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3.1 h. Characterization of dislocations in natural and synthetic majorite garnets (P. Cordier and F. Langenhorst, in collaboration with V. Voegelé/Lille and S. Heinemann/Potsdam)

Majorite garnet is a major mineralogical component in petrologic models of the Earth's mantle transition zone (depth 400-660 km). In order to understand the rheological behavior of the transition zone, we are studying the deformation mechanisms of majorite through investigations of the dislocation microstructures of deformed samples.

Because natural samples of high-pressure minerals from the Earth's mantle are almost non-existent, shocked meteorites are the main natural source of such minerals. However when applying observations of microstructures of shocked meteorites to the Earth's mantle, possible effects of the extreme pressure-temperature-time histories of shocked meteorites have to be considered. In the present transmission electron microscope (TEM) study, we have characterized the dislocation microstructures in majorite which occurs as clasts in shock veins of the ordinary chondrite Acfer 90072 L5-6 (S6) (Algerian Sahara). The microstructures have been compared to those in majoritic garnets synthesized at 19 GPa, 2000°C in a multianvil apparatus. Samples along the join Mg4Si4O12 (majorite end member) - Mg3Al2Si3O12 (pyrope) have been studied. These samples exhibit both cubic (meteoritic majorite and synthetic Ma50-Py50) and tetragonal (synthetic Ma96-Py4 and Ma100) symmetries. In the case of synthetic majorite-rich samples (Ma96-Py4 and Ma100), run durations at 19 GPa, 2000°C have been varied from 7 min to 1 hour.

Majorite is observed in two different occurrences in the shock veins of Acfer 90072: (a) small, defect-free grains in the matrix of the veins and (b) within large polymineralic fragments in which majorite is usually associated with ringwoodite. Only majorite in this latter occurrence is considered here. Microanalysis in the TEM shows that majorite grains from the fragments have the same chemical composition as the orthopyroxene in the bulk meteorite. This observation suggests that the majorite formed from the pyroxene by a solid-state phase transformation (the associated ringwoodite is also believed to have a similar origin). Most of the grains contain numerous dislocations that form sub-grain boundaries organized in cells (Fig. 3.1-10). The sizes of the cells are of the order of a micrometer and the misorientation between adjacent cells is small (< 5-10°). Some cells contain a few free dislocations that sometimes appear to be slightly dissociated in weak-beam dark field images.

Fig. 3.1-10: Acfer meteorite. Majorite grain from a fragment in a shock vein. The microstructure consists primarily of dislocations arranged in subgrain boundaries. TEM Weak Beam Dark field micrograph. (g: 400).

The microstructures of synthetic samples are very similar to those from the meteorite. Most of the grains contain dislocations, many of which are arranged in subgrain boundaries which usually form cells (Fig. 3.1-11). Dissociated dislocations have been found in some grains. Systematic variations in microstructure with composition and symmetry have so far not been identified. However, tetragonal majorite-rich garnets show pervasive occurrence of merohedral and pseudomerohedral twins (see 1995 Annual Report) and dislocations are found to interact elastically with the pseudomerohedral twins.

Detailed characterization of dislocation Burgers vectors have been performed on these samples by Large Angle Convergent Beam Electron Diffraction (LACBED) in the TEM. 1/2<111> and <100> dislocations are found in all samples, both meteoritic and synthetic.

Fig. 3.1-11: MgSiO3 majorite synthesized at 19 GPa, 2000°C for 1 hour. Dislocations are arranged in subgrain boundaries. TEM Weak Beam Dark field micrograph (g: 400).

In summary, the microstructures (subgrain boundaries and dislocations) observed in majorite garnets are comparable to those observed in low-pressure garnets (e.g. pyrope) deformed at high temperature. Also, the study of synthetic majorite samples annealed for times ranging from 7 min to 1 hour did not reveal any change in the microstructures with time. The recovery behavior of majorite is thus very uniform over a wide range of pressure-temperature-time conditions. On the basis of the experimental data, it is thus difficult to infer the P-T-t history of the meteoritic samples.

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