M2, 2011 David Mainprice Geosciences, Montpellier, France Structure et minéralogie du manteau 2011 M2 Dynamique de la Terre Interne
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M2, 2011 David Mainprice Geosciences, Montpellier, France
Structure et minéralogie du manteau���2011 M2 Dynamique de la Terre Interne
Documentation
ftp://www.gm.univ-montp2.fr/mainprice/ Dossier « Master Documents M2 »
Chemin : [www.gm.univ-montp2.fr][mainprice] [Master_M2_Documents]
Plan 1. Geodynamic motivation 2. Seismic discontinuities 3. Equation of state (P,T,volume) 4. Mg2SiO4 - ‘low’ pressure (upper mantle) 5. MgSiO3 - ‘high’ pressure (lower mantle) 6. Phase transitions and thickness of the TZ 7. 300 km X discontinuity 8. D” layer, perovskite and post-perovskite
Geodynamic motivation -
Understanding the composition and dynamics of the Earth’s deep interior
Multi-disciplinary approach ….
van der Hilst, 2004
Geodynamics
Plume or Plate Tectonics ?
Mantle Overview
Exploring the deep Earth using Seismology
Velocity discontinuities
Structure interne de la Terre
Vitesse de propagation des ondes sismiques
dans la Terre
Discontinuité à 410 km Discontinuité à 660 km Changements minéralogiques (pression, température)
(Kennett, 1991)
CMB 2900 km
Graine, solide
Noyau externe, liquide
Mantle Phase Transitions and Mineralogy
Pyrolite - New experimental data
Hirose 2002
MORB - New experimental data
Most recent compilation
Mantle & Core : petrology & seismic anisotropy
Panning & Romanowicz 06 Ono & Oganov 05 Perrillat et al. 06 Irifune et al. 94 modif. by Poli & Schmidt 02
Mantle PT conditions
Subduction zone & Phase Transitions
Seismic discontinuities Seismic velocity jumps : phase transitions changes in composition
- au dessus de 410 km : α-(Fe,Mg)2SiO4 ou alpha-(Fe,Mg)2SiO4 =OLIVINE - au dessous de 410 km : α--(Fe,Mg)2SiO4 --> beta β-(Fe,Mg)2SiO4 β-(Fe,Mg)2SiO4 beta modified spinel = WADSLEYITE Changement de structure cristalline - au dessous de 520 km : β-(Fe,Mg)2SiO4 --> gamma γ-(Fe,Mg)2SiO4 γ-(Fe,Mg)2SiO4 gamma spinel = RINGWOODITE Ces 3 (Fe,Mg)2SiO4 «olivines » sont formées de tétraèdres silicatés dont la structure cristalline se densifie...
Changement de la structure de (Fe,Mg)2SiO4 dans le manteau supérieur
Pv+Mv
Equation of state (P,T,volume) EOS - Describes relations between P,T and crystal cell volume Seismic wave speed - function of elastic constants and density For the isotropic case: Compressional velocity Vp2 = (bulk modulus + 4 shear modulus/3) /density Vp2 = M/density (P-wave modulus , M = 4 shear modulus/3) or Vp=√(M/density) Bulk sound velocity Vb2 = bulk modulus/density or Vb=√(bulk modulus/density) Shear velocity Vs2 = shear modulus /density or Vp=√(shear modulus/density) Bulk modulus = 1/Compressibility = (module de incompressibility) = -dP/(dV/V)
Mg2SiO4 - ‘low’ pressure
60% of the volume fraction for Pyrolite • Olivine (orthorhombic) - Upper mantle • Wadsleyite (orthorhombic) - Upper Transition Zone • Ringwoodite (cubic) - Lower Transition Zone
Pure Mg2SiO4
Olivine -Wadsleyite - Ringwoodite
Mg2SiO4 - Fe2SiO4
Olivine-Ringwoodite Transition TEM
Orientation Relationship
Crystallographic relationships
Cij : Mg2SiO4 polymorphs
Vp ,Vs & PREM : ���Mg2SiO4
polymorphs
Anisotropy : ���Mg2SiO4
polymorphs
Effect of temperature
olivine
Sinogeikin et al 03 PEPI
wadsleyite
ringwoodite
Mantle Overview
Mantle composition
Saut de vitesse sismique (densité) assez brusque vers 670 km Saut = Passage de minéraux silicatés à structure tétraèdrique (α,β,γ) à un mélange de minéraux silicaté octaédrique (Perovskite) et oxyde (Ferropericlase) : la PEROVSKITE (Fe,Mg)SiO3 la MAGNESIOWUSTITE (Fe,Mg)O (Ferropericlase)
Plus simplement en prenant le pôle magnésien le
plus courant dans le manteau
Mg2SiO4 <--> MgSiO3 + MgO Rw <--> Pv + Mw
LIMITE MANTEAU SUP / MANTEAU INF. = DECOMPOSITION de la PHASE g
MgO
• MgO Periclase , reference material for physical properties.
