5:48 PM - 5:51 PM
[SCG59-P07] Rheological weakening due to phase mixing of olivine + orthopyroxene
3-min talk in an oral session
The formation of well-mixed, fine-grained, poly-phase rocks may lead to strain localization and play a key role in the development of the lithosphere asthenosphere boundary (LAB). To understand the mixing process in the olivine + orthopyroxene rocks, we have conducted torsion experiments on samples of iron-rich olivine + orthopyroxene aggregates at a temperature of 1200℃ and a pressure of 300MPa. We fabricated the samples with grain sizes significantly larger than the steady state grain size. The samples were deformed to total shear strains up to γ = 17. We conducted two series of torsion experiments, the first at fixed strain rate to different strains and the second at different strain rates to the same strain.
The stress exponent of n ≈ 3 and grain size exponent of p ≈ 1 were determined from a least-squares fit to the strain rate, stress and grain size data using a power-law creep equation; these values of n and p indicate that our samples deformed by dislocation-accommodated grain boundary sliding. Dynamic recrystallization occurred with significant grain size reduction of both phases in deformed samples. Well-mixed microstructures develop in samples deformed to higher strains at faster strain rates, whereas elongated olivine and pyroxene grains without a mixed texture are observed at lower strain and strain rate. Mixing of the olivine and orthopyroxene phases occurs due to a contribution of interface-reaction-limited diffusion (IRLD) creep [Sundberg and Cooper, 2008]. This IRDL creep process involves diffusion of metal oxides along phase boundaries oriented perpendicular to σ1 to boundaries parallel to σ1 resulting in the formation of new pyroxene grains along boundaries perpendicular to σ1 and olivine grains along boundaries parallel to σ1. Grain size reduction due to dynamic recrystallization of olivine and orthopyroxene enhance the rate of this process.
The stress exponent of n ≈ 3 and grain size exponent of p ≈ 1 were determined from a least-squares fit to the strain rate, stress and grain size data using a power-law creep equation; these values of n and p indicate that our samples deformed by dislocation-accommodated grain boundary sliding. Dynamic recrystallization occurred with significant grain size reduction of both phases in deformed samples. Well-mixed microstructures develop in samples deformed to higher strains at faster strain rates, whereas elongated olivine and pyroxene grains without a mixed texture are observed at lower strain and strain rate. Mixing of the olivine and orthopyroxene phases occurs due to a contribution of interface-reaction-limited diffusion (IRLD) creep [Sundberg and Cooper, 2008]. This IRDL creep process involves diffusion of metal oxides along phase boundaries oriented perpendicular to σ1 to boundaries parallel to σ1 resulting in the formation of new pyroxene grains along boundaries perpendicular to σ1 and olivine grains along boundaries parallel to σ1. Grain size reduction due to dynamic recrystallization of olivine and orthopyroxene enhance the rate of this process.