Abstract:
The kinetics of Fe oxidation-reduction in two hydrous rhyolitic melts, one metaluminous and the other peralkaline, have been studied at 800 °C, 2kb, for melt water contents from ~5 wt% to saturation and fO2 between NNO-2 and NNO + 3 (NNO = nickel-nickel oxide redox buffer). The metaluminous melt (~1 wt% FeOT) reached redox equilibrium after 10 hours and the peralkaline one (~3 wt% FeOT) after 3 hours. The kinetics of Fe oxidation and reduction are similar and unaffected by the presence or absence of a hydrous fluid phase. No redox front is observable in the glass as the Fe3+/Fe2+ ratio evolves, implying that the Fe redox kinetics in hydrous silicic melts is rate-limited neither by the diffusion of H2 nor by the mobilities of divalent cations, as observed for anhydrous basaltic melts. We propose a two-step reaction mechanism that involves: (1) virtually instantaneous diffusion of H2 in the sample, followed by (2) slower structural/chemical reorganizations around Fe atoms. The overall redox process involving iron and hydrogen in Fe-poor, H2O-rich melts is thus reaction-limited and obeys a first-order logarithmic rate law. The relatively slow kinetics of oxidation/reduction explains why melt Fe3+/Fe2+ can be readily quenched in laboratory experiments. Simulation of oxidation of magmas due to H2 exchange with wall rocks is performed using these new kinetics laws and two DH2values extracted from the literature. We demonstrate that the metaluminous composition is not significantly modified whereas the peralkaline composition undergoes important and fast changes of Fe3+/Fe2+ during short processes such as ascent prior to Plinian-style eruptions.
