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High‐Temperature Deformation Behavior of Synthetic Polycrystalline Magnetite

High‐Temperature Deformation Behavior of Synthetic Polycrystalline Magnetite

Title: High‐Temperature Deformation Behavior of Synthetic Polycrystalline Magnetite
Author: Till, Jessica L   orcid.org/0000-0002-6982-6973
Rybacki, E.
Morales, L.F.G.
Naumann, M.
Date: 2019-03
Language: English
Scope: 2378-2394
University/Institute: Háskóli Íslands
University of Iceland
School: Verkfræði- og náttúruvísindasvið (HÍ)
School of Engineering and Natural Sciences (UI)
Department: Jarðvísindastofnun (HÍ)
Institute of Earth Sciences (UI)
Series: Journal of Geophysical Research: Solid Earth;124(3)
ISSN: 2169-9356
2169-9313 (eISSN)
DOI: 10.1029/2018JB016903
Subject: Flow laws; Magnetite; Experimental deformation; Oxide minerals; Creep equations; Bergfræði; Jarðefni; Jarðvísindi
URI: https://hdl.handle.net/20.500.11815/1208

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Till, J. L., Rybacki, E., Morales, L. F. G., & Naumann, M. (2019). High-Temperature Deformation Behavior of Synthetic Polycrystalline Magnetite. Journal of Geophysical Research: Solid Earth, 124(3), 2378-2394. 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.


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