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Earth's Upper Mantle Rheology
Projet financé par l'ANR MANTLE
RHEOLOGY
Partners: O. Castelnau (PIMM), P. Cordier (UMET), R.A. Lebensohn
(LANL), S. Merkel (UMET), P. Raterron (UMET), W. Crichton (ESRF)
Earth's plate tectonics generates major haz(~30%)ards
for human societies (earthquakes, volcanic eruptions, tsunamis, etc.).
It is largely constrained by the rheology of the Earth upper mantle
(Figure 1, top 410 km) composed of polycrystalline
aggregates comprising olivine (~60 vol.%) mixed with
pyroxenes (~30%) and other minor phases such as garnets, and
possibly melt.
Figure 1: Earth
cut (image by Colin Rose)
The
complex behavior of this material comes from the extreme viscoplastic
anisotropy at the crystal scale, so that the simultaneous activation of
several deformation mechanisms (dislocation glide, dislocation climb,
grain boundary diffusion ...) is probably necessary. Despite decades of
experimental work, rheology of Earth mantle minerals is not well
understood for several reasons:
 until recently, deformation experiments could not reach the
pressure (~14 GPa) and temperature (~1500K) of the mantle;
 the pressure has a strong influence on dislocation mobility;
 data extrapolation to the extremely small in situ
strainrates (~10^{15 }s^{1}) requires a multiscale
approach coupling several techniques and modeling;
 the extreme local anisotropy of all mineral phases requires
the use of accurate homogenization models, which have almost never been
applied in the context of deep Earth minerals.
The aim of this project is to achieve an accurate
modeling of uppermantle plasticity, which accounts for highpressure
experimental data on olivine and olivinepyroxenesgarnetmelt
aggregates obtained in stateofthe art high pressure equipments set at
synchrotron facilities on mono and polycrystals, ab initio calculations for the
dislocation properties at the nanometer scale, and polycrystal
plasticity inferred from micromechanical
modeling approaches.
The resulting multiscale model (from nanometer to kilometer scales)
will be a firstofakind and, while applied in specific geodynamical
context, will provide crucial information for the interpretation of
uppermantle seismic data and the modeling of mantle convection
(Figure 2).
Figure 2:
in earth's mantle
The used polycrystal model will be based on meanfield homogenization methods
(selfconsistent scheme associated with the "SecondOrder"
linearization procedure of Ponte Castañeda).
This secondorder selfconsistent approach is very efficient for
minearl materials. The effective behavior predicted by this approach
are compared with ensemble averages of the the fast Fourier transform
FFTbased fullfield solutions. This comparison show that the
predictions obtayned by means of the secondorder approach are in
better agreement with the FFTbased fullfield solutions (Figure 3).
Figure 3 Right:
Stress field compute by FFT in olivine polycrystal
Left:
Comparison between FFTbased fullfield solutions (FFT) and several
meanfield approaches (SO: secondorder linearization; AFF: affine
linearization; TGT: tangent lenarization) for different values of
plastic anisotropic parameters M.
