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1.
The first data are reported on the carbon isotopic composition of diamond crystals from the Grib pipe kimberlite deposit of the Archangelsk diamond province (ADP). The δ13C value of the crystals ranges from ?2.79 to ?9.61‰. The isotopic composition of carbon was determined in three zoned crystals (δ13C of ?5.8 ?6.96 ‰, ?5.64/ ?5.85 ‰, and ?5.94/ ?5.69 ‰), two “diamond in diamond” samples (diamond inclusion with δ13C of ?4.05 and ?6.34 ‰ in host diamond crystals with δ13C of ?8.05 and ?7.54 ‰, respectively), and two samples of coated diamonds (cores with δ13C of ?6.98 and ?6.78‰ and coats with δ13C of ?7.51 and ?8.01 ‰, respectively). δ13C values were obtained for individual diamond crystals from bort-type aggregates (δ13C of ?4.24/ ?4.05 ‰, ?6.58/ ?7.48 ‰, and ?5.48/ ?6.08 ‰). Correlations were examined between the carbon isotopic composition of diamonds and their crystal morphology; the color; the concentration of nitrogen, hydrogen, and platelet defects; and mineral inclusions content. It was supposed that the observed δ13C variations in the crystals are most likely related to the fractionation of carbon isotopes rather than to the heterogeneity of carbon sources involved in diamond formation. The isotopic characteristics of diamonds from the Grib pipe were compared with those of previously investigated diamonds from the Lomonosov deposit. It was found that diamonds from these relatively closely spaced kimberlite fields are different; this also indicates the existence of spatially localized peculiarities of isotope fractionation in processes accompanying diamond formation.  相似文献   

2.
《Applied Geochemistry》1991,6(5):477-494
In the past decade, the isotopic compositions of C in > 600 inclusion-bearing diamonds have been determined. Such analyses have revealed the following isotopic characteristics: (1) peridotitic diamonds, which typically contain garnet, chromite, olivine and/or orthopyroxene inclusions with refractory compositions (high Mg, Cr), have δ13C values predominantly between −10 and −1‰, with a sharp peak in the distribution near −5‰; (2) eclogitic diamonds, which commonly contain inclusions of omphacitic clinopyroxene, Cr-poor pyrope, and/or eclogitic accessory minerals such as rutile, kyanite, coesite or sanidine, have δ13C values between −34 and +3‰, with a smaller peak near −5‰; (3) the isotopic compositions obtained for suites of diamonds from individual occurrences are, in general, unique and do not resemble the range and distribution obtained by amalgamating the diamond isotope data from a number of localities; (4) isotopic zoning patterns and heterogeneities are found in some diamonds; cores of coated diamonds tend to be depleted in13C relative to the rims, and within single octahedral diamonds δ13C variations of nearly 6‰ have been reported.Because expected C isotope fractionations at mantle temperatures are small, attempts to model the full range of diamond isotope values through fractionating a homogeneous mantle C source have been unsuccessful. Nevertheless, fractionation is probably responsible for some of the observed variation in δ13C values. Two other models have also been proposed to account for the diamond characteristics outlined above. The “primordial model” suggests that the range and distribution of C isotope compositions are inherited from primordial C in the mantle which has an inhomogeneous isotopic composition, such as that found in meteorites. The “subduction model” suggests that subducted, crustal C is the source of C in diamonds, as organic and inorganic C compounds in the crust exhibit a range of δ13C values similar to that observed in diamonds. This paper reviews the C isotope characteristics of diamonds and compares the models which have been proposed to explain the origins of these characteristics.  相似文献   

