Experimental investigation of amphibole rheology and deformation mechanisms at deep crustal conditions
Abstract
Amphiboles occur in a variety of tectonic settings from subduction and collision zones to arcs, transform faults, and rifts, which makes their deformation behavior a significant contributor to deep crustal rheology. However, we currently have few constraints on amphibole rheology and deformation mechanisms. Work on naturally deformed amphibole-rich rocks suggest amphibole can be a strain localizing phase. These studies often document strong crystallographic preferred orientation (CPO) in amphibole, yet the exact mechanisms of CPO formation remain enigmatic and may range from dislocation creep, rigid body rotation, or synkinematic metamorphic growth.
To quantify the strength of amphibole and investigate its deformation mechanisms, we conducted a suite of deformation experiments on a Griggs-type apparatus at deep crustal P-T conditions. We ran pure, nominally dry pargasite-tschermakite experiments at grain sizes of <5 µm and 5-10 µm. Aggregates were hot-pressed at P = 1.5 GPa, T= 750˚C for 16 - 72 h and subsequently deformed at a pressure of 1.5 GPa, temperatures of 750, 800, and 850°C, and strain rates of 10-6-10-3 s-1. Mechanical results reveal that (1) amphibole is relatively strong at 750°C with a peak shear stress (𝛕 )of ~1000 MPa, (2) with increasing temperature 𝛕 decreases significantly to 350 MPa at 800°, and ~100 MPa at 850°C, and (3) there does not appear to be a grain-size dependence of strength at these conditions. Stress stepping at 750° C indicates that amphibole deformed by dislocation accommodated grain boundary sliding (n ≤ 2) at low stresses (𝛕 ≤ 500 MPa), and switched to dislocation creep (n=4-5) at above 500 MPa. These mechanical data are coupled with electron beam imaging (SEM-BSE, EBSD) of deformation products to investigate the microstructures and mechanisms that facilitated deformation. Preliminary EBSD data from a sample deformed at 750°C show a strong CPO coupled with intragranular misorientations and subgrain formation, documenting dislocation creep microstructures. CPO development during this deformation is consistent with slip on (100)<001>, an easy slip system observed in some naturally deformed amphibole. Our experimental results suggest relatively dry amphibole can be quite strong at moderate temperatures but weakens at high temperatures and slow strain rates.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.T43D0457C
- Keywords:
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- 3902 Creep and deformation;
- MINERAL PHYSICS;
- 8159 Rheology: crust and lithosphere;
- TECTONOPHYSICS;
- 8160 Rheology: general;
- TECTONOPHYSICS;
- 8164 Stresses: crust and lithosphere;
- TECTONOPHYSICS