• Cubic space group Fm-3m , Halite structure, density 3.585 at room P & T.
• In the lower mantle Ferropericlase-magnesowüstite (Mg,Fe)O is present (20% by volume).
Wave velocities in MgO Hama & Suito (1999)
Single Crystal MgO-FeO - Vp & Vs
MgO high velocity & anisotropic FeO low velocity & isotropic Hence varing Fe/Mg ratio changes the velocity and seismic anisotropy
MgSiO3 - ‘high’ pressure
(Mg,Fe)SiO3 Perovskite (orthorhombic) - 80% of the lower mantle, the most abundant mineral in the Earth !
MgSiO3
0
200
400
600
800
1000
1200
1400
0 50 100 150
MgSiO3 Perovskite elastic constants as a function of pressure
C11C22C33C12C13C23C44C55C66
C ij
Pressure (GPa)
Oganov et al. (2001)
C22C33
C11
C12
C23C13C22C66C55
Mg-Perovskite Cij at lower mantle PT
Wentzcovitch_et_al 2004
Lower Mantle phases: Vp & Vs
Birch’s Law ���at high
pressure
Seismic anisotropy versus Pressure : SiO2, MgO & MgSiO3 SiO2 - very anisotropic with strong variations with pressure due to phase transitions. MgO - moderately anisotropic and very pressure dependent, but NO phase transitions in this pressure range MgSiO3 - moderately anisotropic with no variation with pressure no phase transitions in this figure published in by Karki_et_al Rev.Geophys.2001, now a new phase ‘post-Pv’ discovered in 2004 at about 120 GPa.
Perovskite Composition K0 ρ Vφ=√(Ko/ ρ)
(GPa) (Mg/m3) (km/s) MgSiO3 262 4.12 7.974 MgSiO3~10% FeO 262 4.25 7.851 MgSiO3 ~3.25% Al2O3 261 4.123 7.956 MgSiO3 ~3.25% Al2O3+H2O 256 4.088 7.913 K= incompressibility Hyp=liquid
Interpretation of Tomography: Compositional Variations
Phase Transitions - Consequences
Seismic velocity changes (elastic moduli & density) Vs = √(G/ρ) Seismic anisotropy changes (symmetry & elastic moduli) Compositional changes Mg/Fe… Density changes Grain size changes (reduction or increase ?) Deformation mechanism changes
Effect of temperature on Transition zone thickness
410 km depth : exothermic reaction : dP/dT = positive = +ve 660 km depth : endothermic reaction : dP/dT = negative = -ve
The form of the Clapeyron or Clausius-Clapeyron equation most often used is dP/dT = ΔS/ΔV The slope of an univariant equilibrium plotted on a P-T diagram is equal to the entropy change (ΔS) of the reaction divided by the volume change (ΔV) of the reaction
Transition zone discontinuities
* discontinuities are caused by olivine phase changes thin transition zone in hotspot/plume regions
Seismic Structure of TZ
• 410 km:P-vel. increase ΔVp 5-6% S-vel. increase ΔVs 4-7% first order discontinuity sharp: 2-4km beneath oceans 35 km beneath continents Α olivine to β wadselyite exothermic with positive Clapeyron slope 3MPa/K • 520 km:Controversial discussion of existence artefact? There in some regions, absent in others? few seismological observations contrasts ΔVp = 1% ΔVs=0.8-1.5% Δρ= 2.5-3%
• 670 km: barrier to convection? increase in velocity and density: 6-11% could be explained by phase change or change in chemical composition. if chemical: either: Fe content or Al content => discontinuity complex P’P’ observations => <4km but long period P-SV conversions: 20-30 km Clapeyron slope negative, endothermic, depression of 670 disc. may hinder convection
Depth of phase transitions
Discontinuities Deuss et al. 2005
* using SS precursors, only large scale structure
* thin transition zone, correlation with hotspots/plumes?
Subduction:��� cold
temperatures
Regional Study - South Pacific ‘hot spot’
Niu et al. 2002 EPSL
Reflections at 410 and 670 km
HotSpot ?
