Microstructural evidence of dislocation creep and diffusion accommodated deformation of glaucophane in naturally deformed lawsonite and epidote blueschists

The rheology and deformation mechanisms of mafic blueschists play a key role in the mechanical behavior of subducting oceanic crust in subduction zones. While mafic blueschists are often ubiquitous along the plate interface from the base of the seismogenic zone (~35 km) to the sub-arc depths (~100 km), the strength of this lithology still remains poorly constrained. Observations of blueschists from exhumed subduction terranes suggests that blueschist can accommodate significant strain, largely partitioned into the sodic amphibole glaucophane. However, it remains an open question whether the observed deformation is accommodated by dislocation or diffusion deformation processes.

We present microstructural and textural analyses to investigate the glaucophane fabric and deformation mechanisms in three naturally deformed blueschists exhumed from variable P-T conditions: (1) a lawsonite blueschist from the Catalina Schist (Santa Catalina Island, CA, USA), (2) higher-grade epidote blueschist from the Bandon blueschist (Bandon, OR, USA) and (3) an epidote-blueschist from the Cycladic Blueschist Unit (Tinos, GR). We used electron backscatter diffraction (EBSD), scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDS) to interpret the textural and geochemical record of deformation mechanisms that were active during the subduction history of these exhumed blueschists. All three blueschists display well-developed foliations and lineations which are defined by interconnected layers of glaucophane. EBSD microstructural analysis of glaucophane in the samples reveals evidence of dislocation accommodated deformation including: (1) strong crystallographic preferred orientation (CPO) development, (2) intragranular orientation gradients, (3) activity of dislocation motion on multiple slip systems, and (4) subgrain boundary formation. Core-mantle structures in which the daughter grains display evidence of a weakened CPO inherited from the mother (core) grains imply the activity of subgrain boundary recrystallization in the samples. Taken together, this microstructural evidence implies that dislocation creep accommodated deformation was active in all three blueschists during their deformation history. SEM images and EDS maps of glaucophane reveal evidence of chemical zoning in grains with higher Ca and Al concentrations in the rims and along the walls of  (micro)fractures within the grains (Bandon, OR Sample). The Catalina lawsonite blueschist displays interspersed evidence of microfractures with higher concentrations of Fe and lower Al and Mg concentrations. This chemical zoning and microfractures suggest micro-boudinage and/or coupled dissolution-precipitation occurred in these samples, and that potential fluid-mediated diffusion accommodated deformation processes may be preserved in these two mafic blueschists. We leverage the relationships between the textural and chemical evidence in concert with P-T estimates for their host terranes to interpret the deformation histories of these samples during subduction and exhumation. Crosscutting relationships between the chemical zoning and intragranular orientation gradients in the samples suggests that dislocation-related deformation was prograde and predates diffusion-related processes which became active in the Catalina and Bandon samples at or near peak conditions and during retrogression. Together, these results suggest that glaucophane can readily deform by dislocation creep, and also record fluid-mediated processes during deformation.