High-pressure, high-temperature deformation of CaGeO3 (perovskite)±MgO aggregates: Implications for multiphase rheology of the lower mantle

Sintered polycrystalline rocks of two-phase aggregate CaGeO3 perovskite (GePv)+MgO and single-phase GePv were deformed at pressure, temperature, and strain rates of 4-10 GPa, 600-1200 K, and approximate to 1-3 x 10(-5) s(-1), respectively, with maximum bulk strains up to approximate to 20%. The as-synthesized two-phase aggregate, produced from the reaction CaMgGeO4 (olivine) GePv+MgO (MgO occupying approximate to 28% in volume), possessed a load-bearing framework (LBF) texture indistinguishable from that of (Mg,Fe)(2)SiO4 (Mg,Fe)SiO3 perovskite+(Mg,Fe)O reported in previous studies. Stress states of the two phases in the deforming aggregate were evaluated based on systematic distortion of lattice spacings over the entire 360 degrees diffraction azimuth angle. Compared with the single-phase GePv sample, stresses of GePv in the two-phase composite were about 10-20% higher at similar strain and strain rates. Stresses of MgO are about a factor of approximate to 2 lower than GePv in the same two-phased rock. Volumetrically averaged bulk stresses in the two samples were therefore virtually identical. Texture analyses showed that both samples deformed by dislocation glide, with the dominant slip systems {1 0 0}<1 1 0> (in cubic setting) for both GePv and MgO. These results show that, at low bulk strains up to approximate to 20%, the two-phase aggregate remains a LBF fabric, with rheological properties of GePv controlling those of the bulk. These experimental findings are in quantitative agreement with previous numerical simulations. Implications of the results to the silicate counterparts and dynamics of the lower mantle are discussed.