Rheology of Partially Molten Spinel Lherzolite at High Temperature

Uniaxial creep experiments were performed on synthetic spinel lherzolite samples with grain sizes of 25- 50μm in a gas medium dead-load creep rig to investigate the deformation behavior of partially molten upper mantle. The room pressure experiments were conducted at conditions of elevated temperatures of 1140-1200 °C and well-controlled oxygen fugacities of 10-6-10-10atm. Differential stresses of 4-74MPa were applied to yield strain rates between 10-8 and 10-5 s-1. By fitting the steady-state creep strength with empirical power law, we obtain an oxygen fugacity exponent of 0.05 ± 0.02, a stress exponent of 1.5 ± 0.4, and an activation energy of 1428 ± 48 KJ/mol for the high-temperature deformation of a partially molten spinel lherzolite. The oxygen fugacity exponent, m, is comparable to those obtained for deformation of olivine single crystal, which suggests that deformation of pyroxene doesn't depend significantly on oxygen fugacity as olivine. The creep activation energy obtained in this study is about twice of those from previous studies. SEM observations show about 2-4% melt existing as melt pockets in triple junctions and melt films between grain boundaries in deformed spinel lherzolite. The stress exponent of 1.5 suggests that both dislocation creep and melt-enhanced diffusion creep contribute significantly to the deformation of lherzolite. Our results provide constrains on the rheology and deformation microstructures of partially molten upper mantle.