Deformation mechanisms and rheology of serpentines in experiments and in nature

Microstructures in serpentine samples recovered from deformation experiments performed at high pressure (1-8 GPa), high temperature (150-500 degrees C), and laboratory strain rates (4 10(-4)-10(-6)s(-1)) were studied using transmission electron microscopy on thin sections prepared by focused ion beam. Lizardite crystals deform by easy glide along the basal planes associated with microkink. This mechanism is associated with a plastic strength of similar to 100MPa that defines the upper bound of lizardite strength in natural conditions. Antigorite crystals deform essentially by conjugated slip along (101) and (10 (1) over bar) planes observed in sections close to (010). This conjugate system results in an apparent global slip system akin to 100. In all samples, delamination and comminution produce fine-grained interconnected regions at grain boundaries because intracrystalline deformation mechanisms are insufficient to satisfy the von Mises criterion. The deformation laws of lizardite (plastic flow) and antigorite (strain rate dependent) differ because of their differing intracrystalline deformation mechanisms. Subgrain boundary geometry in natural samples and 100 crystal preferred orientations indicate the activation of slip systems similar to those observed in experimentally deformed samples, suggesting that the strain rate-dependent rheology of antigorite derived from experiments applies to subduction zone conditions. Delamination of antigorite crystals of a few tens to hundreds of nanometers is not observed in natural samples from subduction zones, suggesting plastic deformation of serpentinites at natural low strain rates and stresses is likely accompanied by recrystallization through dissolution-precipitation processes that act as a low-temperature equivalent of dynamic recrystallization through diffusion at high temperature.