Methane-related diamond crystallization in the Earth's mantle: Stable isotope evidences from a single diamond-bearing xenolith

Abstract

Mineralogical studies of deep-seated xenoliths and mineral inclusions in diamonds indicate that there is significant variability in oxygen fugacity within the Earth's upper mantle. This variability is consistent both with the occurrence of reduced (methane-bearing) or oxidized (CO 2/carbonate-bearing) fluids. Invariably, direct sampling of reduced deep fluids is not possible as they are unquenchable and re-equilibrate with either the surrounding mantle or are affected by degassing. Key information about the nature of such fluids might be found in diamond if it were possible to study a population related to a single source. Usually, diamonds within a kimberlite pipe have different parageneses and can be shown to have formed at different times and depths. We studied 59 diamonds extracted from a single diamondiferous peridotite xenolith (with a volume of only 27 cm 3), from the Cullinan mine (formerly called the Premier mine) in South Africa. Diamond sizes range from 0.0005 to 0.169 carats (0.1 to 33.8 mg). A correlation between the nitrogen contents of the diamonds (range 40 to 1430 ppm) and their nitrogen aggregation state (varying from 10 to 85% of IaB defects) is compatible with a single growth event. δ 13C-values range from − 4.2‰ to − 0.1‰, with slight internal variability measured in the largest diamonds. Nitrogen isotope measurements show δ 15N ranging from − 1.2‰ to + 7.2‰. On the centimeter scale of this upper mantle rock, the variations for nitrogen content, nitrogen aggregation state, carbon and nitrogen isotopic compositions, respectively, cover 64%, 75%, 15% and 23% of the ranges known for peridotitic diamonds. In spite of such large ranges, N-content, δ 13C and δ 15N within this diamond population are distinctly coupled. These relationships do not support a mixing of carbon sources, but are best explained by a Rayleigh distillation within the sub-continental mantle at depths > 150 km and T > 1200 °C, which precipitates diamonds from methane-bearing fluid(s). The involvement of this reduced metasomatic agent also suggests that the heterogeneous redox state of Archean cratons may mostly result from the heterogeneous nature of percolating fluids. The striking variability of the four determined parameters at cm scale may also account for the difficulty in interpreting these parameters in larger productions, such as those from a mine, because in these cases, the diamonds are mixed and sub-populations cannot be disentangled.