Oxidation state of Mayotte magmatic series: insights from Fe and S K-edge XANES spectroscopy

Charles Le Losq, Roberto Moretti, Etienne Médard, Carole Berthod, Federica Schiavi, Nicolas Trcera, Élodie Lebas

Following the 2018-2020 Fani Maoré submarine eruption near Mayotte Island, Indian Ocean, multiple oceanographic expeditions provide unprecedented access to fresh alkaline volcanic glasses spanning basanite to phonolite compositions from the East-Mayotte Volcanic Chain (EMVC). We applied Fe and S K-edge X-ray Absorption Near-Edge Spectroscopy (XANES) to determine iron and sulfur oxidation states in 13 glass samples, providing the first comprehensive redox characterization of this submarine volcanic system. Fe$^{3+}$/Fe\textsuperscript{TOT} ratios range from 0.19 to 0.51, while S$^{6+}$/S\textsuperscript{TOT} ratios span 0.02-0.17, with more evolved compositions that tend to show higher oxidation states. Beam damage monitoring revealed significant photo-oxidation effects on sulfur measurements, requiring analysis of initial spectra only. Comparison of redox estimates from multiple independent oxybarometers based on the Fe$^{3+}$/Fe\textsuperscript{TOT} and S$^{6+}$/S\textsuperscript{TOT} as well as the olivine-spinel-$a_{SiO_2}^{melt}$ reveals systematic discrepancies between Fe-based and S-based estimates, likely due to Fe-Ti nanolite contamination along X-ray beam paths. Integration of S$^{6+}$/S\textsuperscript{TOT} measurements with the olivine-spinel-$a_{SiO_2}^{melt}$ oxybarometer indicates $\Delta$FMQ = +0.3 $\pm$ 0.2 for basanite to tephriphonolite magmas, suggesting slightly more oxidized conditions than previously estimated for EMVC. These results provide essential constraints for thermodynamic modeling of alkaline submarine volcanism and highlight the importance of multi-proxy approaches in determining magmatic redox conditions. The oxidation state is consistent with other regional volcanic systems in the SW Indian Ocean, supporting a moderately oxidized mantle source beneath the Comoros archipelago.