Recent Progress in Experimental Studies on RheologyDeformation Microstructures Under HighPressures

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Recent Progress in Experimental Studies on
Rheology/Deformation Microstructures Under
High-Pressures

• Shun-ichiro Karato
• Yale University
• Department of Geology Geophysics
• (Xu, Nishihara, Yamazaki)
• (Weidner, Durham, Wang, Green, Burnley)

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Outline

• Geodynamic motivations
• Background
• Why high-pressure rheology
• What are the issues? (what do we need to worry
  about?)
• Apparatus development (initial results)
• A combination of X-ray stress (strain)
  measurements and controlled stress (strain)
  generation device
• D-DIA (deformation DIA)
• RDA (rotational Drickamer apparatus)
• Perspectives

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Mantle convection controls the evolution of this
planet. Convection pattern is controlled largely
by rheological properties of mantle materials.
Ritsema et al. (1999)
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Trampert and van Heijst (2002)
Transition zone
Upper mantle
Pattern of anisotropy is completely
different Between the upper mantle and the
transition zone
What does it tell us?
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Reliable quantitative rheological data from
currently available apparatus are limited to
Plt0.5 GPa (15 km depth Rheology of more than 95
of the mantle is unconstrained!).
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Why high-pressure?

• Pressure dependence is important only at
  high-pressure experiments and can be determined
  only by high-P experiments.
• Rheology of high-pressure phases, or rheology
  associated with a phase transformation
  (transformation plasticity) can only be studied
  under high-P.

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Pressure effects
Pressure effects are large at high P. Depends
strongly on V (activation volume).
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30-100 for P2-P1lt0.5 GPa 3-10 for P2-P1lt15
GPa
Although uncertainties in each measurements are
larger at higher-P experiments, the pressure
effects (V) can be much better constrained by
higher-P experiments.
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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).

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Different mechanisms of deformation operate at
different conditions yielding different flow
laws. Results obtained for one mechanism cannot
be extrapolated to another regime.
low-T regime
high-T regime
(parameters B and C depend on strain-rate)
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Various methods of deformation experiments under
high-pressures
Rotational Drickamer Apparatus (RDA)
Multianvil apparatus stress-relaxation tests
DAC
D-DIA
Very high-P Mostly at room T Unknown strain
rate (results are not relevant to most regions
of Earths interior.)
Stress changes with time in one
experiment. Complications in interpretation
Constant shear strain-rate deformation
experiments Large strain possible High-pressure
can be achieved. Stress (strain) is
heterogeneous. (complications in stress
measurements)
Constant displacement rate deformation
experiments Easy X-ray stress (strain) measuremen
ts Strain is limited. Pressure may be limited.
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D-DIA (Deformation DIA) High-P and T, homogeneous
stress/strain
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Rotational Drickamer Apparatus large strain
(radial distribution), high P-T
(Yamazaki, Xu, Nishihara)
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Sample assembly for a rotational
Drickamer Apparatus (torsion tests on a thin
disk-shaped sample to large strain)
Zirconia
Alumina
MgO
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Pressure calibration
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(Mg,Fe)O, 12 GPa, 1473 K
zirconia
heater (TiCdiamond)
Deformation occurred mostly in the sample
and the magnitude of strain is consistent with
the angle of rotation.
sample
strain marker
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Large-strain shear deformation of
wadsleyite (Mg,Fe)2SiO4
10 mm
10 mm
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Whats next?

• Technical development
• Stress measurements, strain history in RDA
• (at synchrotron facility)
• Benchmark for stress measurements
• Better characterization of chemical environment
  (oxygen fugacity, water content)
• T (P)-distribution in RDA
• Scientific goals
• Rheology and fabric developments in
  transition-zone and lower mantle minerals
• What is the geodynamic significance of seismic
  anisotropy
• (what is the pattern of mantle convection)?
• How strong are the deep mantle minerals?
• Why (how) do deep and intermediate earthquakes
  occur?

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stress-distribution in a RDA
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A change in geometry of a strain-marker
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Bench-marking(planned)

• Deform the exactly same sample (olivine
  polycrystal) at nearly identical conditions
  (strain-rate, water fugacity (water content), T,
  P, oxygen fugacity)
• Compare the stress measurements from 3 different
  techniques (i) X-ray, (ii) load-cell reading,
  (iii) dislocation density

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Stress measurement from X-ray diffraction
d-spacing becomes orientation-dependent under
nonhydrostatic stress. Strain (rate) can also
be measured from X-ray imaging.
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Characteristics of study of plastic deformation
in Earth sciences

• Time scale
• How to apply results of laboratory studies to
  Earth?
• Multiple mechanisms
• scaling laws (theoretical models)
• Extreme conditions
• High pressure
• Very large or very small strains
• Complications
• Chemical reactions
• Various feed-back mechanisms

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Immediate scientific goals(next 1-2 years RDA)

• What is the flow pattern in the transition zone
  (what is the geodynamic significance of seismic
  anisotropy in the transition zone)?
• How strong are the deep mantle minerals (compared
  to the upper mantle minerals)?
• Why do deep and intermediate earthquakes occur?