Spatial heterogeneities in the dissolved water content within volcanic glasses are generated by diffusive degassing and crystallization of melts, as well as subsequent posteruptive hydration of quenched glasses. Characterization of
the resultant water diffusion gradients using micro-analytical techniques such as SIMS or synchrotron FTIR allows us to model the timescale of water diffusion and thus key pre-, synand post-eruptive processes. We present a variety of microanalytical data to illustrate three sources of water content heterogeneity within rhyolitic glasses.
Firstly, enrichment of water in glass surrounding spherulites reflects its expulsion during growth of anhydrous mineral phases. Our diffusion models have placed new constraints on spherulite growth rates within obsidian flows . Secondly, we have found strong water enrichment adjacent to perlitic fractures in subglacially erupted obsidian lavas . Diffusion and cooling models show that perlitisation
starts at ~400 °C and occurs over timescale of days .
Finally, we have used SIMS analysis to overcome the hydration problem and successfully measure the dissolved magmatic water content of pumices . This provides new insight into magma storage and degassing prior to an
exceptionally violent eruption.