Shear-driven formation of olivine veins by dehydration of ductile
serpentinite: a numerical study with implications for transient
Serpentinite subduction and the associated formation of dehydration
veins is 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 ductile
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 exponentially with porosity. The total and fluid pressures are
initially homogeneous and in the antigorite stability field. Initial
perturbations in porosity, and hence viscosity, cause fluid pressure
perturbations. Dehydration nucleates where the fluid pressure decreases
locally below the thermodynamic pressure defining the reaction boundary.
Dehydration veins grow during progressive simple-shearing in a direction
parallel to the maximum principal stress, without involving fracturing.
The porosity evolution associated with dehydration reactions is
controlled to approximately equal parts by three mechanisms: volumetric
deformation, solid density variation and reactive mass transfer. The
temporal evolution of dehydration veins is controlled by three
characteristic time scales for shearing, mineral-reaction kinetics and
fluid-pressure diffusion. The modelled vein formation is self-limiting
and slows down due to fluid flow decreasing fluid pressure gradients.
Mineral-reaction kinetics must be significantly faster than
fluid-pressure diffusion to generate forsterite during vein formation.
The self-limiting feature can explain the natural observation of many,
small olivine veins and the absence of few, large veins. We further
discuss implications for transient weakening during metamorphism and
episodic tremor and slow-slip in subduction zones.