Hydroxyl and molecular H2O diffusivity in a haploandesitic melt

Abstract

H2O diffusion in a haploandesitic melt (a high-silica and Fe-free andesitic melt, NBO/T=0.173) has been investigated at 1GPa in a piston-cylinder apparatus. We adopted a double diffusion couple technique, in which one couple was composed of a nominally anhydrous glass with 0.01wt.% H2O and a hydrous glass with 5.7wt.% H2O, and the other contained the same nominally anhydrous glass and a hydrous glass with 3.3wt.% H2O. Both couples were annealed in a single experimental run and hence experienced exactly the same P–T history, which is crucial for constraining the dependence of H2O diffusivity on water content. H2O concentration profiles were measured by both Fourier transform infrared (FTIR) microspectroscopy and confocal Raman microspectroscopy. Nearly identical profiles were obtained from Raman and FTIR methods for profile length >1mm (produced at 1619–1842K). By contrast, for profile lengths <100μm (produced at 668–768K), FTIR profiles show marked convolution effects compared to Raman profiles. A comparison between the short FTIR and Raman profiles indicates that the real spatial resolution (FWHM) of FTIR analyses is about 28μm for a 7μm wide aperture on ∼200μm thick glasses.While the short profiles are not reliable for quantitative modeling, the long diffusion profiles at superliquidus temperatures can be fit reasonably well by a diffusivity model previously developed for felsic melts, in which molecular H2O (H2Om) is the only diffusive species and its diffusivity (DH2Om) increases exponentially with the content of total water (H2Ot). However, there is noticeable misfit of the data at low H2Ot concentrations, suggesting that OH diffusivity (DOH) cannot be neglected in this andesitic melt at high temperatures and low water contents. We hence develop a new fitting procedure that simultaneously fits both diffusion profiles from a single experimental run and accounts for the roles of both OH and H2Om diffusion. With this procedure, DOH/DH2Om is constrained to be 0.1–0.2 at 1619–1842K as H2Ot concentration approaches zero. The obtained OH diffusivity is similar to fluorine diffusivity but is much higher than Eyring diffusivity.