The Effect of Slab-Derived Hydrous Fluids on the Oxidation State of Mantle Beneath the Cascade Arc: Constraints from Mössbauer-Determined Fe3+/Fe2+ in Spinel

David S. Draper

Key Centre for Geochemical Evolution and Metallogeny of Continents (GEMOC), School of Earth Sciences, Macquarie University, Sydney NSW 2109 Australia

Alan D. Brandon

Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road, NW, Washington DC 20015 USA

Type I spinel peridotite xenoliths from Simcoe Volcano, southern Washington (USA), are from lithospheric mantle approximately 65 km inboard from the axis of the subduction-related Cascade Range. The xenoliths are primarily refractory spinel harzburgite with minor websterite, but show evidence for metasomatism such as the presence of phlogopite and trace element enrichments in clinopyroxene. Oxygen fugacities calculated from contents of Fe3+/ _Fe in Simcoe spinels, determined by Möessbauer spectroscopy, are up to 1.4 log units more oxidizing than the FMQ buffer. These are among the most oxidized mantle xenoliths reported, with oxygen fugacities substantially higher than those calculated for mantle beneath most of western North America. These results, together with those from amphibole-bearing spinel peridotites from Ichinomegata, Japan (Wood and Virgo, 1989), provide evidence that the mantle above subduction zones is more oxidized than is oceanic or ancient cratonic mantle. We suggest that oxidation was accomplished by an agent ranging in composition from solute-rich hydrous fluid to water-bearing silicate melt, and we develop a semi-quantitative model relating extent of oxidation, duration of the oxidation process, and proportion of the available water derived from subducting slabs that oxidizes Fe in spinel. This model suggests that such an agent can easily produce the observed extents of oxidation over timescales similar to the typical lifespans of subduction zones. For the Cascade arc, where subduction has taken place for at least 50 Ma, the observed oxidation in the Simcoe peridotites can be achieved by reaction of 2% to 8% of the available water, assuming that 50% of the water flux from subducting slabs is transported into the mantle. These results demonstrate that water can be an efficient oxidizing agent, and that such a mechanism is a viable way to cause large-scale oxygen fugacity heterogeneity in the mantle.