HotSpot ?
HotSpot ?
410 km
670 km
Interpretation
Global tomography Shear wave velocity model S20RTS: * body waves * surface waves * normal mode splitting functions
Ritsema, van Heijst & Woodhouse (1999)
Global and Regional Study : reflections from S20RTS
Stack for North America (strong and weak)
(Deuss & Woodhouse, GRL, 2002)
220
800
1050
1150
410
520
660
Global versus Regional data
Deuss and Woodhouse (2002) GRL
Splitting of the Mid–Transition Zone Discontinuity
Varing chemical composition Mg/(Mg+Fe) changing olivine to wadselyite depth ?
Deuss and Woodhouse (2002) Science
Regional Study : L & X discontinuities
Bagley and Revenaugh JGR 2008
Pacific Plate
Bagley and Revenaugh JGR 2008
X discontinuity – OEn -> HP Cen ?
Jacobsen et al. 2010 PEPI
The 300 km (X) seismic discontinuity & Coesite to Stishovite SiO2 phase transition���
���
Global distribution of 300 km (X) seismic discontinuity
Williams & Revenaugh Geology 2005
Williams & Revenaugh Geology 2005
Williams & Revenaugh Geology 2005
The 1200 km seismic discontinuity & Stishovite to CaCl2 structure of SiO2 ���
���
Si02 in the mantle
Multianvil apparatus 14 GPa - 1300°C - 10 hours
Crystal by Patrick Cordier (Lille)
SiO2 Polymorphs : Cij theory
Structure along c axis Stishovite - Tetragonal a=b CaCl2 type - Orthorhombic b>a
a
b
Effect on a,b & c axes
3,7
3,8
3,9
4,0
4,1
4,2
0 20 40 60 80 100 120
a Landau theory b Landau theory a experimental datab experimental data
a a
nd b
cell
par
amat
ers
(ang
strom
s)
Pressure (GPa)
StishoviteP4
2/mnm
Tetragonal
CaCl2-type
PnnmOrthorhombic
b
a
a & b
2,45
2,50
2,55
2,60
2,65
2,70
0 20 40 60 80 100 120
c Landau theoryc experimental data
c ax
is ce
ll pa
ram
eter
(ang
strom
s)
Pressure (GPa)
StishoviteP4
2/mnm
Tetragonal
CaCl2-type
PnnmOrthorhombic
36
38
40
42
44
46
0 20 40 60 80 100 120
Landau theory Experimental data
Unit
Cell
Volum
e (a
ngstr
oms3 )
Pressure (GPa)
CaCl2-type
PnnmOrthorhombic
StishoviteP4
2/mnm
Tetragonal
4,2
4,4
4,6
4,8
5,0
5,2
5,4
5,6
0 20 40 60 80 100 120
Landau theoryExperimental data
Dens
ity (g
/cm3 )
Pressure (GPa)
CaCl2-type
PnnmOrthorhombic
StishoviteP4
2/mnm
Tetragonal
Effect on volume & density
Effect on single crystal elastic constants Cij using Landau theory
Carpenter et al. 2000
0
200
400
600
800
1000
1200
0 20 40 60 80 100 120
C ij (GPa
)
Pressure (GPa)
StishoviteP4
2/mnm
Tetragonal
CaCl2-type
PnnmOrthorhombic
C11
= C22
C11
C
22
C33
C33
C12
C12
C66
C66
C44
= C55
C44
C55
C13
= C23
C23
C13
Effect on isotropic elastic constants
0
100
200
300
400
500
600
700
800
0 20 40 60 80 100 120
Mod
ulus
(GPa
)
Pressure (GPa)
K, Bulk Modulus
G, Shear Modulus
Voigt
Reuss
StishoviteP4
2/mnm
Tetragonal
CaCl2-type
PnnmOrthorhombic
VRH
0
4
8
12
16
0 20 40 60 80 100 120
Velo
city
(km
/s)
Pressure (GPa)
Vp
V s
Voigt
Voigt
Reuss
Reuss
VRH
VRH
StishoviteP4
2/mnm
Tetragonal
CaCl2-type
PnnmOrthorhombic
Isotropic Vp and Vs
Anisotropy Stishovite - CaCl2 transition
D” layer and Perovskite (Pv) to Post-Peroskite (PPv) phase
transition
Large Igneous Provinces (LIPs)
RED – LIPs BLUE – Plume heads (hot spots)
LIPs, plumes and D’’
2D View of the Earth
The density profile
Kellogg et al 1999 Science
Schematic models of D’’
Lay et al EPSL 04
Heterogeneous Layer
Joint Vp & Vs Tomography heterogeneity
D’’ topography & ULVZ (plumes?)
