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1.
Superdeep diamonds from the Juina area, Mato Grosso State, Brazil   总被引：4，自引：1，他引：3  
Alluvial diamonds from the Juina area in Mato Grosso, Brazil, have been characterized in terms of their morphology, syngenetic mineral inclusions, carbon isotopes and nitrogen contents. Morphologically, they are similar to other Brazilian diamonds, showing a strong predominance of rounded dodecahedral crystals. However, other characteristics of the Juina diamonds make them unique. The inclusion parageneses of Juina diamonds are dominated by ultra-high-pressure ("superdeep") phases that differ both from "traditional" syngenetic minerals associated with diamonds and, in detail, from most other superdeep assemblages. Ferropericlase is the dominant inclusion in the Juina diamonds. It coexists with ilmenite, Cr-Ti spinel, a phase with the major-element composition of olivine, and SiO2. CaSi-perovskite inclusions coexist with titanite (sphene), "olivine" and native Ni. MgSi-perovskite coexists with TAPP (tetragonal almandine-pyrope phase). Majoritic garnet occurs in one diamond, associated with CaTi-perovskite, Mn-ilmenite and an unidentified Si-Mg phase. Neither Cr-pyrope nor Mg-chromite was found as inclusions. The spinel inclusions are low in Cr and Mg, and high in Ti (Cr2O3<36.5 wt%, and TiO2>10 wt%). Most ilmenite inclusions have low MgO contents, and some have very high (up to 11.5 wt%) MnO contents. The rare "olivine" inclusions coexisting with ferropericlase have low Mg# (87-89), and higher Ca, Cr and Zn contents than typical diamond-inclusion olivines. They are interpreted as inverted from spinel-structured (Mg, Fe)2Si2O4. This suite of inclusions is consistent with derivation of most of the diamonds from depths near 670 km, and adds ilmenite and relatively low-Cr, high-Ti spinel to the known phases of the superdeep paragenesis. Diamonds from the Juina area are characterized by a narrow range of carbon isotopic composition ('13C=-7.8 to -2.5‰), except for the one majorite-bearing diamond ('13C=-11.4‰). There are high proportions of nitrogen-free and low-nitrogen diamonds, and the aggregated B center is predominant in nitrogen-containing diamonds. These observations have practical consequences for diamond exploration: Low-Mg olivine, low-Mg and high-Mn ilmenite, and low-Cr spinel should be included in the list of diamond indicator minerals, and the role of high-Cr, low-Ti spinel as the only spinel associated with diamond, and hence as a criterion of diamond grade in kimberlites, should be reconsidered.  相似文献   

2.
金刚石产地来源的确定是国际性难题,其中由于砂矿金刚石可能存在搬运过程的混合,其产地来源的确定更加具有争议性。巴西金刚石绝大部分来源于砂矿,从成矿地质背景、形貌和颜色、内部结构、矿物包裹体、微量元素、C同位素组成分布等六个要素总结分析了巴西金刚石研究的相关成果,为国际砂矿来源金刚石产地辨识提供参考。研究结果显示,巴西大部分矿区金刚石的特征与世界范围其它矿区的相似,以橄榄岩型为主,难以相互区分。但其中Juina地区和Machado河产地的金刚石具有独特的氮含量（无氮的Ⅱ型为主）和聚集状态,内部结构、矿物包裹体组合及温压环境特征均显示出深部地幔来源的特点,和巴西其它产地具有一定的区分度。与世界范围不同产地砂矿来源金刚石的基本特征对比显示,不同国家具体地区砂矿金刚石在上述不同要素的组合上存在一定的差异性,但总体上要借助现有资料进行产地来源的准确判断还非常困难,需要进行更深入细致的工作。  相似文献   

