PRESSURE EFFECT ON THE DIVALENT-CATION DISTRIBUTION IN NONIDEAL SOLID-SOLUTION OF FORSTERITE AND FAYALITE

The divalent cation distribution in olivine (Mg, Fe)2SiO4 under high pressure and temperature was studied to clarify the detailed state of olivine in the mantle. Single crystal samples were heated for a sufficient period of time for the cations to migrate and quenched fast enough to preserve the equilibrated state under high pressures, and the crystal structure was determined with X-ray method. The pressure effect on the distribution coefficient K(D)[=(Fe/Mg)M1/(Fe/Mg)M2] was determined for the first time; dK(D)/dP congruent-to 0.02 GPa-1. A set of five thermodynamic parameters required to describe the regular solution model was determined from data concerning the pressure dependence and the known temperature and compositional effects. As a result we have shown how K(D) depends on pressure, temperature, and composition. The notable feature clarified is the.very large contribution of nonideality in the olivine solid solution. The K(D) of olivine crystals in the mantle is predicted; K(D) increases to approximately 2.2 at the depth of 400 km, in contrast to 0.9 approximately 1.2 of natural samples available at the surface of the Earth.