Oxidation state of Fe and occurrence of precipitates within olivine in basaltic scoria from Kami-Kometsuka, Aso, Kumamoto Prefecture, Japan

Abstract

In this study, 1) olivine phenocrysts and associated minerals in the black and red-brown scorias from Kami-Komesuka, Aso, Kumamoto Prefecture, Japan were described in detail, 2) their chemical compositions were determined, and 3) the oxidation state of Fe in olivine phenocrysts were examined using electron microbeam method to evaluate effect of high temperature oxidation on the scoria. The Kami-kometsuka scoria cone consists of black and reddish-brown scorias. The reddish-brown scoria is covered by the black scoria. In the olivine phenocrysts from the reddish-brown scoria, Fo content attains up to 99 mol%, and Fe-rich precipitate minerals, less than few mm in size, occur on the rims and along fractures. Such occurrence of the Kami-Kometsuka scoria cone consisting of the red-brown and black scorias, and the extremely Mg-rich olivine phenocrysts and Fe-rich precitates indicate that the Kami-Kometsuka scoria subjected to high temperature oxidation after the eruption.