1. Models of Crust Composition
Roberta L. Rudnick
Geochemistry Laboratory
Department of Geology
University of Maryland
Apollo 17 view of Earth
Appolo image of Earth

2. Plate tectonics gives rise to two types of crust: oceanic and continental
Start by review of plate tectonics.

3. Young (on average 80 Ma, <200 Ma) ~7 km thick
Oceanic Crust:
Young (on average 80 Ma, <200 Ma)
~7 km thick
High density: ~3.0 g/cm3
Low standing (-4000 m)
Composition: Basalt (SiO2 ~50 wt.%)
Properties of oceanic crust -- young, thin, dense basalt.

4. Intrusion and differentiation of mantle-derived basalt
Generation of the Earth’s Crust
Oceanic
From Press & Siever
Intrusion and differentiation of mantle-derived basalt

5. Ancient (on average 2 Ga, <4 Ga) ~40 km thick
Continental Crust:
Ancient (on average 2 Ga, <4 Ga)
~40 km thick
Low density: ~2.7 g/cm3
High standing (+800 m)
Compositionally stratified
Diverse rock types
Composition: Andesite (SiO2 ~60 wt.%)
Properties of continental crust -- old, thick, buoyant andesite.

6. Convergent margin processes? Intraplate processes?
Generation of the Earth’s Crust
Continental ?
Convergent margin processes? Intraplate processes?

7. Upper Crust Lower Crust http://www.ub.es/ggac/research/piris
Crust is vertically stratified. Upper and lower crust (sometimes “middle” crust) defined on the basis of physical properties (i.e. seismic wave speeds).

8. Continental crust: Lots of heterogeneity!
Every rock type known on Earth occurs in continental crust
Shuttle view of granite intruding metamorphic basement, northern
Chile.

9. How is crust composition determined?

10. Models of Crust Composition
Crustal growth scenarios (Taylor & McLennan, 1985)
Empirical models (Christensen & Mooney, 1995; Wedepohl, 1995, Rudnick & Fountain, 1995; Rudnick & Gao, 2003)

11. Taylor & McLennan Recipe
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
*A special type of granite called tonalite, with relatively low K, Th and U

12. Empirical Models
Upper crust: grid sampling & sedimentary rocks
Deep crust: determined from seismic velocities, heat flow

13. Upper crust major elements: Grid sampling
Space shuttle view of Thunder Bay, Ontario

14. Upper continental crust
is granitic (67 wt.% SiO2)

15. Trace elements: analyses of sedimentary rocks
Quantitative transport of insoluble elements from site of weathering to deposition.

16. K U log t Th La (REE) log K Insoluble elements:
10.0
Soluble
Moderately soluble Na
8.0 K Mg
Insoluble
Insoluble elements:
Transferred from source of weathering to sediments U B Re Sr Au Li
6.0 Ca Rb Se Mo Sb W As Cs Cd
4.0 Bi
log t Si Ag V Tl Ta Ge Ba Cr Hf Ni
(residence time) In Zn Nb Ga Cu Zr
2.0 Sn Ti Be Y Pb Mn Th Al Sc Co
REE
La (REE)
0.0 Fe
-2.0
-10.0
-8.0
-6.0
-4.0
-2.0
0.0
log K sw y
(sea water partition coefficient)
After Taylor & McLennan, 1985

17. Loess: samples of averaged upper crust?2 4 6 8 10 12 14 15 20 25 30 35 40 Th
r2 = 0.82
r2 = 0.15
K2O
0.0
1.0
2.0
3.0
4.0 10 15 20 25 30 35 40
3.5 U
Rudnick & Gao, 2003
Taylor & McLennan, 1985
Gao et al., 1998
3.0
2.5
2.0
r2 = 0.48
1.5
1.0 10 15 20 25 30 35 40 45
La (ppm)
La (ppm)

18. Upper crustal estimates: Major elements
1.4
Shaw et al. G
Eade & Fahrig
& R
Taylor & McLennan C
1.2 U o t d 1 e z i l a m
0.8
Wt. % K2O:
2.7 to 3.4%
Rudnick & Gao: 2.8 wt.% r o N
0.6
1.4 G
& R C
1.2 U o t d e
1.0 z i l a
Borodin m
0.8 r
Condie o N
Gao et al.
Ronov & Yaroshevsky
0.6 K Si Al Fe Mg Ca Na

