Local redox buffering by carbon at low pressures and the formation of moissanite - natural SiC

Andrei A. Shiryaev (1, 2) , Fabrice Gaillard (3)

1 Institute of Physical Chemistry and Electrochemistry RAS
2 Institute of Geology of Ore Deposits
3 ISTO - Institut des Sciences de la Terre d'Orléans - UMR7327

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 iron-carbide inclusions in association with moissanite grains.

Mots clés: moissanite; silicon carbide; carboreduction; mantle redox state; thermodynamic modeling

Domaines: Volcanologie; Milieux et Changements globaux

Andrei A. Shiryaev, Fabrice Gaillard. Local redox buffering by carbon at low pressures and the formation of moissanite - natural SiC. European Journal of Mineralogy, 2014, 26, pp.53-59. ⟨10.1127/0935-1221/2013/0025-2339⟩. ⟨insu-00985690⟩