Iron speciation in olivine-hosted melt inclusions inferred from Mössbauer spectroscopy

International audienceXANES spectroscopy is widely used to provide the oxidation state of magmas by determining their Fe3+/ΣFe ratios. This technique applied to glasses and olivine-hosted melt inclusions suggests that arc basalts are generally much more oxidized (Fe3+/ΣFe ratios from 0.190 to 0.344; [1-4]) than MORBs (Fe3+/ΣFe ratio of 0.14 ± 0.01; [5]).These resultsimply thatthe mantle beneath arcs ismore oxidized by one log unit relative to the QFM buffer than the mantle source of MORBs [1-5]. However, a recent study demonstrates that hydrous glasses canbe affected by beam-induced oxidation during XANES analysis that can lead to an over-estimation of their Fe3+/ΣFe ratios [6]. In this study, and for the first time, Fe3+/ΣFe ratios in olivine-hosted melt inclusions from various arcs, OIB and MORB localities were analyzed by synchrotron Mössbauer spectroscopy. Fe3+/ΣFe ratios obtained with this method allow us to constrain the oxidation state of those magmas by avoiding the effect of photo-oxidation that occurs during XANES analysis. A comparison between Fe3+/ΣFe ratios obtained by XANES and Mössbauer is carried out to determine whethera correction for beam-induced oxidation can be applied. Then, Fe3+/ΣFe ratios in these magmas will be used to constrain the oxidation state of the primary magmas formed in these different geological settings.[1] Brounce et al. (2014) J. Petrol. 55, 12, 2513‑2536 [2] Brounce et al. (2015) Geology43, 9, 775‑778 [3] Kelley & Cottrell (2009) Science325, 605‑607 [4] Kelley & Cottrell (2012) Earth Planet. Sci. Lett. 329‑330, 109‑121 [5] Zhang et al. (2018) Chem. Geol. 479, 166‑175 [6] Cottrell et al. (2018) Am. Mineral. 103, 4, 489‑50