Serpentinisation of the Mantle Wedge Controls the Chemistry of Arc Magmas
Yardley, B W D
Arc magmas are derived from mantle wedge source regions that are more highly oxidised than other parts of the mantle but the cause of the oxidation is not well understood. Conventionally it has been attributed to influx of water derived from dehydration of the underlying slab, but this possibility has been comprehensively debunked by Frost et al. (1998). A direct link to the introduction of the fluids that flux melting fails to explain the ubiquitous high oxygen fugacity of xenoliths from this region, which point to extensive oxidation of the wedge. If it is not possible to introduce excess oxygen to the mantle wedge, it is likely that the oxidation arises from residual enrichment in oxygen due to dissociation of water and subsequent loss of hydrogen. Serpentinisation is the only geological process that generates sufficiently reducing conditions for water to dissociate, and it allows hydrogen fugacities to rise to sufficiently high levels for hydrogen to be lost as a migrating vapour phase. Because it creates magnetite, serpentinisation results in an increase in the Fe3+ content of the serpentinised rocks, which means that if subsequent heating regenerates less hydrous peridotite assemblages, the magnetite content of their spinel phase will be higher than in the precursor peridotite. For these regenerated peridotites, the greater magnetite content results in a higher oxygen fugacity than for the precursor peridotite. Hence the high Fe3+ signature generated during serpentine growth is carried down by flow in the wedge beyond the temperatures of serpentine breakdown and results in generation of high Fe3+:Fe2+ magmas. The main introduction of slab volatiles to the mantle wedge takes place at low temperatures, probably above 50km depth, where fluids are more abundant than at greater depths. This interpretation is consistent with geological and geophysical evidence for the existence of a serpentine body immediately above the slab in both ancient and modern subduction systems. Serpentinisation has implications for the budgets of S and of some metals in arc magmas, which are of significance for the major geochemical anomalies in S, Cu, Au Mo etc. associated with many arc magmas. It may generate Fe-Ni alloys with the potential to concentrate siderophile elements at shallow levels, while subsequently the more oxidised environment as the wedge melts may result in lower sulfide levels, with less entrapment of chalcophile elements in immiscible sulphide melts and mobilisation of S as sulphate.
B. R. Frost, C. Ballhaus C, Geochim. Cosmochim. Acta 62, 329-331 (1998).
2008 AGU Fall Meeting