Effect of iron content on the creep behavior of Olivine: 2. Hydrous conditions

We have undertaken an experimental investigation of the effect of iron content on the viscosity of Fe-Mg olivine aggregates deformed under hydrous conditions in order to provide a basis for comparing convection models for the mantle of Earth with those for the more iron-rich mantle of Mars. Fine-grained samples of Fe-bearing olivine with fayalite contents, Fa(x), of x = 100, 75, 50, 30 and 10 were deformed in triaxial compressive creep primarily in the dislocation creep regime under water-saturated conditions at temperatures of 1273 to 1473 K and a confining pressure of 300 MPa. Nickel sleeves around the samples of Fa(10), Fa(30) and Fa(50) set the oxygen fugacity at the Ni:NiO buffer and thus the water fugacity at less than or similar to 300 MPa, while Fe sleeves around samples of Fa(75) and Fa(100) set the oxygen fugacity at the Fe:FeO buffer and thus the water fugacity at less than or similar to 200 MPa. Samples were deformed in triaxial compression to a maximum strain of 0.2 at differential stresses from 10 to 300 MPa and strain rates from 10(-7) to 10(-3) s(-1). In the dislocation creep field at a given temperature, the viscosity of samples of Fa(50) is a factor of similar to 10 smaller than the viscosity of samples of Fa(30), while the viscosity of samples of Fa(30) is a factor of similar to 10 smaller than that of samples of Fa10. Our experimental results can be described by the flow law epsilon(disl) = C-disl(sigma/mu)(ndisl)X(Fa)(pdisl)exp((-Q(disl)(0) + alpha X-disl(Fa))/RT)f(H2O)(mdisl) with C-disl = 99.7 MPa-5/4 s(-1), n(disl) = 3.7, P-disl = 0.5, Q(disl)(0) = 510 kJ/mol, alpha(disl) = -120 kJ/mol, and m(disl) = 5/4. This flow law indicates that the viscosity of olivine of a specific Fe:Mg ratio is a factor of similar to 10 smaller than its counterpart deformed under anhydrous conditions. In a hydrous environment at the same thermodynamic conditions, the viscosity of the more Fe-rich mantle (similar to Fa(19)) of Mars is a factor of similar to 5 lower than that of the mantle (similar to Fa(8)) of Earth.