島根県大根島大塚山赤褐色スコリア中のMgに富んだかんらん石斑晶の成因

Occurrence and chemical compositions of olivine phenocrysts in red-brown and black scorias from Otsukayama scoria cone, Daikonjima, Shimane Prefecture, Japan, were investigated to evaluate effect of high temperature oxidation on the scoria. The Otsukayama scoria cone consists of black and red-brown scorias of alkaline olivine basalt. Olivine phenocrysts within the black scoria lack precipitate minerals, but alteration minerals occur along fractures and on the rims of olivine phenocrysts. Olivine phenocrysts in the red-brown scoria contain abundant cryptocrystalline precipitates including hematite and enstatite, which are characteristic in scorias and lavas subjected to high-temperature oxidation. Olivine in the black scoria has normal zoning with Fo_<81.2> cores and Fo_<80.3> rims, whereas those in the red-brown scoria reach 97.7 mol% Fo. The cryptocrystalline precipitates and extremely high Fo contents in olivine phenocrysts in the red-brown scoria are due to high temperature oxidation.