Dept. of Geosciences Colloquium: New perspective on recovery processes in the upper mantle and their geophysical implications from natural and experimental peridotites

Abstract:

The dynamics and kinematics of the upper mantle are affected by and resolved through the rheological and elastic properties of olivine, its most ubiquitous mineral. The interpretation of geophysical data based on peridotite xenoliths relies on the accurate estimation of stress state from pieozmetery (e.g., grain size and dislocation density), elastic anisotropy from crystallographic preferred orientation (CPO), and on relating these measurements with the current/last stress-strain field from the depth of origin. Under mantle’s P-T conditions olivine deforms and flows by plastic, intra-crystalline deformation mechanism/s, in a similar way to the ductile behavior of hot metals. There is a vast body of work on the way deformation affects existing microstructure in the upper mantle - by means of its grain-size (i.e., piezometer) and CPO. However, the way and the conditions in which recovery processes (annealing) will modify these microstructural signatures is overlooked.

I will present evidence of recovery processes in deformed natural peridotites through microstructural (EBSD) and geochemical analysis (EPMA, SIMS) of kimberlitic xenoliths originated in the Wyoming Craton (Montana, USA) and from lab experiments. I focus on a distinctive feature of the xenoliths - tablet-shaped, faceted olivine grains showing cross-cutting relationship with surrounding grains. Intragranular strain in the tablets is negligible, yet the surrounding olivine grains show high intragranular strain. This texture indicates a post-deformation recovery process driven by reduction of strain energy. Both olivine populations (tablet and matrix) are homogeneous with respect to major elements and water contents (7-22 ppm). In addition, the CPO of the tablets appears to be modified with respect to the bulk sample; the CPO of the former is weaker and rotated 10-15° from the original CPO. Remarkably similar observations were shown by annealing experimentally deformed samples (Boneh et al., 2017). The analysis of natural and experimental tablets shows that, contrary to previous studies, the growth mechanism of olivine tablets is not related to fluid-mediated processes but rather driven by the strain energy and CPO in a solid-solid medium. I will discuss the applicability, kinetics, and implications of this overlooked style of recovery mechanism in our interpretation of the upper mantle microstructure and tectonic history.

Event Organizers: Dr. Roy Barkan and Dr. Asaf Inbal