Diffusional fractionation of helium isotopes in silicate melts

Estimating Helium (He) concentration and isotope composition of the mantle requires quantifying He loss during magma degassing. The knowledge of diffusional He isotope fractionation in silicate melts may be essential to constrain the He loss. Isotopic mass dependence of He diffusion can be empirically expressed as (DHe)-He-3/(DHe)-He-4 = (4/3)(beta), where D is the diffusivity of a He isotope. However, no studies have reported any beta values for He in silicate melts due to technical challenges in both experiments and computations. Here, molecular dynamics simulations based on deep neural network potentials trained by ab initio data show that beta for He in albite melt decreases from 0.355 +/- 0.012 at 3000 K to 0.322 +/- 0.019 at 1700 K. beta in model basalt melt takes a smaller value from 0.322 +/- 0.025 to 0.274 +/- 0.027 over the same temperature range. Based on our results, we suggest using (DHe)-He-3/(DHe)-He-4 values of 1.097 +/- 0.006 and 1.082 +/- 0.008 in natural rhyolite and basalt melt, respectively, to interpret measured He concentration and isotope composition of natural samples.