Water diffusion in dacitic melt

H2O diffusion in dacitic melt was investigated at 0.48–0.95 GPa and 786–893 K in a piston-cylinder apparatus. The diffusion couple design was used, in which a nominally dry dacitic glass makes one half and is juxtaposed with a hydrous dacitic glass containing up to ∼8 wt.% total water (H2Ot). H2O concentration profiles were measured on quenched glasses with infrared microspectroscopy. The H2O diffusivity in dacite increases rapidly with water content under experimental conditions, similar to previous measurements at the same temperature but at pressure <0.15 GPa. However, compared with the low-pressure data, H2O diffusion at high pressure is systematically slower. H2O diffusion profiles in dacite can be modeled by assuming molecular H2O (H2Om) is the diffusing species. Total H2O diffusivity DH2OtDH2Ot within 786–1798 K, 0–1 GPa, and 0–8 wt.% H2Ot can be expressed as: DH2Ot=1+2X-14X(X-1)(1-4/K)+1exp-9.423-62.38X-19064-108882X+1476.7PT, where DH2OtDH2Ot is in m2/s, T is temperature in K, P is pressure in GPa, K = exp(1.49 − 2634/T  ) is the equilibrium constant of speciation reaction (H2Om+O⇌2OH)(H2Om+O⇌2OH) in the melt, X = C/18.015/[C/18.015 + (100 − C)/33.82], C is wt.% of H2Ot, and 18.015 and 33.82 g/mol correspond to the molar masses of H2O and anhydrous dacite on a single oxygen basis. Compared to H2O diffusion in rhyolite, diffusivity in dacite is lower at intermediate temperatures but higher at superliquidus temperatures. This general H2O diffusivity expression can be applied to a broad range of geological conditions, including both magma chamber processes and volcanic eruption dynamics from conduit to the surface.