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

Moussallam, Yves and Oppenheimer, Clive and Scaillet, Bruno and Gaillard, Fabrice and Kyle, Philip and Peters, Nial and Hartley, Margaret E. and Berlo, Kim and Donovan, Amy (2014) Tracking the changing oxidation state of Erebus magmas, from mantle to surface, driven by magma ascent and degassing. Earth and Planetary Science Letters, 393. pp. 200-209. DOI 10.1016/j.epsl.2014.02.055

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Official URL: http://www.sciencedirect.com/science/article/pii/S...

Abstract

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.

Item Type: Article
Uncontrolled Keywords: 2014AREP; IA67;
Subjects: 05 - Petrology - Igneous, Metamorphic and Volcanic Studies
Divisions: 05 - Petrology - Igneous, Metamorphic and Volcanic Studies
Journal or Publication Title: Earth and Planetary Science Letters
Volume: 393
Page Range: pp. 200-209
Identification Number: 10.1016/j.epsl.2014.02.055
Depositing User: Sarah Humbert
Date Deposited: 11 Apr 2014 15:34
Last Modified: 15 Apr 2014 20:22
URI: http://eprints.esc.cam.ac.uk/id/eprint/3012

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