Effects of water and iron content on the rheological contrast between garnet and olivine

The effects of water and iron content on the relative creep strengths of garnet and olivine were investigated by shear deformation experiments. arnet and olivine samples were sandwiched together between alumina pistons in a simple shear geometry and were deformed at P= 1–2 GPa, = 1473 K and strain rates ranging from 10−5 to 10−3 s−1 using a Griggs-type solid-medium apparatus. The stressand strain-rate relation, as ell as the deformation microstructures including lattice-preferred orientation and dynamic recrystallization, indicates that the deformation by islocation creep. The creep tests show that the Fe-rich garnet (Alm67Prp29Grs3) was slightly weaker than olivine (Fo90), whereas the Mg-rich arnet (Alm19Prp68Grs12) was significantly stronger than olivine under dry conditions. The wet experiments show that the creep rate of the Mg-rich arnet is more sensitive to water than olivine; the water fugacity exponent on strain rate was estimated to be ∼2.4 for garnet and ∼1.2 for olivine, nd the Mg-rich garnet becomes weaker than olivine in a water-rich environment. The experimental results show that the rheological contrast etween garnet and olivine depends strongly on water content and to a lesser degree on Fe content. Consequently, the geodynamic behavior of eochemical reservoirs can be sensitive to their chemical environments in the upper mantle. 2007 Elsevier B.V. All rights reserved.