Diffusivity and solubility of He in garnet: An exploratory study using nuclear reaction analysis

Abstract

We have used the nuclear reaction 3He(d,p) 4He in an exploratory study of the diffusivity and solubility of He in the mineral garnet. The garnet samples analyzed were single crystals which had been loaded with 3He at 7000 bar pressure and temperatures of 900, 950 and 1000 °C. From the 3He depth profiles measured we could deduce diffusion coefficients and solubility values. For the diffusivity the Arrhenius parameters D 0 = 7.5 × 10 9 cm 2/s and E a = 500 kJ/mol were found. An estimate of 11.5 at ppm was obtained for the apparent solubility, which comprises volume solubility and dislocation solubility, and an upper limit of 3 at ppm for the volume solubility. Furthermore, out-diffusion of He at room temperature occurring faster than to be expected from the high-temperature diffusivity was observed. The results are compared to those of a preceding study in which a bulk technique for measuring 4He had been used. It can be concluded that the NRA technique is well suited to the problem in question.