Abstract
The preferential enrichment of noble gases in amorphous materials are discussed based on Xe in amorphous silica and Ne in natural glasses. For studying the enrichment of Xe in silica, silica samples generated by the leaching of rocks in acid geothermal waters were used in addition to those synthesized by the precipitation on some materials in neutral geothermal waters. Oxygen isotope fractionation factors between these amorphous silicas and geothermal waters indicate that silica samples from the acid waters also are generated in isotopic equilibrium with the water by the processes of dissolution and redeposition. Xe enrichments are observed in both silica samples from neutral and acid waters, in spite of no enrichments of other noble gases. The lower the precipitating temperature of silica, the higher the content of Xe in silica become. These Xe gases can be released at high temperature of 1100-1700°C. These observations seem to suggest that Xe is not adsorbed on amorphous silica but occluded in the framework or structure of the silica, of which the space is suitable for atomic size of only Xe in noble gases. On the other hand, Ne excess is a common feature generally seen in natural glasses such as obsidian, tektite, Darwin glass and submarine glasses. The release of Ne from these glasses can be taken place in the low temperature of 400°Cto 500°C, with a exception of tektite being degassed mainly at 1600°C. It is conceivable that the occurrence of Ne excess can be explained by diffusion into the glasses from the atmosphere based on the Ne isotopic data. The Ne excess has been produced by gas-solid interaction after the glasses solidified, because of high diffusivity of Ne. These Ne and Xe enrichments in silica and natural glasses and the difference between release patterns of noble gases from them indicate that only gas having suitable atomic or molecular size for the the frameworks and structures of materials can be preferentially occluded in them. Though noble gas enrichments have been reported only for the amorphous materials such as silica and glasses, it may be expected that such preferential enrichments of noble gases make a contribution to study the chemical and physical effects caused by gases occluded in crystalline materials.
