Effect of Iron Content on the High-Pressure Plasticity of Olivine

Abstract:

<span">The mantle of terrestrial planets are olivine-rich, with Fe/(Mg+Fe) ratio lower than ~2% for Mercury and up to 25-30% for Mars, with intermediate compositions for the Earth, the Moon and Venus. The range of iron contents may promote a range of mantle viscosities, with implications for planet thermal history and surface structures. Indeed, experiments at low pressure (i.e. at 300 MPa, Zhao et al., 2009, EPSL, 287,229-240) indicate that Fe dramatically decreases olivine viscosity. Thus, the Martian mantle may be ~10 times less viscous than the Earth’s at the same conditions. However, there is no data available on the effect of iron on olivine plasticity at pressures relevant to planetary interiors.

In this study, we deformed polycrystalline olivine specimens with iron ratios of 0%, 10%, 70%, 90% and 100%, at high pressure (P) ranging from 2 to 5 GPa and temperatures (T) in the range 800°-1200°C. Experiments were carried out in the Deformation-DIA apparatus (D-DIA) coupled with x-ray synchrotron radiation - to quantify stress and strain by in-situ diffraction and imaging - at the NSLS (Upton, NY) and the ESRF (Grenoble, France). Stacked cylinders of olivine with different compositions were deformed at steady-state conditions to compare their plastic responses while experiencing identical T, P and differential stresses. Results of these experiments indicate that the weakening effect of Fe on olivine viscosity is not as great at high pressure as the effect observed at low pressure.