Strength Of Olivine Research Articles

Pressure Dependence of Magnesite Creep

We determined the activation volumes (V*) for polycrystalline magnesite with grain sizes of 2 and 80 µm deforming by low temperature plasticity (LTP) mechanisms (kinking and dislocation glide), diffusion creep, and dislocation creep at temperatures of 500, 750, and 900 °C, respectively, and a strain rate of 1–2 × 10−5 s−1 at effective pressures of 2.9–7.5 GPa in a D-DIA and 0.76 GPa in a Griggs apparatus. In each set of experiments performed at a given temperature, the strength of magnesite increases with increasing pressure. Microstructures of fine-grained magnesite deformed at 500 °C and 750 °C are consistent with deformation by LTP mechanisms and diffusion creep, respectively. Microstructures of coarse-grained magnesite deformed at 900 °C are consistent with deformation by dislocation creep. Pressure dependencies of magnesite flow laws for LTP, diffusion creep, and dislocation creep are given by activation volumes of 34 (± 7), 2 (± 1), and 10 (± 5) × 10−6 m3/mol, respectively. Addition of these activation volumes to previously determined flow laws predicts magnesite strength to be much lower than the flow strength of olivine at all subduction zone depths of the upper mantle. Thus, subducting oceanic lithosphere that has been partially carbonated by reaction with CO2-bearing fluids may deform at lowered stresses where magnesite is present, possibly resulting in strain localization and unstable run-away shear.

Effect of pressure on the strength of olivine at room temperature

A fine grained fully-dense olivine aggregate was deformed in a D-DIA press at room temperature and pressures ranging from 3.5 to 6.8GPa, at constant strain rates between 6×10-6 and 2.2×10-5s−1. A weighted non-linear least square fit of a dataset including our results and data from other high-pressure studies to a low-temperature plasticity flow law yields a Peierls stress σP0=7.4(0.5)GPa and an activation energy E∗=232(60)kJ.mol−1. The dependence of the Peierls stress to pressure, σP=σP0(1+0.09P), appears to be larger than the value predicted by the formulation proposed by Frost and Ashby (1982). With such a dependence, the activation volume is very small (V*=1.6(1.7)cm3.mol−1). Extrapolation to natural conditions yields a viscosity of ~1023-1024Pa.s for a cold subducting slab at depths of 50–100km.

The role of hydrogen in geological processes in the earth's interior

Hydrogen (reported as H 2O) affects the physical properties of silicate materials profoundly. Silicates (with Mg and Fe) dominate a significant portion of the earth's crust and mantle (0–2900 km depth). The melting point of the silicate assemblage in the upper mantle (olivine dominated) is decreased by up to 400°C at 3 GPa with the addition of 0.14 wt.% H 2O. The strength of olivine is also reduced by two orders of magnitude with the incorporation of small amounts of water (e.g., 0.001 wt.%). H occurs in the earth's upper mantle (30–670 km depth) in trace proportions (0.05–0.2 wt.% H 2O equivalent). Over the depth range of the upper mantle there is a two orders of magnitude increase with depth in the ability of the dominant mineral (olivine) to accommodate hydrogen. This suggests that silicates derived from deeper (minerals or partial melts of those minerals) may be more hydrous. There is some speculation that Mg-perovskite (stable at 670–2900 km depths) may also accommodate significant hydrogen.