yakutia). Four types of djerfisherite were distinguished in the mantle rocks on the basis of morphology, spatial distribution, and relationships with the rock-forming and accessory minerals in the nodules. Type 1 was found in the rims of polysulfide inclusions in the rock-forming minerals of the xenoliths; there, it was younger than the primary sulfide assemblage pyrrhotite + pentlandite ± chalcopyrite. Type 2 formed rims around large polysulfide segregations (pyrrhotite+ pentlandite) in the xenolith interstices. Type 3 formed individual grains in the xenolith interstices together with other sulfides, silicates, oxides, phosphates, and carbonates. Type 4 was present as a daughter phase in the secondary melt inclusions which occurred in healed cracks in the rock-forming minerals of the xenoliths. Along with djerfisherite, the inclusions contained silicates, oxides, phosphates, carbonates, alkaline sulfates, chlorides, and sulfides. The results indicate that djerfisherite from the xenoliths is consanguine with kimberlite. Djerfisherite both in the sheared-peridotite xenoliths from the Udachnaya-East pipe and in different xenoliths from other kimberlite pipes worldwide formed owing to the interaction between the nodules and kimberlitic melts. Djerfisherite forming individual grains in the melt inclusions and xenolith interstices crystallized directly from the infiltrating kimberlitic melt. Djerfisherite bounding the primary Fe–Ni ± Cu sulfides formed by their replacement as a result of a reaction with the kimberlitic melt.  相似文献   

20.
From Donghai County of Jiangsu Province to Rongcheng County of Shandong Province on the southern border of the Sulu orogen, there exposes an ultramafic belt, accompanied with an ultrahigh-pressure metamorphic zone. It can be further divided into the Xugou belt (the northern belt), and the Maobei-Gangshang belt (the southern belt). One grain of diamond has been discovered from the Zhimafang pyrope peridotite in the southern belt using the heavy mineral method. The diamond grain is 2.13 mm × 1.42 mm × 0.83 mm in size and weighs 9.4 mg. The occurrence of the diamond suggests that the Zhimafang pyrope peridotite xenolith is derived from the lithospheric upper mantle. The tectonic emplacement mechanism of the pyrope peridotite xenoliths in granite-gneisses is obviously different from those in kimberlite. The Sulu orogen was located on the active continental margin of the Sino-Korean craton in the Neoproterozoic. The relatively cold and water-bearing oceanic crustal tholeiite slab subducted beneath the lith  相似文献   

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