models of crust composition roberta l. rudnick geochemistry laboratory department of geology...
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Models of Crust Models of Crust CompositionCompositionModels of Crust Models of Crust CompositionComposition
Roberta L. RudnickGeochemistry LaboratoryDepartment of GeologyUniversity of Maryland
Apollo 17 view of Earth
Plate Plate tectonics tectonics gives rise to gives rise to two types of two types of crust: crust: oceanic and oceanic and continentalcontinental
Plate Plate tectonics tectonics gives rise to gives rise to two types of two types of crust: crust: oceanic and oceanic and continentalcontinental
Oceanic Crust:
Young (on average 80 Ma, <200 Ma) ~7 km thickHigh density: ~3.0 g/cm3
Low standing (-4000 m)Composition: Basalt (SiO2 ~50 wt.%)
Oceanic
Generation of the Earth’s Crust
Intrusion and differentiation of mantle-derived basalt
From Press & Siever
Continental Crust:
Ancient (on average 2 Ga, <4 Ga) ~40 km thickLow density: ~2.7 g/cm3
High standing (+800 m)Compositionally stratifiedDiverse rock types Composition: Andesite (SiO2 ~60 wt.%)
Generation of the Earth’s Crust
?Convergent margin processes?
Intraplate processes?
Continental
Upper Crust
Lower Crust
http://www.ub.es/ggac/research/piris
Shuttle view of granite Shuttle view of granite intruding metamorphic intruding metamorphic basement, northern basement, northern Chile.Chile.
Continental Continental crust:crust:
Lots of heterogeneity!
Every rock type known on Earth occurs in continental crust
How is crust composition determined?
1.Crustal growth scenarios (Taylor & McLennan, 1985)
2.Empirical models (Christensen & Mooney, 1995; Wedepohl, 1995, Rudnick & Fountain, 1995; Rudnick & Gao, 2003)
Models of Crust Composition
25% “Andesite model”75% Archean crust
Archean crust: Mixture of Archean basalt & Archean granite*Assume 50% of 40 mWm-2 surface heat flow
derives from crust: 75% basalt, 25% granite
Taylor & McLennan Recipe
*A special type of granite called tonalite, with relatively low K, Th and U
Upper crust: grid sampling & sedimentary rocks
Deep crust: determined from seismic velocities, heat flow
Empirical Models
Upper crust major elements: Grid sampling
Space shuttle view of Thunder Bay, Ontario
Upper continental crustis granitic (67 wt.% SiO2)
Trace elements: analyses of sedimentary rocks
Quantitative transport of insoluble elements from site of weathering to deposition.
log
-2.0
0.0
2.0
4.0
6.0
8.0
10.0
SolubleModerately soluble
Insoluble
-10.0 -8.0 -6.0 -4.0 -2.0 0.0
Cu
Au
Mo
CaLi
Re Sr
K Mg
B
Na
WSb
SeRb
U
Cs
BiCd
As
SiV
Ag
Ni
BaTl
Fe
Mn
HfTa
GaIn
Ge
Zn
Cr
ThAl Sc Co
TiY
SnZr
Nb
PbBe
REE
log Kswy
Insoluble elements:
Transferred from source of weathering to sediments
After Taylor & McLennan, 1985
(residence time)
(sea water partition coefficient)
La (REE)
r2 = 0.15
K2O
0.0
1.0
2.0
3.0
4.0
10 15 20 25 30 35 40
La (ppm)
2
4
6
8
10
12
14
10 15 20 25 30 35 40
Th
r2 = 0.82
101.0
1.5
2.0
2.5
3.0
3.5
15 20 25 30 35 40 45
U
r2 = 0.48
Rudnick & Gao, 2003
Taylor & McLennan, 1985
Gao et al., 1998
Loess: samples of averaged upper crust?
La (ppm)
Upper crustal estimates: Major elements
0.6
0.8
1
1.2
1.4 Shaw et al.Eade & Fahrig
Taylor & McLennan
Nor
mal
ized
to U
CR
&G
0.6
0.8
1.0
1.2
1.4
Si Al Fe Mg Ca Na K
BorodinCondieGao et al.Ronov & Yaroshevsky
Nor
mal
ized
to U
CR
&G
Wt. % K2O:
2.7 to 3.4%
Rudnick & Gao: 2.8 wt.%
Upper crustal estimates: U & Th
Th ppm:
8.6 to 10.8 (10.5)
U ppm:
1.5 to 2.8 (2.7)
Th/U = 3.9
0.5
1.0
1.5
Tl Pb Bi Th U
Actinides & heavy metals
Shaw
Gao et al.
