Carbon Isotopic Composition Of Diamonds Research Articles

The Nature and Compositional Peculiarities of Volcanogenic Diamonds

Abstract —We have studied volcanogenic diamonds in the context of a discussion of their genesis, including some assumption on their artificial origin. The carbon isotope composition of diamonds collected from the eruption products of Tolbachik volcano (δ13CVPDB from –22 to –29‰) is within the range of the δ13CVPDB values of natural diamonds, including those from kimberlites. The δ15NAir values of the Tolbachik diamonds, measured for the first time (–2.58 and –2.32‰), correspond to δ15NAir of volcanic gases and differ from that of atmospheric nitrogen (δ15NAir = 0‰), which may be expected in synthetic diamonds. In the studied volcanogenic diamonds, as in synthetic ones, the nitrogen impurity is unaggregated. However, such an unaggregated form of nitrogen is specific to many natural diamonds (e.g., variety II diamonds, according to Orlov’s classification). Impurity elements (Cl, F, O, S, Si, Al, Ca, and Na) are locally concentrated in volcanogenic diamonds; they are a constituent of micro- and nanoinclusions in them. The high contents of F and Cl in the studied diamonds are correlated with the composition of volcanic gases; there is no reason to expect a similar correlation in synthetic diamonds. Moreover, the studied cube-octahedral Tolbachik diamonds have a number of accessory forms, some of which are not observed in synthetic diamonds. Their surfaces are frequently covered with films composed of Mg–Fe and Ca–Mg silicates, aluminosilicates, sulfates, metal alloys, and native Al. Mineral inclusions in the studied diamonds are Mn–Ni–Si alloys and silicides varying in composition from (Mn,Ni)4Si to (Mn,Ni)5Si2, Mn5Si2, and pure Mn silicide MnSi. Summing up the obtained data, we conclude that volcanogenic diamonds form in a strongly reducing environment, in which silicides and native metals and their alloys are stable. The predominant cube-octahedral morphology of these diamonds and the unaggregated nitrogen impurity point to their short-term residence under high-temperature conditions. This makes them similar, to some extent, to synthetic diamonds. There are, however, clear differences as well. Volcanogenic diamonds are similar in compositional peculiarities, including isotope compositions, to natural diamonds that form under most unfavorable conditions, such as cuboids, balases, carbonado, and some diamonds of the eclogite paragenesis. They also resemble diamonds found in situ in harzburgite and chromitite of ophiolites. This suggests a specific mechanism of formation of both volcanogenic and ophiolitic diamonds in the oceanic lithosphere.

The inclusions of SiO2 in sublithospheric diamonds

The specific features of the mineralogy of SiO2 inclusions in sublithospheric diamonds are described in this study. Such diamonds are characterized by a complex growth history with stages of growth and dissolution and postgrowth processes of deformation and crushing. The nitrogen content in all studied crystals does not exceed 71 ppm and nitrogen is detected only as B-defects. The carbon isotope composition of diamonds varies widely from -26.5 to -6.7 ‰ of δ13С. SiO2 inclusions associate with omphacitic clinopyroxenes, majoritic garnets, CaSiO3, jeffbenite and ferropericlase. All SiO2 inclusions are coesite, which is often accompanied by micro-blocks of kyanite. These phases are suggested to represent the product of the retrograde transformation of the primary Al-stishovite. Significant internal stresses in the inclusions and deformations around them can be evidence of thise phase transformation. The heavier oxygen isotope composition of SiO2 inclusions in sublithospheric diamonds (up to 12.9 δ18O) indicates the crustal origin of their protoliths. The observed anti-correlation of δ18O of SiO2 inclusions and δ13C of their host diamonds reflects the processes of interaction of slab-derived melts with reduced mantle rocks at depths above 270 km.

Carbon isotopic composition of diamonds in Ukraine and their probable polygenetic nature

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.

