Interaction Between Cerium And H2o In Hydrous Rhyolitic Melts

Cerium (Ce) in silicate melt is considered to provide important information about the nature of silicate magmas in the Earth's and planetary interiors, especially about their oxidation state. However, the behavior of Ce in a silicate melt may not be simple, particularly in hydrous silicate melt, where a strong depression of the Ce4+/Ce3+ ratio occurs. Here we investigate the interaction between cerium and H2O in the structure of hydrous rhyolitic melts. Hydrous rhyolitic glasses quenched from melts at 1 GPa and 1300 degrees C show a significant decrease of H2O solubility by the incorporation of Ce. Pair distribution function analyses show no distinct change in the local structures of hydrous rhyolitic glasses with the incorporation of Ce, while we found a distinct change in the intensity of the T-OH (where T is Si or Al) Raman peak. The incorporation of Ce decreases the proportion of T-OH species and increases those of Q(4) and Q(3) species. These results suggest the occurrence of a direct charge transfer reaction between OH- in the melt and incorporated Ce as Ce-melt(4+) + 2(OH)(melt)(-) -> Ce-melt(3+) + H2O + O-melt. This reaction causes a charge transfer from Ce4+ to Ce3+ in the hydrous rhyolitic melt, and it inevitably produces Ce3+. X-ray absorption near-edge structure (XANES) measurements show almost completely trivalent Ce (Ce3+) in the Ce-incorporated hydrous rhyolitic glasses, which supports the production of Ce3+ through the charge transfer reaction between Ce and OH-. The decrease of Ce4+/Ce3+ ratio reported in hydrous silicate melts would be due to the charge transfer reaction between Ce and H, which implies that the Ce4+/Ce3+ ratio in hydrous silicate melts may bear a complex relationship to the oxidation state of magmas.