High‐Temperature Deformation Behavior of Synthetic Polycrystalline Magnetite

• Published in: Journal of geophysical research. Solid earth 2019-03, Vol.124 (3), p.2378-2394
• Main Authors: Till, J. L., Rybacki, E., Morales, L.F.G., Naumann, M.
• Format: Article
• Language: English
• Subjects: Aggregates; Atmospheric pressure; Confining; Creep (materials); creep equations; Crustal deformation; Deformation; Deformation mechanisms; Dislocation; Dislocations; experimental deformation; Gabbros; Geophysics; Grain growth; Hot isostatic pressing; Laws; Magnetite; oxide minerals; Polycrystals; Porosity; Powder; Pressure; Rheological properties; Silicates; Solifluction; Stresses; Temperature; Temperature effects
• ISSN: 2169-9313; 2169-9356
• DOI: 10.1029/2018JB016903

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. Key Points Creep rates of polycrystalline magnetite aggregates were measured as a function of temperature, stress, grain size, and water content The presence of water resulted in significantly enhanced magnetite diffusion creep rates Flow laws based on the experimental results predict magnetite to be weaker than many silicate minerals under both dry and wet conditions