Deformation microstructures and magnetite texture development in synthetic shear zones Jessica L. Till ⁎, Bruce M. Moskowitz Institute for Rock Magnetism, University of Minnesota, 100 Union St SE, Minneapolis, MN 55455, USA abstract article info Article history: Received 3 October 2013 Received in revised form 5 March 2014 Accepted 17 April 2014 Available online 26 April 2014 Keywords: Experimental deformation Magnetite Magnetic anisotropy Texture Deformation mechanisms 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%. Anisotro- py of magnetic susceptibility and shape preferred orientation (SPO) analysis were combined with electron back- scatter 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 sub- stantial 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 chro- mite and ringwoodite. © 2014 Elsevier B.V. All rights reserved. 1. Introduction Magnetite is a common accessory mineral in a wide range of crustal rocks and is an important component of some iron ore bodies (Barbosa and Lagoeiro, 2010; Morales et al., 2008). Magnetite is also uniquely impor- tant to tectonics and structural geology studies that use magnetic anisotro- py techniques as a petrofabric indicator since it is strongly ferromagnetic and often dominates magnetic anisotropy signatures when present (Borradaile and Jackson, 2004). However, interpretation of magnetic an- isotropy in magnetite-bearing rocks can be improved with better con- straints on magnetite deformation behavior during metamorphism. A limited number of deformation experiments have previously been performed on magnetite aggregates and single crystals (Crouch and Robertson, 1990; Gómez-García et al., 2002; Hennig-Michaeli and Siemes, 1975, 1982; Muller and Siemes, 1972) and Till and Moskowitz (2013) recently proposed a revised set of flow laws for magnetite that can help to predict magnetite deformation behavior over a range of metamorphic conditions. Plastically deformed magnetite in tectonic settings has been reported in a handful of field studies (e.g. Agar and Lloyd, 1997; Ferré et al., 2003; Housen et al., 1995; Mamtani et al., 2007) but the conditions of paleostresses and strain rates can be difficult to determine in natural rocks. Mineral microstructures and crystallo- graphic textures are often used to infer the mechanisms of deformation in naturally deformed rocks, and the development of electron backscatter diffraction (EBSD) techniques in recent decades has enabled rapid and precise determination of mineral textures. While previous experimental deformation studies have examined crystallographic texture develop- ment in magnetite using neutron diffraction methods, no experimental studies have yet used EBSD to document texture development in magne- tite or any other spinel mineral. We present an integrated study combining anisotropy of magnetic susceptibility (AMS), shape-preferred orientation (SPO), and EBSD anal- ysis of intragrain microstructures and crystallographic textures in plas- tically deformed magnetite from high-temperature laboratory shear experiments. We show that despite evidence for magnetite having deformed primarily by dislocation creep in most experiments, no clear crystallographic preferred orientation (CPO) could be detected in the texture measurements for those samples. Tectonophysics 629 (2014) 211–223 ⁎ Corresponding author at: Helmholtz-Zentrum GFZ Potsdam, Telegrafenberg, 14473 Potsdam, Germany. E-mail address: till@gfz-potsdam.de (J.L. Till). http://dx.doi.org/10.1016/j.tecto.2014.04.026 0040-1951/© 2014 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Tectonophysics journal homepage: www.elsevier.com/locate/tecto