Abstract
Cuboid diamonds are particularly common in the placers of the northeastern Siberian platform, but their origin remains unclear. These crystals usually range in color from dark yellow to orange and, more interestingly, are characterized by unusual low aggregated nitrogen impurities (non-aggregated C-center), suggesting a short residence time and/or low temperatures at which they have been stored in the mantle. In order to track possible isotopic signature that could help deciphering cuboid diamond’s crystallization processes, δ13C values, δ15N values, and nitrogen concentrations have been determined in situ in three samples using secondary ion mass spectrometry (SIMS), whereas nitrogen aggregation states have been determined by FTIR spectroscopy. The samples fall out of the δ13C vs. δ15N field of canonical mantle composition. Different scales of carbon and nitrogen fractionation may produce the observed variations. Alternatively, mixing mantle and crustal material would obscure initial co-variations of δ13C values with δ15N or nitrogen content.
Highlights
Natural diamonds are the subject of extensive research because they provide unique information on the geochemistry, mineralogy, and P-T-fO2 regimes of the mantle
According to FTIR data, the three cuboid diamonds are of mixed type IaA/Ib caused by the presence of an A-center and C-center with additional lines of unknown centers described as X and Y [13,14]
The higher nitrogen concentrations as determined by secondary ion mass spectrometry (SIMS) relative to those measured from FTIR are in the central zones
Summary
Natural diamonds are the subject of extensive research because they provide unique information on the geochemistry, mineralogy, and P-T-fO2 regimes of the mantle. Previous investigations of crystalline inclusions in diamonds from the subcontinental lithospheric mantle have shown that diamonds originated in a range of peridotitic (P-type) and eclogitic (E-type) host-rocks [1]. Diamonds from both parageneses occur with diverse morphological and physical properties, which are widely interpreted as reflecting variations in the conditions of diamond formation, within the Earth’s upper mantle, providing important information about growth conditions and subsequent diamond post-growth histories. The carbon and nitrogen isotopic compositions of diamonds formed in the mantle have been widely used as a tool for investigating the origin of diamond-forming carbon in the mantle [3]
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