Presentation on theme: "12/14/2009 MR-14A-06 1 Some remarks on micro-physics of LPO (plastic anisotropy) some tutorials Shun-ichiro Karato Yale University Department of Geology."— Presentation transcript:
12/14/2009 MR-14A-06 1 Some remarks on micro-physics of LPO (plastic anisotropy) some tutorials Shun-ichiro Karato Yale University Department of Geology & Geophysics
12/14/2009 MR-14A-06 2 Visser et al. (2008) Trampert and van Heijst (2002) Panning and Romanowicz (2006) upper mantle transition zone D” layer Why LPO?
12/14/2009 MR-14A-06 3 Seismic anisotropy is controlled mostly by LPO. But the relationships between LPO and flow geometry are poorly known for most part of Earth’s interior. LPO is determined by the dominant slip systems (LPO) that are controlled by a combination of many microscopic processes (physics of LPO is complex!). experimental approach: (i) systematic, well-defined lab experiments + (ii) scaling analysis theoretical approach: (i) key parameters (diffusion, dislocation properties) + (ii) integration of multi-scale physics of deformation
12/14/2009 MR-14A-06 4 olivine wadsleyite  ~1800 K ~1700 K ~1500 K (preliminary results)
12/14/2009 MR-14A-06 5 LPO-flow geometry relationship depends on (i) materials, and (ii) physical/chemical conditions (fabric transitions).
12/14/2009 MR-14A-06 6 Origin of plastic anisotropy (dislocation creep) How do fabric transitions occur? What controls LPO?
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12/14/2009 MR-14A-06 8 Houlier et al. (1981) from Costa and Chakraborty (2008) “dry” Si diffusion in olivine is nearly isotropic. --> diffusion controlled model does not explain large plastic anisotropy
12/14/2009 MR-14A-06 9 Fabric transitions for olivine A B C D E
12/14/2009 MR-14A Observations for olivine Dominant slip direction is b= (or ) consistent with the role of kink/jog Stress-induced fabric transitions inconsistent with the simple diffusion-controlled model Larger water weakening effect for b= cannot be explained by the simple diffusion-controlled model larger weakening effects for dislocations with larger Peierls stress (or longer Burgers vector) anisotropy is largely due to dislocation-related properties (not by diffusion)
12/14/2009 MR-14A A model of fabric transitions Stress/temperature-induced fabric transition (low T, high stress) [kink energy (Peierls stress)] Water/temperature-induced fabric transition ( high T, low stress, high water fugacity) [diffusion (point defects), jog energy]
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12/14/2009 MR-14A Testing the jog (+ diffusion) - controlled model some speculations on post-perovskite phase Eshelby-Foreman theory for dislocation energy with anisotropic elasticity Jog-controlled climb model is consistent with olivine data. (010) or (001) for post-perovskite? diffusion creep? large dislocation energy fast diffusion (Karki-Khanduja (2007))
12/14/2009 MR-14A Conclusions Plastic anisotropy is caused mostly by anisotropic dislocation properties (not much by diffusion anisotropy). Plastic anisotropy depends on T, stress, water content etc. lab studies: well-defined experiments (high-T, low stress) + scaling analysis [Direct applications of lab results without scaling analyses can lead to misleading conclusions.] modeling: test with well-known materials (e.g., olivine) and then apply to not-yet-studied minerals [jog-controlled model (high-T plasticity model) works OK for olivine, and suggests (010) or (001) (or (100))is the easiest slip system in post-perovskite. But deformation in ppv might occur by diffusion creep.]
12/14/2009 MR-14A Plastic anisotropy of post-perovskite ?
12/14/2009 MR-14A Micro-physics of LPO (reminder of ABC of LPO) LPO depends on macroscopic deformation geometry and microscopic deformation mechanisms. Deformation by dislocation creep produces LPO. LPO formed by dislocation creep depends activity of slip systems. LPO is largely controlled by easiest (+ some other) slip system(s). The relative easiness of slip systems is controlled by the relative rate of deformation that is controlled by (i) anisotropy of dislocation energy (kink, jog formation energy), (ii) by anisotropy of diffusion. –These factors will change with T, P, stress, water content etc. –Results at conditions different from Earth’s interior (e.g., low T) cannot be applied to Earth’s interior.
12/14/2009 MR-14A How should we investigate LPO relevant to Earth’s interior? (micro-physics of LPO is complex) Experimental approach: time scales are vastly different between lab and Earth (need extrapolation) what kind of experiments should we conduct ? How should we extrapolate these results ? Theoretical (modeling) approach: creep processes are complex How should we infer the dominant slip system(s)? Diffusion coefficients ? Dislocation properties ? How should we integrate ?
12/14/2009 MR-14A Strong LPO develops by deformation only through certain mechanisms
12/14/2009 MR-14A Classic diffusion-controlled high-T creep model: can it explain fabric transitions? Peierls stress: How does it explain plastic anisotropy at high-T?