Pressure dependence of Ar and Kr diffusion in a jadeite melt

The pressure dependence of the diffusivity of Ar and Kr in a jadeite melt was investigated experimentally. Sorption studies on spheres (∼ 1.6–3.4 mm in diameter) were performed in an internally heated pressure vessel at 800°C and pressures between 200 and 6000 bar. Run times ranged from 24 to 240 hr. Sorbed noble gases were analysed by gas chromatography. From a comparison of a partially saturated sphere and a saturated powder, the diffusivity is calculated. Results indicate decreasing diffusivity of Ar and Kr with increasing pressure. Least-squares fitting of an Arrhenian type equation D = D0exp(−PVRT) to the data yields activation volumes of 11.4 ± 1.3 cm3/mol (log D0 = −9.14 ± 0.02; D0 in cm2/s) and 17.3 ± 4.3 cm3/mol (log D0 = −10.81 ± 0.07; D0 in cm2/s) for Ar and Kr, respectively. Both values are close to the atomic volume. A comparison with diffusivities of charged species in jadeite melts shows that Si4+, Al3+ and O2− diffusivities increase with increasing pressure while MD simulation predicts decreasing diffusivity of Na+. From the activation volumes of Ar and Kr an average jump distance of 1.77 nm can be estimated. For Na+, Ar and Kr a linear relationship between activation volume and activation energy is observed.