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1.
To elucidate the conditions of formation of epigenetic graphite inclusions in natural diamond, we carried out experiments on high-temperature treatment of natural and synthetic diamond crystals containing microinclusions. The crystal annealing was performed in the CO–CO2 atmosphere at 700–1100 °C and ambient pressure for 15 min to 4 h. The starting and annealed diamond crystals were examined by optical microscopy and Raman spectroscopy. It has been established that the microinclusions begin to change at 900 °C. A temperature increase to 1000 °C induces microcracks around the microinclusions and strong stress in the diamond matrix. The microinclusions turn black and opaque as a result of the formation of amorphous carbon at the diamond–inclusion interface. At 1100 °C, ordered graphite in the form of hexagonal and rounded plates is produced in the microcracks. A hypothesis is put forward that graphitization in natural diamond proceeds by the catalytic mechanism, whereas in synthetic diamond it is the result of pyrolysis of microinclusion hydrocarbons. The obtained data on the genesis of graphite microinclusions in diamond are used to evaluate the temperature of kimberlitic melt at the final stage of formation of diamond deposits.  相似文献   

2.
A xenolith of eclogite from the kimberlite pipe Udachnaya–East, Yakutia Grt+Cpx+Ky + S + Coe/Qtz + Dia + Gr has been studied. Graphite inclusions in diamond have been studied in detail by Confocal Raman (CR) mapping. The graphite inclusion in diamond has a highly ordered structure and is characterized by a substantial shift in the band (about 1580 cm–1) by 7 cm–1, indicating a significant residual strain in the inclusion. According to the results of FTIR spectroscopic studies of diamond crystals, a high degree of nitrogen aggregation has been detected: it is present mainly in form A, which means an “ancient” age of the diamonds. In the xenolith studied, the diamond formation occurred about 1 Byr, long before their transport by the kimberlite melt, and the conditions of the final equilibrium were temperatures of 1020 ± 40°C at 4.7 GPa. Thus, these graphite inclusions found in a diamond are the first evidence of crystallization of metastable graphite in a diamond stability field. They were formed in rocks of the upper mantle significantly below (≥20 km) the graphite-diamond equilibrium line.  相似文献   

3.
ABSTRACT

The preservation of metastable diamond in ultrahigh-pressure metamorphic (UHPM) complexes challenges our understanding of the processes taking place during exhumation of these subduction zone complexes. The presence of diamonds in UHPM rocks implies that diamonds remained metastable during exhumation, and within thermodynamic stability of graphite for an extended period. This work studies the influence of pressure on the surface graphitization rate of diamond monocrystals in carbonate systems to understand the preservation of microdiamond during exhumation of UHP subduction complexes. Experiments were performed with 2–3 mm synthetic diamond monocrystals at 2–4 GPa in СаСО3 (1550°С) and К2СО3 (1450°С) melts using a high-pressure multi-anvil apparatus. The highest rate of surface graphitization took place at 2 GPa; diamond crystals were almost completely enveloped by a graphite coating. At 4 GPa, only octahedron-shaped pits formed on flat {111} diamond crystal faces. Our results demonstrate that the surface graphitization rate of diamonds in the presence of carbonate melts at 1450–1550°C increases with decreasing pressure. Decreased pressure alone can graphitize diamond regardless of exhumation rate. Metastable diamond inclusions survive exhumation with little or no graphitization because of excess pressure up to 2 GPa acting on them, and because inclusions are protected from interaction with C-O-H fluid.  相似文献   

4.
An assemblage of Cr-spinels widespread in Carnian (Upper Triassic) diamondiferous deposits in the northeastern Siberian Platform is studied. Analysis of their morphology and chemical composition has revealed two dominant varieties of Cr-spinels and has demonstrated certain regularities in their distribution in the study area. Correlations have been established between the areal distribution of the recognized types of Cr-spinels and diamond varieties typical of kimberlite sources and between the distribution of Cr-spinels and rounded diamond dodecahedrons. The phase and chemical compositions of polyphase inclusions in the Cr-spinels are studied. The spatial arrangement of inclusions along the crystal growth zones indicates their primary genesis and trapping from the melt during crystallization. Compositional features of some minerals in the inclusions—SiO2 impurity in apatite and high CaO contents (0.2-0.8 wt.%) in olivines—point to a nonkimberlite source of these Cr-spinels. The presence of K- and Na-containing phases and calcite in the inclusions indicates saturation of the initial melt with alkalies, Ca, and CO2. The data obtained suggest that the numerous Late Vendian diatremes in K-rich alkaline basites of the Olenek Uplift area are the source of the dominant Cr-spinel variety.  相似文献   

