The constitution of the mantle—II: Further data on the olivine-spinel transition

Abstract

Subsolidus phase equilibria were studied in the system Mg 2SiO 4-Mg 2GeO 4 at 660°C and 30,000 bars in a squeezer apparatus. From the compositions of co-existing phases, the free energy of transition of forsterite from the olivine to the spinel structure is calculated. This enables the total pressure needed to cause this transition in forsterite to be determined. It is found to be about 90,000 bars at 660°C. This result, combined with analogous data obtained in Part I makes it possible to construct a P—T curve defning the olivine-pinel transition in forsterite. A similar study was made in the system Fe 2GeO 4-Fe 2SiO 4 at 700°C and 25,000 bars and it was predicted that a spinel polymorph of fayalite should become stable around 50,000 bars at 700°C. The predicted polymorph was subsequently synthesized and its transition pressure determined as 38,000 bars at 600°C. The spinel has a lattice constant of 8.235 Å and is 12 per cent denser than fayalite. A thermodynamic study of the stability of MgSiO 3 pyroxene in the mantle is made. It is concluded that it will break down into Mg 2SiO 4 spinel and coesite at a maximum pressure not much greater than that required to produce the olivine-spinel transition in forsterite. The pressures and temperatures required to cause the olivine-spinel transition indicate that the transition will occur in the upper 1000 km of the mantle. Because of solid solution effects, the transition will occur over an appreciable depth range.