Helium storage and diffusion in the deformed Earth’s upper mantle
Résumé
Helium (He) provides important information on
chemical and isotopic mantle heterogeneities since it
behaves 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 sites
and diffusion mechanisms) remained poorly under-
stood at high pressure and high temperature. As an
incompatible element, He is preferentially stored in
defects within the crystal structure in mantle rocks,
such as point defects (i.e., Mg vacancies and intersti-
tial sites), linear defects (i.e., dislocations), planar
defects (i.e., grain boundaries), and 3-dimensions
defects (i.e., pores and inclusions). Recent experi-
mental studies were able to constrain He storage in
polycrystalline olivine, settling that He is preferen-
tially stored in grain boundaries rather than in point
defects within the crystal lattice ([1], [2]). It implies
that ~22% of He amount is stored in grain bounda-
ries at typical mantle grain sizes, inducing a signifi-
cant enhancement of bulk diffusivities compared to
lattice diffusivities. Nevertheless, He storage and
transport in planar defects is still poorly understood
as well as the implications of deformation processes
also remain to be determined. Since incompatible
elements are preferentially stored along dislocations
in zircon ([3]), the same behavior is expected to oc-
cur for He in mantle mineral lattice.
In this study, the implications of deformation
processes on He storage and transport have been
tested in deformed fine-grained synthetic polycrys-
talline forsterite. The starting material consisted in
sintered forsterite aggregates with a grain size of ~3
μm. Samples were then deformed in axial compres-
sion at 300 MPa and 950, 1050 or 1200 °C using a
Paterson press. Three deformed samples and one
undeformed sample were subsequently doped in
presence of He source (i.e., uraninite from Mis-
tamisk, Canada) at 1 GPa and 1120 °C in a piston
cylinder apparatus. Helium was then analyzed by
coupling a cycled step heating protocol with a noble
gas 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 textures
and microstructures.
Our results show complex diffusive behaviors
with diffusivities that cannot be fitted by a single
linear regression. Thus, a F-test has been performed
on each individual step heating cycle showing that
diffusivities can be fitted by several linear regres-
sions. It highlights the competition between different
diffusion mechanisms related to different He storage
sites (Mg vacancies, interstitial sites, dislocations,
and grain boundaries). Activation energy (Ea) and
pre-exponential factor (D0) for He grain boundary
diffusion 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 Mg
vacancies (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 included
between those of He diffusion in grain boundaries
and 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 with
the highest dislocation density and millimetric grain
size, a maximum He fraction of only 1.2% can be
stored along dislocations. This value is well below
the He fraction of 22%, which can be stored in grain
boundaries at typical mantle grain sizes. Moreover,
bulk diffusivities are affected by the presence of He
in grain boundaries but the He amount stored along
dislocations is too small to significantly modified
bulk lattice diffusivities, regardless of the dislocation
density. It implies that deformation processes could
only increase He storage capacity and mobility in
mantle rocks by reducing grain size (via dynamic
recrystallisation). This process can implicate an in-
crease of bulk concentrations of the deformed peri-
dotites upon equilibration with nearby undeformed
(or less deformed) peridotites.