An experimental investigation of factors controlling the oxygen content of sulphide melts in the Earth’s upper mantle

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Abeykoon, Sumith ; Laurenz, Vera ; Frost, Daniel J. ; Miyajima, Nobuyoshi ; McCammon, Catherin:
An experimental investigation of factors controlling the oxygen content of sulphide melts in the Earth’s upper mantle.
In: Contributions to Mineralogy and Petrology. Bd. 178 (2023) Heft 2 . - 13.
ISSN 1432-0967
DOI: https://doi.org/10.1007/s00410-023-01992-3

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Abstract

To determine the factors that control the oxygen content of sulphide melts in the upper mantle, mono-sulphides were equilibrated with mantle peridotite assemblages, with varying FeO and NiO contents, between 3 and 13 GPa and 1300 to 1819 °C. Carbonates were added as a flux and the use of graphite capsules allowed oxygen fugacities (fo2) to be estimated for most of the experiments. Sulphide melt O concentrations were in the range 0.2–3.7 wt. %, coexisting with silicate assemblages with olivine Fe/(Fe + Mg) ratios between 0.04 and 0.25. Except in Ni-bearing experiments, variations in the sulphur/metal cation ratio did not affect the sulphide melt O contents, which also appeared to be independent of fo2. The silicate FeO contents, temperature and pressure were found to be the main controls on O contents. Nickel lowers the O content, although all Ni-bearing experiments produced sulphides with lower sulphur/metal ratios, making it difficult to categorically separate the effect of Ni alone. A preliminary geothermometer expression was developed based on the O content of the sulphide melts and the iron oxide concentrations of coexisting olivine and orthopyroxene. With this expression, the experimental temperatures are reproduced to within 74 K for Ni-free experiments and within 135 K for Ni-bearing samples. Using measurements of the O contents of sulphide inclusions of peridotite affinity in diamonds from the Lac de Gras kimberlite field, a plausible average entrapment temperature of 1318 ± 48 °C is calculated. Mantle peridotite assemblages would be expected to contain sulphide melts with approximately 0.4–0.6 wt. % O along a typical mantle adiabat down to 200 km depth.