3.
Forty-one diamonds sourced from the Juina-5 kimberlite pipe in Southern Brazil, which contain optically identifiable inclusions, have been studied using an integrated approach. The diamonds contain <20 ppm nitrogen (N) that is fully aggregated as B centres. Internal structures in several diamonds revealed using cathodoluminescence (CL) are unlike those normally observed in lithospheric samples. The majority of the diamonds are composed of isotopically light carbon, and the collection has a unimodal distribution heavily skewed towards δ13C ~ ?25 ‰. Individual diamonds can display large carbon isotope heterogeneity of up to ~15 ‰ and predominantly have isotopically lighter cores displaying blue CL, and heavier rims with green CL. The light carbon isotopic compositions are interpreted as evidence of diamond growth from abiotic organic carbon added to the oceanic crust during hydrothermal alteration. The bulk isotopic composition of the oceanic crust, carbonates plus organics, is equal to the composition of mantle carbon (?5 ‰), and we suggest that recycling/mixing of subducted material will replenish this reservoir over geological time. Several exposed, syngenetic inclusions have bulk compositions consistent with former eclogitic magnesium silicate perovskite, calcium silicate perovskite and NAL or CF phases that have re-equilibrated during their exhumation to the surface. There are multiple occurrences of majoritic garnet with pyroxene exsolution, coesite with and without kyanite exsolution, clinopyroxene, Fe or Fe-carbide and sulphide minerals alongside single occurrences of olivine and ferropericlase. As a group, the inclusions have eclogitic affinity and provide evidence for diamond formation at pressures extending to Earth’s deep transition zone and possibly the lower mantle. It is observed that the major element composition of inclusions and isotopic compositions of host Juina-5 diamonds are not correlated. The diamond and inclusion compositions are intimately related to subducted material and record a polybaric growth history across a depth interval stretching from the lower mantle to the base of the lithosphere. It is suggested that the interaction of slab-derived melts and mantle material combined with subsequent upward transport in channelised networks or a buoyant diapir explains the formation of Juina-5 diamonds. We conclude that these samples, despite originating at great mantle depths, do not provide direct information about the ambient mantle, instead, providing a snapshot of the Earth’s deep carbon cycle.  相似文献   

4.
Microdiamonds (crystals smaller than 1 mm) of octahedral and cubic habit from Udachnaya kimberlite pipe (Yakutia) have been compared in order to distinguish genetic features inferred from carbon isotopic composition and impurity defects. Microdiamonds of cubic habit from the Udachnaya kimberlite pipe have a fibrous internal structure and a high content of nitrogen impurity (400–3000 ppm). Octahedral microdiamonds from the same deposit are distinguished by a low nitrogen content of 0 to 500 ppm and zoning structure. The isotopic composition of carbon (δ13C is –4.7‰ for octahedra and –4.5‰ for cuboids) suggests a common source of carbon for these morphologic groups. The studied characteristics can be due to crystallization of octahedra from carbon dissolved in the melt, and cuboids, under the conditions of the hampered diffusion of carbon.  相似文献   

5.
Diamonds from high- and low-MgO groups of eclogite xenoliths from the Jericho kimberlite, Slave Craton, Canada were analyzed for carbon isotope compositions and nitrogen contents. Diamonds extracted from the two groups show remarkably different nitrogen abundances and δ13C values. While diamonds from high-MgO eclogites have low nitrogen contents (5-82 ppm) and extremely low δ13C values clustering at ∼−40‰, diamonds from the low-MgO eclogites have high nitrogen contents (>1200 ppm) and δ13C values from −3.5‰ to −5.3‰.Coupled cathodoluminescence (CL) imaging and SIMS analysis of the Jericho diamonds provides insight into diamond growth processes. Diamonds from the high-MgO eclogites display little CL structure and generally have constant δ13C values and nitrogen contents. Some of these diamonds have secondary rims with increasing δ13C values from −40‰ to ∼−34‰, which suggests secondary diamond growth occurred from an oxidized growth medium. The extreme negative δ13C values of the high-MgO eclogite diamonds cannot be produced by Rayleigh isotopic fractionation of average mantle-derived carbon (−5‰) or carbon derived from typical organic matter (∼−25‰). However, excursions in δ13C values to −60‰ are known in the organic sedimentary record at ca. 2.7 and 2.0 Ga, such that diamonds from the high-MgO eclogites could have formed from similar organic matter brought into the Slave lithospheric mantle by subduction.SIMS analyses of a diamond from a low-MgO eclogite show an outer core with systematic rimwards increases in δ13C values coupled with decreases in nitrogen contents, and a rim with pronounced alternating growth zones. The coupled δ13C-nitrogen data suggest that the diamond precipitated during fractional crystallization from an oxidized fluid/melt from which nitrogen was progressively depleted during growth. Model calculations of the co-variation of δ13C-N yielded a partition coefficient (KN) value of 5, indicating that nitrogen is strongly compatible in diamond relative to the growth medium. δ13C values of diamond cores (−4‰) dictate the growth medium had higher δ13C values than primary mantle-derived carbon. Therefore, possible carbon sources for the low-MgO eclogite diamonds include oxidized mantle-derived (e.g. protokimberlite or carbonatite) fluids/melts that underwent some fractionation during migration or, devolatilized subducted carbonates.  相似文献   

