The chemical compositions and oxidation states of Fe in olivine phenocrysts in scorias from Kami-Kometsuka, Aso, Kumamoto Prefecture, Japan, were analyzed to evaluate effect of high temperature oxidation on the scoria. The rocks in the interior of the Kami-Kometsuka scoria cone are red-brown and weakly welded, whereas black scoria occurs on the upper zone of the cone. Olivine phenocrysts within the black scoria lack precipitate minerals, but those exposed on the voids contain small amounts of precipitates at their rims. Olivine phenocrysts in the red-brown scoria contain abundant cryptocrystalline precipitates including hematite, magnetite and enstatite. Olivines in the black scoria have normal zoning with Fo
87 cores and Fo
68 rims and reverse zoning with Fo
66 cores and Fo
70 rims, whereas those in the red-brown scoria reach 99 mol% Fo. By applying the relationship between Fe
Lβ/Fe
Lα-intensity ratios and Fe
3+/∑Fe, the Fe
3+/∑Fe of the olivine phenocrysts in the black scoria were determined to be 0-1(2)% at the cores and 3(2)% at the rims. The Fe
3+ at the rims of the olivine phenocrysts in the black scoria and the cryptocrystalline precipitates and extremely high Fo contents within olivine phenocrysts in the red-brown scoria are due to high temperature oxidation.
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