Experimental deformation of synthetic wet jadeite aggregates

Abstract

Predictions on interplate coupling and shear heating in the deeper levels of subduction zones require an understanding of the rheology of eclogite. The strength of eclogite is probably limited by the flow strength of its major constituent omphacite, representing a solid solution between the clinopyroxene end‐members diopside and jadeite. Here we report the results of deformation experiments on the end‐member jadeite (NaAlSi2O6), carried out on fine‐grained synthetic aggregates crystallized in the stability field of jadeite from a synthetic glass precursor. A standard procedure was established to yield samples with a uniform microstructure and an average grain size of approximately 10 μm. The presence of micropores and the detection of small amounts of freezable water by Fourier transform infrared spectroscopy (FTIR) in both undeformed and deformed samples indicates the presence of a free aqueous fluid, hence wet conditions. Deformation experiments were carried out in a modified Griggs‐type apparatus at a confining pressure of 2.5 GPa, temperatures between 800°C and 1100°C and at strain rates of 4 × 10−4 s−1 to 2 × 10−6 s−1. A molten eutectic CsCl/NaCl mixture was used as pressure medium. The microfabrics of the deformed samples and the mechanical data indicate deformation in the dislocation creep regime. The mechanical data are fit by a power law using a global inversion method, yielding an activation energy of Q = 326 ± 27 kJ mol−1, a stress exponent n = 3.7 ± 0.4, and a preexponential factor of ln A = −3.3 ± 2.0. Extrapolation of this flow law for synthetic jadeitite to low geological strain rates, and comparison with available flow laws for diopside, indicates jadeite to be significantly weaker than diopside in the dislocation creep regime.