Static compression of hydrous silicate melt and the effect of water on planetary differentiation

Abstract

High pressure experiments using the sink/float method have bracketed the density of hydrous iron-rich ultrabasic silicate melt from 1.35 to 10.0 GPa at temperatures from 1400 to 1860 °C. The silicate melt composition was a 50–50 mixture of natural komatiite and synthetic fayalite. Water was added in the form of brucite Mg(OH) 2 and was present in the experimental run products at 2 wt.% and 5 wt.% levels as confirmed by microprobe analyses of total oxygen. Buoyancy marker spheres were olivines and garnets of known composition and density. The density of the silicate melt with 5 wt.% water at 2 GPa and 1500 °C is 0.192 g cm − 3 less than the anhydrous form of this melt at the same P and T. This density difference gives a partial molar volume of water in silicate melt of ∼ 7 cm 3 mol − 1 , which is similar to previous studies at high pressure. The komatiite–fayalite liquids with 0 and 2 wt.% H 2O, have extrapolated density crossovers with equilibrium liquidus olivine at 8 and 9 GPa respectively, but there is no crossover for the liquid with 5 wt.% H 2O. These results are consistent with the hypothesis that dense hydrous melts could be gravitationally stable atop the 410 km discontinuity in the Earth. The results also support the notion that equilibrium liquidus olivine could float in an FeO-rich hydrous martian magma ocean. Extrapolation of the data suggests that FeO-rich hydrous melt could be negatively buoyant in the Earth's D″-region or atop the core–mantle-boundary (CMB), although experiments at higher pressure are needed to confirm this prediction.