Prsentation PowerPoint

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• Linking deformation and anisotropy in the mantle
• Andréa Tommasi
• Géosciences Montpellier
• David Mainprice, Benoit Gibert,
• A.Vauchez, G. Barruol (Montpellier)
• Patrick Cordier, Hélène Couvy,
• Philippe Carrez, Denise Ferré (Lille)

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Anisotropy a tool to probe upper mantle
deformation
electrical conductivity _at_ EPR
shear wave splitting
// ridge
http//garnero.asu.edu
// spreading
Shear wave splitting in the South
Pacific Fontaine et al., GJI in press
resistivity // spreading direction 1/5
resistivity // ridge
Baba et al. JGR 2006
fast EC direction // fast SKS polarisation
How do we translate these observations to flow
patterns?
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anisotropy results from

• layering of materials with very ? properties
• sediments
• strain-induced layering in metamorphic or
  magmatic rocks
• crust
• aligned cracks or lenses filled with gas or
  liquid
• upper crust
• middle to lower crust, upper mantle
• transition zone, D (?)

• strain-induced CPO of anisotropic minerals
• lower crust
• mantle
• inner core (?)

deformation plays an essential role in the
formation of anisotropy
drawing by Luc Mehl
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Refraction profiles in the Pacific P waves
velocity F(propagation direction)
comparison between different ray paths
trade-off between heterogeneity anisotropy

• fast velocities // fractures zones
• fossil spreading direction

Morris et al.(1969),JGR
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Physical properties of Earth materials depend on
their structure at ? scales
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Viscoplastic deformation crystal preferred
orientations
dislocation creep dislocation glide dynamic
recrystallization
polycrystalline ice in-situ deformation pure
shear C. Wilson - Univ. Melbourne, Australia
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70
5
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• dominant 100 slip in the shallow (lithospheric)
  mantle

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HT-LP experimental deformation simple shear
Zhang Karato (1995) Nature
SD

• 100 slip

X
Bystricky et al. (2000) Science
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100 slip in olivine anisotropy upper 210 km

• Observations for horizontal flow
• VPH gtgt VPV
• P wave anisotropy gt 5
• VSH gt VSV
• S-wave anisotropy gt 4

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Until 2001, we "read" seismic anisotropy
observations
B. Holtzman 2004
Fast direction of P Rayleigh propagation, polari
sation fast S-wave flow direction delay time
thickness of the anisotropic layer and
orientation of the flow plane
Can we quantify the anisotropy produced by mantle
flow at different depths or geodynamic
environments (ridges, subductions...)?
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Multi-scale models of mantle deformation and
seismic anisotropy
natural peridotites lab experiments
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Modeling the deformation crystal orientation
evolution
VPSC Molinari et al. 1987, Lebensohn Tomé
1993 Drex Kaminsky Ribe 2001, 2003
within a grain (crystal)
rock (polycrystal) deformation
behavior of the aggregate (rock) average of
crystals' behaviors
strain motion of dislocations on well-defined
crystal planes directions
input parameters slip systems strength, initial
texture, and macroscopic sollicitation (stress
or velocity gradient tensor) output evolution of
crystallographic orientations and macroscopic
response (strain rate or stress tensor)
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Olivine CPO
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Deformation and anisotropy in the upper mantle
new experimental data
shear wave splitting above subduction zones Japan

• effect of fluids (water and melt) and pressure on
  the relation between deformation anisotropy
• changes in dominant olivine slip system
• fast anisotropy directions normal to the shear
  direction?

Nagajima Hasegawa EPSL 2004
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Influence of water (hydrogen) on olivine
orientations
B C type 001 slip
D-type
Bystricky et al. 2000
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High water
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gt90 naturally deformed peridotites A,D E
type 100 slip
low T transition B-C _at_ lower stresses
Low stress Low water
Jung Karato 2001 Katayama et al. 2004 Katayama
Karato 2006
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001 glide olivine CPO only observed in HP
garnet peridotites!
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Can we separate water concentration from pressure
(depth) ?
Hirschmann et al. 2005 EPSL
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Pressure-dependence of water solubility in
olivine melting
Bofan-Casanova 2005 Min.Mag.
Japan shear wave splitting in the wedge
forearc low T water-saturated olivine?
arc melt
Karato Jung 1998 EPSL
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Interactions between reactive fluid transport,
partial melting, and olivine deformation in the
mantle wedge?
Xenoliths in calco-alkaline volcanos from NE
Pacific subduction zones
serpentinite
100 glide only Tommasi Ionov, in prep.
Figure by K. Michibayshi
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Shallow (lt200 km) mantle open questions

• anisotropy in a partially molten mantle
• orientation of melt-rich pockets controlled by
  stress or strain?
• melt- induced strain partition olivine 100
  normal to shear direction?

weak CPO anisotropy controlled by melt
distribution
Holtzman et al. Science 2003
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Flow melt transport // trench in the wedge
strong trench-parallel anisotropy
solid state foliation // dunite bands //
pyroxenite dykes
Canadian Cordillera
100 slip
reactive porous flow (ltlt1) anisotropy only
enhanced if melt in films // foliation
1 melt in vertical dykes 10 S anisotropy
Tommasi et al EPSL 2006
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Anisotropy deformation deeper in the mantle
Lehmann discontinuity decrease in anisotropy
change in olivine deformation?
weak anisotropy in the transition
zone Strain-induced CPO of wadsleyite
ringwoodite?
Mg-Perovskite MgO
Post-Perovskite
lower mantle mainly isotropic
D layer strong anisotropy VsH gtVsV
post-perovskite deformation or layering?
Panning Romanowicz 2004 Science
How do these minerals deform _at_ HP? Experiments
atomic scale models
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Deformation anisotropy in the deep mantle
Atomic Modelling
Crystal Preferred Orientation
Elastic Tensors
Seismology
Plasticity Modelling
Slip Systems CRSS
Experimentation
Geodynamics
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Fast decrease in anisotropy at the bottom of the
upper mantle - 200 to 400 km
Transition from dislocation to diffusion creep
(no CPO -gt no seismic anisotropy) or transition
from 100 slip to 001 slip at HP?
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Deformation of olivine polycrystals _at_ 11GPa
1400C
Technique expérimentale
H. Couvy P. Cordier Bayreuth/Lille
100 olivine simple shear
EBSD olivine CPO
g0.3
001(100) 001(010)
TEM only 001 screw dislocations
Couvy et al. EJM 2004
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Effect of pressure on olivine deformation
bi-crystal

