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