Helium diffusion and solubility in obsidians and basaltic glass in the range 200–300°C

Abstract

Helium diffusion and solubility have been measured by the permeation technique in two natural obsidians and one basaltic glass in the range 200–300°C. Diffusivities for obsidians are of the order of 1 to 5 × 10 −7 cm 2 /s and obey the Arrhenius laws D I = 7.7 × 10 −4 exp[−8.010/ RT ] and D VC = 1.6 × 10 −3 exp[−8.590/ RT ] for Iceland and New Mexico obsidian, respectively. Mean solubilities are S I = 2.2 × 10 −3 and S VC = 2.6 × 10 −3 cm 3 STP/g atm. Diffusivity and solubility in basalt glass are considerably lower and permeation alone could be determined accurately. An estimation of D b ⋍ 10 −3 cm 2 /s and S b ⋍ 4 × 10 −4 cm 3 STP/g atm at 250°C is proposed, with an activation energy for diffusion of ⋍14 ± 2kcal/mole. These results permit a quantitative discussion of the exchange of He between water, gas and glass during the analyses as well as in geological settings. Vesiculation and diffusion loss appear to be the most likely processes in degassing basaltic melts and glasses.