ESTIMATING SYSTEM OXYGEN FUGACITY USING MICRO-XANES SPECTROSCOPY

Oxygen fugacity (fO₂) may exert primary control on key geochemical and geophysical observables including (1) the depth of melt initiation in the asthenosphere by controlling the depth at which reduced carbon oxidizes to carbonate (2) the tendency of magmas to differentiate along calc-alkaline or thoeliitic trends by controlling the onset of magnetite fractionation and (3) the global cycling of key volatile elements such as C, H, S, and O by controlling their speciation and the depth and extent of mantle melting. Iron K-edge X-ray absorption near-edge structure (μXANES) spectroscopy provides Fe³⁺/ΣFe ratios of mantle-derived and experimentally-produced basalts and andesites and serves as a proxy for system fO₂. Here we discuss this technical advance, its many applications to natural and high-pressure experimental systems, and highlight some of the key discoveries enabled by this technique over the last five years.