5.
A multiphase inclusion in a diamond from Liaoning province, China consists of an olivine covered with large plates of graphite. Both phases are enclosed in a thin layer of glass that separates the multiphase inclusion from the host diamond. Microcrystallites of diamond and graphite are embedded in the olivine and graphite plates. The characterization and distribution of all phases has been determined using micro-Raman, infrared and Auger spectroscopy, and electron microprobe analysis. The structural form and morphology of the microcrystallites of diamond and graphite in the olivine suggests they formed contemporaneously with the olivine and the host diamond. An alternative suggestion is that they formed from carbon previously dissolved in the olivine at high pressure and temperature. The genesis of the large graphite plates on the surface of the olivine and beneath the glass film is less easily understood, especially as the composition of the glass is not fully documented. The occurrence of glass associated with other inclusions in diamond has been recognized previously by others although the compositions are varied. This is the first record of diamond and graphite occurring within a silicate inclusion in diamond.  相似文献   

6.
Experimental studies in the Fe3C–SiO2–MgO system (P = 6.3 GPa, T = 1100–1500°C, t = 20–40 h) have been carried out. It has been established that carbide-oxide interaction resulted in the formation of Fe-orthopyroxene, graphite, wustite, and cohenite (1100 and 1200°C), as well as a Fe–C–O melt (1300–1500°C). The main processes occurring in the system at 1100 and 1200°C are the oxidation of cohenite, the extraction of carbon from carbide, and the crystallization of metastable graphite, as well as the formation of ferrosilicates. At T ≥ 1300°C, graphite crystallization and diamond growth occur as a result of the redox interaction of a predominantly metallic melt (Fe–C–O) with oxides and silicates. The carbide–oxide interaction studied can be considered as the basis for modeling a number of carbon-producing processes in the lithospheric mantle at fO2 values near the iron–wustite buffer.  相似文献   

7.
Nanometric solid inclusions in diamond incorporated in garnet and zircon from felsic gneiss of the Kokchetav massif, Kazakhstan, have been examined utilizing electron microscopy and focused ion beam techniques. Host garnet and zircon contain numerous pockets of multiple inclusions, which consist of 1–3 diamond crystals intergrown with quartz, phengite, phlogopite, albite, K‐feldspar, rutile, apatite, titanite, biotite, chlorite and graphite in various combinations. Recalculation of the average chemical composition of the entrapped fluid represented by multiple inclusion pockets indicates that such fluid contained a low wt% of SiO2, suggesting a relatively low‐temperature fluid rather than a melt. Transmission electron microscopy revealed that the diamond contains abundant nanocrystalline inclusions of oxides, rare carbonates and silicates. Within the 15 diamond crystals studied, abundant inclusions were found of SiO2, TiO2, FexOy, Cr2O3, ZrSiO4, and single grains of ThxOy, BaSO4, MgCO3, FeCr2O4 and a stoichiometric Fe‐rich pyroxene. The diversity of trace elements within inclusions of essentially the same stoichiometry suggests that the Kokchetav diamond crystallized from a fluid containing variable amounts of Si, Fe, Ti, Cr, Zr, Ba, Mg and Th and other minor components such as K, Na, P, S, Pb, Zn, Nb, Al, Ca, Cl. Most of the components in crystals included in diamond appear to have their origin in the subducted metasediments, but some of them probably originate from the mantle. It is concluded that Kokchetav diamond most likely crystallized from a COH‐rich multicomponent supercritical fluid at a relatively low temperature (hence the apparently low content of rock‐forming elements), and that the diversity of major and minor components suggests interactions between subducted metasediments and mantle components.  相似文献   

8.
采用快速急冷工艺制备含碳粉末触媒,并以此触媒为原料,在国产DS6×800A型铰链式六面顶压机上进行金刚石合成实验;实验结果表明:在高速冷却条件下(冷速为104℃/s～106℃/s),大量石墨碳被固溶在触媒材料内部,固溶在触媒材料内部的石墨碳形态有球形、长条形及其它不规则形状;固溶石墨碳的存在有利于石墨碳源在高温高压金刚石合成过程中的溶解与传输,从而,缩短了触媒溶剂中溶解碳达到过饱和的时间,提高了金刚石的形核率和合成单产。  相似文献   