3.
Three major suites of silicate inclusions in sublithospheric diamonds show evidence of formation at depths > 250 km, and for each suite there is evidence of their formation from subducted material. Two of these are the well known basic (majoritic garnet) and ultrabasic (MgSi-perovskite + ferropericlase) suites. The third, the recently recognised Ca-rich suite, is characterised by carbonate, Ca-Si-Ti minerals and some aluminous material. Carbon isotope ratios in the host diamonds and geochemical-petrological features of the inclusions themselves provide evidence for their derivation from subducted lithosphere materials. The diamonds hosting the basic and ultrabasic suites are suggested to form in fluids/melts resulting from the release of water caused by dehydration reactions affecting both the crustal and mantle portions of a subducting slab of ocean lithosphere. Conversely, the diamonds containing the Ca-rich suite are linked with the formation of carbonatitic melts. In the Juina kimberlite province of Brazil, all three suites have been found in close proximity. A model is presented whereby the formation of the suites occurs progressively during the subduction and stagnation of a single lithospheric slab, with all three suites being transported to the lithosphere by a plume with which the carbonatitic melts of the Ca-rich suite are associated. Nd-Sr isotopic data are presented for the Juina majoritic-garnet inclusions, which supports their formation from oceanic crust of Mesozoic age. In conjunction with published age data for a Ca-Si-Ti inclusion, the Juina (Brazil) sublithospheric inclusions document a series of events involving diamond formation during and following the emplacement of a subducted slab between ca 190 and 90 Ma beneath west Gondwanaland. This slab and related subducted slabs dating from the Palaeozoic at the Gondwanan margin may be the source of the widespread DUPAL geochemical anomaly in the South Atlantic and Indian Oceans. The kimberlites bringing the diamonds to the Earth's surface may have arisen from a superplume, developed from a graveyard of former Gondwanan stagnant slabs, at the Core-Mantle-Boundary.  相似文献   

4.
The enigmatic appearance of cuboctahedral diamonds in ophiolitic and arc volcanic rocks with morphology and infrared characteristics similar to synthetic diamonds that were grown from metal solvent requires a critical reappraisal. We have studied 15 diamond crystals and fragments from Tolbachik volcano lava flows, using Fourier transform infrared spectrometry (FTIR), transmission electron microscopy (TEM), synchrotron X-ray fluorescence (SRXRF) and laser ablation inductively coupled plasma mass-spectrometry (LA-ICP-MS). FTIR spectra of Tolbachik diamonds correspond to typical type Ib patterns of synthetic diamonds. In TEM films prepared using focused ion beam technique, we find Mn-Ni and Mn-Si inclusions in Tolbachik diamonds. SRXRF spectra indicate the presence of Fe-Ni and Fe-Ni-Mn inclusions with Cr, Ti, Cu, and Zn impurities. LA-ICP-MS data show variable but significantly elevated concentrations of Mn, Fe, Ni, and Cu reaching up to 70 ppm. These transition metal concentration levels are comparable with those determined by LA-ICP-MS for similar diamonds from Tibetan ophiolites. Mn-Ni (+Fe) solvent was widely used to produce industrial synthetic diamonds in the former USSR and Russia with very similar proportions of these metals. Hence, it appears highly probable that the cuboctahedral diamonds recovered from Kamchatka arc volcanic rocks represent contamination and are likely derived from drilling tools or other hard instruments. Kinetic data on diamond dissolution in basaltic magma or in fluid phase demonstrate that diamond does not form under the pressures and temperature conditions prevalent within the magmatic system beneath the modern-day Klyuchevskoy group of arc volcanoes. We also considered reference data for inclusions in ophiolitic diamonds and compared them with the composition of solvent used in industrial diamond synthesis in China. The similar inclusion chemistry close to Ni70Mn25Co5 for ophiolitic and synthetic Chinese diamonds scrutinized here suggests that most diamonds recovered from Tibetan and other ophiolites are not natural but instead have a synthetic origin. In order to mitigate further dubious reports of diamonds from unconventional tectonic settings and source rocks, we propose a set of discrimination criteria to better distinguish natural cuboctahedral diamonds from those produced synthetically in industrial environments and found as contaminants in mantle- and crust-derived rocks.  相似文献   

