Oxidation state of Fe in olivine in a lherzolite xenolith from Oku district, Oki-Dogo Island, Shimane Prefecture, Japan

Abstract

The oxidation state and distribution of Fe in olivine in a lherzolite xenolith from the Oku district, Oki-Dogo Islands, Japan, were investigated. A lherzolite xenolith collected from a volcanic neck of alkali olivine basalt contains olivine, orthopyroxene, clinopyroxene, and a small amount of spinel. Anhedral anorthite grains less than 0.01 mm across occur as inclusions in clinopyroxene. The olivine grains are nearly homogeneous in composition, with Fo contents of 80.7-81.9 mol%. Pure olivine grains for X-ray diffraction and ⁵⁷Fe Mössbauer analyses were separated from the fresh core of the lherzolite xenolith under a binocular microscope. The Mössbauer spectrum of the separated olivines consists of two doublets assigned to Fe²⁺ at the M1 and M2 sites and a doublet attributed to Fe³⁺ at the M2 site. The populations of Fe³⁺ in the olivine derived from the Mössbauer method and FeL_β/FeL_α-intensity ratios are 0.02 and 0.03 atoms per formula unite (apfu), respectively. The site occupancies of Fe in the M1 and M2 sites refined using X-ray Rietveld method are 0.151 and 0.206 apfu, respectively. The chemical formula of the olivine based on the refined Fe populations at the octahedral M1 and M2 sites and oxidation state of Fe determined using Mössbauer method is [^M2(Mg²⁺_0.79Fe²⁺_0.19Fe³⁺_0.02)^M1([]_0.01Mg_0.85Fe²⁺_0.14)]_Σ2.00SiO₄.