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

Abstract

Experiments to constrain cassiterite dissolution kinetics in rhyolitic melts with 0.1–5.9wt% H2O were conducted at 1023–1373K and 0.5GPa 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 161kJ/mol in dry rhyolite to 93kJ/mol in hydrous rhyolite containing 5.9wt% H2O. Tin diffusivity increases exponentially with H2O concentration, by about 3.2 orders of magnitude at 1123K from 0 to 6wt% H2O, and decreases exponentially with SiO2 concentration, by about 0.7 orders of magnitude when SiO2 concentration increases by 10wt%. The equation to describe Sn2+ diffusivity in rhyolitic to dacitic melt (64–76wt% SiO2) at about 0.5GPa is: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.