Microstructural evidence of dislocation creep and diffusion accommodated deformation of glaucophane in a naturally deformed lawsonite blueschist
Jason Ott, University of Washington
Cailey Condit, University of Washington
Matej Pec, Massachusetts Institute of Technology
Baptiste Journaux, University of Washington
Abstract
The rheology and deformation mechanisms of mafic blueschists play a key role in the mechanical behavior of subducting oceanic crust in subduction zones. Mafic blueschists are often ubiquitous along the plate interface from the base of the seismogenic zone to the sub-arc depths, yet the strength of this lithology 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 related deformation processes.
We investigated the glaucophane fabric and deformation mechanisms preserved in a sample from a naturally deformed garnet-bearing lawsonite blueschist (LBS) block from the Catalina Schist subduction complex on Pimu'nga (Santa Catalina Island, CA). This block from the mélange within the LBS unit records eclogite facies peak metamorphic conditions with an LBS-facies overprint. 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 active deformation mechanisms in the block during subduction. The blueschist displays a well-developed foliation and lineation defined by interconnected layers of glaucophane. EBSD microstructural analysis of the glaucophane 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 subgrain boundary recrystallization was active. This microstructural evidence implies that dislocation-creep-accommodated deformation was active in the sample. SEM images and EDS maps of glaucophane reveals evidence of chemical zoning with higher Fe and lower Al and Mg concentrations along microfractures. This suggests a fluid-mediated/diffusion-accommodated deformation process such as micro-boudinage and/or coupled dissolution-precipitation. Crosscutting relationships between the chemical zoning and intragranular orientation gradients in the samples suggests that dislocation-related deformation predates diffusion-related processes. Titanite in the matrix and as inclusions in texturally equilibrated glaucophane and lawsonite grains signals significant glaucophane growth and deformation occurred post-peak conditions at lawsonite blueschist conditions. These observations are supported by our pseudosection modelling results for the garnet blueschist block. Together, these results suggest that glaucophane can readily deform by dislocation creep, and also record diffusion-mediated processes during deformation under lawsonite blueschist facies conditions.
Session
Deformation of mafic or ultramafic rocks