New Data on Refractive Indices of Holocene Tephra Glass in Hokkaido with Special Reference to the Hydration Issue.

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It has been known that incomplete hydration in volcanic glass influences its refractive indices to a relatively severe extent. Holocene tephra glass thus shows remarkably wide ranges of the refractive index. The refractive indices of hydrated Tarumae-a tephra (Ta-a: AD1739), Tarumae-b tephra (Ta-b: AD1667), and Tarumae-c tephra (Ta-c: ca. 3ka BP) glass vary widely: standard deviations range from s=0.0033-0.0036. Thus, the refractive index of incompletely hydrated glass is not a practical indicator for Holocene tephra identification.Hydration near the surface of tephra glass shards can be removed by 400°C 12-hour annealing. The refractive indices of dehydrated (annealed) Ta-a, Ta-b, and Ta-c glass are 0.006-0.014 lower than those of incompletely hydrated (non-treated) glass. Standard deviations of the refractive indices of dehydrated glass are small (s=0.0014-0.0018), in contrast to those of non-treated glass.Using the 400°C 12-hour annealing procedure, the refractive indices of twelve Holocene tephras in Hokkaido were obtained. Most of the Holocene tephras in Hokkaido can be easily identified based on the refractive index of dehydrated glass in addition to some other petrographic properties. It is strongly recommended that the refractive index of dehydrated glass is measured for Holocene tephra identification.
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