Effect of deformation on helium storage and diffusion in polycrystalline forsterite

Co-auteur étranger ; International audience ; Although recent studies have investigated He behavior in undeformed mantle minerals, the effect of defects generated by plastic deformation on He storage and transport remains unconstrained. For this purpose, synthetic dense aggregates of fine-grained iron-free forsterite were deformed under 300 MPa confining pressure at 950, 1050, and 1200 °C using a Paterson press. Three deformed samples and one undeformed sample were then doped with He under static high-pressure (1.00 ± 0.02 GPa) and high-temperature (1120 ± 20 °C) conditions for 24 h in a piston cylinder. Uraninite was used as a source of noble gases. The samples were subsequently analyzed using a cycled step heating protocol coupled with noble gas mass spectrometry to investigate He storage and diffusion in the deformed polycrystalline forsterite aggregates. Results show complex diffusive behaviors that cannot be fitted by a single linear regression. Nevertheless, individual step heating cycles can be fitted by several linear regressions determined by a F-test, suggesting that diffusivities follow Arrhenius law within the given temperature ranges. Our results highlight the complex diffusive behavior of He in deformed forsterite aggregates, which is due to the competition between several diffusion mechanisms related to different He storage sites (Mg vacancies, interstitial sites, dislocations, and grain boundaries). Diffusion parameters (activation energy Ea and pre-exponential factor D0) for He diffusion in grain boundaries were refined from literature data (Ea = 36 ± 9 kJ·mol−1 and D0 = 10−10.57 ± 0.58 m2·s−1), and those of He diffusion in interstitials (Ea = 89 ± 7 kJ·mol−1 and D0 = 10−8.95 ± 1.16 m2·s−1) and Mg vacancies (Ea = 173 ± 14 kJ·mol−1 and D0 = 10−5.07 ± 1.25 m2·s−1) were defined from our results and literature data. Furthermore, we determined Ea = 56 ± 1 kJ·mol−1 and D0 = 10−9.97 ± 0.37 m2·s−1 for He diffusion along dislocations. These results suggest that a maximum He fraction of only 1.2% can be ...