Andrei A. Shiryaev^1,2 and and Fabrice Gaillard³

¹Institute of Physical Chemistry and Electrochemistry RAS, Leninsky pr. 31, 119071 Moscow, Russia
²Institute of Geology of Ore Deposits, Petrography, Mineralogy and Geochemistry RAS, Staromonetny per. 35, 119017 Moscow, Russia
³CNRS/INSU, Institut des Sciences de la Terre d’Orléans – UMR 6113, Université d’Orléans, Campus Géosciences, 1A rue de la Férollerie, 41071 Orléans cedex 2, France

Grains of natural SiC, moissanite, are encountered in various geological settings. According to thermodynamic calculations and high-pressure experiments, SiC formation requires very reducing conditions, approx. 6–10 orders of magnitude in fO2 more reducing than the present-day mantle redox state. SiC occurrences have motivated several studies but the required extremely reducing conditions remain a major inconsistency that has not been solved. It is shown here that such reducing conditions can be achieved during the ultimate steps of ascent of carbon-saturated melts, when pressure is lower than 100 bars. At these conditions, the redox buffering by carbon can impose fO2 similar to IW-6. Conditions favorable to SiC growth can therefore be reached around carbonaceous grains during the shallow emplacement of silicate melts or kimberlites and do not necessarily imply extremely localized oxygen-depleted regions in the mantle. Such reduced conditions can also explain the presence of elemental Si and ironcarbide inclusions in association with moissanite grains.

Reference
Shiryaev AA and and Gaillard F (2014) Local redox buffering by carbon at low pressures and the formation of moissanite – natural SiC. European Journal of Mineralogy 26:53.
[doi:10.1127/0935-1221/2013/0025-2339]
Copyright: E. Schweizerbart’sche Verlagsbuchhandlung

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