9.
Graphite-bearing peridotites, pyroxenites and eclogite xenoliths from the Kaapvaal craton of southern Africa and the Siberian craton, Russia, have been studied with the aim of: 1) better characterising the abundance and distribution of elemental carbon in the shallow continental lithospheric mantle; (2) determining the isotopic composition of the graphite; (3) testing for significant metastability of graphite in mantle rocks using mineral thermobarometry. Graphite crystals in peridotie, pyroxenite and eclogite xenoliths have X-ray diffraction patterns and Raman spectra characteristic of highly crystalline graphite of high-temperature origin and are interpreted to have crystallised within the mantle. Thermobarometry on the graphite-peridotite assemblages using a variety of element partitions and formulations yield estimated equilibration conditions that plot at lower temperatures and pressures than diamondiferous assemblages. Moreover, estimated pressures and temperatures for the graphite-peridotites fall almost exclusively within the experimentally determined graphite stability field and thus we find no evidence for substantial graphite metastability. The carbon isotopic composition of graphite in peridotites from this and other studies varies from δ13 CPDB = ? 12.3 to ? ?3.8%o with a mean of-6.7‰, σ=2.1 (n=22) and a mode between-7 and-6‰. This mean is within one standard deviation of the-4‰ mean displayed by diamonds from peridotite xenoliths, and is identical to that of diamonds containing peridotite-suite inclusions. The carbon isotope range of graphite and diamonds in peridotites is more restricted than that observed for either phase in eclogites or pyroxenites. The isotopic range displayed by peridotite-suite graphite and diamond encompasses the carbon isotope range observed in mid-ocean-ridge-basalt (MORB) glasses and ocean-island basalts (OIB). Similarity between the isotopic compositions of carbon associated with cratonic peridotites and the carbon (as CO2) in oceanic magmas (MORB/OIB) indicates that the source of the fluids that deposited carbon, as graphite or diamond, in catonic peridotites lies within the convecting mantle, below the lithosphere. Textural observations provide evidence that some of graphite in cratonic peridotites is of sub-solidus metasomatic origin, probably deposited from a cooling C-H-O fluid phase permeating the lithosphere along fractures. Macrocrystalline graphite of primary appearance has not been found in mantle xenoliths from kimberlitic or basaltic rocks erupted away from cratonic areas. Hence, graphite in mantle-derived xenoliths appears to be restricted to Archaean cratons and occurs exclusively in low-temperature, coarse peridotites thought to be characteristic of the lithospheric mantle. The tectonic association of graphite within the mantle is very similar to that of diamond. It is unlikely that this restricted occurrence is due solely to unique conditions of oxygen fugacity in the cratonic lithospheric mantle because some peridotite xenoliths from off-craton localities are as reduced as those from within cratons. Radiogenic isotope systematics of peridotite-suite diamond inclusions suggest that diamond crystallisation was not directly related to the melting events that formed lithospheric peridotites. However, some diamond (and graphite?) crystallisation in southern Africa occurred within the time span associated with the stabilisation of the lithospheric mantle (Pearson et al. 1993). The nature of the process causing localisation of carbon in cratonic mantle roots is not yet clearly understood.  相似文献   

10.
Experiments at 6.0–7.1 GPa and 1500–1700°C were carried out to explore the boundary conditions of diamond nucleation and growth in pyrrhotite-carbon melt-solutions. Pyrrhotite is one of the main sulfide minerals of the pyrrhotite-pentlandite-chalcopyrite assemblage of mantle rocks and primary inclusions in diamond. Solutions of carbon in sulfide melts oversaturated with respect to diamond at the expense of the dissolution of starting graphite (thermodynamically unstable phase) are formed owing to the difference between the solubilities of graphite and diamond, which increases under the influence of temperature gradients in experimental samples. We determined the fields of carbon solutions in pyrrhotite melt showing labile and metastable oversaturation with respect to diamond, which correspond to the spontaneous nucleation of the diamond phase and diamond growth on seeds, respectively. The linear growth rate of diamond in sulfide-carbon melts is rather high (on average, 10 μ/min during the first 1–2 min from the onset of spontaneous crystallization). The nucleation density is estimated as 180 grains per cubic centimeter. Diamonds crystallized from sulfide melts show octahedral and spinel twin shapes. Diamond polycrystals were synthesized for the first time from a sulfide medium as intergrowths of skeletal (edge) or “cryptocrystalline” microdiamonds, from 1 to 100 μm in size, their spinel twins and, occasionally, polysynthetic (star-shaped) twins. During diamond growth from sulfidecarbon melts on smooth faces of cuboctahedral diamond seeds synthesized in metal systems, smooth-faced layer-by-layer step-like growth was observed on their octahedral (111) faces, whereas growth on the (100) cubic faces produced rough-surfaced layers of intergrown micropyramids, whose axes were oriented normal to the (100) face. The obtained experimental results were applied to the problem of diamond genesis under the conditions of the Earth’s mantle in the framework of the model of carbonate-silicate parental melts with blebs of immiscible sulfide melts.  相似文献   

