Humphreys, M.C.S. and Brooker, R.A. and Fraser, D.G. and Burgisser, A. and Mangan, M.T. and McCammon, C. (2015) 'Coupled interactions between volatile activity and Fe oxidation state during arc crustal processes.', Journal of petrology., 56 (4). pp. 795-814.
Arc magmas erupted at the Earth’s surface are commonly more oxidized than those produced at mid-ocean ridges. Possible explanations for this high oxidation state are that the transfer of fluids during the subduction process results in direct oxidation of the sub-arc mantle wedge, or that oxidation is caused by the effect of later crustal processes, including protracted fractionation and degassing of volatile-rich magmas. This study sets out to investigate the effect of disequilibrium crustal processes that may involve coupled changes in H2O content and Fe oxidation state, by examining the degassing and hydration of sulphur-free rhyolites. We show that experimentally hydrated melts record strong increases in Fe3+/∑Fe with increasing H2O concentration as a result of changes in water activity. This is relevant for the passage of H2O-undersaturated melts from the deep crust towards shallow crustal storage regions, and raises the possibility that vertical variations in fO2 might develop within arc crust. Conversely, degassing experiments produce an increase in Fe3+/∑Fe with decreasing H2O concentration. In this case the oxidation is explained by loss of H2 as well as H2O into bubbles during decompression, consistent with thermodynamic modelling, and is relevant for magmas undergoing shallow degassing en route to the surface. We discuss these results in the context of the possible controls on fO2 during the generation, storage and ascent of magmas in arc settings, in particular considering the timescales of equilibration relative to observation as this affects the quality of the petrological record of magmatic fO2.
|Keywords:||Degassing, Hydration, Oxidation state, Oxygen fugacity, Rhyolite, H2O activity.|
|Full text:||(AM) Accepted Manuscript|
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|Publisher Web site:||http://dx.doi.org/10.1093/petrology/egv017|
|Publisher statement:||This is a pre-copyedited, author-produced PDF of an article accepted for publication in Journal of Petrology following peer review. The version of record Humphreys, M.C.S., Brooker, R.A., Fraser, D.G., Burgisser, A., Mangan, M.T. and McCammon, C. (2015) 'Coupled interactions between volatile activity and Fe oxidation state during arc crustal processes.', Journal of petrology, 56 (4): 795-814 is available online at: http://dx.doi.org/10.1093/petrology/egv017.|
|Date accepted:||10 March 2015|
|Date deposited:||31 March 2015|
|Date of first online publication:||06 May 2015|
|Date first made open access:||01 April 2016|
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