Diffusion Creep of Sodic Amphibole‐bearing Blueschist limited by Microboudinage

Abstract

To investigate the mechanical and microstructural properties of mafic blueschists, we conducted deformation experiments on powdered natural blueschist aggregates using the general shear geometry in the Griggs apparatus. Experiments were performed at ∼1.0 GPa and temperatures ranging from 650 to 700°C. The blueschist starting material consists primarily of sodic amphibole and epidote, with minor amounts of quartz, titanite, albite, and white mica. Strain rate stepping experiments provided mechanical data with stress exponents ranging from 1.8 to 2.2. Microstructural analysis of the deformed samples show that the blueschist aggregates were deforming by microboudinage of the sodic amphibole, with a chemically new sodic-calcic amphibole diffused into the boudin neck. Based on these results, we interpret the samples to have deformed by diffusion creep of the sodic-calcic amphibole, which was rate-limited by diffusion into the boudin neck. We developed a microboudinage diffusion creep flow law using a least square regression, with parameters of A = 2.43e11 MPa−n μm s−1, n = 2.0 ± 0.3, m = 1.0, and Q = 384 ± 15 kJ/mol. Extrapolation of the flow law to the blueschist stability field suggests viscosities that are higher than metasedimentary rocks (quartz dislocation creep flow law) and lower than eclogitic rocks (omphacite dislocation creep flow law) consistent with field observations. We also show that this type of deformation mechanism matches observations of natural rocks in paleosubduction zone environments, supporting the application of this flow law to estimate amphibole rheology in modern subduction zones.