Shear-driven formation of olivine veins by dehydration of ductile
serpentinite: a numerical study with implications for porosity
production and transient weakening
Abstract
Serpentinite subduction and associated dehydration vein formation are
important for subduction zone dynamics and water cycling. Field
observations suggest that en échelon olivine veins in serpentinite
mylonites formed by dehydration during simultaneous shearing of
serpentinite. Here, we test a hypothesis of shear-driven formation of
dehydration veins with a two-dimensional hydro-mechanical-chemical
numerical model. We consider the reaction antigorite + brucite =
forsterite + water. Shearing is viscous and the shear viscosity
decreases with increasing porosity. Total and fluid pressures are
initially homogeneous and in the serpentinite stability field. Initial
perturbations in porosity, and hence viscosity, cause fluid pressure
perturbations during simple shearing. Dehydration nucleates where fluid
pressure decreases locally below the thermodynamic pressure defining the
reaction boundary. During shearing, dehydration veins grow in direction
parallel to the maximum principal stress and serpentinite transforms
into olivine inside the veins. Simulations show that the relation
between compaction length and porosity as well as the ambient pressure
have a strong impact on vein formation, while the orientation of the
initial porosity perturbation and a pressure-insensitive yield stress
have a minor impact. Porosity production associated with dehydration is
controlled by three mechanisms: solid volumetric deformation, solid
density variation and reactive mass transfer. Vein formation is
self-limiting and slows down due to fluid flow decreasing fluid pressure
gradients. We discuss applications to natural olivine veins as well as
implications for slow slip and tremor, transient weakening, anisotropy
generation and the formation of shear-driven high-porosity bands in the
absence of a dehydration reaction.