19. Upper crustal estimates: U & Th
Actinides & heavy metals
1.5
Th ppm:
8.6 to 10.8 (10.5)
U ppm:
1.5 to 2.8 (2.7)
Th/U = 3.9
1.0
0.5 Tl Pb Bi Th U
Shaw
Taylor & McLennan
Eade & Fahrig
Gao et al.
Condie

20. Deep Crustal Samples Granulite Facies Terrains Granulite Facies
Ross Taylor, KSZ, Ontario, 1983
Granulite Facies
Terrains
Granulite Facies
Xenoliths

21. The great xenolith hunt
Shukrani Manya, Univ. Dar es Salaam, Tanzania
Profs. Gao and Wu, Shanxi, China
Bill McDonough, Queensland, Australia
Xenoliths -- carried by volcanoes from great depths and tossed out onto the Earth’s surface.

22. Granulite Facies Terranes Archean Post-Archean Lower crustal xenoliths10 20 30 40 50 60 70 80 90
Granulite Facies
Terranes
Archean
Post-Archean
Mg# 30 40 50 60 70 80 90 10 20 30 40 50 60 70 80 90
Lower crustal
xenoliths
Mg# 30 40 50 60 70 80 90
SiO2 (wt. %)

23. Middle and Lower Crust -- Seismic evidence
Paleozoic
Orogen
Rifted Margin
Rift
Arc
Contractional
Shield & Platform
Extensional
Forearc 20 40 Vp 60 Km
From Rudnick & Fountain, 1995

24. Upper Mantle Basalt Granite Vp (m/s) Density (g/cm3) Ultramafic rocks8 . 5
Ultramafic rocks
Upper Mantle 8 .
Eclogites 7 . 5 Vp
(m/s)
Mafic rocks
Basalt 7 . 6 . 5
Granite
Felsic
rocks 6 .
m=22 2 . 6 2 . 8 3 . 3 . 2 3 . 4 3 . 6
Density (g/cm3)

25. Comparison of middle crustal models:
Major elements
2.0 G
1.5
& R o t d
1.0 e z i l a m r
Weaver & Tarney
0.5 o
Shaw et al. N
Gao et al.
Rudnick & Fountain
0.0 K Si Al Fe Mg Ca Na
Wt. % K2O: 2.1 to 3.4%
Rudnick & Gao: 2.3 wt.%

26. Comparison of middle crustal models:
Alkali, alkaline Earth & Actinides
2.0
2.6
1.5
1.0
0.5
Weaver & Tarney
Shaw et al.
Gao et al.
Rudnick & Fountain Li Li Rb Rb Cs Cs Sr Sr Ba Ba Pb Pb Th U
Th ppm: 6.1 to 8.4 (6.5)
U ppm: 0.9 to 2.2 (1.3)
Th/U = 5.0

27. Comparison of lower crustal models:
Major elements
2.0
Terrains and models F
&
1.5 R o t d e z
1.0 i l a m r
Weaver & Tarney o
0.5
Shaw et al. N
Gao et al.
Add Christensen & Mooney to this plot.
Wedepohl
Taylor & McLennan
0.0 K Si Al Fe Mg Ca Na
Wt. % K2O: 2.1 to 3.4%
Rudnick & Gao: 2.3 wt.%

29. Composition of the Continental Crust
Christensen
Rudnick &
Wedepohl
Taylor &
Rudnick &
& Mooney
Fountain
1995
McLennan
Gao, 2003
1995
1995
1985, 1995
SiO
62.4
60.1
62.8
57.1
60.6 2 Al O
14.9
16.1
15.4
15.9
15.9 2 3
FeOT
6.9
6.7
5.7
9.1
6.7
MgO
3.1
4.5
3.8
5.3
4.7
CaO
5.8
6.5
5.6
7.4
6.4 Na O
3.6
3.3
3.3
3.1
3.1 2 K O
2.1
1.9
2.7
1.3*
1.8 2
Mg#
44.8
54.3
54.3
50.9
55.3
*Updated by McLennan and Taylor, 1996

30. Composition of the Continental Crust
Rudnick &
Clarke*
Gao, 2003
1889
SiO
60.6
60.2 2
TiO
0.7
0.6 2 Al O
15.9
15.3 2 3
FeO
6.7
7.3 T
MnO
0.10
0.10
MgO
4.7
4.6
CaO
6.4
5.5 Na O
3.1
3.3 2 K O
1.8
3.0 2
F.W. Clarke, P O
0.13
0.23 2 5
Mg#
55.3
53.0
*Clarke, Frank Wigglesworth, for whom the Clarke medal is named