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Iron‐rich olivine is mechanically weaker than olivine of mantle composition, ca. Fo₉₀, and thus is more amenable to study under a wide range of laboratory conditions. To investigate the effects of iron content on deformation‐produced crystallographic preferred orientation (CPO) and grain size, we analyzed the microstructures of olivine samples with compositions of Fo₇₀, Fo₅₀, and Fo₀that were deformed in torsion under either anhydrous or hydrous conditions at 300 MPa. Electron backscatter diffraction (EBSD) observations reveal a transition in CPO from D‐type fabric, induced by dislocation glide on both the (010)[100] and the (001)[100] slip systems, at low strains, to A‐type fabric, caused by dislocation glide on the (010)[100] slip system, at high strains for all of our samples, independent of iron content and hydrous/anhydrous conditions. A similar evolution of fabric with increasing strain is also reported to occur for Fo₉₀. Radial seismic anisotropy increases with increasing strain, reaching a maximum value of ∼1.15 at a shear strain of ∼3.5 for each sample, demonstrating that the seismic anisotropy of naturally deformed olivine‐rich rocks can be well approximated by that of iron‐rich olivine. Based on EBSD observations, we derived a piezometer for which recrystallized grain size decreases inversely with stress to the ∼1.2 power. Also, recrystallized grain size increases with increasing iron content. Our experimental results contribute to understanding the microstructural evolution in the mantle of not only Earth but also Mars, where the iron content in olivine is higher.

We calibrate a subgrain‐size piezometer using electron backscatter diffraction (EBSD) data collected from experimentally deformed samples of olivine and quartz. Systematic analyses of angular and spatial resolution test the suitability of each data set for inclusion in calibration of the subgrain‐size piezometer. To identify subgrain boundaries, we consider a range of critical misorientation angles and conclude that a 1° threshold provides the optimal piezometric calibration. The mean line‐intercept length, equivalent to the subgrain‐size, is found to be inversely proportional to the von Mises equivalent stress for data sets both with and without the correction of Holyoke and Kronenberg (2010,https://doi.org/10.1016/j.tecto.2010.08.001). These new piezometers provide stress estimates from EBSD analyses of polymineralic rocks without the need to discriminate between relict and recrystallized grains and therefore greatly increase the range of rocks that may be used to constrain geodynamic models.

To understand the effects of secondary minerals on changes in the mechanical properties of upper mantle rocks due to phase mixing, we conducted high‐strain torsion experiments on aggregates of iron‐rich olivine + orthopyroxene (opx) with opx volume fractions off_opx = 0.15, 0.26, and 0.35. For samples with larger amounts of opx,f_opx = 0.26 and 0.35, the value of the stress exponent decreases with increasing strain fromn ≈ 3 for γ ≲ 5 ton ≈ 2 for 5 ≲ γ ≲ 25, indicating that the deformation mechanism changes as strain increases. In contrast, for samples withf_opx = 0.15, the stress exponent is constant atn ≈ 3.3 for 1 ≲ γ ≲ 25, suggesting that no change in deformation mechanism occurs with increasing strain for samples with smaller amounts of opx. The microstructures of samples with larger amounts of opx provide insight into the change in deformation mechanism derived from the mechanical data. Elongated grains align subparallel to the shear direction for samples of all three compositions deformed to lower strains. However, strain weakening with grain size reduction and the formation of a thoroughly mixed, fine‐grained texture only develops in samples withf_opx = 0.26 and 0.35 deformed to higher strains of γ ≳ 16. These mechanical and associated microstructural properties imply that rheological weakening due to phase mixing only occurs in the samples with largerf_opx, which is an important constraint for understanding strain localization in the upper mantle of Earth.