Determination of Fe3+/ΣFe of olivine-hosted melt inclusions using Mössbauer and XANES spectroscopy

Abstract

Iron speciation is linked to oxygen fugacity; hence the Fe3+/ΣFe ratio of glasses can reveal the oxygen fugacity of the system where they last equilibrated. Previous studies using X-ray Absorption Near Edge Structure (XANES) spectroscopy in silicate glasses and olivine-hosted melt inclusions have shown that arc basalts are more oxidized than mid-ocean ridge basalts (MORB) and oceanic island basalts (OIB). However, Cottrell et al. (2018) recently demonstrated that hydrous glasses can experience beam-induced oxidation during XANES analysis, leading to an overestimation of their Fe3+/ΣFe ratios. Here, we determined Fe3+/ΣFe ratios in olivine-hosted melt inclusions from various arc, MORB and OIB localities by Mössbauer and XANES spectroscopy. We carried out a careful evaluation of beam damage during XANES measurements that included detailed analysis of consecutive XANES spectra and collection of time series on hydrous basaltic and basanitic glasses using different radiation doses. Fe3+/ΣFe ratios obtained using both spectroscopy methods on the same sample set allowed us to confirm the oxidation state of those magmas and the reliability of our approach to avoid the effect of beam-induced oxidation during XANES analysis. Our results show that melt inclusions from MORBs, OIBs and arc basalts display mean Fe3+/ΣFe ratios of 0.10 ± 0.05 (2σ; n = 5), 0.13 ± 0.05 (2σ; n = 2) and 0.25 ± 0.15 (2σ; n = 19), respectively, while Mount Etna melt inclusions display a mean Fe3+/ΣFe ratio of 0.26 ± 0.05 (2σ; n = 7). These results confirm that arc magmas are more oxidized than those from hot spots and mid-ocean ridges.