ワイラカイトの骨格酸素同位体比の測定法

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Dehydration techniques were developed for the analysis of isotopic ratios of framework oxygen of wairakite, one of calcium zeolites often encountered in geothermal systems. Channel water in wairakite were separated from aluminosilicate framework by dehydration in vacuum at 300°, 400°, 450°, 500°, 550°, 650°, 750°, 850°, and 950°C, and by stepwise heating at temperatures from 300° to 700°C. The oxygen isotopic analyses of the separated channel water and the residual aluminosilicate framework of wairakite indicated that dehydration at temperatues higher than 400°C is accompanied by isotopic exchang between the framework oxygen and dehydrating water vapor. The isotopic exchange during the high temperature dehydration makes the δ18O of framework oxygen lower and that of channel water higher than those obtained by dehydration at 300°C. These results are consitent with dehydration behavior of wairakite under vacuum that the maximum rate of dehydration of channel water is attained at about 400°C. Consequently it is recommended to dehydrate wairakite at a temperature as low as possible in order to avoid the effect of the isotopic exchange. Time required to attain complete dehydration becomes longer with lowering the temperature of dehydration. To compromise these conflicting effects, the optimum conditions of dehydration have been found that most of the channel water is dehydrated at 300°C for 24 hours, followed by stepwise heating for additional 17 hours up to 700°C. We obtained a better than ±0.1 reproducibility for the framework oxygen isotopic determinations with this technique.
日本質量分析学会の論文

ワイラカイトの骨格酸素同位体比の測定法

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