11.
Experimental studies of diamond formation in the alkaline silicate-carbon system Na2O–K2O–MgO–CaO–Al2O3–SiO2–C were carried out at 8.5 GPa. In accordance with the diamond nucleation criterion, a high diamond generation efficiency (spontaneous mass diamond crystallization) has been confirmed for the melts of the system Na2SiO3–carbon and has been first established for the melts of the systems CaSiO3–carbon and (NaAlSi3O8)80(Na2SiO3)20–carbon. It is shown that in completely miscible carbonate-silicate melts oversaturated with dissolved diamond-related carbon, a concentration barrier of diamond nucleation (CBDN) arises at a particular ratio of carbonate and silicate components. Study of different systems (eclogite–K-Na-Mg-Ca-Fe-carbonatite–carbon, albite–K2CO3–carbon, etc.) has revealed a dependence of the barrier position on the chemical composition of the system and the inhibiting effect of silicate components on the nucleation density and rate of diamond crystal growth. In multicomponent eclogite-carbonatite solvent, the CBDN is within the range of carbonatite compositions (<50 wt.% silicates). Based on the experimental criterion for the syngenesis of diamond and growth inclusions in them, we studied the syngenesis diagram for the system melanocratic carbonatite–diamond and determined a set of the composition fields and physical parameters of the system that are responsible for the cogeneration of diamond and various mineral and melt parageneses. The experimental results were applied to substantiate a new physicochemical concept of carbonate-silicate (carbonatite) growth media for most of natural diamonds and to elaborate a genetic classification of growth mineral, melt, and fluid inclusions in natural diamonds of mantle genesis.  相似文献   

12.
We report the first finding of diamond and moissanite in metasedimentary crustal rocks of Pohorje Mountains (Slovenia) in the Austroalpine ultrahigh‐pressure (UHP) metamorphic terrane of the Eastern Alps. Microscopic observations and Raman spectroscopy show that diamond occurs in situ as inclusions in garnet, being heterogeneously distributed. Under the optical microscope, diamond‐bearing inclusions are of cuboidal to rounded shape and of pinkish, yellow to brownish colour. The Raman spectra of the investigated diamond show a sharp, first order peak of sp3‐bonded carbon, in most cases centred between 1332 and 1330 cm?1, with a full width at half maximum between 3 and 5 cm?1. Several spectra show Raman bands typical for disordered graphitic (sp2‐bonded) carbon. Detailed observations show that diamond occurs either as a monomineralic, single‐crystal inclusion or it is associated with SiC (moissanite), CO2 and CH4 in polyphase inclusions. This rare record of diamond occurring with moissanite as fluid‐inclusion daughter minerals implies the crystallization of diamond and moissanite from a supercritical fluid at reducing conditions. Thermodynamic modelling suggests that diamond‐bearing gneisses attained P–T conditions of ≥3.5 GPa and 800–850 °C, similar to eclogites and garnet peridotites. We argue that diamond formed when carbonaceous sediment underwent UHP metamorphism at mantle depth exceeding 100 km during continental subduction in the Late Cretaceous (c. 95–92 Ma). The finding of diamond confirms UHP metamorphism in the Pohorje Mountains, the most deeply subducted part of Austroalpine units.  相似文献   

