Olivine-spinel solid solution equilibria in the system Mg₂SiO₄-Fe₂SiO₄

Abstract

Olivine-spinel solid solution equilibria in the system Mg2SiO4-Fe2SiO4 have been studied over the pressure range 43 to 96 kb at 800°, 1000°, and 1200°C with the aid of tetrahedral anvil-type high-pressure apparatus. From the compositions of coexisting olivine and spinel solid solutions, the transition pressure of Mg2SiO4 from olivine to spinel structure was calculated to be 127 kb at 800°C, 140 kb at 1000°C, and 151 kb at 1200°C. The olivine-spinel transition in the mantle was concluded to begin at 370 km at a temperature of about 1150°C, when composition of the mantle olivine is assumed at (Mg0.9 Fe0.1)2SiO4. Spinel and olivine solvus curves suggest that the two-phase region ranges from 18 to 28 kb (50- to 80-km width) in the mantle.