6.
The geochemical evolution of the fluids migra- ting at the Variscan thrust front in eastern Belgium has been investigated by a petrographic, mineralogical and geoche-mical study of ankerite, quartz and ferroan calcite veins hosted by lower Devonian rocks. Three vein generations have been recognized. The first generation consists of quartz, chlorite and ankerite filling pre- to early Variscan extensional fractures. The second generation is present as shear veins of Variscan age, and contains quartz, chlorite and ferroan calcite. The third generation consists of ankerite filling post-Variscan fractures. The oxygen and carbon isotopic composition of the two ankerite phases and of the ferroan calcites are respectively between –16.4 and –11.4‰ PDB between –17.8 and –1.7‰ PDB. This range is greater than that of calcite nodules in the lower Devonian siliciclastic sediments (δ18O= –15.6 to –11.1‰ PDB and δ13C= –13.4 to –10.2‰ PDB). This suggests precipitation of the carbonate veins from a fluid which was at most only partly isotopically buffered by the calcite nodules in the host rock. The calculated oxygen isotopic composition of the ambient fluid from which the calcite veins formed is between +7.8 and +10.0‰ SMOW. Two main fluid types have been recognized in fluid inclusions in the quartz and carbonates. The first fluid type is present as secondary fluid inclusions in the first and second vein generations. The fluid has a salinity of 0.5–7.2 eq. wt.% NaCl and a high, but variable, homogenization temperature (Th=124–188°C). Two origins can be proposed for this fluid. It could have been expelled from the lower Devonian or could have been derived from the metamorphic zone to the south of the area studied. Taking into account the microthermometric and stable-isotope data, and the regional geological setting, the fluid most likely originated from metamorphic rocks and interacted with the lower Devonian along its migration path. This is in agreement with numerical simulations of the palaeofluid and especially the palaeotempera-ture field, which is based on chlorite geothermometry and vitrinite reflectance data. The second fluid type occurs as secondary inclusions in the shear veins and as fluid inclusions of unknown origin in post-Variscan ankerite veins. Therefore, it has a post-Variscan age. The inclusions are characterized by a high salinity (18.6–22.9 eq. wt.% CaCl2). The composition of the fluid is similar to that which caused the development of Mississippi Valley-type Pb–Zn deposits in Belgium.  相似文献   

7.
Here, we compare nitrogen aggregation characteristics and carbon isotopic compositions in diamonds from Mesoproterozoic (T1) and Jurassic (U2) kimberlites in the Attawapiskat area—the first diamond-producing area on the Superior craton. The T1 kimberlite sampled diamonds from the lithospheric mantle at 1.1 Ga, at the same time as the major Midcontinent Rift event. These diamonds have a narrow range in δ13C (mode of ?3.4 ‰), with compositions that overlap other diamond localities on the Superior craton. Some diamond destruction must have occurred during the Mesoproterozoic in response to the thermal impact of the Midcontinent Rift—the associated elevated geotherm caused a narrow diamond window (<30 km) close to the base of the lithosphere, compared to a wide diamond window of ~85 km following thermal relaxation (sampled by Jurassic kimberlites, such as U2). T1 diamonds have highly aggregated nitrogen, possibly due to the thermal effect of the rift. Diamond-favourable conditions were re-established in the lithospheric mantle after the thermal impact of the Midcontinent Rift dissipated. The poorly aggregated nature of nitrogen in U2 diamonds—compared to highly aggregated nitrogen in diamonds from T1—indicates that renewed diamond formation must have occurred only after the thermal impact of the Midcontinent Rift at 1.1 Ga had subsided and that these newly formed diamonds were subsequently sampled by Jurassic kimberlites. The overall δ13C distribution for U2 diamonds is distinct to T1 and other Superior diamonds, further suggesting that U2 diamonds are not related to the older pre-rift diamonds.  相似文献   

