Oxidation state of La Reunion Island basalts from the pre- and post-caldera 2007 eruption samples: observations from sulfur X-ray absorption micro-spectroscopy

The magma redox state of large mafic ocean island volcanoes is influenced by a range of processes, from the mantle source through the transport and degassing. We focus on a well-studied suite of samples erupted prior and after the 2007 caldera collapse in Piton de la Fournaise to gain insights into the oxidation state of basaltic magmas. We investigated olivine-hosted melt inclusions with sulfur X-ray absorption near-edge spectroscopy at the micrometric length scale (micro-XANES). The micro-XANES signals allow estimating fO2 from integrated abundance of S6+/(Sum(S)). We find that the melt inclusions in pre-caldera samples show more variable S oxidation states, with reduced S coexisting with pyrrhotite as well as mixed of sulfide and sulfate species coexisting in the melt, than in the post-caldera samples. Measurements across and within inclusions reveal small micron-sized areas with significant variations in oxidation state. We estimated fO2 values and most values are close to the FMQ (fayalite-magnetite-quartz oxygen buffer assembly). In addition, we observe the highest “abnormally" oxidized individual inclusions with an fO2 as high as FMQ +0.9. Such high oxidation values are commonly close to sulfur-poor areas near a bubble within an inclusion, at the 10s um length scale. The post-caldera samples appear more homogenous with much less variability in oxidation state within melt inclusions. Our results illustrate the complexity of oxidation/reduction processes at the microscopic length. This may explain the diversity of previously reported results on oxidation state of similar volcanic samples from Piton de la Fournaise. Zoom link : https://u-paris.zoom.us/j/84120207937?pwd=U2JMYzJSR0hnbWNTMExnbGpHZFNrdz09 Meeting ID: 841 2020 7937 Secret Code: 138667