The effect of water on trace element diffusivities in silicic melts: A case study on Li, Rb, and Cs

Trace element diffusion is a powerful tracer for the kinetics of magmatic processes such as recharge and fluid exsolution. However, the dependence of diffusivities on melt water contents is poorly understood, and most trace element diffusivities have been determined in water-free melts unrealistic for most natural magmatic systems. Here, we investigate diffusion of the alkali trace elements Li, Rb, and Cs as a function of water content in metaluminous rhyodacite, high-silica rhyolite, and peralkaline rhyolite.

Homogenous hydrous glass cylinders were produced from trace element-doped and undoped powdered glass with 1-8 wt% H2O. For the diffusion experiments, glass cylinders with the same water content but different trace element concentrations are paired along polished contact surfaces. Diffusion experiments were conducted between 720-1100 °C for 5-25 min using gas pressure vessels and a piston cylinder. Diffusion profiles were analyzed by LA-ICPMS and evaluated by a Monte Carlo iterative fitting procedure for full error propagation.

Measured logD values correlate linearly with melt water content, with one order of magnitude increase for DLi, two orders for DRb and three orders for DCs from driest (1 wt%) to wettest (8 wt%) experiment. Variations in major element composition only have a minor effect. Comparing the dependence of Arrhenius parameters D0 and Ea on melt water content, activation energies correlate linearly, while pre-exponential factors suggest negligible correlation. Quantifying these effects facilitates using the diffusion of alkali trace elements for tracking magmatic processes, such as the exsolution of fluids from long-lived shallow mushy magma reservoirs in the Earth’s crust.