Strength contrast between plagioclase and olivine at water-rich Moho depths

Abstract

Strength contrast between plagioclase and olivine was tested utilizing two-layer deformation experiments under hydrous conditions at the pressure and temperature corresponding to the Moho (P = 1.0 GPa and T = 400-800 °C). Deformation microstructures characterized by lattice-preferred orientation and dislocation density indicate that both minerals were plastically deformed via dislocation-controlled creep. Our experimental results show that olivine is weaker than plagioclase at T = 400 °C, whereas plagioclase is weaker than olivine at T = 800 °C. Consequently, strength contrast between plagioclase and olivine is sensitive to temperature, and olivine may be weaker than plagioclase, or display almost no difference in strength between these materials under the wet continental Moho. This suggests that the "crème brulee"model, in which the upper mantle is weak and the strength is limited to the crust, is expected as a rheological layer model of the water-rich Moho conditions.