Combined C isotope and geochemical evidence for a recycled origin for diamondiferous eclogite xenoliths from kimberlites of Yakutia

An integrated petrographic-geochemistry and stable isotope study of 15 diamondiferous eclogite xenoliths is presented from the Udachnaya pipe, Russia. Carbon isotope determinations were performed by combustion and SIMS while in situ mineral major and trace element concentrations were determined using LA-ICP-MS and microprobe. Carbon isotope compositions of diamonds range from − 13.7 to − 2.8‰. Although this range is less than that reported from all diamonds from kimberlites (− 38.5 to + 5.0‰), the large variation in a small sample suite establishes that the C sources in the eclogite parental rocks were extremely variable. The carbon isotope composition of diamonds from a single sample records little variation, with δ 13C variations usually close to ± 1‰ excluding samples where two generations of diamonds are present. Core-rim zonations in individual crystals are in the order of 1 per mil. Major and trace elements have been obtained for garnet and clinopyroxene from most of the samples studied for diamonds. About 60% of garnets and clinopyroxenes show Eu anomalies that suggest possible formation from recycled oceanic lithosphere. Additional evidence in support of an origin from subduction of ancient oceanic crust includes the presence of coesite and high δ 18O values of garnets. The minor C isotope variations recorded within individual crystals and individual xenoliths imply that diamond was formed from a C-bearing reservoir that was large enough not to undergo marked C isotope fractionation during diamond formation. This contrasts markedly with the large but coherent variations in N and C isotopes and N aggregation state reported for 59 diamonds extracted from a single peridotite xenolith from the Cullinan Mine, South Africa. The extreme heterogeneity recorded in trace elements of the Udachnaya eclogitic suite, coupled with the homogeneity of C isotopes in individual diamonds, does not conform to a Rayleigh distillation model. Mineralogy and geochemistry suggest that most of these eclogite xenoliths were formed from rocks produced by crystallization of gabbroic rocks in abyssal conditions that underwent subsequent subduction. The wide variation in carbon isotope compositions possibly implies the involvement of both carbonate and carbonaceous sources in the petrogenesis of the eclogitic xenoliths. Notably, however, there is clear evidence that a limited number of eclogites record multiple diamond forming events.

Carbon isotopic composition of diamonds from the Archangelsk diamond province

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.

Carbon, nitrogen, argon and helium study of impact diamonds from Ebeliakh alluvial deposits and Popigai crater

Abstract— Nineteen diamond aggregate specimens (1–2 mm in size) from impactites of Popigai crater and five diamond samples (5–7 mm in size) from Ebeliakh river placers were studied. Our investigations indicate that samples from Ebeliakh were formed in an impact event with the exception of one specimen (Y7).The carbon isotopic composition of diamonds from Popigai varies within the previously reported limits (δ13C, −8 to −22%); whereas, diamonds from Ebeliakh placers show heavier values of δ13C (−7 to −10%). All the specimens studied contain very low amounts of N, mostly <20 ppm, but a few contained up to 60 ppm. For specimens, where the quantity of N allowed reliable analysis, δ15N values were found to be in the range of −3.9 to +11.9%. On the basis of combined Ar and N study, it was concluded that impact diamonds studied here can be a mixture of at least two types of gas carriers (e.g., different diamond components). A possible explanation would be involvement of a carbon vapour deposition (CVD) process or diamond growth in the impact melt in addition to the direct graphite‐diamond shock transformation. The δ13C distributions and different N/36Ar correlations have indicated a difference between impact diamonds from Ebeliakh and diamonds extracted from Popigai crater. This could be explained by the existence of different diamond populations formed during the Popigai impact event. On the other hand, Ebeliakh diamonds could have resulted from a separate impact event to Popigai and an alternative crater is yet to be found.

The carbon isotopic composition of diamonds: relationship to diamond shape, color, occurrence and vapor composition

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 δ 13 C 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 δ 13 C 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 H 2, H 2O, CH 4, CO, CO 2 and O 2 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 δ 13 C observed was inherited from the source carbon.