Rheology of Partially Molten Rocks

Abstract

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.