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 Fa₁₀, Fa₃₀ and Fa₅₀ set the oxygen fugacity at the Ni:NiO buffer and thus the water fugacity at ≲300 MPa, while Fe sleeves around samples of Fa₇₅ and Fa₁₀₀ set the oxygen fugacity at the Fe:FeO buffer and thus the water fugacity at ≲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 Fa₅₀ is a factor of ∼10 smaller than the viscosity of samples of Fa₃₀, while the viscosity of samples of Fa₃₀ is a factor of ∼10 smaller than that of samples of Fa₁₀. Our experimental results can be described by the flow law ε̇_disl=C_disl[Formula presented]^n_disl X_Fa ^p_disl exp-[Formula presented]f_H2O ^m_disl with C_disl = 99.7 MPa^−5/4 s⁻¹, n_disl = 3.7, p_disl = 0.5, Q_disl ⁰ = 510 kJ/mol, α_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 ∼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 (∼Fa₁₉) of Mars is a factor of ∼5 lower than that of the mantle (∼Fa₈) of Earth.