8.
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   

9.
The carbon isotope composition of microdiamonds found in products of the Tolbachik Volcano eruption, Kamchatka (porous lavas and ash), was studied. The isotope composition of microdiamonds (with an average value of δ13C =–25.05‰) is close to that of microsized carbon particles in lavas (from–28.9 to–25.3‰). The general peculiarities of the diamond-forming environment include (1) no evidence for high pressure in the medium; (2) a reduced environment; and (3) mineralogical evidence for the presence of a fluid. The geochemical data characterizing the type of diamonds studied allow us to suggest that they were formed in accordance with the mechanism of diamond synthesis during cavitation in a rapidly migrating fluid, which was suggested by E.M. Galimov.  相似文献   

10.
He Pozanti‐Karsanti ophiolite (PKO) is one of the largest oceanic remnants in the Tauride belt, Turkey. Micro‐diamonds were recovered from the podiform chromitites, and these were investigated based on morphology, color, cathodoluminescence, nitrogen content, carbon and nitrogen isotopes, internal structure and inclusions. The diamonds recovered from the PKO are mainly mixed‐habit diamonds with sectors of different brightness under the cathodoluminescence images. The total δ13C range of the PKO diamonds ranges between ?18.8 ‰ and ?28.4 ‰, with a principle δ13C mode at ?25 ‰. Nitrogen contents of the diamonds range from 7 to 541 μg/g with a mean value of 171 μg/g, and the δ15N values range from ?19.1 ‰ to 16.6 ‰, with a δ15N mode of ?9 ‰. Stacking faults and partial dislocations are commonly observed in the Transmission Electron Microscopy foils whereas inclusions are rather rare. Combinations of (Ca0.81Mn0.19)SiO3, NiMnCo‐alloy and nano‐size, quenched fluid phases were observed as inclusions in the PKO diamonds, confirming a natural origin of these diamonds. We believe that the δ13C‐depleted carbon signature of the PKO diamonds is a remnant of previously subducted crustal matter. These diamonds may have crystallized in metal‐rich melts in the asthenospheric mantle at depth below 250 km which were subsequently carried rapidly upward by asthenospheric melts/fliuds. We concluded that diamond‐bearing asthenospheric melts were likely involved in the formation of the Pozanti‐Karsanti podiform chromitite.  相似文献   

11.
《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   

12.
An extensive study of peridotitic sulfide inclusion bearing diamonds and their prospective harzburgitic host rocks from the 53 Ma Panda kimberlite pipe, Ekati Mine, NWT Canada, has been undertaken with the Re–Os system to establish their age and petrogenesis. Diamonds with peridotitic sulfide inclusions have poorly aggregated nitrogen (<30% N as B centers) at N contents of 200–800 ppm which differs from that of chromite and silicate bearing diamonds and indicates residence in the cooler portion of the Slave craton lithospheric mantle. For most of the sulfide inclusions, relatively low Re contents (average 0.457 ppm) and high Os contents (average 339 ppm) lead to extremely low 187Re/188Os, typically << 0.05. An age of 3.52 ± 0.17 Ga (MSWD = 0.46) and a precise initial 187Os/188Os of 0.1093 ± 0.0001 are given by a single regression of 11 inclusions from five diamonds that individually provide coincident internal isochrons. This initial Os isotopic composition is 6% enriched in 187Os over 3.5 Ga chondritic or primitive mantle. Sulfide inclusions with less radiogenic initial Os isotopic compositions reflect isotopic heterogeneity in diamond forming fluids. The harzburgites have even lower initial 187Os/188Os than the sulfide inclusions, some approaching the isotopic composition of 3.5 Ga chondritic mantle. In several cases isotopically distinct sulfides occur in different growth zones of the same diamond. This supports a model where C–O–H–S fluids carrying a radiogenic Os signature were introduced into depleted harzburgite and produced diamonds containing sulfides conforming to the 3.5 Ga isochron. Reaction of this fluid with harzburgite led to diamonds with less radiogenic inclusions while elevating the Os isotope ratios of some harzburgites. Subduction is a viable way of introducing such fluids. This implies a role for subduction in creating early continental nuclei at 3.5 Ga and generating peridotitic diamonds.Electronic Supplementary Material Supplementary material is available for this article at and is accessible for authorized users.  相似文献   

