The glass transition in hydrous granitic melts

The influence of water on the glass transition temperature of hydrous granitic melts is estimated on the basis of a variety of available sources of data that provide phenomenological data for the liquid-glass transition and for which the relaxation timescale can be accurately estimated. These sources include (a) dilatometric micropenetration viscosity determinations (b) temperature-dependent spectroscopic investigations of hydrous melts and (c) densities of synthetic fluid inclusions in glasses. All available evidence on the glass transition temperature of hydrous granitic melts, when corrected for equivalent timescales, yields a single consistent trend of decreasing T(g) with increasing water content that has powerful applications in the rheology and kinetics of processes involving late-stage silicic intrusives and eruptive products. The brittle-ductile transition in hydrous silicic liquids can be accurately predicted using this trend. Combination of the presently derived relationship for the influence of water on the glass transition of granitic melts with presently available data for the high temperature viscosities of such systems permits the estimation of the temperature dependence of the viscosity of hydrous melts over the entire range of temperature relevant to granite petrogenesis. Such a comparison reveals that the viscosity-temperature relationships of hydrous granitic melts are not Arrhenian, as is assumed by most existing calculational schemes for the prediction of hydrous melt viscosities. The increasing deviation of viscosity-temperature relationships of hydrous granitic melts from Arrhenian with increasing water content is broadly consistent with similar trends accompanying the addition of excess alkalies to metaluminous granitic melt base compositions. In detail, however, the influence of water on the deviation of viscosity-temperature relationships of hydrous granitic melts from Arrhenian is not as strong as would be predicted from a molar comparison of the effects of H(2)O vs, alkali oxide or alkaline earth oxide added to these melts. The answer to this apparent discrepancy may lie in the incomplete dissociation of water in the melt structure. (C) 1998 Elsevier Science B.V. All rights reserved.