5.
 Sulfide inclusions in diamonds may provide the only pristine samples of mantle sulfides, and they carry important information on the distribution and abundances of chalcophile elements in the deep lithosphere. Trace-element abundances were measured by proton microprobe in >50 sulfide inclusions (SDI) from Yakutian diamonds; about half of these were measured in situ in polished plates of diamonds, providing information on the spatial distribution of compositional variations. Many of the diamonds were identified as peridotitic or eclogitic from the nature of coexisting silicate or oxide inclusions. Known peridotitic diamonds contain SDIs with Ni contents of 22–36%, consistent with equilibration between olivine, monosulfide solid solution (MSS) and sulfide melt, whereas SDIs in eclogitic diamonds contain 0–12% Ni. A group of diamonds without silicate or oxide inclusions has SDIs with 11–18% Ni, and may be derived from pyroxenitic parageneses. Eclogitic SDIs have lower Ni, Cu and Te than peridotitic SDIs; the ranges of the two parageneses overlap for Se, As and Mo. The Mo and Se contents range up to 700 and 300 ppm, respectively; the highest levels are found in peridotitic diamonds. Among the in-situ SDIs, significant Zn and Pb levels are found in those connected by cracks to diamond surfaces, and these elements reflect interaction with kimberlitic melt. Significant levels of Ru (30–1300 ppm) and Rh (10–170 ppm) are found in many peridotitic SDIs; SDIs in one diamond with wustite and olivine inclusions and complex internal structures have high levels of other platinum-group elements (PGEs) as well, and high chondrite-normalized Ir/Pd. Comparison with experimental data on element partitioning between crystals of monosulfide solid solution (MSS) and sulfide melts suggests that most of the inclusions in both parageneses were trapped as MSS, while some high-Cu SDIs with high Pd±Rh may represent fractionated sulfide melts. Spatial variations of SDI composition within single diamonds are consistent with growth histories shown by cathodoluminescence images, in which several stages of growth and resorption have occurred within magmatic environments that evolved during diamond formation. Received: 5 July 1995 / Accepted: 21 February 1996  相似文献   

6.
This classification of gem corundum (ruby and sapphire) deposits takes into consideration petrographic data, and the mode of genesis of the deposits. It is aimed at being practical and useful in the field, particularly for prospecting purposes.Deposits are classified into primary and secondary deposits. Primary deposits include igneous and metamorphic deposits. Igneous gem corundum deposits are rare and include sapphire-bearing syenites from Kenya. Emphasis is put on metamorphic deposits, that are subdivided into metamorphic s.s., metasomatic, and anatectic deposits. Many gem corundum deposits result from metasomatic processes. Small-scale metasomatism mostly involves desilication reactions between silico-aluminous rocks (pegmatites, gneisses, etc) and silica-poor rocks (ultramafites, meta-carbonates), and leads to the formation of limited-size deposits. Large-scale (diffuse) metasomatism is usually more difficult to characterize, and therefore is not separated from isochemical metamorphism in this classification. In metamorphic deposits, gem corundum results from transformation of an Al-rich and/or Si-poor protolith. Such deposits include ruby-bearing mafic granulites, ruby-bearing meta-limestones, and ruby/sapphire-bearing gneisses and granulites. An intermediate category includes anatectic deposits. Secondary deposits encompass sedimentary and volcanic (xenoclastic) occurrences. In sedimentary deposits, gem corundum occurs as clasts originating from other lithologies. In volcanic deposits, gem corundums are xenocrysts that have a range of origins.The proposed classification outlines geological environments favorable to the crystallization and distribution of gem corundum, thus facilitating prospecting and mining of this gemstone.  相似文献   

7.
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   

8.
Three new mineral associations have been discovered within diamonds from the Juina district of Brazil. These include a previously unrecorded Na-Al-(Mg, Fe)SiO3 phase associated with ferropericlase and the tetragonal almandine pyrope phase, TAPP. Also reported are an association of corundum with aluminous-pyroxene and an olivine composition phase associated with ferropericlase. The minerals in each association often occurred within the same diamonds in addition to being recovered from individual diamonds in different combinations. High-pressure experimental data indicate that these associations formed at different depths within a region ca. 60 km on either side of the upper-mantle/lower-mantle boundary. Mineral compositions show that for the regions sampled, the deep transition zone and lower mantle are chemically distinct and inhomogeneous. Importantly, in the shallow lower mantle, Al is not solely accommodated within perovskite-structured (Mg, Fe)SiO3 as some recent experimental studies have suggested.  相似文献   

