The kinetics of Ar and CO2 diffusion in simplified Fe-free rhyolitic to hawaiitic melts were analysed by using the diffusion couple technique. The concentration-distance profiles of Ar and CO2 were measured by electron microprobe analysis and Fourier transform infra-red spectroscopy, respectively. Error functions were fitted to the symmetrical concentration-distance profiles in order to extract the diffusion coefficients. At 1373 to 1773K, the activation energies for Ar diffusion ranged from 169 to 257kJ/mol. The Ar diffusivity increased exponentially with the degree of de-polymerisation. The mobility of total CO2, that was identical to Ar mobility in rhyolitic melts, remained constant upon changing the bulk composition from rhyolite to hawaiite. The CO2 speciation at 1623K and 500MPa was modelled, for the range of compositions studied, by using diffusion data on Ar and total CO2 combined with network-former diffusion calculated from viscosity data. This model was in excellent agreement with CO2 speciation data extrapolated from temperatures near to the glass transition temperature for dacitic melt compositions. This model showed that, even in highly de-polymerised hawaiitic and tholeiitic melts, molecular CO2 was a stable species and contributed 70 to 80% to the total CO2 diffusion, respectively.

Argon and CO2 on the Race Track in Silicate Melts - a Tool for the Development of a CO2 Speciation and Diffusion Model. M.Nowak, D.Schreen, K.Spickenbom: Geochimica et Cosmochimica Acta, 2004, 68[24], 5127-38