Elasticity of glaucophane, seismic velocities and anisotropy of the subducted oceanic crust

Upon subduction, the oceanic crust transforms to blueschists and eclogites, with seismic properties that gradually become similar to those of the surrounding mantle. In order to evaluate the anisotropy of blueschists and glaucophane-bearing eclogites, the elastic constants of glaucophane single-crystal plates from the Sesia–Lanzo Zone (Aosta Valley, Western Alps) were measured using Brillouin spectroscopy at ambient conditions. The mean P- and S-wave velocities are 7.8 and 4.6 km s−1 respectively, and the anisotropy is high (38.1% (AVP) and 27.3% (AVS)). Glaucophane develops strong LPO, characterized by the [001]-axes concentrated sub-parallel to the lineation, and the {110} poles concentrated sub-perpendicular to the foliation in both blueschist and eclogite rocks. The measured LPO is in good agreement with viscoplastic self-consistent numerical models. Seismic properties of glaucophane-bearing blueschist and eclogite are calculated by combining measured LPO and the single-crystal elastic moduli of glaucophane with the other main mineral constituents of the rock: mostly epidote for blueschist, and omphacite and garnet for eclogite. Blueschists present stronger anisotropies (AVP = 16.1% and AVS = 10.3%) than eclogites (AVP = 2.9% and AVS = 1.7%). The shear-wave splitting and resulting delay times for a 7-km thick layer of eclogite or blueschist are low for the eclogite (< 0.03 s), but significant for blueschist (0.16 s). Application to the subducted oceanic crust yields a decrease of velocity contrast with the surrounding mantle and of anisotropy at depth depending on the temperature of the slab. The details of the velocity variations in subducted oceanic crust are difficult to reconcile with the blueschist–eclogite transition as probed by exhumed metamorphic rocks and may require additional phases such as lawsonite or chemical variations such as oxidation state.

Research Highlights
► Elastic constants of glaucophane single-crystal were measured by Brillouin spectroscopy.
► Anisotropy of glaucophane single-crystal is of 38.1% for AVP and 27.3% for AVS.
► Anisotropies are stronger for blueschists (AVP = 16.1% and AVS = 10.3%) than eclogites (AVP = 2.9% and AVS = 1.7%).
► Shear-wave splitting and resulting delay times are low for eclogite (< 0.03 s) but significant for blueschist (0.16 s).