Dmitry A. Zedgenizov

Carbon isotopes and nitrogen contents in placer diamonds from the NE Siberian craton: implications for diamond origins

Diamonds from placer deposits in the northeastern Siberian platform were examined for variations in the isotopic composition of carbon and concentrations of nitrogen. The diamonds display large variability in N concentrations, ranging from below detection to 3500 ppm. Nitrogen levels in diamonds with eclogitic inclusions are generally high (average of 950 ppm) compared to diamonds of the ultramafic suite (average of 513 ppm). Diamonds belonging to variety V (classification scheme of Orlov, 1977) have relatively high N levels (from 1500 to 3500 ppm, average of 2549 ppm). The average nitrogen level is five times higher than the worldwide average. Values of δ 13 C in the diamonds range from −27.2 to −3‰ ( n = 28) in eclogitic diamonds and from −7.1 to −0.5‰ ( n = 16) in peridotitic diamonds. Diamonds of variety V range in δ 13 C from −24.1 to −17.4‰. The distributions of nitrogen and its aggregation state in some diamond crystals imply the occurrence of multiple growth events. This is especially evident in the nitrogen content, which decreases and then increases from core to rim. The results indicate that there is no correlation between local variations in the isotopic composition of carbon and either the content or degree of aggregation of nitrogen. The diamonds of variety V differ from most diamonds worldwide in their high N content and light C-isotope composition. The large ranges of δ 13 C and the lack of correlation with nitrogen levels are inconsistent with the open-system Rayleigh isotopic fractionation of carbon species. The nature of the variations in the carbon isotope composition and the nitrogen concentrations indicate that the diamond growth medium had at least two sources of fluids/melts (mantle and recycled Earth crust via subduction). Mantle carbon was involved in the process of diamond formation during the final stages of diamond growth.

Nature of type IaB diamonds from the Mir kimberlite pipe (Yakutia): evidence from spectroscopic observation

In this study, the specific features of structural defects of type IaB diamonds from the Mir kimberlite pipe (Yakutian diamondiferous province) have been characterized using FTIR and photoluminescence spectroscopy. Mineral inclusions in these diamonds [olivine (Ol), orthopyroxene (OPx), chromite (Chr), sulphide (Sf)] correspond to associations of peridotite rocks at the base of the lithosphere. Nitrogen content in type IaB diamonds shows significant variations, suggesting different growth media and/or several growth stages. A specific feature of these diamonds is the absence or very small amount of platelets, which may be related to annealing during their long-term residence at the temperatures of the base of the lithosphere. All studied diamonds show the presence of hydrogen defects that are active in IR spectra with an intense line at 3107xa0cm−1, and additional weaker lines at 3085 and 3237xa0cm−1, which correlated with high nitrogen content. Type IaB diamonds are also characterized by the presence of nitrogen–nickel luminescence centres S2, S3 and 523.2xa0nm. This feature distinguishes them from superdeep diamonds with extreme nitrogen aggregation states, which clearly attest to different growth conditions and crystallization media of type IaB diamonds from the Mir kimberlite pipe.

The characteristic photoluminescence and EPR features of superdeep diamonds (São-Luis, Brazil)

AbstractnPhotoluminescence (PL) spectroscopy and electron paramagnetic resonance (EPR) were used for the first time to characterize properties of superdeep diamonds from the São-Luis alluvial deposits (Brazil). The infrared measurements showed the low nitrogen content (>50 of 87 diamonds from this locality were nitrogen free and belonged to type IIa) and simultaneously the extremely high level of nitrogen aggregation (pure type IaB being predominant), which indicates that diamonds under study might have formed under high pressure and temperature conditions. In most cases, PL features excited at various wavelengths (313, 473, and 532xa0nm) were indicative of different growth and post-growth processes during which PL centers could be formed via interaction between vacancies and nitrogen atoms. The overall presence of the 490.7xa0nm, H3, and H4 centers in the luminescence spectra attests to strong plastic deformations in these diamonds. The neutral vacancy known as the GR1 center has probably occurred in a number of crystals due to radiation damage in the post-growth period. The 558.5xa0nm PL center is found to be one of the most common defects in type IIa samples which is accompanied by the EPR center with g-factor of 2.00285. The 536 and 576xa0nm vibronic systems totally dominated the PL spectra of superdeep diamonds, while none of “normal” diamonds from the Mir pipe (Yakutia) with similar nitrogen characteristics showed the latter three PL centers.

Diamond formation during metasomatism of mantle eclogite by chloride-carbonate melt

A xenolith of bimineralic eclogite from the Udachnaya kimberlite pipe provides a snapshot of interaction between mantle rocks and diamond-forming fluids/melts. The major-element composition of the eclogite is similar to that of N-MORB and/or oceanic gabbros, but its trace-element pattern shows the effects of mantle metasomatism, which resulted in diamond formation. The diamonds are clustered in alteration veins that crosscut primary garnet and clinopyroxene. The diamonds contain microinclusions of a fluid/melt dominated by carbonate and KCl. Compared to the worldwide dataset, the microinclusions in these diamonds fall in middle of the range between saline fluids and low-Mg carbonatitic melts. The fluid/melt acted as a metasomatic agent that percolated through ancient eclogitic rocks stored in the mantle. This interaction is consistent with calculated partition coefficients between the rock-forming minerals and diamond-forming fluid/melt, which are similar to experimentally-determined values. Some differences between the calculated and experimental values may be due to the low contents of water and silicates in the chloride-carbonate melt observed in this study, and in particular its high contents of K and LILE. The lack of nitrogen aggregation in the diamonds implies that the diamond-forming metasomatism took place shortly before the eruption of the kimberlite, and that the microinclusions thus represent saline carbonate-rich fluids circulating in the basement of lithospheric mantle (150–170xa0km depth).