The effect of water and fO(2) on the ferric-ferrous ratio of silicic melts

New experiments on the effect of dissolved water on the ferric-ferrous rati o of silicic melts have been performed at 200 MPa, between 800 degreesC and 1000 degreesC and for fO(2) between NNO - 1.35 and NNO + 6.6. Water-satura ted conditions were investigated. Compositions studied include six metalumi nous synthetic melts, with FeOtot progressively increasing from 0.47 to 4.2 5 wt.%, two natural obsidian glasses (one peraluminous and the other peralk aline) and a synthetic rhyolitic glass having the composition of the matrix glass of the June 15, 1991 Pinatubo dacite. Ferrous iron was analyzed by t itration and FeOtot by electron microprobe. Variation of quench rate was fo und to have no detectable effect on the ferric-ferrous ratio of the hydrous silicic melts investigated. At NNO, no dependence of the ferric-ferrous ra tio with temperature is observed. Al fO(2) < NNO + 1, metaluminous melts ha ve ferric-ferrous ratios higher than calculated from the Kress and Carmicha el (KC) (Kress, V.C., Carmichael, I.S.E., 1991. The compressibility of sili cate liquids containing Fe2O3 and the effect of composition: temperature: o xygen fugacity and pressure on their redox states. Contrib. Mineral. Petrol . 108, 82-92.) empirical equation (i.e., water has an oxidizing effect rela tive to KC), whereas at fO(2) > NNO + 1, the experimental ferric-ferrous ra tios are equal or lower than calculated. The peralkaline samples show the s ame type of behaviour. A non-linear relationship between X-Fe2O3/X-FeO and fO(2) implies that a term for dissolved water must be added to the KC equat ion if it is to be applied to the calculation of ferric-ferrous ratios of h ydrous silicic melts. Above NNO + 1, the ferric-ferrous ratio is essentiall y controlled by the anhydrous melt composition and fO(2). However, differen ces exist between measured and calculated ferric-ferrous ratios of silicic melts that are not all attributable to the effect of dissolved water, Addit ional work is needed to describe more precisely the dependence of the ferri c-ferrous ratio on anhydrous melt composition. The oxidizing effect of wate r is restricted to relatively reduced magmatic liquids. In oxidized calk-al kaline magma series, the presence of dissolved water will not largely influ ence melt ferric-ferrous ratios. (C) 2001 Elsevier Science B.V. All rights reserved.