9.
The Venetia kimberlites in the Northern Province of South Africa sampled diamonds from the lithosphere underlying the Central Zone of the Limpopo Belt. Given the general correlation of diamond-bearing kimberlites with old stable cratons, this tectonic setting is somewhat anomalous and, therefore, it is desirable to characterise the diamonds in terms of their infrared characteristics. A suite of diamonds of known paragenesis from the Venetia mine spans a large range of nitrogen concentrations from less than the detection limit to 1,355 ppm. Diamond nitrogen contents are, on average, higher in the eclogitic diamond population relative to the websteritic and peridotitic diamonds. Nitrogen aggregation states are variable, ranging from almost pure type IaA diamond (poorly aggregated nitrogen) to pure type IaB diamond (highly aggregated nitrogen). On a nitrogen aggregation diagram two distinct groups can be identified based on nitrogen content and nitrogen aggregation state. These are a minor population of diamonds with nitrogen contents generally higher than 500 ppm and nitrogen aggregation states of less than 40% IaB, and another, dominant population that is characterised by higher and more variable nitrogen aggregation. The unusually aggregated nature of the majority of the diamonds analysed is unique to Venetia relative to other intrusives on the Kaapvaal-Kalahari craton, but is similar to aggregation states observed for diamonds from other craton margin or adjacent mobile belt settings such as the Argyle lamproite and the George Creek kimberlite. This could be a consequence of diamond mantle residence at mantle temperatures higher than the norm for other kimberlites from the interior of cratons. Deformation of the mantle, associated with dynamic processes such as orogenesis or subduction, might also be responsible for accelerating the rate of nitrogen aggregation in these diamonds. Low numbers of diamonds with degradation of platelets at the Venetia kimberlite, relative to diamonds from the Argyle lamproite, indicate that deformation was at a significantly lower level. The comparatively low value of diamonds from Argyle (at approximately US8/carat) as opposed to Venetia (US8/carat) as opposed to Venetia (US90/carat) is in large part because of the very high abundance of brown diamonds at Argyle. Therefore, it is apparent that deformational history of the mantle in which the diamonds were resident prior to or during sampling by the host may have an important role to play in the profitability of a primary diamond deposit. The apparently consistent association of diamonds with unusually aggregated nitrogen with kimberlites, or lamproites intruded into craton margin or mobile belt settings suggests that it may be possible to recognise such contributory sources in alluvial diamond deposits, through the study of the infrared characteristics of the diamonds. Electronic supplementary material to this paper can be obtained by using the Springer Link server located at http://dx.doi.org/10.1007/s00410-002-0385-2  相似文献   

10.
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   


11.
天然红宝石和蓝宝石的包裹体中常见典型的含水矿物包裹体,这些含水矿物包裹体容易受外界环境升温而发生改变。微量含水矿物包裹体变化会对红宝石和蓝宝石的物理和化学性质产生明显影响,该性质为宝石热处理的鉴定提供了检测思路。本文采集了天然红宝石和蓝宝石样品,用显微镜放大观察包裹体特征,结合红外光谱与显微共焦激光拉曼光谱测试研究了含水矿物包裹体的特征。结果表明:天然红宝石和蓝宝石样品中含水矿物包裹体的外观轮廓清晰,晶形完整;红外光谱在2000～3700cm~(-1)附近显示出2105～2110cm~(-1)和1977～1985cm~(-1)硬水铝石和3619cm~(-1)和3696cm~(-1)高岭石等水(H_2O或—OH等)的特征吸收峰;拉曼光谱中可见角闪石、云母、磷灰石和长石等结晶度较好的典型含水矿物包裹体的特征拉曼位移。该系列特征揭示了红宝石和蓝宝石样品中含有水的特征,可作为红宝石和蓝宝石天然成因且未经过热处理的鉴定依据。  相似文献   

12.
Rubies and sapphires are of both scientific and commercial interest. These gemstones are corundum colored by transition elements within the alumina crystal lattice: Cr3+ yields red in ruby and Fe2+, Fe3+, and Ti4+ ionic interactions color sapphires. A minor ion, V3+ induces slate to purple colors and color change in some sapphires, but its role in coloring rubies remains enigmatic. Trace element and oxygen isotope composition provide genetic signatures for natural corundum and assist geographic typing. Here, we show that V can dominate chromophore contents in Mogok ruby suites. This raises implications for their color quality, enhancement treatments, geographic origin, exploration and exploitation and their comparison with rubies elsewhere. Precise LA-ICP-MS analysis of ruby and sapphire from Mogok placer and in situ deposits reveal that V can exceed 5,000 ppm, giving V/Cr, V/Fe and V/Ti ratios up to 26, 78, and 97 respectively. Such values significantly exceed those found elsewhere suggesting a localized geological control on V-rich ruby distribution. Our results demonstrate that detailed geochemical studies of ruby suites reveal that V is a potential ruby tracer, encourage comparisons of V/Cr-variation between ruby suites and widen the scope for geographic typing and genesis of ruby. This will allow more precise comparison of Asian and other ruby fields and assist confirmation of Mogok sources for rubies in historical and contemporary gems and jewelry.  相似文献   

13.
This paper reports on the petrology and geochemistry of a diamondiferous peridotite xenolith from the Premier diamond mine in South Africa.

