Incorporation et diffusion de l’hélium et de l’argon dans l’olivine polycristalline

Abstract : Noble gases are key tracers of mantle geochemical and isotopic heterogeneities and can constrain our understanding of mantle geodynamics. Nevertheless, the basic mechanisms of noble gas storage and transport in mantle minerals remain poorly understood. In this PhD thesis, I focused on helium and argon to constrain their storage sites and the diffusive mechanisms, which occur in mantle rocks. Polycrystalline olivine was doped with helium and argon at high temperature (1150 ± 25 and 1050 ± 25 °C) and high pressure (0.30 ± 0.01 GPa), followed by step heating extraction experiments. I also tested the effect of heterogeneous initial concentrations on the extracted diffusivities, and demonstrate the robustness of diffusion parameters obtained in this study. My results show that two diffusion domains are present in polycrystalline olivine: (i) a high temperature domain with high activation energy (Ea) where diffusion is only controlled by lattice diffusion, and (ii) a lower temperature domain with lower Ea where diffusion is controlled by both grain boundary and lattice diffusion. These two domains are separated by a transition temperature that depends on the depletion of helium or argon hosted in grain boundaries, i.e., the amount of helium or argon stored at grain boundaries and the temperature and duration of the step heating sequence. The results confirm that grain boundaries can represent a significant storage site for helium and argon. Moreover, I constrained argon and helium diffusion in olivine lattice. For helium, I report two different populations of Ea in the lattice diffusion domain, which are interpreted as diffusion in interstitials (Ea = 95 ± 15 kJ.mol-1) and Mg vacancies (Ea = 168 ± 19 kJ.mol-1). For argon, a mean value of diffusion parameters in olivine lattice (Ea = 166 ± 44 kJ.mol-1 and logD0 = −7.04 ± 1.13 with D0 in m2.s-1) is obtained for data from literature and this study. Furthermore, I determine grain boundary diffusion parameters: Ea = 45 ± 12 kJ.mol-1 and D0 = 5.30 ± 1.53 * 10-13 m2.s-1 for helium, and Ea = 22 ± 5 kJ.mol-1 and log(D0) = -12.33 ± 0.3 for argon with D0 in m2.s-1. Applying these results to the upper mantle reveals that high content of helium and argon can be stored at grain boundaries. As a consequence, bulk diffusivities can be significantly higher than lattice diffusivities, inducing important implications for mantle geochemistry and geodynamics
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Rémi Delon. Incorporation et diffusion de l’hélium et de l’argon dans l’olivine polycristalline. Sciences de la Terre. Université de Lorraine, 2017. Français. ⟨NNT : 2017LORR0207⟩. ⟨tel-01816003⟩

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