DI13C-05
Deformation and Crystallographic Preferred Orientation of Two-phase Lower Mantle Mineral Analogs: Implications for Seismic Anisotropy in the Lower Mantle

Monday, 14 December 2015: 14:40
301 (Moscone South)
Pamela M Kaercher, University of Liverpool, Liverpool, L69, United Kingdom, Lowell M Miyagi, University of Utah, Salt Lake City, UT, United States, Waruntorn Kanitpanyacharoen, Chulalongkorn University, Bangkok, Thailand, Eloisa Zepeda-Alarcon, University of California Berkeley, Berkeley, CA, United States, Yanbin Wang, University of Chicago, Chicago, IL, United States, Dilworth Parkinson, Lawrence Berkeley National Laboratory, Advanced Light Source, Berkeley, CA, United States, Ricardo Lebensohn, Los Alamos National Laboratory, Los Alamos, NM, United States, Francesco DeCarlo, Argonne National Laboratory, Argonne, IL, United States and Hans-Rudolf Wenk, University of California Berkeley, Department of Earth and Planetary Science, Berkeley, CA, United States
Abstract:
Geodynamic models predict large strains due to convection in the mantle, and polycrystal plasticity simulations suggest strong crystallographic preferred orientation (CPO), yet much of the lower mantle is observed to be mostly isotropic. However, these models ignore interaction among phases, which is important for the lower mantle, estimated to be composed of ~25% soft ferropericlase (Mg,Fe)O and ~70% harder bridgmanite (MgSiO3). Here we consider deformation of these two lower mantle mineral phases together and look at the microstructure to find whether soft ferropericlase becomes interconnected and acts as a lubricant between harder bridgmanite grains or if ferropericlase is largely disconnected leaving deformation to be absorbed by bridgmanite. We quantify how the volume percent of ferropericlase might affect deformation and CPO development in the lower mantle.

We deformed lower mantle mineral analogs halite (NaCl, ferropericlase structure) and neighborite (NaMgF3, perovskite structure) together in the D-DIA. Development of CPO was recorded in situ with radial X-ray diffraction, and information on microstructural evolution was collected using X-ray microtomography. We performed self-consistent polycrystal plasticity modelling to infer likely slip systems and strain rates in each phase.

Results show that when present in as little as 15% volume, the soft halite becomes interconnected during deformation, surrounding the harder neighborite grains. The change in microstructure during deformation coincides with a decrease in differential stress, i.e. weakening of the aggregate, and a reduction in CPO, likely due to a switch in deformation mode. Furthermore, polycrystal plasticity models imply much higher strain rates in the softer halite, suggesting it is absorbing the bulk of deformation. The halite does not develop significant CPO, and CPO in the neighborite is reduced by nearly half with addition of 15% volume halite. The results suggest that ferropericlase controls deformation in the lower mantle through a mechanism that does not produce significant CPO. This implies a more viscous lower mantle and helps to explain why the bulk of the lower mantle is fairly isotropic.

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