Hydration of rhyolitic glass during weathering as characterized by IR microspectroscopy

The mechanism and rate of hydration of rhyolitic glass during weathering were studied. Doubly polished thin sections of two rhyolites with different duration of weathering (Ohsawa lava: 26,000 yr, Awanomikoto lava: 52,000 yr) were prepared. Optical microscope observation showed that altered layers had developed along the glass surfaces. IR spectral line profile analysis was conducted on the glass sections from the surface to the interior for a length of 250 μm and the contents of molecular H₂O (H₂O_m), OH species (OH) and total water (H₂O_t) were determined. The diffusion profile of H₂O_m in Ohsawa lava extends beyond the layer observed by optical microscope. The content of H₂O_m in the hydrated region is much higher than that of OH species. Thus, the reaction from H₂O_m to OH appears to be little and H₂O_m is the dominant water species moving into the glass during weathering. Based on the concentration profiles, the diffusion coefficients of H₂O_m(D_{H2 Om}) and H₂O_t(D_{H2 Ot}) were determined to be 2.8 × 10^-10 and 3.4 × 10^-10 μm² s^-1 for Ohsawa lava, and 5.2 × 10^-11 and 4.1 × 10^-11 μm² s^-1 for Awanomikoto lava, respectively. The obtained D_{H2 Om} during weathering are more than 2-3 orders of magnitude larger than the diffusion coefficient at ∼20 °C that is extrapolated from the diffusivity data for >400 °C. This might suggest that the mechanism of water transport is different at weathering conditions and >400 °C.