The solubility of H2O and CO2 under predicted magma genesis conditions and some petrological and geophysical implications

Abstract

Available data on the solubility of H2O and CO2 in silicate melts at high pressures and temperatures reveal that (1) the solubility of H2O is several times greater than that of CO2 and (2) the solubility of H2O depends strongly on pressure and, compared to that of CO2, depends only to a small extent on temperature. It has been suggested that the species in silicate melts can be chosen so that the molar solubility of H2O may not depend on the bulk composition of the melt. The solubility of CO2, on the other hand, varies significantly with pressure, temperature, and bulk composition of the melt. Solution of volatiles at high pressure affects the structure of the silicate melts. Water depolymerizes the melt, the result being lowered viscosity. The same depolymerization is manifested in the enhanced stability of silicate minerals on the liquidus, which are less polymerized than the minerals precipitating from the same melt composition at the same pressure under volatile‐free conditions. Carbon dioxide, on the other hand, enhances polymerization of the melt, the result being increased viscosity and increased stability of liquidus minerals which are more polymerized than those that would precipitate under volatile‐free conditions. Because of the large difference in the solubilities of CO2 and H2O in silicate melts, partial melting of an (H2O + CO2)‐bearing mantle results in enrichment of H2O in the liquid, whereas the residual mantle becomes, enriched in CO2. At P ≲ 20 at the CO2 may be retained in a vapor phase. At higher pressures, carbonate is likely to be the stable phase. Therefore it would be expected that as the result of partial melting throughout geological history the upper mantle would be heterogeneous with respect to vapor components.