Grain growth in the system forsterite–diopside–water

Abstract

Grain growth experiments in dunite, clinopyroxenite, and wehrlites having various forsterite/diopside ratios under water-saturated conditions were performed to investigate the effect of water on grain growth kinetics in the Earth's upper mantle. The experiments were conducted at 1200 °C and 1.2 GPa for 1.5–763 h using a piston-cylinder apparatus. Both normal grain growth (NGG) and abnormal grain growth (AGG) proceeded in the rocks. The addition of water to the rocks resulted in the inhibition of NGG in dunite and clinopyroxenite, whereas it resulted in the acceleration of NGG of both forsterite and diopside in wehrlites. AGG of the first phase in the rocks tended to be promoted by increasing the amount of water added. The values of the growth exponent n for forsterite and diopside in wehrlites were in the range 1.8–4.8 and 1.8–4.5, respectively. The n-values for the first phase in wehrlites tended to show low values (1.8–2.5), whereas higher n-values (>3) have been reported under dry conditions. This suggests that faster NGG of the first phase with n ∼ 2 can proceed in peridotites under water-saturated conditions, whereas much slower NGG of the first phase with n > 3 proceeds under dry conditions. This could have the consequences that the effective viscosity of the water-saturated peridotites can be higher than that under dry conditions in the Earth's upper mantle when the creep mechanism is grain size-sensitive creep.