The effect of water on alkali trace element diffusion (Li, Rb, Cs) in silicic melts

Abstract

Trace element diffusion is a powerful tracer of kinetic magmatic processes such as recharge and fluid exsolution. However, the dependence of diffusivities (D) on melt water contents is poorly quantified, and most trace element diffusivities were determined in water-free melts unrealistic for most natural magmatic systems. Here, we investigate the diffusion of the alkali trace elements Li, Rb, and Cs as a function of water content in rhyodacite, high-silica rhyolite, and peralkaline rhyolite. For the diffusion experiments, homogenous hydrous glass cylinders were produced from trace element-doped and undoped powdered glass with 1–8 wt% H2O. Glass cylinders with the same water content but different trace element concentrations were paired along polished contact surfaces. Diffusion experiments were conducted between 720 and 1100 °C at 100–700 MPa for 5–25 min using gas pressure vessels or a piston cylinder apparatus. Resulting diffusion profiles were analyzed by LA-ICPMS and evaluated by a Monte Carlo iterative fitting procedure for full error propagation. At a given experimental temperature, measured log10D values increase linearly with melt water content, with one order of magnitude increase for Li, two orders for Rb, and three orders for Cs from least hydrous (1 wt%) to wettest (8 wt%) experiment, due to a linear decrease in activation energies. Variations in major element composition only have a minor effect. We illustrate the impact of differential diffusion in hydrous silicic magmas on their trace element budget by coupling our data to a two-phase mechanical model of channelized fluid transport in magmatic systems. Quantifying water-dependent diffusivities provides an important tool to track the exsolution, mass and rate of fluid transport within long-lived shallow mushy magma reservoirs in the Earth’s crust.