Newtonian flow of heterogeneous synthetic gabbros at high strain: Grain sliding, ductile failure, and contrasting local mechanisms and interactions

Abstract

We combined experimental and numerical approaches, in order to constrain the rheology of ultramylonitic rocks characteristic of gabbroic high-temperature shear zones. Synthetic samples containing 70 vol% fine-grained anorthite matrix (<5 μm) and 30 vol% coarse diopside inclusions (<55 μm) were deformed in torsion at T = 1150 °C and P = 370 MPa. At modest shear stresses (<40 MPa) the data evidenced linear-viscous flow with a stress exponent n ≈ 1. The outermost rim of the samples exhibited substantial cavitation coalescence leading to ductile damage and ultimately to ductile failure at shear strains between γ = 2–3.5. The mechanical data and the microstructures indicate dominant grain boundary sliding (GBS) and diffusional mass transfer. However, extensive crystal slip plasticity (CSP), dislocation creep and dynamic recrystallization operated within and in the vicinity of the diopside inclusions. For instance, the inclusions presented an extremely fine-grained interfacial layer, resulting from their intense peripheral recrystallization. The latter crystal plasticity mechanism require differential stresses exceeding by far the overall flow stress supported by our specimens, which indicates substantial local stress enhancement related to the heterogeneous nature of the two-phase rock. In order to characterize the local mechanical responses we performed finite element numerical modelling of the shear deformation process, considering elastoviscoplastic behavior based on the constitutive laws for GBS and CSP of the constituent phases. We emphasized the effects of inclusion shapes and interactions. We show that even at relatively low concentrations (25 vol%) and for overall Newtonian flow, the important strength contrasts and the interactions between neighbouring and irregular inclusions rapidly induce significant local stress amplifications, allowing for twinning and dislocation creep. On the other hand, the simulations suggest that the presence of fine-grained recrystallized interfacial layers deforming by GBS allows for very efficient local stress relaxation. As a result, we suggest that whilst the overall material sustains fairly low stresses and presents GBS related Newtonian flow, locally the diopside inclusions and the surrounding matrix experience highly fluctuating stresses in relation with cycles of dislocation creep, peripheral recrystallization and interfacial GBS. On the one hand, from a stochastic point of view, the cyclic nature of the process may ensure a stable overall flow stress. On the other hand, at the local scale the sequential mechanism allows for continuous size reduction of the stronger inclusions, a process that we call “ductile abrasion”.