THE TEMPERATURE-DEPENDENCE OF THE SPECIATION OF WATER IN NAALSI3O8-KALSI3O8 MELTS - AN APPLICATION OF FICTIVE TEMPERATURES DERIVED FROM SYNTHETIC FLUID INCLUSIONS

The speciation of water dissolved in glasses along the join NaAlSi3O8- KAlSi3O8 has been investigated using infrared spectroscopy. Hydrous me lts have been hydrothermally synthesized by chemical equilibration of cylinders of bubble-free anhydrous start glasses with water at 1040 de grees C and 2 kbar. These melts have been isobarically and rapidly (20 0 degrees C/s) ''drop''-quenched to room temperature and then subseque ntly depressurized. The speciation of water in the quenched glasses re flects the state of water speciation at a temperature (the so-called f ictive temperature) where the quenched-in structure of the glasses clo sely corresponds to the melt structure at equilibrium. This fictive te mperature is detectable as the macroscopically measureable glass trans ition temperature of these melt compositions. A separate set of experi ments using vesicular samples of the same chemistry has precisely defi ned the glass transition temperature of these melts (+/-5 degrees C) o n the basis of homogenization temperatures for water-filled fluid incl usions (Romano et al. 1994). The spectroscopic data on the speciation of water in these quenched glasses has been quantified using experimen tally determined absorptivities for OH and H2O for each individual mel t composition. The knowledge of glass transition temperatures, togethe r with quantitative speciation data permits an analysis of the tempera ture dependence of the water speciation over the 113 degrees C range o f fictive temperatures obtained for these water-saturated melts. The v ariation of water speciation, cast as the equilibrium constant K where K = [H2O][O-m]/[OH](2) is plotted versus the fictive temperature of t he melt to obtain the temperature dependence of speciation. Such a plo t describes a single linear trend of the logarithm of the equilibrium constant versus reciprocal temperature, implying that the exchange of K for Na has little influence on melt speciation of water. The enthalp y derived from temperature dependence is 36.5 (+/-5) kJ/mol. The resul ts indicate a large variation in speciation with temperature and an in sensitivity of the speciation to the K-Na exchange.