Water solubility in orthopyroxene: Dependence on temperature and Al content

Abstract

The water solubility in Al-Fe-Mg orthopyroxene [(Mg,Fe,Al)(Si,Al)O 3 : 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)SiO 3 -H 2 O-Al 2 O 3 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 Al 2 O 3 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 relationwhere A = 0.0024 ± 0.0015 ppm/bar 0.5 , 1bar H = −103.348 ± 9.768 kJ/mol, and solid V = 9.2 ± 1.1 cm 3 /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. orthopyroxene, water solubility, infrared spectroscopy, asthenosphere Citation:Zhang B H, Matsuzaki T, Wu X P. Water solubility in orthopyroxene: Dependence on temperature and Al content. Chin Sci Bull, 2013Bull, , 58: 38953902, doi: 10.1007 The presence of water or hydroxyl defects in nominally anhydrous minerals (NAMs) has a dramatical influence on their geophysical properties such as solidus temperature [1], electrical conductivity [2-6] and viscosity [7][8][9], as well as on mantle dynamics such as melt generation [10] and mantle convection [7]. Critical to understanding the role of water in the Earth's mantle, a key step is to estimate the distribution of water inside the mantle and the incorporation mechanism of hydrogen in mantle minerals by combining experimentally determined mineral properties with geophysical observations. Two principal approaches can provide significant constraints on the water content in the Earth's interior. One approach is water solubility experiments in mantle minerals [11][12][13], and the other infers the water content based on the electrical conductivity of the NAMs [2-6,14-19].Orthopyroxene is considered to be one of the main constituent minerals in the Earth's upper mantle and is believed to constitute approximately 20-40 vol% [20]. Previous studies indicate that the water solubility in pure MgSiO 3 enstatite increa...

Origins of water content variations in the suboceanic upper mantle: Insight from Southwest Indian Ridge abyssal peridotites

“…Experiments have shown that the water incorporation in pyroxene also depends on P-T conditions [Rauch and Keppler, 2002;Mierdel and Keppler, 2004;Mierdel et al, 2007;F erot and Bolfan-Casanova, 2012;Zhang et al, 2013] and water activity [Stalder et al, 2008]. In addition, oxygen fugacity is also believed to control the incorporation of water in mantle peridotite NAMs [Peslier et al, 2002;Peslier, 2010;Yu et al, 2011].…”

Effect of rare earth element Gd on the mechanical Properties of AZ91D alloy

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