Taylor & McLennan
Eade & Fahrig
Condie
Granulite Facies Terrains
Granulite FaciesXenoliths
Deep Crustal Samples
Ross Taylor, KSZ, Ontario, 1983
Shukrani Manya, Univ. Dar es Salaam, Tanzania
The great xenolith hunt
Profs. Gao and Wu, Shanxi, China
Bill McDonough, Queensland, Australia
10
20
30
40
50
60
70
80
90
30 40 50 60 70 80 90
10
20
30
40
50
60
70
80
90
30 40 50 60 70 80 90
Mg#Mg#
Mg#Mg#
Granulite Facies Granulite Facies TerranesTerranes
ArcheanArchean Post-ArcheanPost-Archean
Lower crustal Lower crustal xenolithsxenoliths
SiOSiO22 (wt. %) (wt. %)
Rifted MarginContractional Shield & Platform
Paleozoic OrogenRift
ExtensionalArc
Forearc0
20
40
60
KmVp
6.4 6.6 6.8 7.0 7.2
Middle and Lower Crust -- Seismic evidence
From Rudnick & Fountain, 1995
22..66 22..88 33..00 33..22 33..44 33..66
66..00
66..55
77..00
77..55
88..00
88..55
Mafic rocks
Felsic rocks
Eclogites
m=21
m=22
Ultramafic rocks
VVpp
(m/s)(m/s)
Density (g/cmDensity (g/cm33))
Upper Mantle
Basalt
Granite
No
r ma
liz
ed
to
R&
G
Weaver & Tarney
Shaw et al.
Gao et al.
Rudnick & Fountain
0.0
0.5
1.0
1.5
2.0
Si Al Fe Mg Ca Na K
Comparison of middle crustal models:Major elements
Wt. % K2O: 2.1 to 3.4%
Rudnick & Gao: 2.3 wt.%
Th ULi Rb Cs Sr Ba Pb
0.5
1.0
1.5
2.0
Li Rb Cs Sr Ba Pb
2.6
Comparison of middle crustal models:Alkali, alkaline Earth & Actinides
Weaver & Tarney
Shaw et al.
Gao et al.
Rudnick & Fountain
Th ppm: 6.1 to 8.4 (6.5)
U ppm: 0.9 to 2.2 (1.3)
Th/U = 5.0
No
rmal
ized
to
R&
F
0.0
0.5
1.0
1.5
2.0
Si Al Fe Mg Ca Na K
Weaver & Tarney
Shaw et al.
Gao et al.
Wedepohl
Taylor & McLennan
Terrains and models
Comparison of lower crustal models:Major elements
Wt. % K2O: 2.1 to 3.4%
Rudnick & Gao: 2.3 wt.%
Composition of the Continental Crust
Christensen Rudnick & Wedepohl Taylor & Rudnick && Mooney Fountain 1995 McLennan Gao, 2003
1995 1995 1985, 1995
SiO2 62.4 60.1 62.8 57.1 60.6Al2O3 14.9 16.1 15.4 15.9 15.9FeOT 6.9 6.7 5.7 9.1 6.7MgO 3.1 4.5 3.8 5.3 4.7CaO 5.8 6.5 5.6 7.4 6.4Na2O 3.6 3.3 3.3 3.1 3.1K2O 2.1 1.9 2.7 1.3* 1.8
Mg# 44.8 54.3 54.3 50.9 55.3
*Updated by McLennan and Taylor, 1996
Rudnick & Clarke*Gao, 2003 1889
SiO 2 60.6 60.2TiO 2 0.7 0.6Al 2O 3 15.9 15.3FeO T 6.7 7.3MnO 0.10 0.10MgO 4.7 4.6CaO 6.4 5.5Na 2O 3.1 3.3K 2O 1.8 3.0P 2O 5 0.13 0.23
Mg# 55.3 53.0
*Clarke, Frank Wigglesworth, for whom the Clarke medal is named
F.W. Clarke, 1847-1931
Composition of the Continental Crust
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