High-pressure, high-temperature deformation of dunite, eclogite, clinopyroxenite and garnetite using in situ X-ray diffraction

Abstract

The rheology of eclogite, garnetite and clinopyroxenite in the peridotitic upper mantle was experimentally investigated in a large volume press combined with in situ synchrotron X-ray diffraction techniques to study the impact on mantle convection resulting from the subduction of oceanic lithosphere. Experiments were carried out over a range of constant strain rates (2×10−6–3×10−5 s−1), pressures (4.3 to 6.7 GPa) and temperatures (1050 to 1470 K). Results show substantial strength variations among eclogitic garnet and clinopyroxene and peridotitic olivine. At low temperatures (<1200 K), eclogite is over 1 GPa stronger than dunite. On the other hand, at high temperatures (>1400 K) eclogite is weaker than dunite by 0.2 GPa or more. Garnetite and clinopyroxenite exhibit higher strength than dunite at approximately 1200 K. However, at higher temperature (1370 K), clinopyroxenite is significantly weaker than garnetite (and dunite) by more than a factor of five. We explain these observations by transitions in deformation mechanisms among the mineral phases. In clinopyroxene, high temperature dislocation creep resulting in a strength reduction replaces low temperature twinning. Whereas garnet remains very rigid at all experimental conditions when nominally anhydrous (‘dry’). Microstructural observations show phase segregation of clinopyroxene and garnet, development of a crystallographic and shape preferred orientation in the former but not in the latter, suggesting an overall weak seismic anisotropy. Detection of eclogite bodies in the peridotite-dominated mantle may only be possible via observation of high VP/VS1 ratios. A comparable or weaker rheology of eclogite to dunite suggests effective stirring and mixing of eclogite in the convecting mantle.