GEOLOGY | September 2013 | www.gsapubs.org 959 INTRODUCTION A switch from grain size–insensitive, or dis- location, creep to grain size–sensitive, or grain boundary sliding (GBS), creep due to grain size reduction in deforming rocks is considered to be a major cause of strain localization in ductile shear zones (e.g., Jin et al., 1998; Hansen et al., 2012). The details of the localization process are still elusive but important to understanding how deformation is accommodated in the lithosphere (e.g., Braun et al., 1999). Geologists often associate evidence of GBS with fine-grained microstructures observed in intensely deformed rocks such as ultramylonites (e.g., Boullier and Gueguen, 1975; Fliervoet et al., 1997). These rocks are often polymineralic with a homog- enous distribution of mineral phases such that GBS has been assumed to mix recrystallized grains of different minerals, which retain fine grain size because grain growth is inhibited by grain boundary pinning (e.g., Warren and Hirth, 2006; Skemer et al., 2010). GBS can produce superplasticity of solid crystalline materials (e.g., Ashby and Verrall, 1973) defined by the achievement of tensile strains of >>100% with- out failure, such that geologists often refer to GBS as superplasticity irrespective of its rate- controlling process (e.g., Boullier and Gueguen, 1975; Goldsby and Kohlstedt, 2001). Recently, superplasticity of geomaterials was demonstrated experimentally for the first time on a forsterite (Fo)-bearing system (Hiraga et al., 2010a). In these experiments, deforma- tion-enhanced grain growth and aggregation of the same phase were observed. Here we focus on the aggregation microstructure using Fo + Ca-bearing pyroxene (Px) aggregates, which allow us to analyze the structure easily using scanning electron microscopy (SEM). The aggregate was deformed in various deformation geometries so that we are able to determine the aggregation mechanism in terms of stress and strain geometries. The microstructure develops as a consequence of the phase redistribution during GBS-dominant deformation. We com- pare these to similar microstructures identified in ultramylonites, which are thought to have deformed with a large contribution of GBS, to evaluate the role of GBS in phase mixing and further in strain localization in nature. Here, interphase boundary sliding is also considered as part of GBS. EXPERIMENTS We prepared samples with Fo plus 20–30 vol% Px, a composition that has previously exhibited superplasticity (Hiraga et al., 2010a) and in which it is easy to analyze the phase distribution using SEM because of the den- sity contrast between the phases. We used a vacuum sintering technique to synthesize our samples from nano-sized powders into fully dense aggregates that are pore and crack free (Koizumi et al., 2010). All the samples exhibit an equilibrium microstructure with a homo- geneous distribution of phases and a larger Fo grain size relative to that of Px (Fig. 1A) Comparison of microstructures in superplastically deformed synthetic materials and natural mylonites: Mineral aggregation via grain boundary sliding Takehiko Hiraga 1 , Tomonori Miyazaki 1 , Hidehiro Yoshida 2 , and Mark E. Zimmerman 3 1 Earthquake Research Institute, University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-0032, Japan 2 Advanced Ceramics Group, National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan 3 Department of Geology and Geophysics, University of Minnesota, Minneapolis, Minnesota 55455, USA ABSTRACT We conducted compressional, tensile, and torsional creep experiments on fine-grained forsterite plus Ca-bearing pyroxene aggregates. A distinct microstructure with aggregation of the same phase in the direction of compression was formed in our samples after all the experiments. The stress–strain rate relationship, grain-size dependent flow strength, and the achievement of large tensile strain all indicate that samples underwent creep due to grain boundary sliding (GBS). As a result of GBS, grain-switching events allow dispersed phases to contact grains of the same phase and orient in the direction of compression. We identify simi- lar aggregated microstructures in previously reported micrographs of polymineralic granite- origin ultramylonites. Mineral phase mixing through GBS, which helps to retain fine grain size in rocks due to grain boundary pinning, has been speculated to occur during formation of mylonites. However, our results contradict this hypothesis because mineral aggregation through GBS promotes demixing rather than mixing of the mineral phases. GBS processes alone will not promote a transformation of well-developed monomineralic bands to polymin- eralic bands during mylonitization. GEOLOGY, September 2013; v. 41; no. 9; p. 959–962; Data Repository item 2013269 doi:10.1130/G34407.1 | Published online 22 July 2013 © 2013 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or editing@geosociety.org. A μ μ E F μ μ μ μ μ μ C μ μ μ μ B D μ μ Figure 1. Backscattered- electron images of deformed and undeformed forsterite (gray colored) + 20 or 30 vol% Ca-bearing pyroxene (light colored) (30 vol% in D; 20 vol% in A–C and E–F) samples. All samples were thermally etched except for the sample in D. Pairs of arrows indicate deforma- tion direction. A: Starting sample. B: Sample with tensile strain of 0.6 (KS-13). C: Sample with tensile strain of 1.5 (KS-17). D: Sample with tensile strain of 0.8 (tD1). E: Sample with com- pressional strain of 0.7 (KF-125). F: Sample with shear strain of 4.1 (PI-0629).