Solubility behavior of alkaline earth and alkali aluminosilicate components in aqueous fluids in the Earth's upper mantle
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 CaSi 4O 9-Ca(Ca 0.5Al) 4O 9 with 0, 3, and 6 mol % Al 2O 3. The aluminosilicate solubility, X sil, 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 X sil is also positive but nonlinear. The solubility decreases with increasing Al 2O 3 content. The solubility was fitted to the expression: Xsil(mol %) = -2.6±0.7-0.20±0.02·X Al 2O 3(mol %)+0.002±0.001·T (°C)+0.86±0.04·P 2(GPa)R 2=0.96 This relationship is qualitatively similar to that of equivalent compositions in the Na 2O-Al 2O 3-SiO 2-H 2O system (2Na + is exchanged for 1Ca 2+). 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 H 2O in the Ca silicate-saturated aqueous fluids, V̄ H 2O fluid, ranges between ∼17 and ∼27 cm 3/mol depending on pressure, temperature, and compositions so that, V̄H 2O fluid=20.1±0.7-0.0019±0.015X Al 2O 3+7.0±0.5·10 -4T(°C)-6.97±0.08P, R2=0.995 where T is in K, P in GPa, and X Al 2O 3 in mol %. The molar volume of silicate-saturated aqueous fluid is nearly identical to V̄ H 2O fluid because H 2O is the dominant component. The isochors of Ca-aluminosilicate-saturated aqueous fluids differ from those of pure H 2O. 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 Na 2O-Al 2O 3-SiO 2-H 2O 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.