Water solubility in orthopyroxene: Dependence on temperature and Al content

Abstract

The water solubility in Al-Fe-Mg orthopyroxene [(Mg,Fe,Al)(Si,Al)O3: X Fe = 0.1] was investigated as a function of temperature and Al contents. Experiments were performed at 10 kbar with temperatures ranging from 800 to 1200°C under water-saturated conditions. Water contents in the (Mg,Fe)SiO3-H2O-Al2O3 system were determined using unpolarized Fourier transform infrared spectroscopy. The present results show that water solubility in Al-bearing orthopyroxene decreases systematically with temperature from approximately 1 weight % at 800°C to 568 ± 58 ppm at 1200°C and increase significantly with increasing Al2O3 contents under the same annealing temperature and pressure. Combined with published results on the dependence of hydroxyl solubility on water fugacity and pressure, the present results can be described by the relation $$C_{OH} = A(T)f_{H_2 O}^{n = 0.5} \exp \left( { - \frac{{\Delta H^{1bar} + \Delta V^{solid} P}} {{RT}}} \right)$$ where A = 0.0024 ± 0.0015 ppm/bar0.5, ΔH 1bar = −103.348 ± 9.768 kJ/mol, and solid ΔV solid = 9.2 ± 1.1 cm3/mol. This equation implies that the incorporation mechanism of water in aluminous orthopyroxene involves the isolated OH groups. Based on the experimentally established solubility model used in this study, it is suggested that water solubility decreases with increasing temperature under typical upper mantle pressure. The predicted temperature dependence of water solubility is in good agreement with the previous experimental observations in Al-bearing orthopyroxene, but the opposite dependence is observed in Al-free systems. Moreover, our estimation of the water solubility in upper-mantle minerals as a function of depth for a typical oceanic geotherm might be of potential importance in interpreting the geophysical observations.