Two-phase deformation of lower mantle mineral analogs

Abstract

Editor: J. BrodholtKeywords: crystallographic preferred orientation seismic anisotropy two-phase deformation lower mantle rheology 3D X-ray microtomographyThe lower mantle is estimated to be composed of mostly bridgmanite and a smaller percentage of ferropericlase, yet very little information exists for two-phase deformation of these minerals. To better understand the rheology and active deformation mechanisms of these lower mantle minerals, especially dislocation slip and the development of crystallographic preferred orientation (CPO), we deformed mineral analogs neighborite (NaMgF 3 , iso-structural with bridgmanite) and halite (NaCl, isostructural with ferropericlase) together in the deformation-DIA at the Advanced Photon Source up to 51% axial shortening. Development of CPO was recorded in situ with X-ray diffraction, and information on microstructural evolution was collected using X-ray microtomography. Results show that when present in as little as 15% volume, the weak phase (NaCl) controls the deformation. Compared to single phase NaMgF 3 samples, samples with just 15% volume NaCl show a reduction of CPO in NaMgF 3 and weakening of the aggregate. Microtomography shows both NaMgF 3 and NaCl form highly interconnected networks of grains. Polycrystal plasticity simulations were carried out to gain insight into slip activity, CPO evolution, and strain and stress partitioning between phases for different synthetic two-phase microstructures. The results suggest that ferropericlase may control deformation in the lower mantle and reduce CPO in bridgmanite, which implies a less viscous lower mantle and helps to explain why the lower mantle is fairly isotropic.

The anisotropic signal of topotaxy during phase transitions in D″

“…Wenk et al (2011) use a similar procedure and assume that 50% of the strain is accommodated by deformation mechanisms such as diffusion creep, and that this does not contribute to the development of CPO. It is unlikely that all strain is accommodated by dislocation motion, but the fraction is probably stress and temperature dependent, and is unconstrained by experiment or theory in lowermost mantle phases (although recent work on bridgmanite analogues by Wang et al, 2013 andKaercher et al, 2016 represents a step in this direction). One approach would be to attempt to tune the partitioning of strain between dislocation glide and other "invisible" mechanisms such that the strength of the anisotropy in models matches that in the observations.…”

Ferropericlase Control of Lower Mantle Rheology: Impact of Phase Morphology

“…However, it is not yet clear to which degree the assumption of perfectly interconnected ferropericlase layers is valid. Laboratory experiments (Girard et al, ; Kaercher et al, ) indicate that ferropericlase may indeed control lower mantle rheology due to the formation of interconnected layers, thus potentially resulting in strain localization in the lower mantle.…”

“…In a lower mantle of pyrolitic composition, bridgmanite is expected to compose the largest part of the lower mantle (with estimates ranging from approximately 70-90%), followed by ferropericlase and calcium silicate perovskite (e.g., Hirose et al, 2017;Stixrude & Lithgow-Bertelloni, 2012). It has been suggested that the rheology of the Earth's lower mantle is governed by the rheology of its two main constituents bridgmanite and ferropericlase; however, both rheologies are not very well known at lower mantle conditions and are subject of active research (e.g., Girard et al, 2016;Immoor et al, 2018;Kaercher et al, 2016;Merkel et al, 2003;Miyagi & Wenk, 2016;Reali et al, 2019). The rheology of calcium silicate perovskite is even less known due to experimental difficulties (Miyagi et al, 2009;Nestola et al, 2018).…”

Texture Development and Stress–Strain Partitioning in Periclase + Halite Aggregates

“…If the stress exponent of the strong phase is similar or smaller than that of the weak phase, the stress level in the former will slightly exceed that of the latter [4]. As shown in Kaercher et al (2016) [1], the stress difference between NaMgF 3 and NaCl decrease from a high value of~1.7 GPa to a low value of 0.5 GPa with increasing NaCl proportion from 15% to 70%, consistent with a transition from LBF to IWL. When strong and weak phases have comparable phase proportions, both phases could be interconnected and 'percolate' each other, leading to mechanical behavior that is intermediate to LBF and IWL end members.…”

“…In multiphase deformation experiments at high pressure, one can utilize synchrotron X-ray diffraction to directly measure lattice strains and estimate the flow stress in each phase. Kaercher et al (2016) [1] performed in-situ deformation experiments on NaCl + NaMgF 3 two-phase aggregates with a series of phase proportions and reported the flow stress in each phase. The flow stress in the strong NaMgF 3 phase decreases nonlinearly with increasing weak NaCl phase proportion.…”