Torsion experiments on coarse-grained dunite: implications for microstructural evolution when diffusion creep is suppressed

Large strain deformation experiments in torsion were conducted on a coarse-grained natural dunite with a pre-existing lattice preferred orientation (LPO). Experiments were conducted at conditions where deformation by diffusion creep is initially negligible. Microstructural evolution was studied as a function of strain. We observe that the pre-existing LPO persists to a shear strain of at least 0.5. At larger strains, this LPO is transformed. Relict deformed grains exhibit LPO with [100] crystallographic axes at high angles to the shear plane. Unlike previous experimental studies, these axes do not readily rotate into the shear plane with increasing strain. Partial dynamic recrystallization occurs in samples deformed to moderate strains (g . 0.5). Recrystallized material forms bands that mostly transect grain interiors. The negligible rate of diffusion creep along relict grain boundaries, as well as the limited nature of dynamic recrystallization, may account for the relatively large strains required to observe evolution of microstructures. Our data support hypotheses based on natural samples that microstructures may preserve evidence of complex deformation histories. Relationships between LPO, seismic anisotropy and deformation kinematics may not always be straightforward. As the most abundant mineral in the upper mantle, olivine plays a key role its rheological behaviour (Karato & Wu 1993; Hirth & Kohlstedt 2003). Moreover, olivine is essential to our interpretation of mantle kinematics because of its strong influence on seismic anisotropy (Nicolas & Christensen 1987; Karato et al. 2008). The generation and evolution of olivine lattice preferred orientation (LPO) has been well documented in laboratory studies (Carter & Ave Lallemant 1970; Nicolas et al. 1973; Zhang & Karato 1995; Bystricky et al. 2000; Zhang et al. 2000; Jung & Karato 2001), naturally deformed peridotites (Nicolas et al. 1971; Mercier 1985; Ben Ismail & Mainprice 1998; Warren et al. 2008; Skemer et al. 2010) and numerical simulations (Wenk et al. 1991; Wenk & Tome 1999; Tommasi et al. 2000; Kaminski & Ribe 2001; Blackman & Kendall 2002; Castelnau et al. 2009). However, there are still substantial differences between the microstructures generated or observed in these studies. Notably, studies of peridotite mylonites suggest that somewhat larger strains may be required to rotate LPO into concordance with deformation kinematics, in comparison to experimental studies and numerical simulations that have an initially random texture (Warren et al. 2008). This difference in the influence of strain on LPO may be a consequence of factors such as the initial microstructure and the conditions of deformation. Furthermore, different numerical models show distinct patterns and rates of LPO development which may differ from experimental and geological observations (Castelnau et al. 2009). Variations between models arise from differences in numerical techniques, but also the inclusion or exclusion of specific dynamic recrystallization mechanisms (Wenk et al. 1991; Wenk & Tome 1999). To expand on existing experimental studies and provide further bases for numerical investigations, we conducted a series of deformation experiments on coarse-grained natural dunite. The objective of these experiments is to improve our understanding of microstructural processes in realistic mantle materials and to help bridge the gap between the laboratory and nature. For technical reasons, many of the experimental studies of olivine deformation have been conducted on relatively fine-grained synthetic materials (e.g. Zhang & Karato 1995; Bystricky et al. 2000). As a consequence, deformation in these experiments typically includes contributions from several parallel mechanisms including dislocation creep, diffusion creep and grain-boundary sliding (Fig. 1). In contrast, many geologically observable mantle rocks are interpreted to have deformed at conditions where dislocation creep is strongly dominant. It is therefore important to understand from an From: Prior, D. J., Rutter, E. H. & Tatham, D. J. (eds) Deformation Mechanisms, Rheology and Tectonics: Microstructures, Mechanics and Anisotropy. Geological Society, London, Special Publications, 360, 211–223. DOI: 10.1144/SP360.12 # The Geological Society of London 2011. Publishing disclaimer: www.geolsoc.org.uk/pub_ethics experimental perspective how mantle materials deform when grain-size-sensitive creep is more completely suppressed. To this end, we conducted laboratory deformation experiments on samples cored from a block of Balsam Gap Dunite, which was chosen for two reasons. First, it has a moderately large grain size which allows us to suppress activity of the diffusion creep mechanism. Secondly, it has a moderately strong pre-existing LPO. This allows us to consider how LPO might evolve in a situation where kinematics are changing, for example in a corner flow regime such as a mantle wedge or mid-ocean ridge.