Quantitative determination of Fe site occupation in olivine using electron channeling X-ray spectroscopy: A preliminary study of its relation to cooling history

The distribution of Fe between the M1 and M2 sites in olivine deviates slightly from equality depending on its thermal history. In this study, Fe site fluctuations during cooling from high temperatures (HTs) were investigated using high angular resolution electron channeling X-ray spectroscopy (HARECXS). HARECXS analyses successfully determined Fe site occupancies while minimizing impurity effects from Mn, Ni, and Ca. The measured distribution coefficients KDFe (=[XFeM1/XMgM1]/[XFeM2/XMgM2], where Xelementsite represents a site occupancy) after HT holding–cooling experiments were higher than 1, indicating a slight preference of Fe for the M1 site. Morever, KDFe increased with increasing cooling rate. Among the investigated samples, a fresh olivine extracted from a peridotite from the Horoman complex, Japan, showed a KDFe value sufficiently lower than 1; however, subsequent HT holding and rapid cooling experiments increased KDFe to values similar to those of the other samples—the San Carols xenolith olivine from the United States and the olivine phenocryst in gray pumice from the 2021 Fukutoku-Oka-no-Ba eruption in Japan. Empirical kinetic parameters for Fe–Mg exchange between M1 and M2 for the studied natural olivines were determined, allowing the simulation of Fe fluctuations during cooling. Using the empirical kinetic parameters and the KDFe measurement results, potential cooling paths of the natural olivines were evaluated. These results provide proof-of-concept for a new analytical strategy based on HARECXS, which has the potential to be applied to evaluations of the past cooling histories of olivine.