Low strength of Earth’s uppermost mantle inferred from tri-axial deformation experiments on dry olivine crystals

• Dry olivine crystals were deformed in tri-axial compression at lithospheric T.
• Stresses are weaker than predicted by both high T and low T flow laws.
• TEM show that [0 0 1] glide dominates at low T and EBSD maps show than strain localized.
• New semi-empirical flow law is provided for a T range 500–1000 °C.
• Small melt fraction or H in olivine is not required to explain a weak lithosphere.

We have performed tri-axial compression experiments on single crystals of San Carlos olivine with various orientations at temperatures relevant for the uppermost mantle, between 800 and 1090 °C. The experiments were carried out at a confining pressure of 300 MPa in a high-resolution gas-medium mechanical testing apparatus at various constant strain rates (from 7 × 10−6 s−1 to 1 × 10−4 s−1); they yield differential stresses ranging from 88 to 754 MPa. Unpolarized infrared spectroscopy analyses indicate that hydrogen concentration in the olivine lattice is very low (<0.5 ppm wt H2O) both before and after deformation. Transmission electron microscopy confirms plastic deformation by dislocation glide. [0 0 1] glide dominates regardless of the orientation of the crystals, even if very marginal [1 0 0] glide has also been activated in some samples. Orientation mapping by electron backscatter diffraction highlights significant local deviations from the original orientation in some samples associated with bending and deformation bands. These strain localizations suggest a heterogeneous mechanical behavior in this temperature range, which is favored by the strong mechanical anisotropy of the olivine crystal. The present experiments confirm that previous published high-temperature flow laws (i.e., power flow law) overestimate the strength of lithospheric mantle, since all samples deformed under stresses significantly lower than predicted by these flow laws. Based on the present results and on a compilation of previously published data on olivine deformation between 500 and 1000 °C, we propose a new semi-empirical exponential flow law applicable to the uppermost mantle: ε̇=1±0.2×106exp-450±60×103RT1-σ15±41/22where T is the absolute temperature, R is the gas constant, and σ is the differential stress in GPa. Subsequently, small fractions of melt or incorporation of hydrogen in olivine may not be required to reconcile natural observations indicating a limited strength of Earth’s uppermost mantle lithosphere (<1 GPa) and experimental data on the rheology of olivine.