熊本県阿蘇市上米塚産スコリアのかんらん石斑晶における鉄酸化数と噴出後の高温酸化の影響

抄録

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 Fo₈₇ cores and Fo₆₈ rims and reverse zoning with Fo₆₆ cores and Fo₇₀ rims, whereas those in the red-brown scoria reach 99 mol% Fo. By applying the relationship between FeL_β/FeL_α-intensity ratios and Fe³⁺/∑Fe, the Fe³⁺/∑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 Fe³⁺ 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.