conditions remain sparse. Significant advances have been made in documenting texture development and slip system activities in lower mantle phases and some general trends are observed with respect to differential stress measurements in these phases. Based on the experimental body as a whole, at mantle pressure Fp appears to be significantly weaker that Brg; however, the strength contrast as a function of pressure, temperature, and strain remains poorly constrained. As would be expected the strength of both of these phases decreases with increased temperature and with decreased strain rates. Room‐temperature strengths of Brg and MgSiO3 pPv seem to indicate that MgSiO3 pPv is weaker than Brg, but there is a large pressure gap between measurements in these two phases. Texture development in two‐phase aggregates also exhibits complex behavior with high plastic anisotropy hard phases disrupting texture development in softer more highly strained phases. Bulk behavior of lower mantle polyphase aggregates (i.e., rocks) remains relatively unstudied but has important implications for mechanical properties and anisotropy development in the deep Earth. Challenges exist both to interpretation of bulk rheology of these materials and texture development in the various phases. Systematic studies of polyphase deformation documenting the effects of strength contrast, plastic anisotropy, and microstructure will be vital moving forward. Additional complications may occur at high temperatures where possibilities exist for phases to deform by different deformation mechanisms. Recent technical advances in experimental deformation as well as theoretical developments have provided new capabilities to tackle these problems in the near future.
ACKNOWLEDGMENTS
LM acknowledges support from the National Science Foundation through EAR‐1654687 and from the Capitol DOE Alliance Center (CDAC). LM would also like to thank two anonymous reviewers and editor H. Marquardt, whose comments greatly improved the manuscript.
REFERENCES
1 Allègre, C. J., & Turcotte, D. L. (1986). Implications of a two‐component marble‐cake mantle. Nature, 323(6084), 123–127. https://doi.org/10.1038/323123a0
2 Ammann, M. W., Brodholt, J. P., Wookey, J., & Dobson, D. P. (2010). First‐principles constraints on diffusion in lower‐mantle minerals and a weak D′′ layer. Nature, 465(7297), 462–465. https://doi.org/10.1038/nature09052
3 Amodeo, J., Carrez, P., & Cordier, P. (2012). Modelling the effect of pressure on the critical shear stress of MgO single crystals. Philosophical Magazine, 92(12), 1523–1541. https://doi.org/10.1080/14786435.2011.652689
4 Amodeo, J., Dancette, S., & Delannay, L. (2016). Atomistically‐informed crystal plasticity in MgO polycrystals under pressure. International Journal of Plasticity, 82(March), 177–191. https://doi.org/10.1016/j.ijplas.2016.03.004
5 Amodeo, J., Merkel, S., Tromas, C., Carrez, P., Korte‐Kerzel, S., Cordier, P., et al. (2018). Dislocations and Plastic Deformation in MgO Crystals: A Review. Crystals, 8(6), 240. https://doi.org/10.3390/cryst8060240
6 Andrault, D., Muñoz, M., Bolfan‐Casanova, N., Guignot, N., Perrillat, J. P., Aquilanti, G., & Pascarelli, S. (2010). Experimental evidence for perovskite and post‐perovskite coexistence throughout the whole D′ region. Earth and Planetary Science Letters, 293(1–2), 90–96. https://doi.org/10.1016/j.epsl.2010.02.026
7 Appel, F., & Wielke, B. (1985). Low temperature deformation of impure MgO single crystals. Materials Science and Engineering, 73, 97–103. https://doi.org/10.1016/0025‐5416(85)90299‐X
8 Arrhenius, S. (1889). Über die Reaktionsgeschwindigkeit bei der Inversion von Rohrzucker durch Säuren. Zeitschrift Für Physikalische Chemie, 4(1), 226–248. https://doi.org/10.1515/zpch‐1889‐0416
9 Azuma, S., Nomura, R., Uesugi, K., Nakashima, Y., Kojima, Y., Doi, S., & Kakizawa, S. (2018). Anvil design for slip‐free high pressure deformation experiments in a rotational diamond anvil cell. High Pressure Research, 38(1), 23–31. https://doi.org/10.1080/08957959.2017.1396327
10 Barreiro, J. G., Lonardelli, I., Wenk, H. R., Dresen, G., Rybacki, E., Ren, Y., & Tomé, C. N. (2007). Preferred orientation of anorthite deformed experimentally in Newtonian creep. Earth and Planetary Science Letters, 264(1–2), 188–207. https://doi.org/10.1016/j.epsl.2007.09.018
11 Boehler, R. (2000). Laser heating in the diamond cell: techniques and applications. Hyperfine Interactions, 128(1/3), 307–321. https://doi.org/10.1023/A:1012648019016
12 Boioli, F., Carrez, P., Cordier, P., Devincre, B., Gouriet, K., Hirel, P., et al. (2017). Pure climb creep mechanism drives flow in Earth’s lower mantle. Science Advances, 3(3), e1601958. https://doi.org/10.1126/sciadv.1601958
13 Bons, P. D., & den Brok, B. (2000). Crystallographic preferred orientation development by dissolution–precipitation creep. Journal of Structural Geology, 22(11–12), 1713–1722. https://doi.org/10.1016/S0191‐8141(00)00075‐4
14 Brokmeier, H. G., Böcker, W., & Bunge, H. J. (1988). Neutron Diffraction Texture Analysis in Extruded Al‐Pb Composites. Textures and Microstructures, 8, 429–441. https://doi.org/10.1155/TSM.8‐9.429
15 Brown, J. M., & Shankland, T. J. (1981). Thermodynamic parameters in the Earth as determined from seismic profiles. Geophysical Journal International, 66(3), 579–596. https://doi.org/10.1111/j.1365‐246X.1981.tb04891.x
16 Burnley, P. C., & Kaboli, S. (2019). Elastic plastic self‐consistent (EPSC) modeling of San Carlos olivine deformed in a D‐DIA apparatus. American Mineralogist, 104(2), 276–281. https://doi.org/10.2138/am‐2019‐6666
17 Burnley, P. C., & Zhang, D. (2008). Interpreting in situ x‐ray diffraction data from high pressure deformation experiments using elastic–plastic self‐consistent models: an example using quartz. Journal of Physics: Condensed Matter, 20(28), 285201. https://doi.org/10.1088/0953‐8984/20/28/285201
18 Bystricky, M., Heidelbach, F., & Mackwell, S. (2006). Large‐strain deformation and strain partitioning in polyphase rocks: Dislocation creep of olivine–magnesiowüstite aggregates. Tectonophysics, 427(1–4), 115–132. https://doi.org/10.1016/J.TECTO.2006.05.025
19 Canova, G. R., Wenk, H. R., & Molinari, A. (1992). Deformation modelling of multi‐phase polycrystals: case of a quartz‐mica aggregate. Acta Metallurgica Et Materialia, 40(7), 1519–1530. https://doi.org/10.1016/0956‐7151(92)90095‐V
20 Carrez, P., Ferré, D., & Cordier, P. (2007a). Implications for plastic flow in the deep mantle from modelling dislocations in MgSiO3 minerals. Nature, 446(7131), 68–70. https://doi.org/10.1038/nature05593
21 Carrez, P., Ferré, D., & Cordier, P. (2007b). Peierls‐Nabarro model for dislocations