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 650‐700°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 both the hydrostatic and 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.

Dislocation creep near the frictional-viscous transition in blueschist: experimental constraints

“…9a). This change in deformation mechanisms at these conditions is also confirmed by the change in stress exponent, from greater than 20 in Okazaki and Hirth (2020) to ∼3 from our data and Hacker and Christie (1990) to less than 3 in Tokle et al (2023a) and Getsinger and Hirth (2014) (Fig. 9b).…”

“…The mixed Type 1/3 CPOs present in the hydrostatic and peak stress samples indicate that this fabric can be produced by rigid rotation and cataclasis. We also note that a Type 1/3 fabric was observed by Tokle et al (2023a) for diffusion creep in experimentally deformed blueschist samples. The transition to a more definitive Type 1 CPO in our mechanical steady-state samples that reached γ ∼2 suggests that this CPO is also consistent with dislocation creep.…”

“…Types 1-3 have been produced in experiments to shear strains <3 and are interpreted to correspond to different temperatures and differential stresses (Ko and Jung, 2015). The Type 4 CPO has only been documented in deformation experiments in fine-grained samples deformed to shear strains >3 (Kim and Jung, 2019) (Hacker and Christie, 1990;Getsinger and Hirth, 2014;Okazaki and Hirth, 2020;Tokle et al, 2023a) data were not presented in those studies. Additionally, the differences among Types 1, 2, and 3 are subtle--i.e.…”

“…Several studies, for example, suggest that sodic amphibole deforms via dislocation mechanisms such as dislocation glide or creep, based on the presence of undulose extinction, subgrain development, and/or dynamic recrystallization (Reynard et al, 1989;Kim et al, 2013;Cao et al, 2013;Kim et al, 2015;Kotowski and Behr, 2019). Other studies discuss evidence for dissolution-precipitation creep processes based on the presence of micro-boudinage with new amphibole growth in boudin necks, or new amphibole overgrowths oriented parallel to the foliation (Misch, 1969;Tokle et al, 2023a;De Caroli et al, 2024). Some studies have also documented brittle deformation and cataclasis (Ildefonse et al, 1990;Muñoz-Montecinos et al, 2023).…”

“…Previous experimental work on sodic amphibole has focused on its bulk modulus (Jenkins et al, 2010), development of crystallographic preferred orientations (CPO) (Park et al, 2020;Park and Jung, 2022), its seismic velocity and anisotropy (Ha et al, 2019;Park and Jung, 2022), and the role of dehydration embrittlement (Kim et al, 2015;Incel et al, 2017). Tokle et al (2023a) provided the first flow law for mafic polymineralic blueschists, describing conditions under which they deform via diffusion creep associated with microboudinage. In this paper, we build on these existing experimental constraints by focusing on the mechanical and microstructural properties of sodic amphibole (glaucophane) aggregates.…”