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 (OpxOl -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 Fo 68.5−74.9 cores and Fo 64.9−72.9 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: 3+ -ratio, small amounts of Fe 3+ (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, Fe 3+ 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.

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

“…The precipitates in olivine phenocrysts in the Kasayama oxidized scorias are similar to those previously described in oxidized lavas by Ejima et al (2013). Precipitates consisting of vermicular rods and radial ribs of hematite, typical of the Kasayama red-brown scoria, as well as the precipitate assemblage of hematite and enstatite, are characteristically generated by oxidation at subsolidus temperatures (Haggerty and Baker, 1967;Blondes et al, 2012;Ejima et al, 2013). Well-defined crystallographic relationships between host olivine and precipitate hematite crystals have been reported previously in oxidized olivine containing precipitates elsewhere (Champness, 1970;Kohlstedt and Vander, 1975;Mckernan and Carter, 1989;Hwang et al, 2008;Blondes et al, 2012;Ejima et al, 2015).…”

“…The precipitates in olivine phenocrysts in the Kasayama oxidized scorias are similar to those previously described in oxidized lavas by Ejima et al (2013). Precipitates consisting of vermicular rods and radial ribs of hematite, typical of the Kasayama red-brown scoria, as well as the precipitate assemblage of hematite and enstatite, are characteristically generated by oxidation at subsolidus temperatures (Haggerty and Baker, 1967;Blondes et al, 2012;Ejima et al, 2013).…”

“…13 in Ejima et al, 2015). However, the olivine phenocrysts in the scoria tend to have a euhedral morphology and are less variable in shape than those in lava described by Haggerty and Baker (1967) and Ejima et al (2013Ejima et al ( , 2015. This feature is also documented in oxidized scoria of the Otsukayama scoria cone on Daikon-jima Island, Shimane Prefecture (Ejima and Akasaka, 2016), and the Kometsuka scoria cone at Aso volcano, Kumamoto Prefecture (Ejima, 2015).…”

“…Conversely, the volume of the precipitates is smaller in the red-brown scoria than in the red-brown lava from Kasayama. This difference may be attributed to the total Fe content of olivine phenocrysts, which are 17-19 wt% in black scoria (Table 1) and 23-31 wt% in black lava from Kasayama (Ejima et al, 2013).…”

Oxidation States of Fe in Constituent Minerals of a Spinel Lherzolite Xenolith from the Tariat Depression, Mongolia: The Significance of Fe3+ in Olivine

“…In fact, Fe 2+ ions diffuse to the edge of the olivine grain, as Mackwell [45] demonstrated in an annealing experiment of fayalite. This mechanism seems to apply reasonably to the generation of Fe 3+ within olivine in the upper mantle if the redox conditions are suitable for oxidation of Fe 2+ to Fe 3+ in the olivine, as well as the case of high temperature oxidation of olivine in lavas and scoria [46][47][48][49].…”