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 sou rces of data that provide phenomenological data for the liquid-glass t ransition and for which the relaxation timescale can be accurately est imated. These sources include (a) dilatometric micropenetration viscos ity determinations (b) temperature-dependent spectroscopic investigati ons of hydrous melts and (c) densities of synthetic fluid inclusions i n glasses. All available evidence on the glass transition temperature of hydrous granitic melts, when corrected for equivalent timescales, y ields a single consistent trend of decreasing T-g with increasing wate r content that has powerful applications in the theology and kinetics of processes involving late-stage silicic intrusives and eruptive prod ucts. The brittle-ductile transition in hydrous silicic liquids can be accurately predicted using this trend. Combination of the presently d erived relationship for the influence of water on the glass transition of granitic melts with presently available data for the high temperat ure viscosities of such systems permits the estimation of the temperat ure dependence of the viscosity of hydrous melts over the entire range of temperature relevant to granite petrogenesis. Such a comparison re veals 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 increasin g deviation of viscosity-temperature relationships of hydrous granitic melts from Arrhenian with increasing water content is broadly consist ent with similar trends accompanying the addition of excess alkalies t o metaluminous granitic melt base compositions. In detail, however, th e influence of water on the deviation of viscosity-temperature relatio nships of hydrous granitic melts from Arrhenian is not as strong as wo uld be predicted from a molar comparison oi the effects of H2O vs, alk ali oxide or alkaline earth oxide added to these melts. The answer to this apparent discrepancy may lie in the incomplete dissociation of wa ter in the melt structure. (C) 1998 Elsevier Science B.V. All rights r eserved.