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 (similar to 1.6-3.4 mm in diameter) were performed in an internally heated pressure vessel at 800 degrees 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 = D-0 exp(-PV/RT) to the data yields activation volumes of 11.4 +/- 1.3 cm(3)/mol (log D-0 = -9.14 +/- 0.02; D-0 in cm(2)/s) and 17.3 +/- 4.3 cm(3)/mol (log D-0 = -10.81 +/- 0.07; D-0 in cm(2)/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.