Water speciation and diffusion in haploandesitic melts at 743–873 K and 100 MPa

Abstract

Water is an important volatile component in andesitic eruptions and deep-seated andesitic magma chambers. We report an investigation of H 2O speciation and diffusion by dehydrating haploandesitic melts containing ⩽2.5 wt.% water at 743–873 K and 100 MPa in cold-seal pressure vessels. FTIR microspectroscopy was utilized to measure species [molecular H 2O (H 2O m) and hydroxyl group (OH)] and total H 2O (H 2O t) concentration profiles on the quenched glasses from the dehydration experiments. The equilibrium constant of the H 2O speciation reaction H 2 O m + O ⇌ 2 OH , K = ( X OH) 2/( X H 2 O m X O ) where X means mole fraction on a single oxygen basis, in this Fe-free andesite varies with temperature as ln K = 1.547–2453/ T where T is in K. Comparison with previous speciation data on rhyolitic and dacitic melts indicates that, for a given water concentration, Fe-free andesitic melt contains more hydroxyl groups. Water diffusivity at the experimental conditions increases rapidly with H 2O concentration, contrary to previous H 2O diffusion data in an andesitic melt at 1608–1848 K. The diffusion profiles are consistent with the model that molecular H 2O is the diffusion species. Based on the above speciation model, H 2O m and H 2O t diffusivity (in m 2/s) in haploandesite at 743–873 K, 100 MPa, and H 2O t ⩽ 2.5 wt.% can be formulated as D H 2 O m = exp - 12.273 - 13.905 X - 18172 - 73136 X T and D H 2 O t = D H 2 O m 1 + 2 X - 1 4 X ( X - 1 ) ( 1 - 4 / K ) + 1 , where T is in K, X = C/18.015/[ C/18.015 + (100 − C)/33.84] is mole fraction of H 2O t with C being H 2O t in wt.%, and K is the equilibrium constant of H 2O speciation reaction. By comparison with previous water diffusion studies, H 2O diffusivity at T < 873 K in calc-alkaline silicate melts is found to increase with degree of polymerization (andesite < dacite < rhyolite), opposite to the trend at superliquidus temperatures.