Coupled Interactions between Volatile Activity and Fe Oxidation State during Arc Crustal Processes

JOURNAL OF PETROLOGY Journal of Petrology, 2015, Vol. 56, No. 4, 795–814 doi: 10.1093/petrology/egv017 Advance Access Publication Date: 6 May 2015 Original Article Coupled Interactions between Volatile Activity and Fe Oxidation State during Arc Crustal Processes M. C. S. Humphreys1,2*, R. A. Brooker3, D. G. Fraser2, A. Burgisser4, M. T. Mangan5 and C. McCammon6 1 Department of Earth Sciences, Durham University, Science Labs, Durham DH1 3LE, UK, 2Department of Earth Sciences, University of Oxford, South Parks Road, Oxford OX1 3AN, UK, 3School of Earth Sciences, University of Bristol, Wills Memorial Building, Queen’s Road, Bristol BS8 1RJ, UK, 4CNRS, ISTerre, Universite de Savoie, F-73376 Le Bourget du Lac, France and Université Savoie Mont Blanc, ISTerre, F-73376 Le Bourget du Lac, France, 5US Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025-3561, USA and 6Bayerisches Geoinstitut, Universität Bayreuth, D-95440 Bayreuth, Germany *Corresponding author. Telephone: þ44 191 334 2300. Fax: þ44 191 334 2301. E-mail: Received March 5, 2014; Accepted March 25, 2015 ABSTRACT 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 P hydrated melts record strong increases in Fe3þ/ 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 f O2 might develop within arc crust. Conversely, degassing experiments produce an inP crease in Fe3þ/ 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 f O2 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 f O2. Key words: degassing, hydration, oxidation state, oxygen fugacity, rhyolite, H2O activity INTRODUCTION A fundamental question in the Earth Sciences concerns the distribution of oxygen within the solid Earth, its cycling through subduction zones and, through volcanic degassing, its effects on the atmosphere. In particular, the controls on the oxidation state of melts as they pass through the mantle and crust are challenging to constrain. It is clear that transfer of hydrous fluids from the subducting slab into the mantle is crucial for generating some of the typical geochemical signatures of subduction zone magmas, such as enrichment in large ion lithophile elements (LILE) and volatiles. Comparison P of Fe3þ/ Fe in lavas arriving at the Earth’s surface shows that arc lavas also tend to be more oxidized than C The Author 2015. Published by Oxford University Press. All rights reserved. For Permissions, please e-mail: V 795 796 In Part II, we tackle the second problem, presenting new data to explore the effect of disequilibrium or opensystem changes in H2O content on the oxidation state of sulphur-free rhyolite melts and glasses, using a new P series of coupled H2O and Fe3þ/ Fe measurements from existing hydration and decompression experiments. We discuss the implications of our results for processes operating in natural magmas, highlighting the need to consider kinetics in arc systems where there is ample evidence of disequilibrium at a variety of temporal and spatial scales. PART I: A REVIEW OF THE EVIDENCE LINKING VOLATILES, MELT CHEMISTRY AND OXIDATION STATE The effects of variations in anhydrous melt composition on ferric–ferrous ratios at constant fO2 are well known (e.g. Paul & Douglas, 1965; Sack et al., 1980; Kress & Carmichael, 1991; Toplis, 2005), but there has been considerable debate over the effect of H2O, as a chemical P component of the melt, on Fe3þ/ Fe. In particular, it has been suggested that the same process that causes oxidation when alkalis are added to silicate melts at constant fO2 (e.g. Paul & Douglas, 1965) should also operate during dissolution of volatile species (Fraser, 2005; Moretti, 2005; Toplis, 2005), by altering the relative activity coefficients of Fe2þ and Fe3þ species within the melt. This relies on the quasi-chemical theory defined by the Lux–Flood ‘basicity’ of different oxide components, which considers equilibrium between bridging oxygens (O0), non-bridging oxygens (O–) and ‘free oxide’ anions that are not bonded to the tetrahedral silicate polymer network (O2–, Toop & Samis, 1962). In this model, the melt is a molten ionic solution dominated by oxide ions (Flood & Förland, 1947; Fraser, 1975, 2005; Duffy, 1993; Ottonello et al., 2001; Moretti, 2005). Interaction between oxygen in different structural positions O0 þ O2– ¼ 2O– (1) defines the basicity of the solution. Basic oxides such as alkalis (Na2O or K2O) dissociate to supply O2– to the system and hence drive depolymerization [breaking of oxo-bridges; equilibrium (1) moves to the right], whereas acidic oxides (such as SiO2) react with O2– to form polymer chains. Amphoteric oxides, including Fe2O3, Al2O3, H2O and CO2 (Fraser, 1977), can act as an acid or a base depending on the composition of the silicate solution (Kushiro, 1975). Fe oxide components in the silicate melt therefore have the following possible reactions (neglecting the acidic behaviour of FeO; Fraser, 1975, 2005): basic : FeO ¼ Fe2þ þ O2– basic : Fe2 O3 ¼ 2Fe3þ þ 3O2– acidic : Fe2 O3 þ O2– ¼ 2FeO–2 (2a) (2b) (2c) where FeO2– is part of the structural network, analogous to AlO4–. those produced at mid-ocean ridges or in other settings (e.g. Carmichael, 1991; Ballhaus, 1993; Lee et al., 2010; Evans et al., 2012). Mantle xenoliths entrained by arc magmas also appear to be more oxidized than those in other tectonic environments (e.g. Wood et al., 1990; Brandon & Draper, 1996; Parkinson & Arculus, 1999; Frost & McCammon, 2008). It is commonly suggested that increased fO2 in arc systems compared with other tectonic settings is a primary feature related to mass transfer from the subducting slab to the mantle wedge (e.g. Brandon & Draper, 1996; Parkinson & Arculus, 1999; Kelley & Cottrell, 2009), either direct (...truncated)