Low Temperature Bulging Recrystallization in Olivine Aggregates: a Mechanism for Strain Weakening and Localization

Evidence of strain localization in olivine aggregates has been observed in both obducted slices of upper mantle and in high stress experiments (Post, 1977). Most experimental deformation studies of olivine aggregates have been done in gas apparatus at low P (<300 MPa) and thus at low flow stress; both axial compression and shear experiments involving climb-accommodated dislocation creep show steady state flow, with little if any strain weakening. However, experimental studies of quartz and feldspar at high P have shown the existence of a low T, high stress dislocation creep regime in which climb is very limited and creep is accommodated by bulging recrystallization; in this regime extreme strain weakening and localization occur. To explore high stress dislocation creep in olivine, we have deformed natural and synthetic aggregates using a modified molten salt assembly in a Griggs apparatus at a P of 1.5 GPa, T of 950 to 1100° C, and strain rates of 5x10^-5/s to 10^-6/s, in both axial compression and general shear. For starting materials we used both Balsam Gap dunite (d ∼500 μ m) and synthetic aggregates hot pressed from San Carlos olivine powders (10-20 and 25-38 μ m). Prior to weld sealing in Ni and outer Pt, each sample was dried for 24 hrs at 900° C in a CO/CO₂ atmosphere. The yield stress of the samples ranged from 1300 to 380 MPa and all samples strain weakened. The highest stress sample shows evidence of semi-brittle flow, with high densities of linear dislocations and a few very small recrystallized grains. A sample with an intermediate yield stress (780 MPa) strain weakened to 600 MPa by 16% strain and shows very high densities of tangled dislocations in the porphyroclasts, and small (1-2 μ m) dislocation-free recrystallized grains along their boundaries. The sample with the lowest yield stress (380 MPa) strain weakened to a steady state flow stress of 120 MPa at γ = 4.5. The few remaining porphyroclasts contain subgrains, and the recrystallized grain size is consistent with the recrystallized grain size piezometer for olivine deformed by climb-accommodated dislocation creep. The microstructures and mechanical behavior of these olivine samples are consistent with those previously observed in quartz and feldspar aggregates, and indicate a transition from climb-accommodated dislocation creep at lower stresses to recrystallization-accommodated dislocation creep at higher stresses. It appears that bulging recrystallization in low T, high stress dislocation creep is capable of producing significant strain weakening and strain localization in the upper mantle.