We performed a series of deformation experiments on synthetic magnetite aggregates to characterize the high‐temperature rheological behavior of this mineral under nominally dry and hydrous conditions. Grain growth laws for magnetite were additionally determined from a series of static annealing tests. Synthetic magnetite aggregates were formed by hot isostatic pressing of fine‐grained magnetite powder at 1,100 °C temperature and 300‐MPa confining pressure for 20 hr, resulting in polycrystalline material with a mean grain size around 40 μm and containing 2–4% porosity. Samples were subsequently deformed to axial strains of up to 10% under constant load conditions at temperatures between 900 and 1,150 °C in a triaxial deformation apparatus under 300‐MPa confining pressure at applied stresses in the range of 8–385 MPa or in a uniaxial creep rig at atmospheric pressure with stresses of 1–15 MPa. The aggregates exhibit typical power‐law creep behavior with a mean stress exponent of 3 at high stresses, indicating a dislocation creep mechanism and a transition to near‐Newtonian creep with a mean stress exponent of 1.1 at lower stresses. The presence of water in the magnetite samples resulted in significantly enhanced static grain growth and strain rates. Best‐fit flow laws to the data indicate activation energies of around 460 and 310 kJ/mol for dislocation and diffusion creep of nominally dry magnetite, respectively. Based on the experimentally determined flow laws, magnetite is predicted to be weaker than most major silicate phases in relatively dry rocks such as oceanic gabbros during high‐temperature crustal deformation.

The deformation of rocks in the Earth's middle and lower crust is often localized in ductile shear zones. To better understand the initiation and propagation of high-temperature shear zones induced by the presence of structural and material heterogeneities, we performed deformation experiments in the dislocation creep regime on Carrara marble samples containing weak (limestone) or strong (novaculite) second phase inclusions. The samples were mostly deformed in torsion at a bulk shear strain rate of approximate to 1.9 x 10(-4) s(-1) to bulk shear strains gamma between 0.02 and 2.9 using a Paterson-type gas deformation apparatus at 900 degrees C temperature and 400 MPa confining pressure. At low strain, twisted specimens with weak inclusions show minor strain hardening that is replaced by strain weakening at gamma > 0.1-02. Peak shear stress at the imposed conditions is about 20 MPa, which is approximate to 8% lower than the strength of intact samples. Strain progressively localized within the matrix with increasing bulk strain, but decayed rapidly with increasing distance from the inclusion tip. Microstructural analysis shows twinning and recrystallization within this process zone, with a strong crystallographic preferred orientation, dominated by {r} and (c) slip in < a >. Reaystallization-induced weakening starts at local shear strain of about 1 in the process zone, corresponding to a bulk shear strain of about 0.1. In contrast, torsion of a sample containing strong inclusions deformed at similar stress as inclusion-free samples, but do not show localization. The experiments demonstrate that the presence of weak heterogeneities initiates localized creep at local stress concentrations around the inclusion tips. Recrystallization-induced grain size reduction may only locally promote grain boundary diffusion creep. Accordingly, the bulk strength of the twisted aggregate is close to or slightly below the lower (isostress) strength bound, determined from the flow strength and volume fraction of matrix and inclusions. (C) 2014 Elsevier B.V. All rights reserved.

Samples of fine-grained (approximately 9 mu m) and coarse-grained (approximately 45 mu m) hematite ores with almost random crystallographic preferred orientation (CPO) were deformed in high pressure, high temperature (400 MPa, 850-1 000 degrees C) torsion experiments up to shear strains of 4.7. Samples with large initial grain size, preferably deformed by dislocation creep attended by dynamic recrystallization, showed grain size reduction and a weak CPO (J similar to 1.4 at 950 degrees C). In contrast, fine-grained ores, deformed mainly by grain boundary sliding accompanied by dislocation activity with slip on the dominant basal glide system, showed grain growth and a strong CPO (J similar to 2.9). At high strain both ores attained similar strength and grain size, but different CPOs. The experiments demonstrate that the texture intensity of highly deformed rocks strongly depends on initial microstructure and may not reflect unambiguously the prevailing deformation mechanism and strength as often assumed in field studies.

Polycrystalline hematite samples cored perpendicular and parallel to the foliation, i.e. perpendicular and parallel to a weak c-axis maximum, respectively, were deformed in triaxial compression experiments at confining pressures of 300 and 400 MPa, temperatures T between 600 and 1100 degreesC, and strain rates between 10(-4) and 10(-6) s(-1). The measured strength of hematite ranges from 890 to 67 MPa.The grain size of the main starting material is up to 175 mum. Grain boundaries are intensely serrated. At T less than or equal to 800 degreesC the grain boundaries become increasingly lobate and the number of r-twins decreases. Dynamic recrystallization starts above 800 degreesC and a foam structure with grain sizes up to 150 mum develops at T greater than or equal to 900 degreesC.Neutron diffraction measurements show a distinct change of the preferred orientation (texture) in compression parallel to the foliation. Below 800 degreesC a {300}-maximum developed due to {a} slip. At 800 and 900 degreesC a c-axis maximum evolved, probably due to (c) slip. The original texture is preferentially preserved but with lower densities at T greater than or equal to 1000 degreesC, presumably caused by increasing diffusional flow processes. Perpendicular to the foliation only minor changes of the texture occurred.The experimentally determined textures allow a better understanding of the natural preferred orientation of hematite ores. The extrapolation of experimental strength data of hematite, quartz, and carbonates to geological conditions are compatible with field observations. (C) 2003 Elsevier Science Ltd. All rights reserved.