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Of the measurements on Brg, one measurement was made in the DAC on a presynthesized sample using ruby fluorescence and the pressure gradient technique (Meade & Jeanloz, 1990). Two measurements have been made in the DAC with angle dispersive diffraction on a presynthesized sample (Merkel et al., 2003) or samples synthesized in‐situ from enstatite, olivine, and ringwoodite (Miyagi & Wenk, 2016). All of these DAC experiments were performed at room temperature, aside from two data points from Miyagi & Wenk (2016) where samples were annealing at ~1600K for 30 minutes. Two studies using large volume apparatus have also been performed. Chen et al. (2002) performed stress relaxation experiments using a DIA type apparatus and determined stress levels from x‐ray line broadening. More recently, Girard et al. (2016) performed high shear strain experiments using the RDA in conjunction with energy dispersive x‐ray diffraction to determine stress levels in a two‐phase aggregate of Brg and Fp. This is to date the only high pressure and high temperature controlled strain rate deformation experiment on Brg. For the differential stresses plotted in Figure 2.3 for Girard et al. (2016), only data collected after ~20% strain was used, as this is where steady‐state stress levels are achieved. One should note that in Miyagi & Wenk (2016), a moment pole stress model was used to fit lattice strains, and standard deviations from this model (smaller than the symbols in Figure 2.2) represent errors in fitting and likely underestimates errors in the measurement and those due to plastic anisotropy. Thus, this study has smaller error bars than other measurements.

When looking at differential stress measurements on Brg, the slope of differential stress versus pressure is steep, with the exception of the data of Meade & Jeanloz (1990), which is much flatter (Figure 2.3, green Xs). Stresses are highest in the experiment of Merkel et al. (2003) (Figure 2.3, blue squares). This sample, like Meade & Jeanloz (1990), was mostly compressed outside of the Brg stability field (P<~23 GPa), and it is unclear if this may affect the flow strength of the material. The slope of the room temperature data of Miyagi & Wenk (2016) is the steepest but data points at ~30 GPa are immediately after synthesis and prior to deformation (Figure 2.3, green triangles) so are not reflective of the materials flow strength. If we assume that the highest measured stresses in the DAC are reflective of the room temperature flow strength, then Brg is significantly stronger than Fp, particularly at lower pressures. However Fp becomes much closer in strength to Brg at higher pressures (Figure 2.2, 3). Differential stresses measured in Brg in the large‐volume apparatus at high temperature are lower than room temperature DAC experiments (Figure 2.3). Stress levels measured in Chen et al. (2002) (Figure 2.3, light red X) and Girard et al. (2016) (Figure 2.3, red diamonds) are 2–3 GPa lower than measurements of Meade & Jeanloz (1990) and Merkel et al. (2003) at similar pressures, though the large error bars of Girard et al. (2016) do overlap with the DAC data. Interestingly, the stress levels measured in the study of Chen et al. (2002) at 1073 K are very similar to those of Girard et al. (2016) at ~2000 K. It is unsurprising that stress relaxation provides a lower value than steady state measurements. Likewise, the DAC annealing points of Miyagi & Wenk (2016) at ~50 GPa are quite low. Recently Kraych et al. (2016) used molecular dynamics simulations to study temperature dependence of the strength of Brg. At 30 GPa, the calculations of Kraych et al. (2016) show reasonably good agreement with both room temperature DAC measurements and the high‐temperature measurements of Girard et al. (2016), showing the great potential for theory to link room‐temperature measurement to high‐temperature measurements and also the potential to link laboratory strain rates to mantle strain rates.