Relative strengths of orthopyroxene and olivine at asthenospheric conditions

Abstract:

Orthopyroxene is the second most common mineral in the Earth’s upper mantle. However, very little is known about its strength relative to olivine and clinopyroxene, both of which are well studied. Analyses of microstructures in peridotites containing orthopyroxene and olivine that were deformed at lithospheric conditions (relatively low temperatures and pressures) indicate that the orthopyroxene is stronger than surrounding olivine. In contrast, analyses of microstructures in asthenospheric peridotite xenoliths indicate that olivine and orthopyroxene have similar strengths.

In order to better determine the pressure, temperature and strain rate sensitivity of the strength of orthopyroxene aggregates, we have performed an experimental study on stacked cylinders of orthopyroxene aggregates and olivine aggregates in the D-DIA. Cylinders of Bamble orthopyroxene (d~5-30 microns) or San Carlos olivine (d~25 microns) were hot-pressed in-situ at 1300^oC for 1 hour prior to deformation. Although the assemblies and powders were dried at >100^oC for >12 hours prior to installation in the apparatus, minor concentrations of water were observed (OPx ~ 500 H/10⁶ Si; Ol ~ 200 H/10⁶ Si). Multiple deformation steps were performed in each experiment over a range of strain rates (5x10-6 to 2x10-4/s) at a single temperature and pressure (T = 1000 – 1400^oC and P = 2 – 5 GPa).

At almost all conditions tested in these experiments, the orthopyroxene aggregates deformed at the same strain rate as the olivine cylinders, indicating both materials have the same stress exponent and very similar activation enthalpy. The microstructures observed in both the orthopyroxene and olivine cylinders are consistent with dislocation creep and lattice preferred orientations consistent with those observed in naturally deformed peridotites. These results indicate that at asthenospheric mantle conditions, the strengths of orthopyroxene and olivine in the dislocation creep field are very similar.