THE SOLUBILITY OF QUARTZ IN H2O IN THE LOWER CRUST AND UPPER-MANTLE

The solubility of quartz in H2O has been determined experimentally from 5 to 20 kb and 500 to 900-degrees-C. The results double the pressure range over which the molality of aqueous silica (m(SiO)2(aq) has been determined and lead to more accurate estimates of quartz solubility in H2O below 5 kb because of the rapid-quench methods employed. At constant temperature, log M(SiO)2(aq) increases with increasing pressure and (partial derivatives log M(SiO)2(aq)/partial derivatives P)T decreases with increasing pressure. Comparison of the new data with previous low-pressure experiments demonstrate that isothermal values of log M(SiO)2(aq) increase linearly with increasing log rho(H)2O between 200 and 900-degrees-C. This observation was used to derive the following expression for the equilibrium constant (K) of the reaction quartz = SiO2(aq): log K = 4.2620 - 5764.2/T + 1.7513 x 10(6)/T2 - 2.2869 x 10(8)/T3 + [2.8454 - 1006.9/T + 3.5689 x 10(5)/T2] log rho(H)2O, where log K = log M(SiO)2(aq). The equation agrees well with previous results, while accurately reproducing measured quartz solubilities over a much wider range in pressure and temperature, from 25-degrees-C and 1 bar to the conditions of this study. If the isothermal variation of log M(SiO)2(aq) with log rho(H)2O is assumed to be linear, the results can be extrapolated to > 20 kb. The equation allows evaluation of aqueous silica transport in Barrovian metamorphic belts, subduction zones, and metasomatized magma source-regions in the mantle.