Shenghua Mei

Influence of melt on the creep behavior of olivine–basalt aggregates under hydrous conditions

The influence of melt on the creep behavior of olivine–basalt aggregates under hydrous conditions has been investigated by performing a series of high-temperature triaxial compression experiments. Samples with melt fractions of 0.02≤φ≤0.12 were deformed under water-saturated conditions at temperatures between 1373 and 1473 K and a confining pressure of 300 MPa in a gas-medium apparatus. At constant differential stress and temperature, the rate of deformation increased rapidly but systematically with increasing melt fraction. In the diffusion creep regime, at a given differential stress, samples with melt fractions of 0.02 and 0.12 deformed a factor of ∼2 and ∼20, respectively, faster than a melt-free sample. In the dislocation creep regime, a sample with a melt fraction of 0.12 deformed a factor of ∼40 faster than a melt-free sample. For partially molten olivine–basalt aggregates deformed under hydrous conditions, the dependence of creep rate on melt fraction can be expressed in the form ϵ(φ)=ϵ(0) exp(αφ), where α≈26 for diffusion creep and α≈31 for dislocation creep. The results of this study, combined with reasonable estimates for the spatial variation in the concentrations of water and melt (as well as for the geotherm and the activation volume for creep), provide constraints on the viscosity structure of Earth’s upper mantle. As an example, we present a viscosity profile for the mantle wedge above a subducting plate, demonstrating that the viscosity in that region can vary by ∼3 orders of magnitude over a depth of ∼60 km due to the combined effects of water and melt weakening.

Rheology of Partially Molten Rocks

Melt and water are two of the most important elements governing the viscosity of the rocks in regions of Earth’s upper mantle such as beneath a mid-ocean ridge and in the mantle wedge above a subducting plate. Over the past five years, laboratory deformation experiments under controlled thermodynamic conditions have yielded quantitative relationships describing the dependence of strain rate, \( \dot{\varepsilon } \) and, thus, viscosity, η, on melt fraction, ф, and hydrogen or hydroxyl concentration, C OH, as well as on differential stress, σ, grain size, d, temperature, T, pressure, P, oxygen fugacity, f O2, and silica or pyroxene activity, a opx. These constitutive equations provide a critical part of the framework necessary for modeling processes such as convective flow in the mantle and melt extraction from partially molten environments. To extend flow laws to low differential stresses important in the mantle and to compare the high-temperature rheological behavior of partially molten rocks at total pressures 0.1 and 300 MPa, recent creep experiments were carried out on samples of olivine plus 3 vol% basalt with an average grain size of ~30 µm under anhydrous conditions in compressive creep. In experiments performed at 0.1 MPa, relatively small differential stresses of 0.5 to 3 MPa were used in order to minimize microcracking that can occur in rock samples at low confining pressures. These 0.1-MPa experiments yield a stress exponent of n = 1.0, an f O2 exponent of 1/7 and an activation energy of 530 kJ/mol. To eliminate the cavitation and microcracking that can occur during deformation at 0.1 MPa, creep tests were performed at 300 MPa; in this case the differential stresses were in the range 14 to 224 MPa. At 1250°C, a transition from diffusion creep (n ≈ 1.0.) to dislocation creep (n ≈ 3.5) occurs at a differential stress of ~70 MPa. The f O2 exponent determined at 300 MPa agrees well with that measured at 0.1 MPa. Creep rates obtained in experiments at 0.1 MPa are in good agreement with those determined at 300 MPa when normalized to the same T-σ- f O2 conditions, indicating that contributions due to cavitation and microcracking are, at most, minor in the lower pressure experiments. The viscosities measured for partially molten olivine-basalt aggregates with 3 vol% melt deformed in both the diffusion and the dislocation creep regime are 3 to 5 time smaller than values published for melt-free samples. These results imply that, if the melt fraction remains small in the upwelling source rock beneath mid-ocean ridges, partial melting will not dramatically modify the rheological behavior of this region of the mantle except as the melt depletes the hydroxyl content of the host minerals and thereby eliminates water-weakening of the rock.

Diffusion Creep of Enstatite at High Pressures Under Hydrous Conditions

Mantle convection and large-scale plate motion depend critically on the nature of the lithosphere-asthenosphere boundary and thus on the viscosity structure of Earths upper mantle, which is determined by the rheological properties of its constituent minerals. To constrain the flow behavior of orthopyroxene, the second most abundant constituent of the upper mantle, deformation experiments were carried out in triaxial compressive creep on fine-grained (similar to 6m) samples of enstatite at high pressures (3.8-6.3GPa) and high temperatures (1323-1573K) using a deformation-DIA apparatus. Based on results from this study, the deformation behavior of enstatite is quantitatively presented in the form of a flow law that describes the dependence of deformation rate on differential stress, water fugacity, temperature, and pressure. Specifically, the creep rate depends approximately linearly on stress, indicating deformation in the diffusion creep regime. A least squares regression fit to our data yielded a flow law for diffusion creep with an activation energy of similar to 200kJ/mol and an activation volume of similar to 14x10(-6)m(3)/mol. The magnitude of the water-weakening effect is similar to that for olivine with a water fugacity exponent of r approximate to 0.7. This strong dependence of viscosity on water fugacity (concentration) indicates that the viscosity of an orthopyroxene-bearing mantle varies from one geological setting to another, depending on the large-scale water distribution. Based on the rheology contrast between olivine and enstatite, we conclude that olivine is weaker than enstatite throughout most of the upper mantle except in some shallow regions in the diffusion creep regime.