Precipitates within olivine phenocrysts in oxidized andesitic scoria from Kasayama volcano, Hagi, Japan

Abstract

Mineral precipitates within olivine grains are a sensitive recorder of the oxidation conditions of scoria. The crystallization process of precipitates within olivine phenocrysts in andesitic scoria from Kasayama volcano, Hagi, Yamaguchi Prefecture, Japan was investigated. Electron microprobe analysis and Raman spectroscopy were used for mineral identification and electron back–scattered diffraction to determine the crystallographic orientation of the precipitate minerals and host olivine phenocrysts. The scoria in the interior of the Kasayama scoria cone is red–brown, and the outer surface of the cone is black or black with red–brown tint. The olivine phenocrysts (Fo_79–81) within the black scoria lack precipitate minerals, but those in the black scoria with red–brown tint (Fo_82–85) contain small amounts of precipitates at their rims, and those in the red–brown scoria (Fo₉₉) contain abundant cryptocrystalline precipitates, including hematite and enstatite. Vermicular rods of hematite and enstatite form symplectite zones on the rims, and symplectite domains in the cores of the phenocrysts. The host olivine and the hematite precipitates have a crystallographic relationship of [100]_Ol//[0001]_Hem, [010]_Ol//[1010]_Hem, and [001]_Ol//[2130]_Hem, which is characteristic of olivine and precipitate minerals generated by high–temperature oxidation. The maximum oxidation temperature is estimated to be >800 °C. High temperature oxidizing conditions may have been maintained in the inner wall of the scoria cone because the scoria that erupted early in the sequence was deposited in the presence of air and was subsequently covered by black scoria making up the outer wall of the cone.