Effects of Al content on water partitioning between orthopyroxene and olivine: Implications for lithosphere-asthenosphere boundary

To investigate the partitioning coefficient of water between orthopyroxene (Opx) and olivine (Ol) (D_(Opx/Ol)) under low-water concentrations (3 ~ 387wt ppm) similar to the Earth's mantle conditions, high-pressure experiments have been conducted at pressures of 1.5-6 GPa and a temperature of 1573 K. The experiments were performed with Kawai-type multi-anvil and piston-cylinder apparatus by using starting materials of natural Ol and synthetic Opx with various Al contents. The water contents were obtained with a vacuum type Fourier transform infrared spectrometer (FT-IR6100, IRT5000). IR-spectra of Ol and Al-bearing Opx in this study are similar to those obtained by high-pressure experiments (Bai and Kohlstedt, 1993) and natural rocks (Grant et al., 2007), respectively. It is believed that broad bands in IR spectra of natural Opx are due to effect of crystal distortion by large Al substitution. On the contrary, IR-spectra of Al-free Opx are not consistent with those reported by Rauch and Keppler (2002) likely because of the large difference of water fugacity. D_{(Al-free Opx/Ol)} is ~1 at all pressure conditions or decreases with increasing pressure. However, the water contents of Al-bearing Opx are significantly larger than those of Ol at the same conditions. In addition, the effect of Al concentration in Opx on D_(Opx/Ol) becomes larger with increasing pressure. The high Al content in Opx significantly increases D_(Opx/Ol) and the trend increases with increasing pressure. D_(Opx/Ol) drops sharply at the pressure at which the Al concentration of Opx becomes nearly 0 in the Earth's mantle conditions. This conclusion indicates that viscosity of the upper mantle decreases sharply at depths larger than those in which orthopyroxene contains no Al. The dramatic change of D_(Opx/Ol) may explain the lithosphere-asthenosphere boundary beneath oceans and continents.