THE REDOX STABILITY OF MOISSANITE (SIC) COMPARED WITH METAL-METAL OXIDE BUFFERS AT 1773 K AND AT PRESSURES UP TO 90 KBAR

Moissanite (SiC) has been accepted as naturally occurring in some mant le and meteorite samples. The thermochemical data bases have allowed c alculations of the stability of SiC, moissanite, at 1 bar in such reac tions as: SiC + O-2 = C + SiO2 (MsCQ) and SiC + Mg2SiO4 + O-2 = C + 2M gSiO(3) (MsOCP) The fO(2) values of these reactions at igneous petroge netic temperatures and 1 bar are as much as six to eight orders of mag nitude too reduced for Earth upper mantle models that place the log fO (2) value between Delta(FMQ) = +2 and Delta(FMQ) = -3. Utilizing a ZrO 2 membrane technique, in both piston-cylinder and in multi-anvil exper iments, the fO(2) of the (MsOCP) buffer was compared with various meta l-metal oxide buffers at 15 kbar and at 90 kbar at 1773 K. Results can be summarized as: [GRAPHICS] This order of redox potentials does not appear to change from 1 bar to 90 kbar. While we cannot measure the ab solute values of these buffers at high pressure, we have demonstrated that high pressure does not stabilize SiC anywhere near the value of t he (IW) buffer. The oxygen fugacity of the MsCOP assemblage is similar to 1-2 log units lower than the (Mn-MnO) buffer on the geotherm, at l east to depths equivalent to 90 kbar at 1773 K. The thermodynamic calc ulations are thus closely verified by the results of the experiments. The implications for the state of the mantle are discussed.