The xenolith is altered with pervasive serpentinisation of olivine and orthopyroxene. Garnets are in an advanced state of kelyphitisation but partly fresh. Electron microprobe analyses of the garnets are consistent with a lherzolitic paragenesis (8.5 wt.% Cr2O3 and 6.6 wt.% CaO). The garnets show limited variation in trace element composition, with generally low concentrations of most trace elements, e.g. Y (<11 ppm), Zr (<18 ppm) and Sr (<0.5 ppm). Garnet rare earth element concentrations, when normalised against the C1 chondrite of McDonough and Sun (Chem. Geol. 120 (1995) 223), are characterised by a rare earth element pattern similar to garnet from fertile lherzolite.

All diamonds recovered are colourless. Most crystals are sharp-edged octahedra, some with minor development of the dodecahedral form. A number of crystals are twinned octahedral macles, while aggregates of two or more octahedra are also common. Mineral inclusions are rare. Where present they are predominantly small black rosettes believed to consist of sulfide. In one instance a polymineralic (presumably lherzolitic) assemblage of reddish garnet, green clinopyroxene and a colourless mineral is recognised.

Infrared analysis of the xenolith diamonds show nitrogen contents generally lower than 500 ppm and variable nitrogen aggregation state, from 20% to 80% of the ‘B’ form. When plotted on a nitrogen aggregation diagram a well defined trend of increasing nitrogen aggregation state with increasing nitrogen content is observed. Carbon isotopic compositions range from −3.6 ‰ to −1.3 ‰. These are broadly correlated with diamond nitrogen content as determined by infrared spectroscopy, with the most negative C-isotopic compositions correlating with the lowest nitrogen contents.

Xenolith mantle equilibration temperatures, calculated from nitrogen aggregation systematics as well as the Ni in garnet thermometer are on the order of 1100 to 1200 °C.

It is concluded that the xenolith is a fertile lherzolite, and that the lherzolitic character may have resulted from the total metasomatic overprinting of pre-existing harzburgite. Metasomatism occurred prior to, or accompanied, diamond growth.  相似文献   


14.
Manganoan ilmenite was identified in Juina, Brazil kimberlitic rocks among other megacrysts. It forms oval, elongated, rimless grains comprising 8–30 wt.% of the heavy fraction. Internally the grains are homogeneous. The chemical composition of Mn-ilmenite is almost stoichiometric for ilmenite except for an unusually high manganese content, with MnO = 0.63–2.49 wt.% (up to 11 wt.% in inclusions in diamond) and an elevated vanadium admixture (V2O3 = 0.21–0.43 wt.%). By the composition, Mn-ilmenite megacrysts and inclusions in diamond are almost identical. The concentrations of trace elements in Mn-ilmenite, compared to picroilmenite, are much greater and their variations are very wide. Chondrite-normalized distribution of trace elements in Mn-ilmenite megacrysts is similar to the distribution in Mn-ilmenites included in diamond. This confirms that Mn-ilmenite in kimberlites is genetically related to diamond. The finds of Mn-ilmenite known before in kimberlitic and related rocks are late- or postmagmatic, metasomatic phases. They either form reaction rims on grains of picroilmenite or other ore minerals, or compose laths in groundmass. In contrast to those finds, Mn-ilmenite megacrysts in Juina kimberlites are a primary mineral phase with a homogeneous internal structure obtained under stable conditions of growth within lower mantle and/or transition zone. In addition to pyrope garnet, chromian spinel, picroilmenite, chrome-diopside, and magnesian olivine, manganoan ilmenite may be considered as another kimberlite/diamond indicator mineral.  相似文献   