13.
Diamond crystallization in multicomponent melts of variable composition is studied. The melt carbonates are K2CO3, CaCO3?MgCO3, and K-Na-Ca-Mg-Fe-carbonatites, and the melt silicates are model peridotite (60 wt.% olivine, 16 wt.% orthopyroxene, 12 wt.% clinopyroxene, and 12 wt.% garnet) and eclogite (50 wt.% garnet and 50 wt.% clinopyroxene). In the experiments carried out under the PT-conditions of diamond stability, the carbonate-silicate melts behave like completely miscible liquid phases. The concentration barriers of diamond nucleation (CBDN) in the melts with variable proportions of silicates and carbonates have been determined at 8.5 GPa. In the system peridotite–K2CO3–CaCO3?MgCO3–carbonatite they correspond to 30, 25, and 30 wt.% silicates, respectively, and in the analogous eclogite–carbonate system, 45, 30, and 35 wt.%. In the silicate-carbonate melts with higher silicate contents seed diamond growth occurs, which is accompanied by the crystallization of thermodynamically unstable graphite phase. In the experiments with melts compositionally corresponding to the CBDN at 7.0 GPa and 1200–1700 °C, a full set of silicate minerals of peridotite (olivine, orthopyroxene, clinopyroxene, garnet) and eclogite (garnet, clinopyroxene) parageneses was obtained. The minerals occur as syngenetic inclusions in natural diamonds; moreover, the garnets contain an impurity of Na, and the pyroxenes, K. The experimental data indicate that peridotite-carbonate and eclogite-carbonate melts are highly effective for the formation of diamond (or unstable graphite) together with syngenetic minerals and melts, which agrees with the carbonate-silicate (carbonatite) model for the mantle diamond formation.  相似文献   

14.
Mineralogical structures of carbon phases within the ureilite North West Africa 4742, a recent find, are investigated at various scales by high-resolution transmission electron microscopy (HRTEM), Raman microspectrometry and X-ray diffraction. Ureilites are the most carbon-rich of all meteorites, containing up to 6 wt.% carbon. Diamond, graphite and so-called “amorphous carbon” are typically described, but their crystallographic relationships and respective thermal histories remain poorly constrained. We especially focus on the origin of “amorphous carbon” and graphite, as well as their relationship with diamond.Two aliquots of carbon-bearing material were extracted: the insoluble organic matter (IOM) and the diamond fraction. We also compare the observed structures with those of laboratory-shocked graphite.Polycrystalline diamond aggregates with mean coherent domains of about 40 nm are reported for the first time in a ureilite and TEM demonstrates that all carbon phases are crystallographically related at the nanometre scale.Shock features show that diamond is produced from graphite through a martensitic transition. This observation demonstrates that graphite was present when the shock occurred and is consequently a precursor of diamond. The structure of what is commonly described as the “amorphous carbon” has been identified. It is not completely amorphous but only disordered and consists of nanometre-sized polyaromatic units surrounding the diamond. Comparison with laboratory-shocked graphite, partially transformed into diamond, indicates that the disordered carbon could be the product of diamond post-shock annealing.As diamond is the carrier of noble gases, whereas graphite is noble gas free, graphite cannot be the sole diamond precursor. This implies a multiple-stage history. A first generation of diamond could have been synthesized from a noble gas rich precursor or environment by either a shock or a condensation process. Thermally-induced graphitization of chondritic-like organic matter could have produced the graphite, which was then transformed by shock processes into polycrystalline nanodiamond aggregates. The formation of the disordered carbon occurred by diamond post-shock back-transformation during post-shock heating. The noble gases in the first generation diamond could then be incorporated directly into the disordered carbon during the transformation.  相似文献   

15.
《International Geology Review》2012,54(13):1658-1667
The identification of syngenetic inclusions in diamond (i.e. inclusions of minerals that crystallized at the same time and by the same genesis as their host) has long been of paramount importance in diamond studies. However, the widespread assumption that many or most inclusions in diamonds are syngenetic is based on qualitative morphological criteria and few direct measurements. In order to provide statistically significant information on inclusion–host genetic relations for at least one kimberlite, we have determined the crystallographic orientations of 43 olivine inclusions with diamond-imposed morphology, a feature generally interpreted to indicate syngenesis, in 20 diamonds from the Udachnaya kimberlite (Siberia). Our unprecedented large data set indicates no overall preferred orientation of these olivines in diamond. However, multiple inclusions within a single diamond frequently exhibit similar orientations, implying that they were derived from original single monocrystals. Therefore, regardless of the possible chemical re-equilibration during diamond-forming processes, at least some of the olivines may have existed prior to the diamond (i.e. they are protogenetic). Our results imply that a diamond-imposed morphology alone cannot be considered as unequivocal proof of syngenicity of mineral inclusions in diamonds.  相似文献   

