『Abstract
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
(H2Om), OH species (OH)
and total water (H2Ot)
were determined. The diffusion profile of H2Om in Ohsawa lava extends beyond the layer observed
by optical microscope. The content of H2Om in the hydrated region is much higher than that
of OH species. Thus, the reaction from H2Om to OH appears to be little and H2Om is the dominant water species moving into the
glass during weathering. Based on the concentration profiles,
the diffusion coefficients of H2Om
(DH2Om) and H2Ot
(DH2Ot) were determined to be 2.8×10-10
and 3.4 × 10-10 μm2s-1 for Ohsawa
lava, and 5.2×10-11 and 4.1×10-11μm2s-1
for Awanomikoto lava, respectively. The obtained DH2Om
during weathering are more than 2-3 orders of magnitude larger
than the diffusion coefficient at 〜20℃ that is extrapolated from
the diffusivity data for ＞400℃. This might suggest that the mechanism
of water transport is different at weathering conditions and ＞400℃.』
1. Introduction
2. Samples and methods
3. Results
4. Discussion
5. Conclusions
Acknowledgments
References