HR: 0800h
AN: V31C-0604    [Abstracts]
TI: Experimental Characterization of Redox Changes During Degassing of a Vapor Saturated Magma: Redox Exchanges Between H-O-Fe Species
AU: * Mollard, E
EM: edima38@hotmail.com
AF: ISTO CNRS, 1A rue de la Ferollerie, Orleans, 45071, France
AU: Gaillard, F
EM: gaillard@cnrs-orleans.fr
AF: ISTO CNRS, 1A rue de la Ferollerie, Orleans, 45071, France
AU: Scaillet, B
EM: bscaille@cnrs-orleans.fr
AF: ISTO CNRS, 1A rue de la Ferollerie, Orleans, 45071, France
AB: The redox state of a magma reaching the surface is generally thought to be buffered by its iron redox ratio during ascent, reflecting therefore that of its source region. Only recently, the role of volatiles degassing on redox state of silicic magmas has been quantitatively addressed using numerical modelling. In these iron-poor magmatic systems, oxygen fugacity (fO2) is almost dominated by the chemical potential of the H2 and H2O volatile components. Because water is several orders of magnitude more soluble than molecular hydrogen in molten silicate, it was found that the ratio of their chemical potential dramatically changes during closed system degassing leading to an increase in fO2 of 2 orders of magnitude. We present here the results of an experimental test of such an oxidation event associated to the decompression of silicic melts saturated in volatiles. A peralkaline synthetic composition containing 2-4wt percent of dissolved iron oxides is used as starting material. Experiments are performed in cold seal pressure vessels pressurized with pre-mixed argon and hydrogen bottles. All experiments are equilibrated under water-saturated conditions at 800°C and variable pressures between 200 MPa and 25 MPa. Experiments were ended by rapid drop quench. Three oxygen fugacity conditions were investigated by using pure Argon (NNO+3) and two Ar-H2 mixtures buffering fO2 conditions at NNO+1.5 and NNO. Water content was determined using infrared spectroscopy and Karl-Fisher titration and the iron redox ratio was measured by wet chemistry. All run products are free of crystals. Time series experiments at fixed pressure were performed to determine the equilibrium dependence of iron redox ratios on pressure. Decompression experiments were performed from an equilibrated vapor-saturated melt at 200 MPa, lasted from few minutes to few hours, and were quenched at 100 MPa to 25 MPa. The measured iron redox ratios after decompression do not considerably differ from the ones before decompression. A slight oxidation is however noticed for some experiments, which can correspond to 0.7 log- units of fO2 increase in the most favorable cases. We conclude that a "Le Chatelier effect" most likely dominates and restricts the fO2 increase that is otherwise expected to be of ~2 log units in an iron-free melt: The increase in the fugacity ratios of H2O/H2 due to degassing upon decompression is restricted by H2 produced during oxidation of ferrous iron by water. The importance of this "Le Chatelier effect" strongly depends on the fO2 conditions prior to degassing.
DE: 0370 Volcanic effects (8409)
DE: 3612 Reactions and phase equilibria (1012, 8412)
DE: 3630 Experimental mineralogy and petrology
DE: 8430 Volcanic gases
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting