Cerium oxidation state in silicate melts: Combined fO2, temperature and compositional effects

Abstract

To quantify the relative proportions of Ce3+ and Ce4+ in natural magmas, we have synthesized a series of Ce doped glasses ranging in composition from basalt to rhyolite (±H2O) at 0.001 and 1GPa, under fO2 conditions varying from FMQ −4.0 to FMQ +8.4, and temperatures from 1200 to 1500°C. The Ce4+/Ce3+ ratio in the experimental run products was determined both potentiometrically and in situ, using Ce M4,5-edge X-ray absorption near-edge structure (XANES) spectroscopy. For a given melt composition, the change in Ce4+/Ce3+ ratio with fO2 follows the trend predicted from the reaction stoichiometry assuming simple oxides as melt species. In addition to fO2, melt composition and water content have been found to be secondary controls on Ce4+/Ce3+, with more depolymerized melts and hydrous compositions favoring the stabilization of Ce3+. The Ce4+/Ce3+ ratio can be expressed through the equation,logCeO2CeO3/2=14logfO2+5705(±257)T-0.8990(±0.0805)NBOT-3.856(±0.083)·xH2O-3.889(±0.037)where T is in Kelvin, NBO/T is the proportion of non-bridging oxygen to tetrahedrally coordinated cations, and xH2O is the mole fraction (calculated using molecular oxides, e.g. Al2O3, Na2O) of water dissolved in the melt. A recent study conducted by Burnham and Berry (2014, Chemical Geology) investigating Ce oxidation state in silicate melts using Ce L3-edge XANES, equilibrated under a subset of the conditions investigated here, showed a similar dependence of Ce4+/Ce3+ on T and fO2, however, melt composition was found to have the opposite effect, with decreasing melt polymerization resulting in an increased abundance of Ce4+. This apparent discrepancy likely arises from the presence of alkalies and H2O in the compositions presented in this study, which were absent in the study of Burnham and Berry (2014). Our results indicate that even at relatively low oxygen fugacity, trace amounts of Ce4+ will be present in most terrestrial igneous systems, suggesting that Ce partitioning could be a sensitive indicator of fO2.