13.
The paper presents newly obtained data on the mineralogical and geochemical characteristics of Ukrainian endogenic and supergenic diamonds, including pioneering data on the carbon isotopic composition of individual diamond crystals. The endogenic diamonds occur as euhedral microcrystals (their morphology varies from octahedral to rhombododecahedral and cubic) with broadly varying concentrations of the structural nitrogen admixture and with mostly low aggregation of nitrogen centers. According to their carbon isotopic composition, Ukrainian diamonds range from kimberlitic–lamproitic to metamorphic and even carbonado-like, i.e., are polygenetic. Our data confirm the earlier conclusion that the supergenic diamonds are of impact genesis.  相似文献   

14.
The In Ouzzal granulitic unit (IOGU) consists predominantly of felsic orthogneisses most of which correspond to granitoids emplaced during the Archaean, plus metasediments, including olivine-spinel marbles, of late Archaean age. All units were metamorphosed at granulite facies during the Eburnean (2 Ga). The stable isotope signature of the marbles (δ13C=–0.8 to –4.2‰/PDB; δ18O = 7.9 to 18.9‰/SMOW) does not record a massive streaming of C-bearing fluids during metamorphism. Most of the isotopic variation in the marbles is explained in terms of pregranulitic features. Metasomatic transformation of granulites into layered potassic syenitic rocks and emplacement of carbonate veins and breccias occurred during retrogressive granulite facies conditions. The chemistry of these rocks is comparable with that of fenites and carbonatites with high contents of (L)REEs, Th, U, F, C, Ba and Sr but, with respect to these elements, a relative depletion in Nb, Ta, Hf, Zr and Ti. The isotopic compositions of Nd (?Nd(T)=–6.3 to –9.9), of Sr (87Sr/86Sr(T)= 0.7093–0.7104), and the O isotopic composition of metasomatic clinopyroxene (δ18O = 6.9 to 8‰), all indicate that the fluid had a strong crustal imprint. On the basis of the C isotope ratios (δ13C =–3.5 to –9.7‰), the fluid responsible for the crystallization of carbonates and metasomatic alteration is thought to be derived from the mantle, presumably through degassing of mantle-derived magmas at depth. Intense interaction with the crust during the upward flow of the fluid may explain its chemical and isotopic signatures. The zones of metasomatic alteration in the In Ouzzal granulites may be the deep-seated equivalents of the zones of channelled circulation of carbonated fluids described at shallower levels in the crust.  相似文献   

15.
Three hundred and thirty new 13C analyses of diamonds are presented, indicating, in conjunction with earlier published work, a range of about 30%. in the carbon isotopic composition of diamonds. The frequency distribution of diamond δ13C analyses shows a very pronounced mode at ?5 to ?6%.vs PDB, a large negative skewness, and a sharp boundary at about ?1%.. Analyses of diamonds from the Premier and Dan Carl mines, South Africa, demonstrate that: (1) differences in 13C content that can be related to diamond color and shape are smaller than 1%.; (2) the mean 13C content of kimberlite carbonates is 1–2%. lower than that of associated diamonds; (3) significant differences in 13C content exist between the mean isotopic compositions of diamonds from these two pipes; (4) the variability in δ13C differs from one mine to the other.Computations were carried out evaluating the effect on the 13C content of diamonds of: (i) various precipitation processes; (ii) the abundance of the species H2, H2O, CH4, CO, CO2 and O2 in the vapor; (iii) the initial isotopic composition variability of the source carbon; (iv) variations of the carbon isotope effects resulting from changes in pressure and temperature and (v) reservoir effects (Rayleigh fractionation). Fifty-eight genetic models were investigated for compatibility with the 13C distribution in diamonds and associated carbonate. The modeling does not permit an unambiguous answer to the question whether or not a vapor participated in diamond formation, although the presence of methane during diamond formation is compatible with the carbon isotopic composition data, possible oxygen fugacities in the mantle and with the composition of gases liberated from diamonds. In all probability carbon isotope effects in the diamond formation process were small, and the very large range in δ13C observed was inherited from the source carbon.  相似文献   

