Christopher A. Thom, Lars Hansen, David L Goldsby, Kathryn M Kumamoto

Recent work has identified the importance of strain hardening and backstresses among dislocations in the deformation of geologic materials at both high and low temperatures, but very few experimental measurements of such backstresses exist. Using a nanoindentation load-drop method and a self-similar Berkovich tip, we measure backstresses in single crystals of olivine, quartz, and plagioclase feldspar at a range of indentation depths from 100–1750 nm, corresponding to densities of geometrically necessary dislocations (GND) of order 1014–1015 m-2. Our results reveal a power-law relationship between backstress and GND density, with an exponent ranging from 0.44 to 0.55 for each material, in close agreement with the theoretical prediction (0.5) from Taylor hardening. This work provides experimental evidence of Taylor hardening in geologic materials and supports the assertion that backstress must be considered in both high- and low-temperature deformation.