THE MECHANISMS OF WATER DIFFUSION IN POLYMERIZED SILICATE MELTS

Diffusion of water was experimentally investigated for melts of albiti c (Ab) and quartz-orthoclasic (Qz(29)Or(71), in wt %) compositions wit h water contents in the range of 0 to 8.5 wt % at temperatures of 1100 to 1200 degrees C and at pressures of 1.0 and 5.0 kbar. Apparent chem ical diffusion coefficients of water (D-water) were determined from co ncentration-distance profiles measured by FTIR microspectroscopy. Unde r the same P-T condition and water content the diffusivity of water in albitic, quartz-orthoclasic and haplogranitic (Qz(28)Ab(38) Or(34), N owak and Behrens, this issue) melts is identical within experimental e rror. Comparison to data published in literature indicates that anhydr ous composition only has little influence on the mobility of water in polymerized melts but that the degree of polymerization has a large ef fect. For instance, D-water is almost identical for haplogranitic and rhyolitic melts with 0.5-3.5 wt% water at 850 degrees C but it is two orders of magnitude higher in basaltic than in haplogranitic melts wit h 0.2-0.5 wt% water at 1300 degrees C. Based on the new water diffusiv ity data, recently published in situ near-infrared spectroscopic data (Nowak 1995; Nowak and Behrens 1995), and viscosity data (Schulze et a l. 1996) for hydrous haplogranitic melts current models for water diff usion in silicate melts are critically reviewed. The NIR spectroscopy has indicated isolated OH groups, pairs of OH groups and H2O molecules as hydrous species in polymerized silicate melts. A significant contr ibution of isolated OH groups to the transport of water is excluded fo r water contents above 10 ppm by comparison of viscosity and water dif fusion data and by inspection of concentration profiles from trace wat er diffusion. Spectroscopic measurements have indicated that the inter conversion of H2O molecules and OH pairs is relatively fast in silicat e glasses and melts even at low temperature and it is inferred that th is reaction is an active step for migration of water. However, direct jumps of H2O molecules from one cavity within the silicate network to another one can not be excluded. Thus, we favour a model in which wate r migrates by the interconversion reaction and, possibly, small sequen ces of direct jumps of H2O molecules. In this model, immobilization of water results from dissociation of the OH pairs. Assuming that the fr equency of the interconversion reaction is faster than that of diffusi ve jumps, OH pairs and water molecules can be treated as a single diff using species having an effective diffusion coefficient D-OHpair,D-H2O The shape of curves of D-water versus water content implies that D-OH pair,D-H2O increases with water content. The change from linear to exp onential dependence of D-water between 2 and 3 wt % water is attribute d to the influence of the dissociation reaction at low water content a nd to the modification of the melt structure by incorporation of OH gr oups.