Re-creating Upper Mantle Conditions in the Lab: DO NOT TRY THIS AT HOME!
Authored by Pamela Speciale.
I just returned from the Rock Deformation Lab at Brown University, where I’ve been searching for evidence of how microstructure development (i.e., grain size, grain boundary morphology) contributes to strain localization, which is essential for the maintenance of plate tectonics. We can learn a lot about strain localization by observing rock formations in the field, but shear zones exhumed to the surface provide only a snapshot of the microstructures, frozen in time. Fortunately, the microstructures of experimentally-deformed samples are remarkably similar to those we see in nature! To help understand the dynamic conditions under which microstructures evolve in olivine, the dominant mineral in the upper mantle, I’m conducting deformation experiments at the same high pressures and temperatures found within the lithospheric mantle. If I can identify the mechanisms that lead to strain localization, in experimental samples with known deformation conditions, we may be able to infer the same mechanisms in naturally-deformed rocks!
The Griggs Rig at Brown University is like a piston-cylinder apparatus but is equipped with a moving piston to exert a differential stress on the sample.
I start with fine-grained olivine sandwiched between alumina pistons inclined at 45° (to simulate a shear zone). Samples deformed to successively increasing strain magnitudes (1-4γ) provide snapshots of microstructure development during each strain increment. Samples deformed either at constant strain rate or at constant stress magnitude allow me to compare the mechanical and microstructural response of highly-deformed olivine under different boundary conditions. To quantify how these microstructures evolved, I measure grain size distributions, aspect ratios, crystallographic orientations, and boundary morphologies using electron backscatter diffraction maps and optical microscopy.
Cross section of experimentally-deformed olivine (green) surrounded by Ni and Pt jackets. The alumina shear pistons (white) are ~5 mm diameter.
So far, the microstructures have a pretty interesting story to tell! Under constant strain rate conditions, where the stress gradually decreases following the yield stress, the microstructures are composed of small, relatively equant grains. But at constant stress conditions, wherein the strain rate accelerates during deformation, many of the grains become highly elongate in the direction of shear. What’s more, at constant stress conditions, additional zones of localized strain developed inside my simulated shear zone! I’m excited to continue analyzing these awesome microstructures, and gain interesting insights into the mechanisms of strain localization.
Inverse pole figure map (IPFx) of sample deformed to <2γ under constant stress conditions. Large, elongate grains are oriented ~45° to the shear direction (top to the left).