Phase Transformation Pv-PPv
MgSiO3 Perovskite ----- Most abundant constituent in the Earth’s lower mantle ----- Orthorhombic distorted perovskite structure (Pbnm, Z=4) ----- Its stability is important for understanding deep mantle (D” layer)
MgSiO3 Perovskite
• MgSiO3 perovskite is the
most abundant mineral in the lower mantle.
Si
Mg
O
But things are more complex…. • Kendall (1998) wrote:
‘…LPO in lower-mantle minerals is an unlikely cause for the anisotropy…. although we must always allow for the possibility that there is an as-yet unknown mineralogy that dominates D”.’
The Post-Perovskite Phase • Motohiko Murakami, Kei Hirose et al
(May 2004) announce new post-perovskite phase.
• Also found by Oganov & Ono (Nature, July 2004)
• Based on “CaIrO3” or “UFeS3” structure.
• Stable at P>120 GPa. • D” will not be made of perovskite at
all!
History of Post-Perovskite���(May to September…2004)
1. Murakami et al publish the first experimental evidence for Post-Perovskite Science 7th May. 2. Tsuchiya et al. ab initio simulations of elastic constants at 0K. GRL 17th July 2004. 3. Iitakata et al ab initio simulations of structure, stability and elastic constants at 0K Nature 22nd July. 4. Organov & Ono ab initio simulations of structure, stability and elastic constants at 0K &
experimental data Nature 22nd July. 5. Tsuchiya et al. ab initio phase transition perovskite-post perovskite and Clapeyron slope. EPSL
August 2004.
6. Stackhouse et al. ab initio simulations of elastic constants at 4000K EPSL submitted.
Post Perovskite X-ray diffraction pattern
b
c a
Lattice system: Bace-centered orthorhombic Space group: Cmcm Formula unit [Z]: 4 (4) Post-Perovskite Perovskite Lattice parameters [Å] a: 2.462 (4.286) [120 GPa] b: 8.053 (4.575)
c: 6.108 (6.286) Volume [120 GPa] [Å
3]: 121.1 (123.3)
68101214162 theta (deg)
Inte
nsity
(arb
itrar
y un
it)
λ = 0.4134 Å120 GPaExpCalc 020
021
002
022
110
111
040
041
023/
130
131
042
132
113
004
Pt
Crystal structure of post-perovskite
Variation in the depth of D” could be due to a effect of temperature on phase transition pressure (from Murakami et al Science 2004)
Phase Diagram
D” Structure • Seismic anisotropy in the D’’ region is variable. • In places, the top of the D’’ layer is bound by a seismic
discontinuity. • There is also mounting evidence that the bottom 40 km has
patches of ultra low velocity zones. • Observations also indicate that in D’’ horizontally polarised
shear waves travel faster than vertically polarised ones (vSH > vSV) by an average of 1 %, except in regions of upwelling streams, such as in the central pacific, where vSV > vSH.
Unexplained seismic properties of D” layer • 1. Presence and magnitude of seismic discontinuity (1% for
Vs, zero for Vp) at the top of the D” layer. • 2. Inferred Clapeyron slope. (Post-Pv 7.5MPa/K) • 3. Anisotropy of S-waves (horizontally polarized SV are 1%
faster than SH). (Crystal or Shape PO ?) • 4. Anticorrelation between bulk and shear moduli or
velocities. The post-perovskite transition has a positive jump of Vs and a negative jump of Vφ. (Unusual shear to bulk modulus ratio, not compatable with Perovskite or MgO)
Compatible with seismic data
+1% Vs Jump near CMB���& Topography of D layer
Sidorin et al 1999 Science
150 km
Hot (slow) and Cold (fast) regions in D”
Sidorin et al Science 1999
Seismic versus Experiment
Helmberger et al PNAS 2005
Denser D” Layer • Masters and Gubbins recently
found the excess density of 0.4% in the bottom 500 km of the lower mantle.
• An expected density increase of 1.0 to 1.2% for the bottom 200- to 300-km layer owing to the post-MgSiO3 perovskite transition is consistent with their observations.
• b-axis is most compressible in Post-Pv.
(from Murakami et al Science 2004)
Suduction goes down to CMB
D’’ Strongly layered
Van der Hilst et al 2007 Science
Layer seismic structure
Double crossing of Pv to Post-Pv phase boundary
Schematic view of D ”
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