An X-ray Absorption Spectroscopy Study of Redox Variable Elements in Silicate Melts

Abstract

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 as if they were different elements. Differences in charge, ionic radius and coordination environment all contribute to the distinct behaviour of each oxidation state. XANES and EXAFS are synchrotron techniques that can be used to investigate the local electronic and geometric structure of an element in geological materials.

Antimony is a moderately chalcophile element, however, it behaves like a REE during basalt petrogenesis. Sb commonly occurs as either Sb3+ or Sb5+ and its behaviour as a chalcophile or lithophile element will be affected by its oxidation state. Sb K-edge XANES spectra were recorded from synthetic mid-ocean ridge basalt (MORB) and CaO-MgO-Al2O3-SiO2 (CMAS) glasses quenched from melts equilibrated over a range of oxygen fugacities (fO2), temperatures, and pressures. Sb3+ was found to be the main oxidation state under terrestrial conditions. EXAFS spectra indicate that Sb3+ has trigonal pyramidal coordination and Sb5+ octahedral coordination in silicate glasses. The distorted coordination environment of Sb3+ means that the lattice-strain theory commonly used to model crystal-melt partitioning is not appropriate. The similarity in oxidation state but difference in ionic radius and coordination environment of Sb3+ and the REE3+ shows that the near constant ratio of Sb with the REE in basaltic melts is not due to a similar affinity for a particular phase but is rather due to a similar lack of affinity.

The siderophile and chalcophile behaviour of Cu depends on its oxidation state (Cu+ or Cu2+). The ability of a sulphide phase to scavenge Cu from a melt has implications for the formation of Cu porphyry deposits. Cu K-edge XANES spectra were recorded from CMAS, MORB, andesite and granite glasses and melts in situ. Cu+/Cu2+ was found to vary with fO2 according to the thermodynamically expected function and Cu+ has an unusual linear coordination. Cu2+ is stabilised by depolymerised basic melts, low temperatures, and high pressures. Cu+ will be the dominant oxidation state in terrestrial silicate melts. An electron exchange reaction between Cu and Fe on cooling means that Cu+/Cu2+ in a melt may not be preserved in a glass.

The oxidation states of S and Fe were determined for a "unique" set of pre-shield stage basanite-nephelinite glasses erupted from Kilauea, Hawaii. Fe3+/Fe (where Fe=Fe3++Fe2+) varied from 0.28-0.46, which is significantly more oxidised than in tholeiitic shield stage melts (009-0.13). The fO2 of the pre-shield melts is estimated to vary from QFM+1 to QFM+2.5 (where QFM is the quartz-fayalite-magnetite buffer) with evolution of the magma. Fe3+/Fe is positively correlated with the S content and S6+/S (~0.22). A model for the genesis of the melts that includes the major element chemistry, redox state changes, and REE patterns indicates that the samples are related by the crystallisation of garnet. This suggest that a magma chamber existed at depths great enough for garnet crystallisation (at least 60 km) during the pre-shield stage of Kilauea.