structure et minéralogie du manteau

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 ”