Recent Progress in High-Pressure Studies on Plastic Properties of Earth Materials COMPRES meeting at lake Tahoe June, 2004

Grand challenge program ( ) PIs P. Burnley (Georgia Tech.) H.W. Green (UC Riverside) S. Karato (Yale University) D.J. Weidner (SUNY Stony Brook) W.B. Durham (LLNL) [Y. Wang (APS)]

Goal of this project To develop new techniques of quantitative rheology experiments under deep Earth conditions (exceeding those in the transition zone: ~15 GPa, ~1800 K) –flow laws –deformation microstructures (fabrics)  global dynamics

Quantitative rheological data at high-T from currently available apparatus are limited to P<3 GPa (80 km depth) Rheology of more than 90% of the mantle is unconstrained!

Why high-pressure? Despite a commonly held view that brittle strength is pressure-sensitive but plastic flow strength is pressure-insensitive, pressure dependence of plastic flow strength is important in Earth’s interior but can be determined only by high-P experiments. Rheology of high-pressure phases, or rheology associated with a phase transformation (transformational plasticity, transformational faulting) can only be studied under high-P.

Pressure effects Pressure effects are large at high P. [Depends strongly on V* (activation volume). V* in many materials is poorly constrained]

Challenges in high-P rheology studies (How are rheological properties different from elasticity?) Controlled generation of stress (strain-rate) Measurements of stress under high-P Plastic deformation can occur by a variety of mechanisms. Large extrapolation is needed in time-scale (extrapolation in stress, temperature).  Careful strategy is needed to choose the parameter space and microscopic mechanisms must be identified. Sensitive to chemical environment and microstructures (large strain is needed to achieve “steady-state” rheology and microstructure).

Pressure effects on creep can be non- monotonic: a simple activation volume formulation may not capture the physics. pressure, GPa strength, GPa

Deformation fabrics (of wadsleyite) depend on conditions of deformation. [MA stress relaxation tests]

Various methods of deformation experiments under high-pressures Multianvil apparatus stress-relaxation tests D-DIA Rotational Drickamer Apparatus (RDA) Very high-P Mostly at room T Unknown strain rate (results are not relevant to most regions of Earth’s interior.) DAC Stress changes with time in one experiment. Complications in interpretation Constant displacement rate deformation experiments Easy X-ray stress (strain) measurements Strain is limited. Pressure may be limited. Constant shear strain-rate deformation experiments Large strain possible High-pressure can be achieved. Stress (strain) is heterogeneous. (complications in stress measurements)

D-DIA (Deformation DIA) High-P and T, homogeneous stress/strain [Pressure exceeding ~15 GPa is difficult. Strain is limited.]

RDA (Rotational Drickamer Apparatus) large strain (radial distribution), high P (because of good support for anvils) [Stress/strain is not uniform. Effects of initial stage deformation must be minimized.]

Picture RDA Incident X-ray RDA at X17B2 in NSLS 13 elements SSD Stage CCD camera

Stress measurement from X-ray diffraction d-spacing becomes orientation-dependent under nonhydrostatic stress. Strain (rate) can also be measured from X-ray imaging.

Lattice strain measured by X-ray is converted to stress using some equations. Equations involve assumptions about stress-strain distribution which is not known apriori. In order to calculate the stress, one needs to understand stress-strain distribution (from the data + modeling). Recent results show significant deviation from elastic model. (Li et al. (2004), Weidener et al. (2004))

Lattice strain Total percent strain 0.5 GPa 1.0 GPa 2.0 GPa 6.4 GPa D0466 D0471 (1.0  ) (9.7  ) (2.5  ) (1.2  ) (9.5  ) MgO: Compression versus extension (D-DIA) 6.4 GPa 1 GPa Uchida et al. 2004, submitted 1000K Strain rate: 0.9x10 -5 s -1

Plastic deformation of olivine (Li et al., 2004)

stress-distribution in a RDA

Sample and diffraction geometry Incident X-ray (50  m wide) Diffracted X-ray (50  m wide) 6.5 º 0.9 mm Effective length for diffraction ~ 0.4 mm ring width Observed part by diffraction

Stress-strain conditions in sample 1 11 33 11 33 Variation of d-spacing (Assuming. elastically isotropic) Principal stress (  1,  3 ) direction  = 0 and 90 º  = +45 and -45 º Y Z  Uniaxial compression Simple shear d 0 : d for hydrostatic condition G: shear modulus t: Uniaxial deviatoric stress  S : Shear deviatoric stress Hydrostatic

By using  = 0, ± 45, ± 90 º geometry, uniaxial deviatoric stress and simple shear deviatoric stress can be determined independently. Annealing at P=15 GPa after compression t = 0.6 GPa,  S = 0.7 GPa 27 º C During rotation at P=15 GPa and T=1600 K 300 K 900 K 1600 K

Pressure dependence of olivine rheology Li et al. (2004), V*< 3 cc/mol Karato and Jung. (2003) Bussod et al. (1993), V*~5-10 cc/mol Green and Borch (1987), V*~27 cc/mol Karato (1977), V*=13.9 cc/mol (theory) Ross et al. (1979), V*~13 cc/mol Karato and Jung. (2003), V*~14, 24 cc/mol Li et al. (2004), V*< 3 cc/mol Karato and Rubie (1997), V*~14 cc/mol

Summary Two new apparati have been developed (D-DIA and RDA). Quantitative high-P and T deformation experiments have been conducted to ~10 GPa ( MgO, olivine ) by D-DIA and ~15 GPa by RDA ( hcp-Fe, olivine, (Mg,Fe)O, wadsleyite, ringwoodite ) Significant discrepancy exists between some of the high-P results and lower-P (better constrained) data (V*): parameterization problem?  harder anvil materials (sintered diamond?) Maximum P with D-DIA is ~10 GPa. How to get higher P?

Summary (cont.) Theoretical problems with stress measurements by X-ray diffraction  stress/strain distribution in deforming polycrystals (heterogeous materials)  sample assembly, better alignment Deformation geometry with RDA is less than ideal. Larger uncertainties in mechanical data. Chemical environment is not well controlled.