15.
C.M. Appleyard  K.S. Viljoen  R. Dobbe 《Lithos》2004,77(1-4):317-332
Previous studies of diamonds from Finsch have shown that eclogitic inclusions are rare at Finsch and that the eclogitic garnet and clinopyroxenes are iron and manganese-rich. In order to expand the current database of information, 93 eclogitic diamonds were selected for this study. Eight diamonds were polished into plates for cathodoluminescence studies and infrared examination of diamond growth and 31 diamonds were cracked to retrieve inclusions. The eclogitic garnets analysed in this study are enriched in Fe and are relatively depleted in Ca and Mg relative to worldwide data. FeO contents for garnet range from 15 to 27 wt.% and MnO contents reach a maximum value of 1.6 wt.%. The eclogitic clinopyroxenes have relatively high FeO contents, up to 14.8 wt.% and K2O contents are low (<0.4 wt.%). Three non-touching garnet–clinopyroxene mineral pairs produce equilibration temperatures of 1138–1179 °C at an assumed pressure of 50 kb. No Type II diamonds were found during this study, all diamonds are of Type IaAB. Total nitrogen contents of Type IaAB diamonds range from 11 to 1520 ppm, with variable aggregation states (up to 84% nitrogen aggregated as B-defects). Distinct infrared characteristics suggest that the Finsch kimberlite sampled either more than one mantle source region of similar age but differing temperature, or two different populations of diamonds with different ages. The diamonds provide evidence of changing mantle conditions during crystallisation. Continuous diamond growth is illustrated by the presence of regular octahedral growth zones, although in some diamonds cubic growth is noted. One diamond shows evidence of platelet degradation, suggesting exposure to high temperatures and/or shearing stresses.  相似文献   

16.
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   

17.
A mineral inclusion, carbon isotope, nitrogen content, nitrogen aggregation state and morphological study of 576 microdiamonds from the DO27, A154, A21, A418, DO18, DD17 and Ranch Lake kimberlites at Lac de Gras, Slave Craton, was conducted. Mineral inclusion data show the diamonds are largely eclogitic (64%), followed by peridotitic (25%) and ultradeep (11%). The paragenetic abundances are similar to macrodiamonds from the DO27 kimberlite (Davies, R.M., Griffin, W.L., O'Reilly, S.Y., 1999. Diamonds from the deep: pipe DO27, Slave craton, Canada. In: Gurney, J.J., Gurney, J.L., Pascoe, M.D., Richardson, S.H. (Eds.), The J. B. Dawson Vol., Proc. 7th Internat. Kimberlite Conf., Red Roof Designs, Cape Town, pp. 148–155) but differ to diamonds from nearby kimberlites at Ekati (e.g., Lithos (2004); Tappert, R., Stachel, T., Harris, J.W., Brey, G.P., 2004. Mineral Inclusions in Diamonds from the Panda Kimberlite, S. P., Canada. 8th International Kimberlite Conference, extended abstracts) and Snap Lake to the south (Dokl. Earth Sci. 380 (7) (2001) 806), that are dominated by peridotitic stones.

Eclogitic diamonds with variable inclusion compositions and temperatures of formation (1040–1300 °C) crystallised at variable lithospheric depths sometimes in changing chemical environments. A large range to very 13C-depleted C-isotope compositions (δ13C=−35.8‰ to −2.2‰) and an NMORB bulk composition, calculated from trace elements in garnet and clinopyroxene inclusions, are consistent with an origin from subducted oceanic crust and sediments. Carbon isotopes in the peridotitic diamonds have mantle compositions (δ13C mode −4.0‰). Mineral inclusion compositions are largely harzburgitic. Variable temperatures of formation (garnet TNi=800–1300 °C) suggest the peridotitic diamonds originate from the shallow ultra-depleted and deeper less depleted layers of the central Slave lithosphere. Carbon isotopes (δ13C av.=−5.1‰) and mineral inclusions in the ultradeep diamonds suggest they formed in peridotitic mantle (670 km). The diamonds may have been entrained in a plume and subcreted to the base of the central Slave lithosphere.

Poorly aggregated nitrogen (IaA without platelets) in a large number of eclogitic (67%) and peridotitic (32%) diamonds, with similar nitrogen contents, indicates the diamonds were stored in the mantle at low temperatures (1060–<1100 °C) following crystallisation in the Archean. Type IaA diamonds have largely cubo-octahedral growth forms, and Type II and Type IaAB diamonds, with higher nitrogen aggregation states, mostly have octahedral morphologies. However, no correlation between these groups and their mineral inclusion compositions, C-isotopes, and N-contents rules out the possibility of unique source origins and suggests eclogitic and peridotitic diamonds experienced variable mantle thermal states. Variation in mineral inclusion chemistries in single diamonds, possible overgrowths of 13C-depleted eclogitic diamond on diamonds with peridotitic and ultradeep inclusions, and Type I ultradeep diamond with low N-aggregation is consistent with diamond growth over time in changing chemical environments.  相似文献   


