Helium incorporation and diffusion in polycrystalline olivine
Résumé
Helium is a key tracer of mantle geochemical and isotopic heterogeneities and can constrain our understanding
of mantle geodynamics. Nevertheless, the mechanisms of helium storage and transport in mantle minerals remain
poorly understood. Polycrystalline olivine was doped with helium at high temperature (1050 ± 25 °C) and
high pressure (0.30 ± 0.01 GPa), followed by step heating extraction experiments to investigate helium storage
and diffusion in Earth's upper mantle. We also tested the effect of heterogeneous initial concentrations on the
extracted diffusivities, and demonstrate the robustness of diffusion parameters obtained in this study. Our results
show that two diffusion processes are acting in polycrystalline olivine: (i) a high temperature process with high
activation energy (Ea) where diffusion is only controlled by lattice diffusion, and (ii) a lower temperature process
with lower Ea where diffusion is controlled by both grain boundary and lattice diffusion. These two diffusion
processes are separated by a transition temperature that depends on the depletion of helium hosted in grain
boundaries, i.e., the amount of helium stored at grain boundaries and the temperature and duration of the step
heating 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 are
interpreted 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 are
observed in literature data after reprocessing the diffusivities. Furthermore, we determine grain boundary diffusion
parameters: Ea=57 ± 14 kJ·mol−1 and log(D0)=−9.20 ± 0.99. Applying these results to the upper
mantle 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 grain
sizes (≤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 applied
directly to the lower mantle, the similar storage sites and diffusion mechanisms are expected in lower mantle
silicates if high pressure does not inhibit diffusion.