Alkali aluminosilicate-saturated aqueous fluids in the earth's upper mantle

The solubility in aqueous fluids of peralkaline alkali aluminosilicate comp onents has been determined in the 0.8 to 2.0 GPa and 1000 to 1300 degreesC pressure and temperature range, respectively. Compositions were along the h aploandesite join, Na2Si4O9-Na-2(NaAl)(4)O-9, with 0, 3, and 6 mol.% Al2O3, denoted NS4Ax, where x represents the mol.% Al2O3. The silicate solubility in aqueous fluid is in the 3 to 20 mol.% range for NS4, 2 to 13 mol.% for NS4A3, and 1.5-8 mol.% for NS4A6 with a linear and p ositive temperature-dependence and a nonlinear and positive pressure-depend ence. The silicate solubility decreases with increasing Al2O3 content. From stepwise regression, the pressure-, temperature; and Al2O3-dependence of t he solubility can be described with the expression: X-silicate (mol.%) = 1.9 - 1.3x(A1203) (mol.%)+D08T(degreesC) + 13P(GPa) 7.3P(2). Partial molar volume of H2O in the silicate-saturated fluids, (V) over bar (fluid)(H2O), is in the range 17 to 25 cm(3)/mol. The (V) over bar (fluid)( H20) was calculated with the assumption that the activity of dissolved sili cate in aqueous fluid equals the mol fraction of dissolved silicate. Compar ed with the molar volume of pure H2O, V-H2O(o), the (V) over bar (fluid)(H2 O) values are 10 to 15% lower at 0.8 GPa and as much as 15% higher than V-H 2O(o) at 2.0 GPa. This volume difference diminishes as the system become mo re aluminous. For all compositions, the (V) over bar (fluid)(H2O) is a linear and negativ e function of pressure with beta[ = - 1/Vo(partial derivative(V) over bar ( fluid)(H2O)/partial derivativeP)(T)] similar to 0.125 GPa(-1) for NS4. The B-NS4A3 increases from 0.141 to 0.172 in the 1000 degrees to 1300 degreesC range, whereas beta (NS4A6) increases from 0.193 to 0.213 GPa(-1). The (V) over bar (fluid)(H2O) is a positive and linear function of temperat ure with thermal expansion coefficients in the range 3-8 x 10(-4) K . This thermal expansivity resembles that of pure H2O although the exact values an d the range in values for pure H2O differ from 1/Vo(partial derivative(V) o ver bar (fluid)(H2O)/partial derivativeT)(P) of silicate-saturated aqueous solution. Copyright (C) 2000 Elsevier Science Ltd.