16.
通过对115粒山东郯城砂矿金刚石样品进行矿物学和光谱学特征研究,结果显示郯城金刚石的粒径集中在1. 0~4. 0mm之间,晶体形态以菱形十二面体为主,其次八面体与菱形十二面体聚形,八面体较少;晶面形貌除倒三角凹坑、塑性变形滑移线、熔蚀沟、生长丘、生长阶梯、叠瓦状蚀象、滴状丘、晕线等原生形貌发育外,小部分发育有次生形貌 绿色色斑,且大多数金刚石的边棱清晰,磨圆程度不高。研究首次测得了郯城金刚石的拉曼特征峰的半高宽数据和金刚石包裹体拉曼谱图,显示郯城砂矿金刚石结晶程度差异较大,暗示其形成的金刚石地质生长条件和环境的复杂性;金刚石包裹体有橄榄石、黄铜矿、针铁矿、石墨矿物,其中橄榄石包裹体占比较高,表明郯城金刚石包裹体类型以橄榄岩型为主,测试结果与华北东部古老克拉通之下的岩石圈地幔大部分由橄榄岩组成的结论一致。对比郯城金刚石与蒙阴金刚石特征的异同,初步探讨了金刚石砂矿的物质来源,为揭示郯城砂矿金刚石的形成及演化提供了金刚石及其包裹体的新的证据。  相似文献   

17.
《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   

18.
We report the first finding of diamond in crustal rocks from the Tromsø Nappe of the North Norwegian Caledonides. Diamond occurs in situ as inclusions in garnet from gneiss at Tønsvika near Tromsø. The rock is composed essentially of garnet, biotite, white mica, quartz and plagioclase, minor constituents include kyanite, zoisite, rutile, tourmaline, amphibole, zircon, apatite and carbonates (magnesite, dolomite, calcite). The microdiamond, identified by micro‐Raman spectroscopy, is cuboidal to octahedral in shape and ranges from 5 to 50 μm in diameter. The diamond occurs as single grains and as composite diamond + carbonate inclusions. Diamond vibration bands show a downshift from 1 332 to 1 325 cm?1, the majority of Raman peaks are centred between 1 332 and 1 330 cm?1 and all peaks exhibit a full width at half maximum between 3 and 5 cm?1. Several spectra show Raman bands typical for disordered and ordered graphite (sp2‐bonded carbon) indicating partial transformation of diamond to graphite. The calculated peak P–T conditions for the diamond‐bearing sample are 3.5 ± 0.5 GPa and 770 ± 50 °C. Metamorphic diamond found in situ in crustal rocks of the Tromsø Nappe thus provides unequivocal evidence for ultrahigh pressure metamorphism in this allochthonous unit of the Scandinavian Caledonides. Deep continental subduction, most probably in the Late Ordovician and shortly before or during the initial collision between Baltica and Laurentia, was required to stabilize the diamond at UHP conditions.  相似文献   

19.
通过能谱和电子探针分析了西藏罗布莎豆荚状铬铁矿石刚玉中的含钛合金和含钛氧化物包裹体特征,分析发现刚玉中含Ti合金矿物包裹体主要有Ti-N、Ti-B、Ti-C、Ti-Si-P和Ti-Si-Fe以及Ti-Al-Zr氧化物.Ti-N合金呈磨圆状、梅花状,粒度约17 μm×35 μm;Ti-B合金呈长柱状,10 μm×58 μm;Ti-C合金呈自形、他形,粒度约40 μm×50 μm;Ti-Si-P和Ti-Si-Fe合金成分不均一,呈一个熔融体包裹在刚玉中;Ti-Al-Zr氧化物成分纯净.结合铬铁矿石中发现大量的微粒金刚石和碳硅石等超高压异常地幔矿物,提出罗布莎铬铁矿石中的刚玉及其中的特殊矿物包裹体组合形成于高压环境的深部地幔.   相似文献   

20.
Silicate inclusions are widespread in natural diamonds, which also may contain rare inclusions of native iron. This suggests that some natural diamonds crystallized in metal-silicate-carbon systems. We experimentally studied the crystallization of diamond and silicate phases from the starting composition Fe0.36Ni0.64 + silicate glass + graphite and calculated the Fe mole fractions of the silicate phases crystallizing under these conditions. The silicates synthesized together with diamond had low Fe mole fractions [Fe/(Fe + Mg + Ca)] in spite of strong Fe predominance in the system. The Fe mole fractions of the silicates decreased in the sequence garnet-pyroxene-olivine, which is consistent with the results of our thermodynamic calculations. The Fe mole fraction of silicates under various redox conditions under which metal-carbon melts are stable drastically decreases with decreasing fo2. The low Fe mole fractions of silicate inclusions in diamond from the Earth’s mantle can be explained by the highly reducing crystallization conditions, under which Fe was concentrated as a metallic phase of the magmatic melts and could be only insignificantly incorporated in the structures of silicates.  相似文献   

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