The goal of this proposal is to investigate the oxidizing potential (oxygen fugacity) of the upper mantle, with special emphasis in subduction zones. Oxygen fugacity is important because it dictates an element's oxidation state, and hence chemical behavior. Of particular interest is how oxygen fugacity varies in subduction environments, beginning with melting within the mantle wedge and ending with emplacement and eruption of the magma at crustal levels. A specific hypothesis to be tested is the widespread notion that the mantle source regions of arc lavas are characterized by oxygen fugacities much higher than that beneath mid-ocean ridges. This view derives largely from the observation that arc lavas themselves appear to be highly oxidized and that slab-derived fluids metasomatizing the mantle wedge are likely to be oxidizing. While estimating the oxygen fugacity of magmas is fairly straightforward, estimating that of the melt source region in the mantle is not so straightforward because a method is required that can "see through" magmatic differentiation processes. This proposal seeks to use V/Sc ratios of primitive arc basalts to see into the mantle. The partitioning behavior of V is redox-sensitive, whereas that of Sc is not. For primitive magmas, this ratio is unlikely to be modified significantly by early crystallization of olivine. Accordingly, the V/Sc ratio may be able to "see through" early magmatic differentiation processes and into the mantle. Preliminary data suggest that V/Sc ratios are slightly correlated with indices of fluid contributions (e.g., Ba/Yb), but that overall, the V/Sc ratios of arc magmas do not significantly differ from that of mid-ocean ridges. Efforts will be made to build on these investigations in more detail on several selected arc series. The proposed study will support 1 PhD thesis and one or two undergraduates over the course of two years.
