Mantle Convection and Surface Expressions. Группа авторов

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СКАЧАТЬ 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.

       Post‐Perovskite.

      For MgSiO₃ pPv only two studies report differential stress as a function of pressure (Merkel et al., 2007; Miyagi et al., 2010). A third high‐temperature study reported a differential stress of 0.25–1.5 GPa at a pressure range of 130–156 GPa and 2500–3000 K (Wu et al., 2017), however, in this study the MgSiO₃ pPv samples were sandwiched between layers of either NaCl or KCl and so it is likely that the measured stress levels reflect the flow strength of the NaCl or KCl pressure medium rather than that of MgSiO₃ pPv.

      There is a large pressure gap between the highest‐pressure measurements on Brg at ~60 GPa and strength measurements on MgSiO₃ pPv at pressures > 130 GPa (Figure 2.3). The two measurements on MgSiO₃ pPv are similar though those of Merkel et al. (2007) (Figure 2.3, blue open squares) are higher than those of Miyagi et al. (2010) (Figure 2.3, green open triangles). For these measurements, Miyagi et al. (2010) used a moment pole stress model and as discussed above the small error bars (smaller than the symbols) only represent fitting errors. In contrast, the study of Merkel et al. (2007) has small errors (smaller than the symbols) but uses the method of Singh et al. (1998). The small error bars in this study are due to that fact that there are only small variations in stresses calculated on different lattice planes.

      Differences in differential stress measurements between Miyagi et al. (2010) and Merkel et al. (2007) could be due to composition. A natural enstatite was used for synthesis in Merkel et al. (2007), whereas a synthetic MgSiO₃ glass was used as starting material in Miyagi et al. (2010). Another possibility is grain size. Visual comparison of raw diffraction images in these two studies shows spottier diffraction rings in Miyagi et al. (2010), which qualitatively indicates larger grain size, and may result in lower flow strength. Interestingly, if room temperature flow strength measurements on Brg are compared to pPv, the highest value measured in Brg is 11.9 GPa at a pressure of 52 GPa (Miyagi & Wenk, 2016). In pPv the highest differential stress value is 11.1 GPa at 177GPa (Miyagi et al., 2010). If one assumes that pressure increases the flow strength of Brg (as would be expected), then at room temperature pPv may be significantly weaker than Brg. A theoretical study by Ammann et al. (2010) suggests that high‐temperature strength of MgSiO₃ pPv may be much lower than Brg based on calculated diffusion coefficients. Likewise, experimental work on pPv analogs found that during high temperature deformation the pPv phase is ~5–10 time weaker than the perovskite (Pv) phase (Dobson et al., 2012; Hunt et al., 2009). If room temperature flow strength is lower and if diffusion is faster in MgSiO₃ pPv, there is a high likelihood that Brg is significantly stronger than MgSiO₃ pPv over a range of pressure and temperature conditions. If the strength contrast between Brg and pPv is as large as the analog studies indicate then pPv may be similar or even weaker than Fp.

      2.4.2 Textures and Slip Systems in Lower Mantle Phases

Recently, Merkel & Cordier (2016) reviewed slip systems in lower mantle and core phases. As such the following discussion will briefly review textures and slip systems in lower mantle phase but will mostly focus on more recent developments. For a more detailed discussion of studies on slip systems prior to 2016 and for a figure showing the relationship between crystal structures and slip systems, the reader is referred to the review by Merkel & Cordier (2016). A summary of experimentally inferred slip systems of lower mantle phases is given in Table 2.1.

Table 2.1 Summary of inferred slip systems in the major lower mantle phases and the conditions under which they are expected to dominate based on experimental data. See text for details of the experiments.

СКАЧАТЬ

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Mantle phase	Slip system	P‐T conditions
Ferropericlase	{110}<1‐0>	High P, low T
	{100}<011>	High P, high T
Ca‐Perovskite	{110}<1‐10>	High P low T