Oxidation states of Fe and precipitates within olivine from orthopyroxene-olivine-clinopyroxene andesite lava from Kasayama volcano, Hagi, Yamaguchi, Japan

Abstract

Oxidation states of Fe and precipitates within olivine in orthopyroxene-olivine-clinopyroxene andesite (Opx-Ol-Cpx andesite) lava from Kasayama volcano, Hagi, Yamaguchi Prefecture, were investigated to reveal the oxidation process of the lava at high temperatures, using electron microprobe analysis, Raman spectroscopy and transmission electron microscopy. Although the Opx-Ol-Cpx andesite lava is generally black in color, in places it has a red-brown surface and reddish-black subsurface. Olivines from the black lava have normal zoning with Fo68.5-74.9 cores and Fo64.9-72.9 rims. Olivine in the black lavas with red-brown tint and red-brown lava contains precipitates of Tirich hematite, hematite, magnesioferrite and enstatite, and tends to be Mg-rich (cores: Fo74.1-78.6; rims: Fo76.4-83.8) in comparison with black lava. Stronger red coloration of the lavas is related to greater volume of cryptocrystalline precipitates within olivine. This results in increased Mg contents in olivine. Olivines in red-brown lava are extremely Mg-rich (Fo91.0-95.4). By applying the correlation between FeLβ- and FeLα-intensity ratio and Fe2+/ Fe3+-ratio, small amounts of Fe3+ (0.05 atoms per formula unit at maximum) were invariably detected in olivine from the black lava with red-brown tint. Even in olivine in the black lava, Fe3+ was detected in the rims, although Fe is ferrous in the cores. These facts on the chemical compositions and oxidation state of Fe within olivine phenocrysts and the occurrence of vermicular rod-form titanohematite and magnesioferrite precipitates in olivine provide the evidence for high temperature oxidation, at temperatures above 800 °C.