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The impact of degassing on the oxidation state of basaltic magmas: A case study of Kīlauea volcano

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Volcanic emissions link the oxidation state of the Earth's mantle to the composition of the atmosphere. Whether the oxidation state of an ascending magma follows a redox buffer – hence preserving mantle conditions – or deviates as a consequence of degassing remains under debate. Thus, further progress is required before erupted basalts can be used to infer the redox state of the upper mantle or the composition of their co-emitted gases to the atmosphere. Here we present the results of X-ray absorption near-edge structure (XANES) spectroscopy at the iron K-edge carried out for a series of melt inclusions and matrix glasses from ejecta associated with three eruptions of Kīlauea volcano (Hawai‘i). We show that the oxidation state of these melts is strongly correlated with their volatile content, particularly in respect of water and sulfur contents. We argue that sulfur degassing has played a major role in the observed reduction of iron in the melt, while the degassing of H2O and CO2 appears to have had a negligible effect on the melt oxidation state under the conditions investigated. Using gas–melt equilibrium degassing models, we relate the oxidation state of the melt to the composition of the gases emitted at Kīlauea. Our measurements and modelling yield a lower constraint on the oxygen fugacity of the mantle source beneath Kīlauea volcano, which we infer to be near the nickel nickel-oxide (NNO) buffer. Our findings should be widely applicable to other basaltic systems and we predict that the oxidation state of the mantle underneath most hotspot volcanoes is more oxidised than that of the associated lavas. We also suggest that whether the oxidation states of a basalt (in particular MORB) reflects that of its source, is primarily determined by the extent of sulfur degassing.



oxygen fugacity, sulfur, degassing, XANES, melt inclusions, CO2

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Earth and Planetary Science Letters

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Elsevier BV
NERC (via University of Leeds) (GA/13M/031)
Natural Environment Research Council (NE/N009312/1)
We thank the Diamond Light Source for access to beamline I18 (proposal number SP11497-1) that contributed to the results presented here and the invaluable support during our analytical sessions from Konstantin Ignatyev. The Smithsonian Institution National Museum of Natural History is thanked for their loan of NMNH 117393. We thank Don Swanson (HVO-USGS) for his help acquiring the samples. YM acknowledges support from the Scripps Institution of Oceanography Postdoctoral Fellowship program. We are grateful to Nicole Métrich and an anonymous reviewer for providing valuable comments improving the quality of the manuscript. ME and CO are supported by the Natural Environment Research Council via the Centre for Observation and Modelling of Earthquakes, Volcanoes and Tectonics (COMET). NP is also funded by the Natural Environment Research Council (grant NE/N009312/1). NERC-funded studentship funded sample collection. ME acknowledges NERC ion probe grant IMF376/0509.