SOLUBILITY BEHAVIOR OF ALKALINE EARTH AND ALKALI ALUMINOSILICATE COMPONENTS IN AQUEOUS FLUIDS IN THE EARTH'S UPPER MANTLE

Abstract

The solubility of aluminosilicate components in aqueous fluids in the 1200 to 1400°C and 0.8 to 2.0 GPa temperature- and pressure-range, respectively, has been determined for three compositions on the join CaSi4O9-Ca(Ca0.5Al)4O9 with 0, 3, and 6 mol % Al2O3. The aluminosilicate solubility, Xsil, ranges from 0.5 to 4.2 mol %. Its temperature dependence is linear and ranges between 0.7 and 4.2.10-3 mol %/°C depending on pressure. The pressure-dependence of Xsil is also positive but nonlinear. The solubility decreases with increasing Al2O3 content.The solubility was fitted to the expression:Xsil(mol %)= -2.6+/-0.7-0.20+/-0.02.XAl2O3(mol %)+0.002+/-0.001.T(°C) +0.86+/-0.04.P2(GPa)R2=0.96 This relationship is qualitatively similar to that of equivalent compositions in the Na2O-Al2O3-SiO2-H2O system (2Na+ is exchanged for 1Ca2+). However, in the latter system, the solubility is 2 to 3 times greater. The magnitude of the pressure, temperature, and composition effects in the latter system is also greater.The partial molar volume of H2O in the Ca silicate-saturated aqueous fluids, VH2Ofluid, ranges between ~17 and ~27 cm3/mol depending on pressure, temperature, and compositions so that,VH2Ofluid=20.1+/-0.7-0.0019+/-0.015XAl2O3+7.0+/-0.5.10-4 T(°C)-6.97+/-0.08P,R2=0.995 where T is in K, P in GPa, and XAl2O3 in mol %. The molar volume of silicate-saturated aqueous fluid is nearly identical to VH2Ofluid because H2O is the dominant component.The isochors of Ca-aluminosilicate-saturated aqueous fluids differ from those of pure H2O. The pressure difference at given temperature ranges between 5 and 10% in the 0.8 to 2.0 GPa and 1000 to 1400°C pressure- and temperature-range. For comparison, in the Na2O-Al2O3-SiO2-H2O system this difference is between 5 and 30% depending primarily on fluid density and pressure. This difference between the Na- and Ca- system reflects the different solubility of Ca- and Na-silicate in aqueous fluids.