Physics of the Earth and Planetary Interiors xxx (2004) xxx–xxx Oxidation state of iron in hydrous mantle phases: implications for subduction and mantle oxygen fugacity C.A. McCammon a,∗ , D.J. Frost a , J.R. Smyth b , H.M.S. Laustsen b , T. Kawamoto c , N.L. Ross d , P.A. van Aken e a Bayerisches Geoinstitut, Universität Bayreuth, D-95440 Bayreuth, Germany b Department of Geological Sciences, University of Colorado, Boulder, CO 80309, USA c Institute for Geothermal Sciences, Kyoto University, Beppu, Japan d Department of Geological Sciences, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061, USA e Institut für Angewandte Geowissenschaften, Technische Universität Darmstadt, D-64287 Darmstadt, Germany Received 29 January 2003; received in revised form 8 June 2003; accepted 15 August 2003 Abstract Samples of olivine, wadsleyite, ringwoodite and majorite with Mg/(Mg + Fe) = 0.85–0.90 were synthesised at high pressure and temperature using a multianvil press under nominally anhydrous conditions as well as hydrous conditions with varying conditions of oxygen fugacity, and examined ex situ using Mössbauer spectroscopy to determine Fe 3+ /Fe, and either Fourier Transform Infrared Spectroscopy or Secondary Ion Mass Spectrometry to determine H concentration. Under oxidising conditions, the relative concentration of Fe 3+ increases with water content in olivine, wadselyite and ringwoodite. Majorite does not incorporate significant water, but shows high Fe 3+ /Fe values when synthesised under oxidising conditions in the presence of water. Fe 3+ /Fe is strongly influenced by oxygen fugacity for all phases studied, and the mechanism of hydration is likely coupled to Fe 3+ incorporation in the crystal structure. Enhanced Fe 3+ /Fe in mantle-derived olivine and orthopyroxene has a negligible effect on oxygen fugacities estimated using olivine–orthopyroxene–garnet oxybarometers, while increased Fe 3+ /Fe in garnet due to dehydrogenation could lead to an overestimation of oxygen fugacity in exhumed samples. Subduction zones are likely oxidised relative to the rest of the mantle, and hence represent regions where not only water is concentrated, but also Fe 3+ . © 2004 Elsevier B.V. All rights reserved. Keywords: Mössbauer spectroscopy; Ferric iron; Water; Subduction 1. Introduction Mantle hydrous phases have been the focus of numerous studies due to the influence of water on ∗ Corresponding author. Tel.: +49-921-553709; fax: +49-921-553769. E-mail address: catherine.mccammon@uni-bayreuth.de (C.A. McCammon). mantle properties, including viscosity, melting be- haviour, rheology, elastic properties and the behaviour of fluids. Many studies have focused on the system MgO–SiO 2 –H 2 O, where a large number of stable hydrous phases have been found (reviewed by Angel et al., 2001), demonstrating that iron is not a nec- essary element for stabilising OH in these phases. Nevertheless, since iron is the most abundant element that adopts multiple valence states, its chemistry is 0031-9201/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.pepi.2003.08.009