Abstract
Helium is a key tracer of mantle geochemical and isotopic heterogeneities and can constrain our understandingof mantle geodynamics. Nevertheless, the mechanisms of helium storage and transport in mantle minerals remainpoorly understood. Polycrystalline olivine was doped with helium at high temperature (1050 ± 25 °C) andhigh pressure (0.30 ± 0.01 GPa), followed by step heating extraction experiments to investigate helium storageand diffusion in Earth's upper mantle. We also tested the effect of heterogeneous initial concentrations on theextracted diffusivities, and demonstrate the robustness of diffusion parameters obtained in this study. Our resultsshow that two diffusion processes are acting in polycrystalline olivine: (i) a high temperature process with highactivation energy (Ea) where diffusion is only controlled by lattice diffusion, and (ii) a lower temperature processwith lower Ea where diffusion is controlled by both grain boundary and lattice diffusion. These two diffusionprocesses are separated by a transition temperature that depends on the depletion of helium hosted in grainboundaries, i.e., the amount of helium stored at grain boundaries and the temperature and duration of the stepheating sequence. Our results confirm that grain boundaries can represent a significant storage site for He.Moreover, we report two different populations of diffusion parameters in the lattice diffusion field, which areinterpreted as diffusion in interstitials (Ea=95 ± 15 kJ·mol−1 and log(D0)=−8.26 ± 2.13) and Mg vacancies(Ea=168 ± 19 kJ·mol−1 and log(D0)=−3.59 ± 2.12). Similar diffusion parameters populations areobserved in literature data after reprocessing the diffusivities. Furthermore, we determine grain boundary diffusionparameters: Ea=57 ± 14 kJ·mol−1 and log(D0)=−9.20 ± 0.99. Applying these results to the uppermantle reveals that an important amount of He can be stored at grain boundaries for typical mantle grain size(22% for a grain size of 1 mm) and that most helium can be stored at grain boundaries for relatively small grainsizes (≤290 μm and ≤10 μm for segregation factors of 1/10−5 and 1/0.0025, respectively). As a consequence,bulk diffusivities can be significantly higher than lattice diffusivities. Although our study cannot be applieddirectly to the lower mantle, the similar storage sites and diffusion mechanisms are expected in lower mantlesilicates if high pressure does not inhibit diffusion.