Diffusion of water in volcanic glasses

The diffusion coefficient of water in silicate melts is critical for understanding degassing phenomena leading to the growth of bubbles and explosive eruptions. Although there have been many measurements of the diffusion of water in felsic melts and glasses, and in the past we have contributed to these measurements and understanding the mechanisms of diffusion, to our knowledge, there has been only one measurement of the diffusivity of water in basaltic melts; i.e., our own measurement reported by Zhang and Stolper in 1991 at a single temperature over a small range in water contents. In collaboration with Youxue Zhang (University of Michigan) and Eduard Persikov (Moscow), Sally Newman and I have been conducting a series of experiments studying the diffusion of water in in basaltic glasses.

These experiments are difficult because of the very high diffusivities involved, the relatively high temperatures required, and problems with quenching of the melt to glass. However, we have succeeded in conducting two different types of experiments: one in which we hydrate an initially dry basaltic glass and measure the concentration profile extending into the initially dry sample, and another in which a diffusion couple is set up between basaltic melts with contrasting water contents. The two figures shown below illustrate representative concentration gradients for these two types of experiments on basaltic melts, and show fits to the data based on the coupled speciation/diffusion model developed with Youxue Zhang in which we assume water molecules are mobile, hydroxyl groups are immobile, and equilibrium between these two dissolved hydrogen-bearing species.

The new results are compared with the single previous measurement on basaltic melt (but with much lower water contents) from Zhang and Stolper and with experiments on other naturally occurring volcanic glasses on the figure shown below. These results demonstrate a high level of consistency between these new results and the slower diffusivities of water in more silicic and polymerized naturally occurring melts.