Cassiterite dissolution and Sn diffusion in silicate melts of variable water content

• Diffusive cassiterite dissolution experiments were conducted.
• Sn diffusion profiles exhibit concentration-dependent diffusivity.
• Dependence of tin diffusivity on temperature and H2O content were obtained.
• Dependence of SnO2 solubility on temperature and H2O content were obtained.

Experiments to constrain cassiterite dissolution kinetics in rhyolitic melts with 0.1–5.9 wt% H2O were conducted at 1023–1373 K and 0.5 GPa in a piston-cylinder apparatus. Care was taken to minimize convection in the experimental charge. Tin diffusivity was found to be concentration dependent. The diffusion profiles were fit well by assuming that tin diffusivity depends exponentially on tin concentration, which is roughly linearly related to SiO2 or SiO2 + Al2O3 concentration. Tin diffusivity was found to increase with temperature following the Arrhenius relation, with activation energy decreasing from 161 kJ/mol in dry rhyolite to 93 kJ/mol in hydrous rhyolite containing 5.9 wt% H2O. Tin diffusivity increases exponentially with H2O concentration, by about 3.2 orders of magnitude at 1123 K from 0 to 6 wt% H2O, and decreases exponentially with SiO2 concentration, by about 0.7 orders of magnitude when SiO2 concentration increases by 10 wt%. The equation to describe Sn2 + diffusivity in rhyolitic to dacitic melt (64–76 wt% SiO2) at about 0.5 GPa is:lnDSn=−18.194+0.17(76–CSiO2)−(19418−1389w)/T,lnDSn=−18.194+0.1776–CSiO2−19418−1389w/T,where DSn is in m2/s, CSiO2 and w are SiO2 and H2O concentrations in wt%, and T is in Kelvin. The solubility of cassiterite (or tin concentration at cassiterite saturation) in rhyolitic melts increases strongly with increasing temperature.