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Kinetics of iron oxidation-reduction in hydrous silicic melts

ISTO, CNRS, 1 A rue de la Férollerie, 45071 Orléans, cedex 02, France

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 f_O2 between NNO-2 and NNO + 3 (NNO = nickel-nickel oxide redox buffer). The metaluminous melt (~1 wt% FeO_T) reached redox equilibrium after 10 hours and the peralkaline one (~3 wt% FeO_T) 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 D_H2values 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.

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