• At high pressure
• higher strain rate in c crystal
• 001(010) slip easier than 100(010)
• very low activation volume
• dislocation creep dominant

Raterron et al, in press
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Ab-initio modeling of dislocation core
properties Ph. Carrez, P. Cordier, D. Ferré
(Lille)
001(010)
8.7 GPa
100 (010)
20.8 GPa
001
100

• olivine easier slip on 100(010) at high
  pressure

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Ideal Shear Stress intrinsic resistance to shear
ISS derivative of the energy barrier
Energy
?max
Stress
shear
Distance
Crystal periodicity Peierls stress
first order approximation of the critical
resolved shear stress of the slip system input
for texture (CPO) models
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Crystal plasticity modeling based on calculated
Peierls stresses for olivine slip systems _at_ 10 GPa
11GPa experiment
Mainprice et al. Nature, 2005
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Global P-wave anisotropy in the deep upper mantle

• Model prediction for horizontal flow
• VPV gt VPH
• VP anisotropy about 1

Montagner Kennett GJI, 1996
1
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Global S-wave anisotropy in the deep upper mantle

• Model prediction for horizontal flow
• VSV gt VSH
• Vs anisotropy 2

2
Montagner Kennett GJI, 1996
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olivine deformation f(P) change in dominant
slip direction from 100 to 001

• strong decrease in seismic anisotropy with depth
• fast P-wave propagation
• fast S-wave polarisation directions
• in the deep upper mantle normal to shallow ones
• global 1D seismic anisotropy data
• horizontal shearing accommodated by dislocation
  creep

Mainprice et al., Nature, 2005
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Transition from 100 slip to 001 slip in the
deep upper mantle?
Experiments P 6-7 GPa (200 km) role of
stress water content? LPO measurements on
naturally deformed peridotites - sp-bearing
samples 100 only (lt70 km) - garnet-bearing
peridotites essentially 100, except
high-pressure massifs (subduction)
cratonic sheared lherzolites
Tgt1300C P 4.4 GPa
001
100
Mizukami et al. Nature 2004
Vauchez et al. EPSL 2005
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Caribean 1s trench //
trench normal flow (drag by the slab) 001
slip dominant
Hikurangi 1-2s trench //
Compilation by M. Long P. Silver
Compilation by M. Long P. Silver some
additional data
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olivine deformation f(P) 001 slip lt 2
anisotropy

• open question strong anisotropy beneath some
  cratons?

Australia other continents oceans
fast SV // APM
also 5 Fennoscandia (Pedersen et al.
2006), Slave Craton (Snieder Bruneton GJI
2007), N. America (Marone Romanowicz Nature
2007)
Debayle et al. Nature 2005
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Anisotropy in transition zone
Montagner Kennett (1996)

• weak anisotropy 2
• SH gt SV
• PH gt PV

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E -gt SH
G -gt SV
Trampert van Heijst, Science 2002
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Strain-induced CPO in Wadsleyite
HT-HP deformation TEM P. Cordier, E. Thurel
H. Couvy Univ. Lille Bayreuth dislocation
creep slip systems
Thurel et al. 2003 Phys Chem Min Couvy 2005
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Wadsleyite VPSC modeling of crystal preferred
orientations
Tommasi et al. JGR 2004
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seismic properties of an aggregate(60
wadsleyite - 40 garnet) _at_ 15 GPa

• seismological observations
• PHgtPV
• SHgtSV
• SV azimuthal anis gt SH's
• SKS isotropic
• horizontal shearing

Tommasi et al. JGR 2004
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Anisotropy deformation in the deep mantle
Lehmann discontinuity change in olivine
deformation
strain-induced anisotropy (CPO) dominant
horizontal flow
lower mantle Mg-Perovskite MgO isotropic?
http//garnero.asu.edu/
D VsH gtVsV,, no SKS splitting, azimuthal
anisotropy?
Panning Romanowicz 2004 Science
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D-Layer anisotropy strain-induced
post-perovskite CPO and/or compositional
layering?
http//garnero.asu.edu/
Van der Hilst et al Science 2007
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(010)100
(010)001
polycrystal plasticity models ? post-perovskite
CPO D" anisotropy
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seismic anisotropy mantle deformation

• upper mantle olivine
• lt 200 km strong anisotropy, SHgtSV, fast
  directions // APM (oceans)
• or // lithospheric
  structure (continents), // fast electrical
  conductivity directions
• dominant 100 slip
• delay times path lenght orientation flow
  plane/direction relative to propagation
• role of fluids (H20, magmas)?
• gt 200 km anisotropy decreases, 001 slip,
  cratons?
• transition zone weak anisotropy wadsleyite CPO
  
• D" strain-induced CPO of post-perovskite ?
• deformation, electrical and thermal conduction in
  olivine are also anisotropic