Rheology of synthetic anorthite‐diopside aggregates: Implications for ductile shear zones

Abstract

We investigated the high‐temperature creep strength of fine‐grained anorthite‐diopside rocks at temperatures ranging from 1323 K to 1523 K and at 300 MPa confining pressure in a Paterson‐type gas‐medium deformation apparatus. Flow stress varied between 20 and 450 MPa resulting in strain rates between 6.1 × 10−7 s−1 and 7.5 × 10−4 s−1. Pure diopside and anorthite samples were hot pressed from crushed natural single crystals and glass powders, respectively. Two‐phase samples were produced by hot isostatic pressing of mechanically mixed powders of anorthite glass with 25, 50 and 75 vol % diopside particles. Arithmetic mean grain size of the anorthite matrix is dAn ≈ 3.5 μm. Three different ranges of diopside particle size were used: dDi < 25 μm, <35 μm, and <45 μm. Water content of as is samples was about 0.05 ± 0.02 wt % H2O, and predried samples contain about 0.004 ± 0.001 wt % H2O. At experimental conditions, as is samples are assumed to be water saturated. Water content of predried samples is about 3 times less than that of starting diopside single crystals. The specimens contain about 1 vol % glass located at fluid inclusions and some multiple grain junctions. Two‐grain boundaries examined by high‐resolution transmission electron microscopy did not show amorphous layers to a resolution of 1 nm. At experimental conditions, pure diopside aggregates are about 2–3 orders of magnitude stronger than pure anorthite samples for as is and predried specimens, respectively. In general, strength of the two‐phase aggregates increases with increasing diopside content but remains between isostress and isostrain rate bounds. Aggregate strengths predicted from continuum mechanics models are in good agreement with the experimental data for dilute diopside particle mixtures and high‐volume fractions, when diopside particles form a load‐bearing framework. At low stresses (<100–200 MPa) the stress exponent is n ≈ 1, suggesting diffusion‐controlled creep. At higher stresses, mechanical data and microstructures suggest that samples deformed in the transition region between diffusion‐controlled creep and dislocation creep. For pure anorthite and diopside aggregates deforming in dislocation creep we estimated stress exponents of n ≈ 3 and n ≈ 5.5, respectively. For the two‐phase aggregates, n is between n ≈ 3 and n ≈ 5, depending on diopside content. At low stresses, deformation microstructures indicate load transfer from a weak anorthite matrix to stronger diopside particles. Creep activation energies for pure diopside and anorthite mixtures range from 286 kJ mol−1 for wet anorthite deformed at low stresses to 691 kJ mol−1 for dry diopside deformed at high stresses. Activation energies of two‐phase mixtures are between or close to those of the end‐members. As is samples have significantly lower activation energies than predried samples.