16.
《Lithos》2007,93(1-2):199-213
Kimberlite pipes K11, K91 and K252 in the Buffalo Head Hills, northern Alberta show an unusually large abundance (20%) of Type II (no detectable nitrogen) diamonds. Type I diamonds range in nitrogen content from 6 ppm to 3300 ppm and in aggregation states from low (IaA) to complete (IaB). The Type IaB diamonds extend to the lowest nitrogen concentrations yet observed at such high aggregation states, implying that mantle residence occurred at temperatures well above normal lithospheric conditions. Syngenetic mineral inclusions indicate lherzolitic, harzburgitic, wehrlitic and eclogitic sources. Pyropic garnet and forsteritic olivine characterize the peridotitic paragenesis from these pipes. One lherzolitic garnet inclusion has a moderately majoritic composition indicating a formation depth of ∼ 400 km. A wehrlitic paragenesis is documented by a Ca-rich, high-chromium garnet and very CaO-rich (0.11–0.14 wt.%) olivine. Omphacitic pyroxene and almandine-rich garnet are characteristic of the eclogitic paragenesis. A bimodal δ13C distribution with peaks at − 5‰ and − 17‰ is observed for diamonds from all three kimberlite pipes. A large proportion (∼ 40%) of isotopically light diamonds (δ13C < −10‰) indicates a predominantly eclogitic paragenesis.The Buffalo Head Terrane is of Lower Proterozoic metamorphic age (2.3–2.0 Ga) and hence an unconventional setting for diamond exploration. Buffalo Hills diamonds formed during multiple events in an atypical mantle setting. The presence of majorite and abundance of Type II and Type IaB diamonds suggests formation under sublithospheric conditions, possibly in a subducting slab and resulting megalith. Type IaA to IaAB diamonds indicate formation and storage under lower temperature in normal lithospheric conditions.  相似文献   

17.
Iron valence state and local environment in a set of fibrous diamonds from Brazilian and Congolese placers were investigated using X-ray absorption and Mössbauer spectroscopies. It is shown that the diamonds could be divided into two main groups differing in the type of dominant Fe-bearing inclusions. In the first group Fe is mostly trivalent and is present in octahedral coordination; diamonds from the second group contain a mixture of Fe2+ and Fe3+, most likely with Fe2+ in dodecahedral coordination. A few other diamonds contain iron in a more reduced state: The presence of metallic Fe and Fe3O4 is inferred from XAS measurements. Spatially resolved XANES and Mössbauer measurements on polished diamond plates show that in some cases the Fe valence state may change considerably between the core and rim, whereas in other cases Fe speciation and valence remain constant. It is shown that Fe valence does not correlate with water and/or carbonate content or ratio, suggesting that iron is a minor element in the growth medium of fibrous diamonds and plays a passive role. This study suggests that, when present, evolution of the C isotopic composition with diamond growth is largely due to changes in chemistry of the growth medium and not due to variations of fO2.  相似文献   

18.
《China Geology》2020,3(4):602-610
Thirty-nine crude oils and twenty-one rock samples from Niger Delta Basin, Nigeria have been characterized based on their isotope compositions by elemental analysis-isotope ratio mass spectrometry and gas chromatography-isotope ratio mass spectrometry. The bulk carbon isotopic values of the whole rock extracts, saturate and aromatic fractions range from –28.7‰ to –26.8‰, –29.2‰ to –27.2 ‰ and –28.5 ‰ to –26.7 ‰, respectively while the bulk carbon isotopic values of the whole oils, saturate and aromatic fractions range from –25.4 ‰ to –27.8 ‰, –25.9 ‰ to –28.4 ‰ and –23.5 ‰ to –26.9 ‰, respectively. The average carbon isotopic compositions of individual alkanes (nC12-nC33) in the rock samples range from –34.9‰ to –28.2‰ whereas the average isotopic values of individual n-alkanes in the oils range from –31.1‰ to –23.8‰. The δ13C isotope ratios of pristane and phytane in the rock samples range from –29.2 ‰ to –28.2 ‰ and –30.2 ‰ to –27.4 ‰ respectively while the pristane and phytane isotopic values range from –32.1‰ to –21.9‰ and –30.5‰ to –26.9‰, respectively. The isotopic values recorded for the samples indicated that the crude oils were formed from the mixed input of terrigenous and marine organic matter and deposited under oxic to sub-oxic condition in lacustrine-fluvial/deltaic environments. The stable carbon isotopic compositions were found to be effective in assessing the origin and depositional environments of crude oils in the Niger Delta Basin.  相似文献   

