Effect of glass composition on the experimental hydration of obsidian between 110 and 230(degree)C

摘要

Chemically characterized high-silica natural glasses were reacted in water vapor atmospheres at 100% relative humidity at temperatures between 110 and 230(degrees)C for up to 400 days. Birefringent hydration layers formed on the glass surfaces and increased in thickness as a function of the square root of time for all glasses, under all experimental conditions, a dependence consistent with a molecular water diffusion reaction mechanism. AEM, SIMS, FTIR and optical microscopy analyses of the birefringent hydration layers further support a molecular water diffusion reaction mechanism. The rate of hydration and its temperature dependence can be quantitatively related to the logarithm of the intrinsic water content of the unreacted glass. This quantification of the process permits estimates of water diffusion coefficients in rhyolitic glasses as a function of temperature and is statistically more precise than previously proposed indices for predicting water diffusion in obsidian. These results may allow obsidian hydration dating to gain more widespread acceptance as an absolute dating technique.