Identification of Particular Tephras by Means of Refractive Indices of Orthopyroxenes and Hornblendes

A Fundamental Study of Tephrochronology

Abstract

This paper deals with my basic study on tephrochronology-an investigation of the availability of refractive indices of orthopyroxenes and hornblendes, contained commonly in the Japanese Quaternary tephras, for exact identification of particular tephras (pumices) which here mean widely spreading tephras so as to be used as stratigraphical key beds.
So far as I am concerned, the phase-contrast technique is the most desirable and efficient in the determination of accurate and semi-statistic range of the highest refractive indices of cleavage flakes of phenocrysts. Some examples of refractive indices measured by this method are shown in Figs. 2-9.
The highest principal refractive indices (γ) of orthopyroxenes contained in the great majority of the Quaternary pumices in Japan are almost invariably within a range of 1.700-1.720 (Fs30-Fs45), except for the special case of Shikotsu tephras in Hokkaido (Fig. 5), while the refractive indices n₂ on hornblende cleavage flakes fall within a rather wide range of 1.665-1.695 (Fig. 9).
In most cases refractive indices of orthopyroxenes and hornblendes in peculiar tephras, however, are confined respectively within a rather definite range of their own. For instance, the notable “Pm-I”(pumice layer), which originated from Ontake volcano in Shinshu, shows a good agreement in the refractive index n₂ of hornblende cleavage flakes in samples collected from the wide area from Shinshu to South Kanto, regardless of their degrees of argillization of glass component of pumices at each locality (Figs. 10-12). Therefore, refractive indices of characteristic minerals can be a great help to identification of some kinds of particular tephra layers.
In some exceptional cases, the top unit of pumice layers such as “Tokyo pumice” in South Kanto and “Imaichi pumice” in North Kanto, characteristically contains En-richer hypersthenes than the lower units (Figs. 6, and 2). It suggests that for some reason somewhat layered magma in chemical composition may have existed in the magma column or chamber prior to the eruption.
On the other hand, in a series of pumice falls which deposited intermittently at intervals of 10-10³ years as an approximate estimation throughout an active stage (10³-10⁵y.) of volcanism, the pumices in later stages abound in Fs-richer orthopyroxenes than the pumices in earlier stages. This case is shown most typically by the tephras of Shikotsu volcano in Hokkaido (Figs. 5, and 8). The same tendency seems to be generally evident through the activity of other volcanoes (Fig. 7). This tendency not only helps the distinction of the pumices with similar mineral composition by making use of the properties of orthopyroxenes, but also provides an interesting subject for volcanology.