Helium diffusion in olivine based on first principles calculations

As a key trace element involved in mantle evolution, the transport properties of helium in the mantle are important for understanding the thermal and chemical evolution of the Earth. However, the mobility of helium in the mantle is still unclear due to the scarcity of measured diffusion data from minerals under mantle conditions. In this study, we used first principles calculations based on density functional theory to calculate the absolute diffusion coefficients of the helium in olivine. Using the climbing images nudged elastic band method, we defined the diffusion pathways, the activation energies (Ea), and the prefactors. Our results demonstrate that the diffusion of helium has moderate anisotropy. The directionally dependent diffusion of helium in olivine can be written in Arrhenius form as follows. D[100]=2.42×10-7exp(-128.33kJ/mol/RT)m2/sD[010]=9.97×10-7exp(-147.62kJ/mol/RT)m2/sD[001]=3.85×10-7exp(-128.33kJ/mol/RT)m2/sOur results are in excellent agreement with previous experiments. We also performed calculations where the pressure reached up to 14 GPa in order to investigate the effect of pressure. As the pressure rose from 0 to 14 GPa, Ea[1 0 0] and Ea[0 0 1] increased to 162.10 kJ/mol, and Ea[0 1 0] increased to 167.89 kJ/mol. Based on the calculated diffusion coefficients, we theoretically estimate that helium can migrate between 2.7 m in the upmost upper mantle (∼3 GPa, 1180 K) and 20.7 m in the deepest upper mantle (∼14 GPa, 1800 K) via the fastest lattice diffusion in 1 Ma. In addition, as a possible thermo-chronometry tool, we found that the closure temperature for helium in olivine in most earth-surface conditions ranged from about 143 to 244 °C.