Deformation microstructures and magnetite texture development in synthetic shear zones

Jessica L. Till, Bruce M. Moskowitz

Research output: Contribution to journalArticlepeer-review

11 Scopus citations

Abstract

We present observations of deformation features in magnetite from synthetic magnetite-bearing silicate aggregates deformed between 1000. °C and 1200. °C in transpressional shear experiments with strains of up to 300%. Anisotropy of magnetic susceptibility and shape preferred orientation (SPO) analysis were combined with electron backscatter diffraction (EBSD) to characterize the magnetite deformation fabrics and intragrain microstructures. Crystallographic preferred orientation (CPO) in magnetite is very weak in all deformed samples and does not vary as a function of either temperature or shear strain. Magnetic anisotropy and SPO increase strongly with both strain and deformation temperature and indicate that strain partitioning between magnetite and the plagioclase matrix decreases at higher temperatures. EBSD orientation mapping of individual magnetite particles revealed substantial dispersions in intragrain orientation, analogous to undulose extinction, after deformation at 1000 and 1100. °C, indicating that dislocation creep processes were active in magnetite despite the lack of a well-developed CPO. Geometrical analysis of crystallographic orientation dispersions from grain map data indicates that low-angle grain boundary formation in magnetite could have been accommodated by slip on {110} or {100} planes, but no evidence for dominant slip on the expected {111} planes was found. Evidence for activation of multiple slip systems was seen in some magnetite grains and could be partially responsible for the lack of CPO in magnetite. These results suggest that, at least in polyphase rocks, crystallographic textures in magnetite may be inherently weak or slow to develop and CPO alone is not an adequate indicator of magnetite deformation mechanisms. These results may aid in the interpretation of deformation textures in other spinel-structured phases such as chromite and ringwoodite.

Original languageEnglish (US)
Pages (from-to)211-223
Number of pages13
JournalTectonophysics
Volume629
Issue numberC
DOIs
StatePublished - 2014

Bibliographical note

Funding Information:
We thank David Kohlstedt and Mark Zimmerman for helpful discussions and assistance in planning and performing deformation experiments. Thanks to Lars Hansen and Steve Reddy for helpful suggestions and comments on an earlier version of this manuscript. Leonardo Lagoeiro and an anonymous reviewer are thanked for constructive reviews. Parts of this work were carried out in the Characterization Facility, University of Minnesota, which receives partial support from NSF through the MRSEC program. This project was funded by a grant from the National Science Foundation EAR division. The Institute for Rock Magnetism is supported by grants from the Instruments and Facilities Program, National Science Foundation EAR division. This is publication 1308 of the Institute for Rock Magnetism.

Publisher Copyright:
© 2014 Elsevier B.V.

Keywords

  • Deformation mechanisms
  • Experimental deformation
  • Magnetic anisotropy
  • Magnetite
  • Texture

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