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 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: X sil(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, V ̄ H 2O fluid=20.1±0.7−0.0019±0.015X Al 2O 3 +7.0±0.5·10 −4T(°C)−6.97±0.08P, R 2=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.