Quick Search:    advanced search

GSW Home   GeoRef Home   My GSW Alerts   Contact GSW   About GSW   Journals List   Help

This Article Figures Only Full Text Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in ISI Web of Science Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via ISI Web of Science (8) Citing Articles via Google Scholar Google Scholar Articles by Gaillard, F. Articles by Pichavant, M. Search for Related Content GeoRef GeoRef Citation

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.

This article has been cited by other articles:

				F. Gaillard, F. Gaillard, M. Pichavant, S. Mackwell, R. Champallier, B. Scaillet, and C. McCammon Chemical transfer during redox exchanges between H2 and Fe-bearing silicate melts American Mineralogist, February 1, 2003; 88(2-3): 308 - 315. [Abstract] [Full Text] [PDF]