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

Water is an important volatile component in andesitic eruptions and deep-seated andesitic magma chambers. We report an investigation of H2O 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 H2O (H2Om) and hydroxyl group (OH)] and total H2O (H2Ot) concentration profiles on the quenched glasses from the dehydration experiments. The equilibrium constant of the H2O speciation reaction H2Om+O⇌2OHH2Om+O⇌2OH, K = (XOH)2/(XH2OmXOXH2OmXO) 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 H2O concentration, contrary to previous H2O diffusion data in an andesitic melt at 1608–1848 K. The diffusion profiles are consistent with the model that molecular H2O is the diffusion species. Based on the above speciation model, H2Om and H2Ot diffusivity (in m2/s) in haploandesite at 743–873 K, 100 MPa, and H2Ot ⩽ 2.5 wt.% can be formulated as DH2Om=exp-12.273-13.905X-18172-73136XT and DH2Ot=DH2Om1+2X-14X(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 H2Ot with C being H2Ot in wt.%, and K is the equilibrium constant of H2O speciation reaction. By comparison with previous water diffusion studies, H2O 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.