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 [1] persisting to considerable depths (100- 250 km).While the transformation from blueschist to eclogite [2] and the presence of glaucophane up to 90-100 km [3] 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 [4]. 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 [5] 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 [6]. 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.

[1] Abers, Earth and Planetary Science Letters, 176, 323-330, 2000

[2] Helffrich et al., Journal of Geophysical Research, 94, 753-763, 1989

[3] Bezacier et al., Tectonophysics, 494, 201-210, 2010

[4] Schmidt & Poli, Earth and Planetary Science Letters, 163, 361-379,1998

[5] Wang et al, Review for Scientific Instruments, 74(6), 3002-3011, 2003

[6] Chantel et al., Earth and Planetary Science Letters, 349-350, 116-125, 2012