Formation de la Terre

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Abundances in the Universe/Crust

Mg Al Si

Melting Temperature

Goldschmidt Classification

Chondrite

Siderophile (Fe, Ni...)

Lithophile (Si, Mg, Ca, Al, K...)

Atmophile (N, He...)

L’interieur de la Terre:proprietes radiales

Geophysical Observations

Seismics:- 1st order spherical- lower BC- material Prop.

" Mass of the Earth M : 6. 1024 kgRadius of the Earth R : 6371 km, of the core Rn=3470 kmAverage density rhobar : 5.5 g/cm3

Inertia I : 0.33 MR2

The average inertia is larger that the density of crustal silicates (around 2.8 g/cm3) and mantle silicates (around 3.2 g/cm3). The deep Earth is thus denser

The inertia is lower than that of an homogeneous Earth (2/5MR2). The deep Earth is thus denser

" Exercise : compute the mass, average density and inertia of a two layer sphere. Estimate the Earth’s mantle and core densities

" solution : rho1 = 12.7 kg m3, rho2=4.2 kg m3. The core mass is 37% of Earth’s mass.

Nature of the core 1)

" The composition of sample mantle rocks is typically

(Mg0.9 Fe0.1)2 Si O4 Olivine

Solid solution of Forsterite Mg2 Si O4 & Fayalite Fe2 Si O4

In chondrites and solar wind a molar ratio Fe/Mg=.84 is observed

" What is the mass of the iron core? (The molar masses of Mg, Fe, Si, O are 24.3, 56, 28 et 16).

The total number of Fe moles is

NFe=Mnoyau/mfe+0.2 Mmanteau/mOl

The total number of Mg moles is

NMg=1.8 Mmanteau/mol

as NFe/NMg is .84 the core mass is 33 % of the Earth’s

Nature of the core 2)

Average Composition Of the Oceanic Crust

Weight %

SiO2 50.0

Al2O3 16.0

CaO 11.3

FeO 10.0

MgO 7.7

Na2O 2.8

TiO2 1.5

K2O 0.15

P2O5 0.12

Average Composition of the Continental Crust

Weight%

SiO2 66.0

Al2O3 15.5

FeO 4.6

CaO 3.7

K2O 3.5

Na2O 3.2

MgO 2.4

TiO2 0.6

P2O5 0.2

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Molar %

" The total Earth’s composition is similar to that of the refractory part of the Universe

" The total Earth’s composition is similar to chondrites (non-differenciated meteorites)

" The siderophile elements went into the Core" The lithophile elements went into the

crust+mantle" The most incompatible elements are in the

crust (oceanic but mostly continental)

Stratification of the Earth: Composition

7 km oceans crust basalt

30 km continents crust granite

R=6371 km

R=3500 km

Mantle pyrolite

Core Iron (+Nickel +O,S,Si…)

Stratification of the Earth: Mineralogy

Upper mantle 410 km depth olivine+pyroxene+garnet

520 km depth olivine beta (wadsleyite)+garnet+pyroxene

610 km depth olivine gamma (ringwoodite)+garnet+pyroxene

Lower mantle oxides magnesiowustite

Mg-Fe perovskite

Ca-perovskite

•Molecular clouds

contraction (0.1-1 ma)

•Disk formation

•Condensation

•Coagulation of dust

•Up to km-size

Orion Nebula

4566 Ma Ca-Al-

4563 Ma Chondrules

t0+5-15Iron meteorites

t0+30 Earth Core

t0+50 Moon formed

t0+100 End of accretion

ChondriteInclusions

Datation: ex U/Pb sur zircons ZrSiO4 (accepte U rejete Pb)

= stable 183W

= radioactif 182Hf

= radiogénique 182W

Half-life 9 ma

W « siderophile » Hf « lithophile »

Core segregation after Hf decay

Same 182/183W ratio in the mantle and in undifferenciated objets

W « siderophile » Hf « lithophile »

Cor formation before Hf decay

Larger 182/183 W ratio in the mantle than in undifferenciated objets

Une planete active

La tectonique des plaques sur le million d’annee

Des mouvements horizontaux…

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La tectonique des plaques sur l’annee

Distance

Pac-Fr

Observations of plate motion

Modèle de la tectonique des plaques

Finland

Hudson

Scotland

Des mvts verticaux…

Bretagne

Estimates of the viscosity:

Observation of heat flux :

Using Earth heat flow, in how much time can I boil the water for my coffee cup??

t~108 s = 3.5 ans

Can we expore the mantle

" KTB, Kola

Hotspots:

seem too:

- exist over long time- being stationary- independent of the

plate motion- are geochemically

distinced

Stratification of the Earth: Rheology

Lithosphere ~100 km ~1026 Pa s solid

Asthenosphere ~1019-22 Pa s solid

« real asthenosphere » 1019-21 Pa s solid

deep mantle 1021-22 Pa s solid

D ’’ <<1021 Pa s ? solid

Outer core 10-3 Pa s liquid

Inner core ? Not so solid

Liquide

Solide

Liquide

Solide

Stratification of the Earth: Temperature

Clapeyron slopes of phase transformations

Olivine-wadsleyite

Ringwoodite-perovskites+oxides 1870 K

Liquid iron/solid iron 5000 K

Modelisation…

Seismic tomography

On a smaller scale:

From Grant/Van der Hilst

La convection

Formation de la Terre

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