Fe–rich olivine from an andesite dike in Miocene Shitara volcanic rocks, central Japan: a revised relationship between Mg/Fe ratio and Raman spectrum in olivine

抄録

Olivine is a major mineral in ultramafic and mafic rocks. Due to the higher Mg/Fe partition coefficient of olivine than the coexisting phases, the occurrences of Mg–poorer olivine grains, especially with Mg#[= Mg/(Mg + Fe²⁺)] = 0.2–0.6, are rarely reported, and thus, their spectroscopic data are limited. Fe–rich olivine phenocrysts showing compositional zoning with Mg# = ~ 0.5 (core) and ~ 0.3 (rim) and microphenocrysts with Mg# = ~ 0.4 (core) and ~ 0.2 (rim) occur in basaltic trachyandesite of Miocene Shitara volcanic rocks in central Japan. These olivine grains were investigated by Raman spectroscopy. Combining our data with the published values, we have revised the equation for Mg#–Raman spectrum relationship proposed by Mouri and Enami (2008) as follows: Mg# = 0.005446ω ² − 0.20259ω + 1.8442 (correlation coefficient r ² = 0.984), where ω is the difference between the doublet peak positions (κ₂ − κ₁).