19.
Diamond from metaultramafic rocks of the Mesoarchean (2.96–3.0 Ga) Olondo greenstone belt, located in the western Aldan–Stanovoy shield, has been studied. Diamonds occur in lenses of olivine–serpentine–talc rocks within metaultramafic rocks of intrusive habit, whose composition corresponds to peridotite komatiites. All diamonds from the metaultramafic rocks are crystal fragments 0.3 to 0.5 mm in size. Morphological examination has revealed laminar octahedra, their transitional forms to dodecahedroids, crystals with polycentric faces, and spinel twins. The crystals vary in photoluminescence color: dark blue, green, yellow, red, or albescent. Characteristic absorption bands in crystals point to nitrogen impurities in the form of A and B1 defects and tabular B2 defects. The crystals studied belong to the IaA/B type, common among natural diamonds. The overall nitrogen content varies from < 100 to 3800 ppm. The relative content of nitrogen in B1 centers varies from 0 to 94%, pointing to long stay in the mantle. The carbon isotope ratio in the diamonds, 13C = ? 26‰, is indicative of involvement of subducted crust matter in diamond formation in the Archean.  相似文献   

20.
The Yinkeng orefield in Yudu County, Jiangxi Province, SE China, is a zone of concentrated Au-Ag-Pb-Zn-Cu-Mn polymetallic ores. Based on summing up basic geology and ore geology of the orefield, the polymetallic deposits in the orefield have been divided into seven major substyles according to their occurring positions and control factors. The ore-forming fluid inclusion styles in the orefield include those of two-phase fluid, liquid CO2-bearing three-phase and daughter mineral-bearing multi-phase. The homogenization temperatures range from 382o to 122oC, falling into five clusters of 370o to 390o, 300o to 360o, 230o to 300o, 210o to 290o and 120o to 200o, and the clusters of 300o to 360o, 230o to 300o and 120o to 200o are three major mineralization stages, with fluid salinity peaks from 4.14% to 7.31%, 2.07% to 7.31% and 0.53% to 3.90%, respectively. The ore-forming fluids are mainly type of NaCl-H2O with medium to high density(0.74–1.02 g/cm3), or CO2-bearing NaCl-H2O with medium to low density(0.18–0.79 g/cm3). The fluid salinity and density both show a decline tendency with decreasing temperature. According to the measurement and calculation of Hand O-isotopic compositions in the quartz of the quartz-sulfide veins, δDV-SMOW of the ore-forming fluid is from-84‰ to-54‰, and δ18OV-SMOW of that is from 6.75‰ to 9.21‰, indicating a magmatic fluid. The δ34SV-CDT of sulfides in the ores fall into two groups, one is from-4.4‰ to 2.2‰ with average of-1.42‰, and the other from 18.8‰ to 21.6‰ with average of 19.8‰. The S-isotopic data shows one peak at-4.4‰ to 2.2‰(meaning-1.42‰) suggesting a simple magmatic sulfur source. The ore Pbisotopic ratios are 206Pb/204Pb from 17.817 to 17.983, 207Pb/204Pb from 15.470 to 15.620 and 208Pb/204Pb from 38.072 to 38.481, indicating characteristics of mantle-derived lead. The data show that the major ore deposits in the orefield have a magmatic-hydrothermal genesis and that the SHRIMP zircon age of the granodiorite porphyry, closely related to the mineralization, is 151.2±4.2 Ma(MSWD = 1.3), which can represent the formation ages of the ores and intrusion rocks. The study aids understanding of the ore-forming processes of the major metallic ore deposits in the orefield.  相似文献   

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