Tracking the changing oxidation state of Erebus magmas, from mantle to surface, driven by magma ascent and degassing

• Published in: Earth and Planetary Science Letters
• Main Authors: Moussallam, Yves, Oppenheimer, Clive, Scaillet, Bruno, Gaillard, Fabrice, Kyle, Philip R., Peters, Nial, Hartley, Margaret, Berlo, Kim, Donovan, Amy
• Other Authors: Department of Geography Cambridge, UK, University of Cambridge UK (CAM), Institut des Sciences de la Terre d'Orléans - UMR7327 (ISTO), Bureau de Recherches Géologiques et Minières (BRGM)-Observatoire des Sciences de l'Univers en région Centre (OSUC), Institut national des sciences de l'Univers (INSU - CNRS)-Observatoire de Paris, Université Paris Sciences et Lettres (PSL)-Université Paris Sciences et Lettres (PSL)-Université d'Orléans (UO)-Centre National de la Recherche Scientifique (CNRS)-Institut national des sciences de l'Univers (INSU - CNRS)-Observatoire de Paris, Université Paris Sciences et Lettres (PSL)-Université Paris Sciences et Lettres (PSL)-Université d'Orléans (UO)-Centre National de la Recherche Scientifique (CNRS)-Institut national des sciences de l'Univers (INSU - CNRS)-Université d'Orléans (UO)-Centre National de la Recherche Scientifique (CNRS), New Mexico Institute of Mining and Technology New Mexico Tech (NMT), Department of Earth Sciences Cambridge, UK, Department of Earth Sciences Oxford, University of Oxford, NSF and United States Antarctic Program (grant ANT1142083), European Project: 279790,EC:FP7:ERC,ERC-2011-StG_20101014,ELECTROLITH(2011)
• Format: Article in Journal/Newspaper
• Language: English
• Published: HAL CCSD 2014
• Subjects: oxygen fugacity; sulfur; degassing; XANES; melt inclusions; CO2; [SDU.STU.VO]Sciences of the Universe [physics]/Earth Sciences/Volcanology; [SDE.MCG]Environmental Sciences/Global Changes; Antarc*; Antarctica
• Online Access: https://insu.hal.science/insu-00966727; https://insu.hal.science/insu-00966727/document; https://insu.hal.science/insu-00966727/file/Mantle_to_surface_tracking_EPSL_Revised_v4.pdf; https://doi.org/10.1016/j.epsl.2014.02.055

International audience The conventional view holds that the oxidation state of a mantle-derived degassed magma reflects its source. During magma ascent and degassing the oxidation state is thought to follow a redox buffer. While this view has been challenged by petrological data, geochemical models and volcanic gas measurements, the fingerprints of such redox changes and their driving forces have not hitherto been captured by an integrated study. Here, we track the redox evolution of an alkaline magmatic suite at Erebus volcano, Antarctica, from the mantle to the surface, using X-ray absorption near-edge structure (XANES) spectroscopy at the iron and sulphur K-edges. We find that strong reduction of Fe and S dissolved in the melt accompanies magma ascent. Using a model of gas-melt chemical equilibria, we show that sulphur degassing is the driving force behind this evolutionary trend, which spans a wide compositional and depth range. Our results explain puzzling shifts in the oxidation state of gases emitted from Erebus volcano, and indicate that, where sulphur degassing occurs, the oxidation states of degassed volcanic rocks may not reflect their mantle source or co-eruptive gas phase. This calls for caution when inferring the oxidation state of the upper mantle from extrusive rocks and a possible re-assessment of the contribution of volcanic degassing to the early Earth's atmosphere and oceans. The relationship between magma redox conditions and pressure (depth) emphasises the value of measuring redox couples in gases emitted from volcanoes for the purposes of operational forecasting.