Externally-driven charge transfer in silicates at high pressure and temperature: A XANES study

Modest perturbations induced by an externally-applied electric field can generate significant variations in effective oxygen fugacity in high temperature silicates. This result has at least two-fold importance: first, it is a new petrologic technique to examine the behavior of a single sample under a large range of effective oxygen fugacity; and second, it is a mechanism for planetary electric fields to generate potentially significant chemical heterogeneities within planetary interiors. The redox state of Fe and V within a partially melted basaltic andesite was manipulated in situ in a piston-cylinder experiment with a DC power supply providing a source and sink of electrons to the sample. A 1 V electrical potential difference was applied across vanadium-doped synthetic basalt samples for 24 h. at 20 kbar and 1400 degrees C in a specially-designed piston cylinder sample assembly. Three experiments were performed: a control sample with no applied voltage, one with bottom ground and top anode (+1 V), and a third with top ground and bottom anode (+1 V). Synchrotron-based X-ray absorption near edge structure spectroscopy (XANES) was used to provide maps of iron and vanadium oxidation states with 5 mu m x 5 mu m spatial resolution throughout the recovered samples. Systematic increasing oxidation states of V and Fe were observed approaching the anode. Oxidation states were mapped to corresponding local oxygen fugacities by comparison with a series of samples synthesized under known oxygen fugacity conditions from previous studies. Both Fe and V markers indicate that the 1 V potential drop across the sample induces effective oxygen fugacity perturbations of 10 orders of magnitude. Therefore, it is possible that the presence of modest poloidal electric fields (similar to 10(-6) V/m) within the Earth's outer core may provide an electrochemical driving force for localized charge transfer reactions in certain overlying mantle areas, generating local order-of-magnitude differences in effective oxygen fugacity. (C) 2007 Elsevier B.V. All rights reserved.