# RheoMan: a five-year, ERC-funded (Advanced Grant), project to model the rheology of the Earth's mantle

## Sep 17, 2015 Deformation of phase A Results

New publication in *Physics of the Earth and Planetary Interiors* with E. Amiguet^{1,2}, N. Bolfan-Casanova^{3}, Y. Wang^{4}, B. Reynard^{1}.

1 Laboratoire de Géologie de Lyon, ENS Lyon.

2 Institute of Condensed Matter Physics, EPFL, Lausanne.

3 Laboratoire Magmas et Volcans, Clermont-Ferrand.

4 Center for Advanced Radiation Sources, Chicago, USA

In this paper, we present a deformation study of an hexagonal hydrous phase that can exist in shear zones within subducting slabs, phase A, after dehydration of serpentine into pyroxene + phase A. Here we combine experimental and theoretical approaches to explore the deformation mechanisms of phase A.

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First, deformation experiments are performed with *in-situ* measurements of stress, strain and lattice preferred orientations (LPO). The observed LPO coupled with viscoplastic self-consistent modelling (VPSC) shown that the active glide plane are the (0001) basal and (01-10) prismatic planes at 580°C, and the (-2110) prismatic, (0001) basal and (11-21) pyramidal planes at 400°C.

*Figure 1 : Experimental results : Lattice strains (proxy for the differential stress) - bulk strain curves and textures refined from in-situ XRD (inverse pole figures for the maximum compressive axis). We attribute the difference in texture between 400°C runs and 580°C run to a change in slip systems contributions on the basis of VPSC modeling (figure 2). The starting texture for run D1333 shows a strong maximum due to spottiness on the diffraction, which is an unstable orientation since this maximum disappears with increasing strain.*

*Figure 2 : Inverse pole figures for the maximum compressive axis resulting from the VPSC models, with run parameters Table 1(see the paper). A and E best reproduce the experimental textures at ca. 400°C and ca. 580°C respectively.*

To complete the experimental results, we use a theoretical approach, by using the Peierls–Nabarro–Galerkin (PNG) model, coupled with first-principles calculations of generalized stacking fault (γ-surface, figures 3, 4 and 5).

*Figure 3 : γ-surface (in J.m ^{-2}) of (0001) plane. The hexagonal base (a_{1},a_{2},a_{3}) is represented, γ_{0}, γ_{1} and γ_{2} are the three stable stacking faults.*

*Figure 4 : γ-surface (in J.m ^{-2}) of (01-10) plane. γ^{I}_{0, }γ^{I}_{1 }γ^{I}_{2 }and γ^{I}_{3} are the stable stacking faults.*

*Figure 5 : γ-surface (in J.m ^{-2}) of (2-1-10) plane. γ^{II}_{0} and γ^{II}_{1} are the two stable stacking faults.*

We highlight the easiness of glide of 1/3<2-1-10> and [01-10] dislocations in the basal plane, due to the dissociation into several partials. This easiness of glide in the basal plane is confirmed by calculation of the Peierls stress, which is compatible with the ubiquity of basal slip in the experiments. The γ-surface calculations also suggest 1/3[2-1-13] and [0-111] dislocations in prismatic and pyramidal planes, which is also consistent with the experimental data.

*Figure 6 : (right) Core structure of 1/3[2-1-10] screw dislocation. Disregistry function f(x) and associated Burgers vector density r=df(x)/dx (dotted line) are plotted in the (0001) plane. The number i corresponds to the i ^{th} partial in the left figure. (left) γ-surfaces (in J.m^{-2}) of (0001) plane. γ_{0}, γ_{1} and γ_{2} are the three stable stacking faults. The dash arrows represent the dissociation path of the dislocation.*

The strong yield strength relative to others minerals, intrinsic weak elastic anisotropy, together with weak deformation textures, suggests that S-wave seismic anisotropy of phase A-bearing rocks is lower than hydrous subduction zone lithologies.

This study illustrates the importance in associating numerical models and experimental data, in order to understand the deformation mechanism of complex materials.

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See the paper just published by our group:

K. Gouriet, N. Hilairet, E. Amiguet, N. Bolfan-Casanova, Y. Wang, B. Reynard, P. Cordier (2015) " Plasticity of the dense hydrous magnesium silicate Phase A at subduction zones conditions ". *Physics of Earth and Planetary Interiors*, doi: *http://dx.doi.org/10.1016/j.pepi.2015.09.004*