Atsushi Yasuda1 (yasuda@magma.eri.u-tokyo.ac.jp),
Toshitsugu Fujii1 (fujii@magma.eri.u-tokyo.ac.jp)
1 Earthq. Res. Inst., Univ. Tokyo, Japan
To constrain the pressure dependence of water diffusivity in silicate melts, diffusion experiments were carried out at 2 and 3 GPa and 1450｡C in basaltic and rhyolitic melts. The total-water concentration profile through the quenched samples were determined with a micro-focused Fourier-transform infrared spectrometer. A window aperture of 27 mm x 27 mm is used to delimit the IR beam. Total water concentration was determined from the net peak heights of 2.8 mm absorption band (fundamental stretch vibrations of molecular H2O and OH).Contrary to the previous studies at low pressures, water diffusivity at 3 GPa is not sensitive to the water content, and the measured total water-concentration profiles can be fitted by a single constant diffusion coefficient for total water. Water diffusivities are 6.2+-1 x 10-10 m2/s in the basaltic melt (0.14 - 2.3 wt% water) and 2+-1 x10-10 m2/s in the rhyolitic melt (1.15 - 3.2 wt% water) at 3 GPa and 1450｡C. On the other hand, water diffusivity at 2 GPa and 1450｡ C in the basaltic melt depends slightly on the water-concentration, 2+-2 x 10-10m2/s at 0.14 wt% water and 8+-2 x 10-10 m2/s at 2 wt% water. This concentration dependence is, however, smaller than the value expected from previous studies at pressure lower than 1 GPa. Provided that both molecular H2O and OH are stable water species in silicate melts under pressures, the extinction of concentration dependence of total water diffusivity found in the present study suggests that the diffusivity of molecular H2O decreases with increasing pressure and becomes comparable to the diffusivity of OH at the experimental pressure range.