Models of Crust
Composition
Roberta L. Rudnick
Geochemistry Laboratory
Department of Geology
University of Maryland
Apollo 17 view of Earth
Plate
tectonics
gives rise to
two types of
crust:
oceanic and
continental
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.%)
Generation of the Earth’s Crust
Oceanic
From Press & Siever
Intrusion and differentiation of
mantle-derived basalt
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.%)
Generation of the Earth’s Crust
Continental
?
Convergent margin processes?
Intraplate processes?
Upper Crust
Lower Crust
http://www.ub.es/ggac/research/piris
Continental
crust:
Lots of heterogeneity!
Every rock type known
on Earth occurs in
continental crust
Shuttle view of granite
intruding metamorphic
basement, northern
Chile.
How is crust composition
determined?
Models of Crust Composition
1.Crustal growth scenarios
(Taylor & McLennan, 1985)
2.Empirical models (Christensen &
Mooney, 1995; Wedepohl, 1995, Rudnick
& Fountain, 1995; Rudnick & Gao, 2003)
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
Empirical Models
Upper crust: grid sampling &
sedimentary rocks
Deep crust: determined from seismic
velocities, heat flow
Upper crust major elements: Grid sampling
Space shuttle view of Thunder Bay, Ontario
Upper continental crust
is granitic (67 wt.% SiO2)
Trace
elements:
analyses of
sedimentary
rocks
Quantitative
transport of
insoluble elements
from site of
weathering to
deposition.
10.0
Soluble
Moderately soluble
8.0
Na
K
Insoluble
B
U
6.0
Re
Au
Se
Rb
W
log t
Si
Ta
Hf
(residence time)
2.0
Th
0.0
-2.0
-10.0
Sc
Al
-8.0
Transferred from
source of
weathering to
sediments
Cd
Bi
Ni
Ti
Y
Pb
Mn
Co
REE
Fe
Insoluble elements:
In Zn Cu
Ga
Nb
Zr
Sn
Be
Ge CrTl
Ag
Li
Ca
As
Cs
V
Ba
Sr
Mo
Sb
4.0
Mg
La (REE)
-6.0
-4.0
-2.0
0.0
log K sw
y
(sea water partition coefficient)
After Taylor & McLennan, 1985
Loess: samples of averaged upper crust?
14
Th
12
10
8
6
r2 = 0.82
4
2
10
15
20
25
30
35
40
4.0
3.5
U
K2O
3.0
3.0
2.5
2.0
2.0
Rudnick & Gao,
2003
Taylor & McLennan,
1985
Gao et al., 1998
1.0
r2 = 0.15
r2 = 0.48
0.0
10
15
20
25
30
La (ppm)
35
40
10
15
20
25
30
La (ppm)
35
40
1.5
1.0
45
Upper crustal estimates: Major elements
1.4
1.2
Shaw et al.
Eade & Fahrig
Taylor & McLennan
1
0.8
Wt. % K2O:
0.6
2.7 to 3.4%
1.4
Rudnick & Gao:
2.8 wt.%
1.2
1.0
0.8
0.6
Borodin
Condie
Gao et al.
Ronov & Yaroshevsky
Si
Al
Fe
Mg
Ca
Na
K
Upper crustal estimates: U & Th
Actinides & heavy metals
Th ppm:
8.6 to 10.8 (10.5)
U ppm:
1.5 to 2.8 (2.7)
1.5
1.0
0.5
Th/U = 3.9
Tl
Pb
Shaw
Eade & Fahrig
Condie
Bi
Th
U
Taylor & McLennan
Gao et al.
Deep Crustal Samples
Ross Taylor, KSZ, Ontario, 1983
Granulite Facies
Terrains
Granulite Facies
Xenoliths
The great xenolith hunt
Shukrani Manya, Univ. Dar es Salaam, Tanzania
Profs. Gao and Wu, Shanxi, China
Bill McDonough, Queensland, Australia
90
80
70
Granulite Facies
Terranes
Archean
Post-Archean
60
Mg#
50
40
30
20
10
30
40
50
60
70
80
90
90
80
70
60
Mg#
Lower crustal
xenoliths
50
40
30
20
10
30
40
50
60
70
SiO2 (wt. %)
80
90
Middle and Lower Crust -- Seismic evidence
Paleozoic
Orogen
Rifted Margin
Rift
Arc
Contractional
Shield & Platform
Extensional
Forearc
0
20
40
Vp
60
Km
6.4
6.6
6.8
7.0
7.2
From Rudnick & Fountain, 1995
m=21
8.5
Ultramafic rocks
8.0
Vp
(m/s)
7.5
Eclogites
Mafic rocks
Basalt
7.0
6.5
Upper
Mantle
Granite
Felsic rocks
6.0
m=22
2.6
2.8
3.0
3.2
Density (g/cm3)
3.4
3.6
Comparison of middle crustal models:
Major elements
2.0
1.5
1.0
0.5
Weaver & Tarney
Shaw et al.
Gao et al.
Rudnick & Fountain
0.0
Si
Al
Fe
Mg
Ca
Na
Wt. % K2O: 2.1 to 3.4%
Rudnick & Gao: 2.3 wt.%
K
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
Rb
Cs
Sr
Ba
Pb
Th ppm: 6.1 to 8.4 (6.5)
U ppm: 0.9 to 2.2 (1.3)
Th/U = 5.0
Th U
Comparison of lower crustal models:
Major elements
2.0
Terrains and models
1.5
1.0
Weaver & Tarney
0.5
Shaw et al.
Gao et al.
Wedepohl
Taylor & McLennan
0.0
Si
Al
Fe
Mg
Ca
Na
Wt. % K2O: 2.1 to 3.4%
Rudnick & Gao: 2.3 wt.%
K
Composition of the Continental Crust
Christensen Rudnick & Wedepohl Taylor &
Rudnick &
& Mooney Fountain
1995
McLennan Gao, 2003
1995
1995
1985, 1995
SiO2
Al2O3
FeOT
MgO
CaO
Na2O
K2O
62.4
14.9
6.9
3.1
5.8
3.6
2.1
60.1
16.1
6.7
4.5
6.5
3.3
1.9
62.8
15.4
5.7
3.8
5.6
3.3
2.7
57.1
15.9
9.1
5.3
7.4
3.1
1.3*
60.6
15.9
6.7
4.7
6.4
3.1
1.8
Mg#
44.8
54.3
54.3
50.9
55.3
*Updated by McLennan and Taylor, 1996
Composition of the Continental Crust
Rudnick &
Gao, 2003
Clarke*
1889
SiO 2
TiO 2
Al 2O 3
FeO T
MnO
MgO
CaO
Na 2O
K 2O
P 2O 5
60.6
0.7
15.9
6.7
0.10
4.7
6.4
3.1
1.8
0.13
60.2
0.6
15.3
7.3
0.10
4.6
5.5
3.3
3.0
0.23
Mg#
55.3
53.0
F.W. Clarke, 1847-1931
*Clarke, Frank Wigglesworth, for whom the Clarke medal is named