Deformation of Lawsonite at High Pressure and High Temperature – Implications for Low Velocity Layers in Subduction Zones
Abstract:During subduction, the hydrated oceanic crust undergoes a series of metamorphic reactions and transform gradually to blueschists and eclogite at depths of 20-50 km. Detailed seismic observations of subduction zones suggest a complex layered structure with the presence of a Low Velocity Layer (LVL) related to the oceanic crust  persisting to considerable depths (100- 250 km).While the transformation from blueschist to eclogite  and the presence of glaucophane up to 90-100 km  could explain some of these observations, the presence of LVL at greater depths could be related to the presence of the hydrous mineral lawsonite (CaAl2(Si2O7)(OH)2 H2O). Its stability field extends to 8.5 GPa and 1100K corresponding to depths up to 250 km in cold hydrous part of subducting slabs . Because these regions undergo large and heterogeneous deformation, lawsonite plasticity and crystal preferred orientation (CPOs) may strongly influence the dynamic of subduction zones and the seismic properties.
We present a deformation study at high presssure and high temperature on lawsonite. Six samples were deformed at 4-10 GPa and 600K to 1000K using a D-DIA apparatus  at 13-BMD at GSECARS beamline, APS, in axial compression up to 30% deformation with strain rates of 3.10-4s-1 to 6.10-6s-1. We measured in-situ lattice strains (a proxy for macroscopic stress), texture and strain using synchrotron radiations and calculated the macroscopic stress using lawsonite elastic properties . Results from lattice strain analysis show a dependence of flow stress with temperature and strain rate. Texture analysis coupled with transmission electron microscopy showed that dislocation creep is the dominant deformation mechanism under our deformation conditions.
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