Aluminosilicate Solubility in Aqueous Fluids in the Earth's Upper Mantle

Bjorn O. Mysen (mysen@gl.ciw.edu)¹ & Kevin Wheeler²

¹ Geophysical Laboratory, 5251 Broad Branch Rd., NW, Washington DC 20015, USA

² Dept. Geol. Sci., Brown University, Providence RI 02912, USA

The solubility of peralkaline alkali aluminosilicate components in aqueous fluids has been determined in the 0.8-2.0 GPa and 1000°-1300°C pressure and temperature range, respectively. Silicate compositions were along the haploandesite join, Na₂Si₄O₉ - Na₂(NaAl)₄O₉, with 0, 3 and 6 mol% Al₂O₃ denoted NS4Ax, where x represents the mol% Al₂O₃.

The aluminosilicate solubility is in the 3-20 mol% range for NS4, 2-13 mol% for NS4A3, and 1.5-8 mol% for NS4A6 with a linear and positive temperature-dependence and a nonlinear and positive pressure-dependence. From stepwise regression, the pressure-, temperature-, and Al₂O₃-dependence can be described with the expression:

X_Al-silicate (mol%) = 1.9 - 1.3XAl₂O₃ (mol%) + 0.008T(°C) - 13P(GPa) + 7.3P². (1)

Partial molar volume of H₂O in the Al-silicate saturated fluids is in the range 25-17 cm³mol%. Compared with pure H₂O, these values are 10-15% lower than the V°_H2O at 0.8 GPa and as much as 15% higher than V°_H2O at 2.0 GPa although this difference diminishes as the system become more aluminous. Thus, H₂O in Al-silicate saturated aqueous solution is considerably less compressible than pure H₂O.

For all compositions, the partial molar volume of H₂O is a linear and negative function of pressure with ß (=1/V_o(V_H2O/P) _T ~ 0.125 GPa^-1 for NS4. The ß_NS4A3 increases from 0.141 to 0.172 in the 1000°-1300°C range, whereas ß_NS4A6 increases from 0.193 to 0.213 GPa^-1.

The partial molar volume of H₂O in Al-silicate-saturated aqueous fluid is a positive and linear function of temperature with thermal expansion coefficients in the range 3-8x10^-4 K^-1. This thermal expansivity resembles that of pure H₂O although the exact values and the range in values for pure H₂O differ from that of H₂O in Al-silicate-saturated solution.

Structural and Thermodynamic Properties of the Synthetic Tremolite-Magnesiohornblende Solid Solutions

Jens Najorka (jnajorka@gfz-potsdam.de), Matthias Gottschalk & Wilhelm Heinrich

GeoForschungsZentrum, Potsdam, D-14473, Germany

The Mg-Tschermaks substitution Mg²⁺ + Si⁴⁺ = Al³⁺ + Al³⁺ is often observed in amphiboles. In order to use this as a petrogenetic indicator we first need structural and thermodynamical information from simplified model systems. Therefore, the tremolite-magnesiohornblende join was experimentally investigated using 6 different exchange reactions in the CMASH system between 600-800°C and 2-20 kbar. The experimental results are best interpreted using ideal mixing in Al-tremolite with random distribution of Mg-Al on M2-octahedra coupled with a Si-Al substitution on the nearest T1-tetrahedra. A value for the standard entropy S°_Hb = 537.0 ± 11.2 J/K/mol and standard enthalpy of formation H°_Hb = -12445.3 ± 14.0 kJ/mol were derived for the magnesiohornblende endmember.

IR investigations in the range of the OH-stretching vibration can also be interpreted using the assumption of Al-ordering on M2-octahedra and T1-tetrahedra. Al-tremolites show 3 main band-systems at 3672 (I), 3652 (II) and 3620 (III) cm^-1. The intensity of the band-system (I) decreases whereas intensities for band-systems (II) and (III) increase with increasing Al-content in tremolite. The band-systems are attributed to configurations with 0 Al (I), 1 Al (II) and 2 Al (III) on the nearest 4 T1-sites around the OH-group. The band broadening of all band systems are the result of different configurations of Mg-Al and Si-Al on the nearest 4 M2-sites and 4 T1-sites around the OH-group. Configuration probabilities were calculated assuming random distribution of Mg-Al on M2 coupled with Si-Al substitution on the nearest T1. The calculated probabilities and observed (absorbance corrected) intensities of the OH-band systems (I), (II) and (III) are in good agreement. Both thermodynamic and IR investigations indicate, that a random distribution of Mg-Al on M2-site coupled with a Si-Al substitution on the nearest T1-sites is a suitable model for Al-ordering in Al-tremolite.