THE ROLE OF WATER IN OXYGEN-ISOTOPE EXCHANGE IN QUARTZ

The role of water in oxygen isotope exchange experiments under hydroth ermal conditions has been modelled mathematically using the results of recent ab initio calculations which provide energy data on the insert ion of water molecules, [H2O](i), within the c-axis channels of alpha- quartz. In the first instance the ab initio results have been used to establish solubility data for molecular water in the channels of alpha over a wide range of both temperature and pressure and it has been po ssible to demonstrate that the solubility of molecular water in Lu-qua rtz is extremely small and of the order of 10(-10)/Si atom at 500 degr ees C and 0.2 Gpa. Since alpha-quartz as host is effectively incompres sible the partial molar free energy of the interstitial water, as mode lled, is independent of hydrostatic pressure and the concentration of molecular water within the quartz is thus linearly dependent on the fu gacity of the external water. The new solubility data have subsequentl y been used to model the self-diffusion coefficient for interstitial w ater in alpha-quartz, D-[H2O](i), using absolute reaction rate theory as applied to molecular diffusion in crystalline solids. Comparison o f these calculated self-diffusion data for interstitial water with exp erimentally determined diffusion coefficients for O-18/O-16 exchange i n alpha-quartz under hydrothermal conditions validates previous sugges tions as to the predominant role of water in the oxygen isotope exchan ge process. The characteristics of the necessary chemical exchange rea ction between O-18 labelled water and O-16 within the quartz structure are also discussed.