Rapid deformation and annealing events recorded in a mantle xenolith from the Wyoming craton; A pathway for understanding microstructure evolution in the mantle

The upper mantle can undergo dynamic flow or experience more static regimes of kinematic quiescence, which will affect its microstructure and various chemical and physical properties. Mantle microstructure, namely grain size, texture, and crystallographic preferred orientation (CPO), evolves according to the ambient P-T-H2O conditions and the stress - strain fields. These microstructural signatures are used to interpret conditions that prevail in the mantle relying on observations from rock deformation/annealing experiments. However, using experiments to infer natural processes often involves large uncertainties due to needed simplification related to the use of synthetic samples with well-mixed, small grains, single or two-phase, with no initial texture, deformed at laboratory time-scales. Here we analyze the microstructures of peridotite samples from the Williams kimberlitic xenoliths (Wyoming craton) that underwent rapid events of deformation and annealing. Our new data constrain the validity of applying experimental flow laws to natural conditions based on a combination of thermobarometry and EBSD-based analysis of microstructural evolution. It is demonstrated that (a) an initially textured mantle recovers by discontinuous static recrystallization (DiSRX), which can shift the grain size and modify the texture strength and geometry, (b) natural deformation at laboratory-like strain rates reproduces experimentally predicted olivine and orthopyroxene (opx) grain size distributions and yields a transitional olivine CPO between A-type and E-type, and (c) microstructural properties suggest deformation mechanisms and relative strength of olivine and opx that generally fits existing flow laws (assuming an activation volume > 20 m3106/mol for dislocation creep of opx). The relative strain rates of olivine and opx evolve with increasing strain and recrystallization, with higher olivine strain rates (prior to recrystallization) transitioning to similar olivine and opx strain rates (after recrystallization). We conclude that previously deformed lithospheric mantle can recover by DiSRX and that predictions based on experimental flow laws and fabric transitions are consistent with natural observations under comparable strain rates and stresses.