18.
For the first time, three-dimensional, high-resolution X-ray computed tomography (HRXCT) of an eclogite xenolith from Yakutia has successfully imaged diamonds and their textural relationships with coexisting minerals. Thirty (30) macrodiamonds (≥1 mm), with a total weight of just over 3 carats, for an ore grade of some 27,000 ct/ton, were found in a small (4 × 5 × 6 cm) eclogite, U51/3, from Udachnaya. Based upon 3-D imaging, the diamonds appear to be associated with zones of secondary alteration of clinopyroxene (Cpx) in the xenolith. The presence of diamonds with secondary minerals strongly suggests that the diamonds formed after the eclogite, in conjunction with meta-somatic input(s) of carbon-rich fluids. Metasomatic processes are also indicated by the non-systematic variations in Cpx inclusion chemistry in the several diamonds. The inclusions in the diamonds vary considerably in major- and trace-element chemistry within and between diamonds, and do not correspond to the minerals of the host eclogite, whose compositions are extremely homogeneous. Some Cpx inclusions possess +Eu anomalies, probably inherited from their crustal source rocks. The only consistent feature for the Cpx crystals in the inclusions is that they have higher K2O than the Cpx grains in the host.

The δ13C compositions are relatively constant at ?5% both within and between diamonds, whereas δ15N values vary from ?2.8% to ?15.8%. Within a diamond, the total N varies considerably from 15 to 285 ppm in one diamond to 103 to 1250 ppm in another. Cathodoluminescent imaging reveals extremely contorted zonations and complex growth histories in the diamonds, indicating large variations in growth environments for each diamond.

This study directly bears on the concept of diamond inclusions as time capsules for investigating the mantle of the Earth. If diamonds and their inclusions can vary so much within this one small xenolith, the significance of their compositions is a serious question that must be addressed in all diamond-inclusion endeavors.  相似文献   

19.
Eclogitic (E-type) and related parageneses of natural diamonds are represented by suites of diamond inclusions and xenoliths of diamondiferous eclogites. Major-element data are presented for 32 coexisting minerals forming 19 bimineralic and trimineralic inclusions from diamonds, including omphacite-orthopyroxene (1 sample), garnet-omphacite (5 samples), garnet-coesite (5 samples), omphacite-coesite (2 samples), garnet-picroilmenite (2 samples), garnet-kyanite (1 sample), omphacite-phlogopite (2 samples), and garnel-omphacite-phlogopite (1 sample). Major-element variations of coexisting minerals are typical of corresponding eclogites. Omphacite with 5.02 wt% Na2O, inter-grown with orthopyroxene with Mg# 83.7, represents the first example of a diamondiferous websterite paragenesis including Na-clinopyroxene. This indicates a broader range in mineral compositions of E-type-related websteritepyroxenite-associated diamonds than known previously. This unique websterite-pyroxenitic mineral assemblage represents a transitional paragenesis between peridotitic or ultramafic (U-type) and E-type parageneses.

Bimineralic eclogites, ilmenite eclogites, coesite + corundum + kyanite eclogites, and grospydites occur not only as sets of inclusions in diamonds but, with a few exceptions (ilmenite and coesite eclogites), also as diamondiferous eclogite xenoliths. The coesite eclogite paragenesis is a significant inclusion suite in diamonds, and was detected in about 15 diamond occurrences worldwide. It represents from 15% to 22% of all E-type diamonds in several occurrences, and thus should not be considered as rare.  相似文献   

20.
The zonal distribution of impurities in six diamonds (2 clear. 1 green-skinned, 2 green-bodied and 1 coated) was studied by neutron activation followed by dissolution of the diamond into a number of fractions. High surface concentrations of impurities found here and by other workers were attributed to both laboratory and natural contaminants. No unusual element distributions were found in the outer layer of the green-skinned diamond, the green skin probably being caused by natural radiation damage. The green-bodied diamonds had very different compositions from each other and from the other diamonds and it is suggested that such stones owe their colour to a high general level of impurities. All the diamonds, including the clear core of the coated diamond, contained impurities thought to be submicroscopic inclusions, either silicates, carbonates or immiscible sulphides derived from the parental magma. Variations in the composition of these inclusions in one diamond suggest changes in the host magma composition during growth. Sulphides apparently occurred in very small amounts throughout all the diamonds. Variations in the concentration of impurities are probably related to changes in growth rate or environment during diamond formation, and could explain some of the zonal variations in the physical properties of diamonds.  相似文献   

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