High‐Temperature Deformation Behavior of Synthetic Polycrystalline Magnetite

Abstract

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.

High-temperature creep of magnetite and ilmenite single crystals

“…Their activation energy of 307 is somewhat lower than our observed values, but is reasonably close considering the different sample sources and experimental apparatus used. It is also worth noting that the strength of magnetite aggregates studied by Till et al (2019) is closest to those of the harder magnetite single-crystal orientations found in this study. Experimentally determined creep rates for polycrystalline hematite by Siemes et al (2003) are comparable to our data for ilmenite compressed perpendicular to <c> (experiment KI05) but with a lower activation energy of 225 kJ/mol.…”

“…The activation energies determined here for magnetite single crystals are notably lower than that reported for dislocation creep of dry polycrystalline magnetite by Till et al (2019) of 463 ± 19 kJ/mol. The discrepancy partly arises from the different oxygen fugacity buffers imposed by the Ni-NiO jacket used in the Paterson rig by Till et al (2019) and the CO 2 -H 2 mixture used in the creep rig here. When both sets of data are plotted at the experimental buffers, creep rates are similar at high temperatures but differ significantly at low temperature (Fig.…”

“…3.5 ± 0.4 −0.7 ± 0.2 690 ± 150 2.5 × 10 8 Ilmenite, <a> 4.3 ± 0.4 0 345 ± 30 1.58 Ilmenite, <c> 3.3 ± 0.5 0 420 ± 35 1.6 × 10 4 Ilmenite, oblique to <c> and <a> 3.9 ± 0.4 0 360 ± 40 1.2 × 10 4 determined in experiment JM07 (Fig. 8a), we adjusted the normalized creep rates from experiment JM04 to oxygen fugacities at the Ni-NiO buffer for comparison to the data of Till et al (2019), as shown as Fig. 9b.…”

“…Values for quartzfayalite-magnetite (QFM) and Ni-NiO are from Myers and Eugster (1983) and Huebner and Sato (1970), respectively, and oxygen fugacity values for the CO 2 -H 2 gas mixture are from Prunier and Hewitt (1981). b Temperature dependence of magnetite creep rates from experiment JM04 before and after adjusting to oxygen fugacities at the Ni-NiO buffer compared with the polycrystalline magnetite flow law of Till et al (2019). The apparent activation energies are indicated for each data set and oxygen vacancy clusters.…”

“…Dislocation and diffusion creep laws for (wet) magnetite were proposed by Till and Moskowitz (2013) based on theoretical considerations. More recently, Till et al (2019) experimentally examined the creep behavior of (dry) polycrystalline magnetite aggregates. The textural development and creep strength of hematite have been studied in detail by Siemes et al (2003Siemes et al ( , 2010Siemes et al ( , 2011, as well as single crystal deformation (Siemes et al 2008).…”