Helium storage and diffusion in the deformed Earth’s upper mantle

International audienceHelium (He) provides important information onchemical and isotopic mantle heterogeneities since itbehaves as both incompatible and volatile element,and his recycling is limited during subduction pro-cesses. Nevertheless, the fundamental physical be-havior of He in mantle minerals (e.g., storage sitesand diffusion mechanisms) remained poorly under-stood at high pressure and high temperature. As anincompatible element, He is preferentially stored indefects within the crystal structure in mantle rocks,such as point defects (i.e., Mg vacancies and intersti-tial sites), linear defects (i.e., dislocations), planardefects (i.e., grain boundaries), and 3-dimensionsdefects (i.e., pores and inclusions). Recent experi-mental studies were able to constrain He storage inpolycrystalline olivine, settling that He is preferen-tially stored in grain boundaries rather than in pointdefects within the crystal lattice ([1], [2]). It impliesthat ~22% of He amount is stored in grain bounda-ries at typical mantle grain sizes, inducing a signifi-cant enhancement of bulk diffusivities compared tolattice diffusivities. Nevertheless, He storage andtransport in planar defects is still poorly understoodas well as the implications of deformation processesalso remain to be determined. Since incompatibleelements are preferentially stored along dislocationsin zircon ([3]), the same behavior is expected to oc-cur for He in mantle mineral lattice.In this study, the implications of deformationprocesses on He storage and transport have beentested in deformed fine-grained synthetic polycrys-talline forsterite. The starting material consisted insintered forsterite aggregates with a grain size of ~3μm. Samples were then deformed in axial compres-sion at 300 MPa and 950, 1050 or 1200 °C using aPaterson press. Three deformed samples and oneundeformed sample were subsequently doped inpresence of He source (i.e., uraninite from Mis-tamisk, Canada) at 1 GPa and 1120 °C in a pistoncylinder apparatus. Helium was then analyzed bycoupling a cycled step heating protocol with a noblegas mass spectrometer. This method permits to de-termine He diffusivity for each temperature step.Additionaly, SEM and TEM analyses were per-formed on pre-doped samples to constrain texturesand microstructures.Our results show complex diffusive behaviorswith diffusivities that cannot be fitted by a singlelinear regression. Thus, a F-test has been performedon each individual step heating cycle showing thatdiffusivities can be fitted by several linear regres-sions. It highlights the competition between differentdiffusion mechanisms related to different He storagesites (Mg vacancies, interstitial sites, dislocations,and grain boundaries). Activation energy (Ea) andpre-exponential factor (D0) for He grain boundarydiffusion have been refined from previous studies(Ea = 36 ± 9 kJ·mol–1 and D0 = 10–10.57 ± 0.58 m2·s–1),while those of He diffusion in intersitial sites (Ea =89 ± 7 kJ·mol–1 and D0 = 10–8.95 ± 1.16 m2·s–1) and Mgvacancies (Ea = 173 ± 14 kJ·mol–1 and D0 = 10–5.07 ±1.25 m2·s–1) are obtained from our results and litera-ture data. A last set of diffusion parameters includedbetween those of He diffusion in grain boundariesand those in interstitials are interpreted as corre-sponding to He diffusion along dislocations (Ea = 56± 1 kJ·mol–1 and D0 = 10–9.97 ± 0.37 m2·s–1).By applying these results to mantle rocks withthe highest dislocation density and millimetric grainsize, a maximum He fraction of only 1.2% can bestored along dislocations. This value is well belowthe He fraction of 22%, which can be stored in grainboundaries at typical mantle grain sizes. Moreover,bulk diffusivities are affected by the presence of Hein grain boundaries but the He amount stored alongdislocations is too small to significantly modifiedbulk lattice diffusivities, regardless of the dislocationdensity. It implies that deformation processes couldonly increase He storage capacity and mobility inmantle rocks by reducing grain size (via dynamicrecrystallisation). This process can implicate an in-crease of bulk concentrations of the deformed peri-dotites upon equilibration with nearby undeformed(or less deformed) peridotites