THE ROLE OF WATER IN OXYGEN ISOTOPE EXCHANGE IN QUARTZ

Abstract

The role of water in oxygen isotope exchange experiments under hydrothermal conditions has been modelled mathematically using the results of recent ab initio calculations which provide energy data on the insertion of water molecules, [H2O]i, within the c-axis channels of α-quartz. In the first instance the ab initio results have been used to establish solubility data for molecular water in the channels of α-quartz over a wide range of both temperature and pressure and it has been possible to demonstrate that the solubility of molecular water in α-quartz is extremely small and of the order of 10-10 atom at 500°C and 0.2 GPa. Since α-quartz as host is effectively incompressible the partial molar free energy of the interstitial water, as modelled, is independent of hydrostatic pressure and the concentration of molecular water within the quartz is thus linearly dependent on the fugacity of the external water. The new solubility data have subsequently been used to model the self-diffusion coefficient for interstitial water in α-quartz, D *[H2O]i, using absolute reaction rate theory as applied to molecular diffusion in crystalline solids. Comparison of these calculated self-diffusion data for interstitial water with experimentally determined diffusion coefficients for 18O16O exchange in α-quartz under hydrothermal conditions validates previous suggestions as to the predominant role of water in the oxygen isotope exchange process. The characteristics of the necessary chemical exchange reaction between 18O labelled water and 16O within the quartz structure are also discussed.