Formation de la Terre
Abundances in the Universe/Crust
Fe
Be
Mg Al Si
Pb
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
mass
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
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La tectonique des plaques sur le million d’annee
Des mouvements horizontaux…
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La tectonique des plaques sur l’annee
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Distance
Pac-Fr
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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