Rheology of Mg2GeO4 olivine and spinel harzburgite: Implications for Earth's mantle transition zone

摘要

Geophysical modeling requires an upper mantle low-viscosity zone (LVZ); recent results suggest that the LVZ could be within the mantle transition zone (MTZ) or above it, depending on boundary conditions. The rheology of the olivine-dominated uppermost mantle is well constrained experimentally but not the MTZ. Here we report first-time measurement of the rheology of Mg2GeO4 spinel (analogue of ringwoodite, the dominant mineral of the lower MTZ) in the dislocation creep regime and compare it with that of Mg2GeO4 olivine. In both cases, we incorporate 15-20% pyroxene, the second most abundant mineral in the upper mantle to provide a realistic comparison of rock rheologies. Under similar conditions, spinel-dominated material is about twice as strong as olivine-dominated material. Our results suggest that unless H2O weakening of ringwoodite or phase-transformation-induced grain size reduction occurs in the lower MTZ, it will be more viscous than above the 410km discontinuity.