Xenoliths play a crucial role in interpretation of mantle deformation and geochemistry. Here, we integrate a suite of interrelated datasets extracted from large area EBSD mapping of entire geological thin sections of peridotite (olivine-rich) xenoliths from the Williams and Homestead kimberlites from the Wyoming Craton, Montana, USA to constrain bulk fabric and intragranular deformation mechanisms (misorientation analysis, “MOA”) in olivine. We compare these results to experimental values of the effect of trace amounts (ppm) of water on olivine slip systems. We also place our results in the context of the classic mantle deformation cycle, originally proposed by Mercier and Nicolas (1975).
The analysis of bulk fabric, misorientations, and subgrain structure reveal a complex picture in the Homestead and Williams xenoliths. We have used predictions for dislocation creep in simple shear deformation as a “baseline” to frame our observations. However, few xenoliths fall neatly within this framework. For example, although the Homestead xenoliths primarily show B-type fabric, the analysis of MOA show evidence for A-type subgrains, and the orientation of the MOA in the sample reference frame are mostly inconsistent with a simple shear kinematics. Homestead xenoliths also contain higher water contents, a diagnostic feature of many B-type fabric olivines. At Williams, the opposite picture emerges, with dominantly A-type bulk CPO and many MOA consistent with E-, or C-type subgrains. Williams xenoliths are also drier on average and show evidence of a rapid cycle of high-temperature annealing. We propose complex lithospheric dynamics played a role in the apparent discrepancies between bulk fabric and intragrain microstructures. Our results indicate that multiple geochemical and tectonic factors contributed to the multi-layered fabric observed in the Wyoming xenoliths.

Invited: Maximizing data extraction from large are EBSD maps of geological thin sections: recent examples using mantle xenoliths

Speakers

Emily Chin