COMPOSITIONS OF AQUEOUS FLUIDS IN EQUILIBRIUM WITH PYROXENES AND OLIVINES AT MANTLE PRESSURES AND TEMPERATURES

Abstract

Solubility experiments were performed at 30 kbars in the system Mg2SiO4-SiO2-H2O, and at 20 and 30 kbars on omphacitic pyroxene-water mixtures. They confirm that the solubility of the forsterite component in aqueous fluids remains rather low (up to 5 wt.%), whereas the solubility of the SiO2 component from solids of appropriate SiO2-rich compositions in the system Mg2SiO4-SiO2-H2O increases with temperature up to some 75% at 1,100° C. At this temperature a simplified harzburgite consisting of forsterite and enstatite coexists with a fluid containing about 35% (MgO+SiO2). Hydrous fluids coexisting with omphacitic clinopyroxenes leach sodium silicate component from the solid leaving less jadeitic pyroxenes behind. Most interestingly, the amount of sodium leached at constant temperature increases with decreasing pressure.

Comparison of the results with previous solubility studies in the system K2O-MgO-Al2O3-SiO2-H2O indicates that hydrous fluids in the mantle must be alkaline rather than silicanormative. Alkali metasomatism caused by such fluids would lead to potassium enrichment in deeper portions of the upper mantle and to sodium enrichment at shallower levels, where amphiboles become stable. This K/Na fractionation in the upper mantle may explain the generation of K-rich or of Na-rich magmas through partial melting at different depths.