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Composition of the Continents. Roberta Rudnick and Bill McDonough Geology, University of Maryland. Oceanic crust <<200 million years old. Continents up to 3500 million years old. ages (Ga). <0.6. 06.-2.6. >2.6. Crustal model 5.1 – 5 º x 5 º grid. Mooney, Lasker and Masters (1998).

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composition of the continents
Composition ofthe Continents

Roberta Rudnick andBill McDonough

Geology, University of Maryland

slide4
Crustal model 5.1 – 5º x 5º grid

Mooney, Lasker and Masters (1998)

slide6
Continental Crust’s contribution . . .

Mass

% Earth’s K, Th & U

>40%

0.57%

. . . is insignificant in terms of mass, but is

a major reservoir for incompatible elements

slide7
How is crust composition determined?

What is its significance?

slide8
Story is in the Upper crust
  • Its composition is constrained from
  • surface sampling (e.g., Canadian Shield)
      • Major elements
      • Soluble elements
  • Eade & Fahrig, 1971, 1973;
  • Shaw et al. 1967, 1976, 1986;
  • Gao et al., 1998
slide9
Insoluble elements from clastic sediments

10.0

Soluble

Moderately soluble

Na

8.0

Mg

K

Insoluble

U

B

Re

Sr

Au

Li

6.0

Ca

Se

Rb

Mo

Sb

t

W

As

Cs

Cd

Tl

Bi

Si

Ag

4.0

log

V

Ge

Ba

Cr

Ta

increasing

solubility

Hf

Ni

Zn

In

Nb

Cu

Ga

Zr

Ti

Sn

2.0

Y

Be

Pb

Mn

Th

Sc

Co

Al

REE

0.0

Fe

-2.0

-10.0

-8.0

-6.0

-4.0

-2.0

0.0

sw

log K

From Taylor & McLennan, 1985

y

slide10
1000

Shale composites

and Loess

Australia

N. America

Europe

Eastern China

100

Chondrite

Normalized

10

loess

1

La

Ce

Pr

Nd

Sm

Eu

Gd

Tb

Dy

Ho

Er

Tm

Yb

Lu

slide11
Loess -- insoluble elements

14

Th

12

Taylor & McLennan

10

Rudnick & Gao

(ppm)

8

Gao et al.

6

4

r2 = 0.82

2

10

15

20

25

30

35

40

La (ppm)

slide12
4.0

3.0

2.0

1.0

0.0

10

15

20

25

30

35

40

Loess -- soluble element (K)

K2O

Taylor & McLennan

Rudnick & Gao

Gao et al.

r2 = 0.15

La (ppm)

slide13
14.0

12.0

10.0

8.0

6.0

4.0

2.0

0.50

1.0

1.5

2.0

2.5

3.0

3.5

4.0

Loess -- soluble element (U)

Th/U =

6

5

4

3

Rudnick & Gao

Taylor & McLennan

Gao et al.

Th

(ppm)

U(ppm)

slide14
Deep crust composition from:
  • Analyses of deep crustal rocks:
    • Crustal cross sections
    • Metamorphic terrains
    • Xenoliths
  • Seismic velocities
  • Surface heat flow
slide15
Granulite Facies

Terrains

Granulite Facies

Xenoliths

slide16
The beauty of xenoliths
  • Direct sampling of deep lithosphere:
    • composition
    • age
    • temperature
    • thickness
    • deformation
    • fluids

“The poor man’s drill hole”

slide17
90

80

Granulite Facies

Terrains

Archean

Post-Archean

70

60

50

40

30

20

10

30

40

50

60

70

80

90

90

80

70

60

50

40

30

20

10

30

40

50

60

70

80

90

Mg#

Lower crustal

xenoliths

Mg#

SiO2 (wt. %)

slide18
Seismic Constraints

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

Rudnick & Fountain, 1995

slide19
Weaver & Tarney

Rudnick & Fountain

heat producing

elements

Rudnick & Gao

Wedepohl

Gao et al.

Taylor & McLennan

1000

100

10

1

Models of the Bulk Continental Crust

mantle normalized

Rb

Th

K

La

Pb

Sr

Zr

Sm

Ti

Ho

Cs

Ba

U

Nb

Ce

Pr

Nd

Hf

Eu

Y

Yb

slide21
Heat Flow Data …

SNO+

  • Qs Tmoho

Perry et al (2006) JGR

Crustal heat production … Canadian Shield

slide22
Heat production

“Moho”

Lithosphere (strong layer)

and Asthenoshpere (weak layer)

Lithosphere: crust + mechanically couple mantle

Lithosphere: sits on the asthenoshpere

slide23
lithospheric

thickness

How thick is the lithospheric lid?

2

Surface heat flow = 40 mW/m

Where are the HPE?

Moho

Mantle

50

adiabat

no HPE in

lithopshere

100

1

150

Depth

(km)

200

2

3

4

5

250

all crust

300

350

500

1000

1500

2000

o

Temperature (

C)

slide24
Jericho

Lac de Gras

Torrie

Grizzly

Archean lithosphere is

thick & cold

Africa

Canada

0

Kalihari

Slave

50

2

Kalihari

geotherm

Best Fit

(44 mWm-2)

100

4

Pressure (GPa)

150

Depth (km)

6

200

Lesotho

Kimberley

8

250

Letlhakane

300

10

200

600

1000

1400

200

600

1000

1400

Temperature (oC)

Temperature (oC)

From Rudnick &Nyblade, 1999

slide25
Heat flow constraints

Crustal Model A (µWm-3)

Shaw et al. (1986) 1.31

Wedepohl (1995) 1.25

Rudnick & Fountain (1995) 0.93

Gao et al. (1998) 0.93

Weaver & Tarney (1984) 0.92

Rudnick & Gao (2003) 0.89

McLennan & Taylor (1996) 0.70

Taylor & McLennan (1985) 0.58

Total Cont.0.79-0.99

slide26
Heat flow constraints

Crustal Age A* % Area

(µWm-3)

Archean 0.56-0.73 9

Proterozoic 0.73-0.90 56

Phanerozoic 0.95-1.21 35

Total Cont. 0.79-0.99

*heat production

Jaupart & Mareschal, 2003

slide27
Bulk Crust K, Th & U

from heat flow

K2O 1.3-2.1 wt.%

Th 4.7-6.8 ppm

U 1.05-1.55 ppm

Assuming: Th/U = 3.8 to 5.0

K/U = 10,000 to 13,000

slide28
0 20 40 60 80 100

K, Th, U in Upper crust

min.

max.

K2O

Th

U

% Total Crust Budget

slide29
Summary:
  • Deep crust composition
  • Uncertainties are great
  • Increasingly more mafic with depth
  • Incompatible element depleted relative to upper crust
slide30
Conclusions: crust composition

Composition of the upper crust is known to ±20% for many elements

Deep crust is more poorly known

Incompatible elements are mainly concentrated in the upper crust

Therefore uncertainties in bulk crust reflect upper crustal uncertainties

Heat flow constrains bulk crust K, Th and U to ± 50%

slide31
b- decay “wish list”

Isotope    Emax (MeV) natural abundance half life

40K 1.31 0.012% 1.3E9 y

1.51 electron capture- monoenergetic nue- BR 11%

87Rb 0.28 28% 4.9E10 y

138La 1.04 <0.1% 1.05E11 y

1.74 electron capture- monoenergetic nue- branching ratio?

176Lu 1.19 2.6% 3.8E10 y

187Re 0.003 63% 4.4E10 y

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