Non-Dilatant Brittle Deformation and Strength Reduction of Olivine Gabbro due to Hydration

Abstract

To investigate the influence of hydration on brittle deformation of oceanic crustal rocks, we conducted triaxial deformation experiments on gabbroic rocks with various degrees of hydration at a confining pressure of 20 MPa and room temperature, measuring elastic wave velocity. Hydrated olivine gabbros reached a maximum differential stress of 225–350 MPa, which was considerably less than those recorded for gabbros (~450 MPa), but comparable to those for serpentinized ultramafic rocks (250–300 MPa). Elastic wave velocities of hydrated olivine gabbros did not show a marked decrease even prior to failure. This indicated that the deformation of hydrated olivine gabbro is not associated with the opening of the stress-induced cracks that are responsible for dilatancy. Microstructural observations of the samples recovered after deformation showed crack damage to be highly localized to fault zones with no trace of stress-induced crack opening, consistent with the absence of dilatancy. These data suggest that strain localization of hydrated olivine gabbro can be caused by the development of shear cracks in hydrous minerals such as serpentine and chlorite, even when they are present in only small amounts. Our results suggest that the brittle behavior of the oceanic crust may considerably change due to limited hydration.

Editorial for Special Issue “Mantle Strain Localization—How Minerals Deform at Deep Plate Interfaces”

“…In addition, this Special Issue highlights new discoveries from deformation experiments. The study of Akamatsu et al [6] first focuses on the role of H 2 O in modifying the strength-and strain-related behavior of the oceanic lithosphere. From measurements of elastic wave velocity during low-pressure rock deformation experiments, they show how hydrated olivine may reduce the strength of olivine gabbro and prevent any dilatancy before rock failure in the brittle regime.…”