The temperature dependence of water solubility in enstatite

The solubility of water in pure enstatite was measured on samples synthesized under water-saturated conditions at 15 kbar and temperatures ranging from 700 to 1,100°C. Polarized FTIR measurements on millimetre-sized, clear crystals showed that water solubility increases strongly with temperature, from 101 ppm by weight at 700°C to 269 ppm by weight at 1,100°C. The position and shape of the infrared bands hardly changes with temperature, with one notable exception: a band close to 3,380 cm^–1 is present in samples synthesized between 700 and 1,000°C, while this band is absent from samples synthesized at 1,100°C. This effect appears to be very reproducible and points towards a slight change in the crystal structure of enstatite between 1,000 and 1,100°C at 15 kbar. The water solubility data of this study as well as those of Rauch and Keppler (Contrib Mineral Petrol 143:525–536, 2002) can be reproduced by the equation where K is water solubility, is water fugacity, A is 0.01354 ppm/bar, V^solid=12.1 cm³/mol is the volume change of enstatite during incorporation of water, and H^{1 bar}=-4,563 J/mol is the reaction enthalpy at 1 bar. This equation predicts the following behaviour of water solubility in enstatite as a function of pressure and temperature: (1) water solubility increases with pressure up to a maximum around 80 kbar; (2) water solubility decreases with temperature at 1 bar; and (3) water solubility increases with temperature between 10 and 100 kbar. If the observed temperature dependence for enstatite were representative for other upper mantle minerals as well, it would have the following implications: (1) Lateral temperature gradients in the upper mantle could cause major variations in water contents at the same depth; in particular, hot mantle plumes may scavenge water from the surrounding shallow upper mantle. (2) The scavenging of water by hot plumes could be a major factor in increasing the mobility of plumes. (3) The predicted temperature dependence of water solubility at the base of the upper mantle may allow plumes to bypass the transition zone water filter postulated by Bercovici and Karato (Nature 425:39–44, 2003).