A comparison between hydrogen and helium diffusion in diamondat Earth’s mantle conditions

Impurities and mineral or fluid inclusions in natural diamonds can provide valuable insights into the evolution of mantle conditions through geologic time (Stachel et al., 2015). In particular, information on the isotopic composition of the early mantle can be inferred by studying helium and hydrogen impurities in diamond, which are usually trapped either as fluid inclusions or as interstitial defects in the diamond lattice. However, a critical aspect to consider is whether or not the isotopic information carried by diamonds is still representative of the original diamond-forming fluid. Processes such as diffusion might occur during prolonged residence time in the Earth’s mantle at high temperature conditions, leading to the re-equilibration of He and/or H with the surrounding mantle, either by loss or gain of He and H themselves. Here we compute the diffusion of He and H in diamond using ab initio molecular dynamics and machine learning molecular dynamics simulations, as implemented in the Vienna Ab Initio Simulation Package (VASP); postprocessing was realized using the UMD package (Caracas et al.,2021). All simulations were performed on a broad range of high pressure and high temperature conditions, compatible with those expected in the Earth’s mantle where diamonds are formed. We determine the diffusion coefficients as a function of both pressure (0, 5 and 10 GPa) and temperature (300 – 3000 K) for He and H. We show that diamonds at greater depths may act as closed systems throughout geological time.MLF, MCD, FI, and MGP acknowledge funding from the European Union (ERC, INHERIT, Starting Grant No. 101041620) Caracas, R., Kobsch, A., Solomatova, N. V., Li, Z., Soubiran, F., & Hernandez, J. A. (2021). Analyzing melts and fluids from ab initio molecular dynamics simulations with the UMD package. JoVE, e61534. doi:10.3791/61534Cherniak, D. J., Watson, E. B., Meunier, V., & Kharche, N. (2018). Diffusion of helium, hydrogen and deuterium in diamond: Experiment, theory and geochemical applications. Geochimica et Cosmochimica Acta, 232, 206-224. https://doi.org/10.1016/j.gca.2018.04.029Stachel, T., & Luth, R. W. (2015). Diamond formation—Where, when and how? Lithos, 220, 200-220. https://doi.org/10.1016/j.lithos.2015.01.028