SEMI-BRITTLE FLOW DURING DEHYDRATION OF LIZARDITE-CHRYSOTILE SERPENTINITE DEFORMED IN TORSION: IMPLICATIONS FOR THE RHEOLOGY OF OCEANIC LITHOSPHERE

Abstract

Serpentinite is a significant component of the oceanic lithosphere and plays an important role in subduction dynamics. However, the rheology of serpentine-bearing rocks is poorly understood, especially at large strains and during the dehydration of serpentine. We have investigated the mechanical behavior and microstructural evolution of serpentinite during dehydration reaction to olivine, talc and water at temperatures of 550 and 600 °C, pressures of 300 and 400 MPa and shear strain rates of 1 × 10- 5 to 1 × 10- 4 s- 1 under drained conditions. The non-coaxial deformation experiments were performed in a gas-medium apparatus equipped with a torsion system to shear strains of up to 3.3. Strong crystallographic preferred orientations (CPO) of partially dehydrated serpentine, shear localization through the development of S-C structures and widespread microfracturing in deformed specimens indicate that deformation took place in the semi-brittle field. Microstructural observations reveal that the CPO results from slip and rotation of (001) planes in partially dehydrated lizardite, possibly assisted by fluid released by the reaction. Despite the development of a strong CPO and shear localization, strain hardening accompanied by repeated transitory stress drops of a few MPa was observed and may be explained by the following combined effects: (1) progressive decrease of excess local pore pressures due to specimen dilatancy and the formation of pore networks during the reaction, leading to an increase in effective pressure, (2) subsequent collapse of this pore space by shear-enhanced compaction, resulting in work hardening, and (3) formation of a mechanically stronger assemblage of reaction products. The experiments imply that the bulk strength of a serpentinized subducting slab increases during dehydration as fluid escapes from the slab, while local embrittlement and shear localization take place. © 2006 Elsevier B.V. All rights reserved.