The redox state of magmas: Cu-porphyry deposits, ocean floor basalts and pre-shield stage Hawaiian volcanism

A thin section containing glass fragments from the pre-shield stage at Kilauea

The oxidation state of an element exerts a significant control on its geochemical behaviour such that two different oxidation states of the same element behave almost as if they were two different elements. Differences in charge, ionic radii and coordination environment all contribute to the distinct behaviour of each different oxidation state of an element. X-ray absorption spectroscopy (XAS) is a synchrotron technique that can be used to investigate the local electronic and geometric structure of an element in geological materials. S, Fe, Cu and Sb K-edge X-ray absorption near edge structure (XANES) spectra have been recorded from experimental and/or natural silicate glasses to investigate the oxidation state and coordination environment of these elements in silicate melts. This provides insights into (1) the geochemical behaviour of Cu with implications for the partitioning of Cu between crystals, melts, and fluids, (2) the correlation of moderately chalcophile elements such as Sb, As and Pb with the light rare earth elements in ocean floor basalts, and (3) the redox state and S content of pre-shield stage melts from Kilauea, Hawaii.