DIFFUSION OF AR IN RHYOLITE, ORTHOCLASE AND ALBITE COMPOSITION GLASSES

The diffusivity of Ar has been determined in a natural rhyolite obsidian (Rhy) and in glasses of NaAlSi3O8 (Ab) and KAlSi3O8 (Or) composition at temperatures between 400 and 900-degrees-C, at pressures from 1150 to 3700 bar. The observed variations in the Ar diffusion coefficient (D) with temperature can be described by the Arrhenius relationship D = D0 exp(- E/RT) with the following results (for pressures from 1100 to 2500 bar; D in cm2/s; R in cal/mole): (Ab) D = 1.62 x 10(-3) . exp(- 32780/RT); (Or) D = 6.64 x 10(-3) . exp(- 29000/RT); (Rhy) D = 6.86 x 10(-3) . exp(- 34490/RT). The data, although of limited extent, also indicate a slight decrease in D with increasing pressure and the calculated activation volume of 36 (+/- 8) cm3/mole for Ar in Rhy at 800-degrees-C agrees with the idea that the volume change associated with a diffusive jump is similar to the size of the diffusing species. Comparison of the Ar results with cation tracer diffusivities indicates that Ar diffusivity is similar to that of rubidium in Rhy and Ab. Argon diffusivity is several orders of magnitude slower than H2O at T < 900-degrees-C and exsolution of H2O to form bubbles might leave a residual rhyolitic melt enriched in Ar (and Kr, Xe) due to kinetic effects. Noble gases show lower activation energies for diffusion than are observed for tracer diffusion of cations with comparable radii. A systematic increase in activation energy for diffusion with increasing size of the noble gas atoms, and similarities in activation energies in rhyolitic through basaltic melts suggests that in basaltic melts, He, Ne, and probably Ar can diffuse faster than CO2, the major volatile component in basalts. In contrasts, Xe and possibly Kr, due to their (estimated) high activation energies may diffuse more slowly than CO2 at temperatures below 1200-degrees-C. This implies that kinetic factors may influence noble gas abundance patterns during separation of a CO2-rich vapor phase from degasing basaltic magma during ascent.