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

Dec 2, 2015 An electron tomography study of deformation in olivine Results

A new publication in European Journal of Mineralogy with M. Nafi (Département de Production - Casting 2 (ACE Line) - Indonésie) and S. Demouchy (Géoscience Montpellier).

This article studies the deformation mechanisms of olivine at the lithospheric temperatures, using the electron tomographic technique. For that purpose, we compare two single crystals deformed at ca. 800°C, along different crystallographic orientations. From a geometrical point of view, the first crystal (PoEM 9) is oriented to solicitate preferentially the (100), (110) and (1-10) planes; and the second crystal (PoEM 11) is oriented to solicitate preferentially the (010), (140), (1-40), (130), (1-30) and, to a lesser extent, the (120) and (1-20) glide planes. Table 1 gives the colour code used to identify the planes containing the indexed dislocation segments.

 

 

Microstructures are totally different (figure 1 and 2). As expected, we note the (100), (110) and (1-10) glide planes in PoEM 9 (figure 1). However, in PoEM 11, the dislocation configuration is more complex (figure 2). The (010), (140), (1-40), (130), (1-30), (120) and (1-20) glide planes are sparsely solicited whereas the the (100), (110) and (1-10) glide planes are highly solicited even if the Schmid factor is low. Consequently, the dislocations glide easily in (100), (110) and (1-10). Moreover, numerous cross-slip configurations are noted in PoEM 11. Actually, many dislocation segments escape from the hard slip planes ((010), (140), (1-40), (130), (1-30), (120) and (1-20)) cross slipping to the easy slip planes ((100), (110) and (1-10)).

 

 

Figure 1 = Indexation of a tilted series of PoEM 9. (a) Micrograph in WBDF conditions, with g: 22-2, showing very few non-screw dislocation segments; (b) corresponding indexed micrograph, where (100), (1-10) glide planes and sessile planes are characterized (yellow dislocations could not be indexed due to their straight-lined configurations).

 

 

Figure 2 = Indexation of a tilted series of PoEM 11. (a) WBDF image observed with g: 004; (b) corresponding indexation; one can notice a dominance of light grey (1-10) glide planes, and red and orange segments are sessile.

 

Numerous sessile segments and prismatic loops are visible in PoEM 11. This article describes the formation source of these segments and loops. Dipoles of dislocations c and -c create prismatic loops by progressive climb annihilation mechanisms (figure 3). These sessile loops interact with mobile dislocations by collinear annihilation mechanisms (figure 4).

 

 

Figure 3 = Sample PoEM 11: break-up of dislocation dipoles into strings of dislocation loops. Micrograph in WBDF condition, with g: 004, where three dislocation dipoles (coloured in yellow) are annihilating by climb and two of them are practically totally annihilated.

 

 

Figure 4 = Collinear interaction between a screw dislocation and a loop in PoEM 11. Micrographs obtained, with g: 004, where a family of three loops (labelled 1, 2 and 3) are in contrast (area extracted from Fig. 2a): projected angle of (a) -56 degree; (b) -36 degree; (c) -2 degree; (d) 30 degree; and (e) 44 degree. The indexed micrograph (f) which corresponds to (c) shows that the red sessile segment, labelled 2, comes from the collinear interaction of a screw dislocation and a sessile loop.

 

Finally, we can trace back the deformation mechanism of olivine at lithospheric temperatures:

-          Dislocation c and -c glide in (100), (110) and (1-10) and form dipoles which strain-harden the material;

-          These dipoles are progressively annihilated by climb and generate prismatic loops;

-          Mobile dislocations interact with these obstacles by collinear annihilation mechanisms;

-          Entanglements are produced

 

See the paper just published by our group:

A. Mussi, M. Nafi, S. Demouchy, P. Cordier (2015): On the deformation mechanism of olivine single crystals at lithospheric temperatures: an electron tomography study. European Journal of Mineralogy, 27, 707-715 doi: doi.org/10.1127/ejm/2015/0027-2481