High pressure phase equilibrium in the system Fe-Mg-Si-O

Recent experimental data on high pressure synthesis of minerals in the Fe-Mg-Si-O system permit the establishment of a thermochemical-thermophysical data set with which phase equilibrium for the earth's mantle can be computed. The results indicate that low pressure (<200 Kbar) silicates (Mg-Fe) spinel and stishovite react to form (Mg-Fe) perovskite and (Mg-Fe) wuestite starting at a pressure close to 200 Kbar. The iron content of perovskite increases with depth a corresponding iron depletion in wuestite. Between 1000 and 1100 Kbar, both minerals begin to lose iron rapidly with pressure. Metal Fe with almost pure Mg-perovskite, MgO and oxygen becomes stable above 1200 Kbar, which pressure corresponds closely to the core-mantle boundary. This is a general feature over a range of temperatures and compositions. Conclusions are a) the present state of the earth is a consequence of thermochemical equilibrium which could have established over a range of post-accretionary temperatures, and b) the oxygen released in the equilibrium reaction could be available as a light element in the core. These results have a very important influence on trace element geochemical models of the mantle and therefore, on models of petrogenetic evolution of the earth.