Composition ofComposition ofthe Continentsthe ContinentsComposition ofComposition ofthe Continentsthe Continents

Roberta Rudnick and Bill McDonoughGeology, University of Maryland

Oceanic crust <<200 million years old

Continents up to 3500 million years old

<0.606.-2.6>2.6

ages(Ga)

Crustal model 5.1 – 5º x 5º grid

Mooney, Lasker and Masters (1998)

Continental Heat Flow : example from Canadian Shield

Perry et al (2006)

Continental Crust’s contribution . . .Continental Crust’s contribution . . .

MassMass % Earth’s K, Th & U% Earth’s K, Th & U

0.57%>40%

. . . is insignificant in terms of mass, but is . . . is insignificant in terms of mass, but is a major reservoir for incompatible elementsa major reservoir for incompatible elements

How is crust composition How is crust composition determined?determined?

What is its significance?What is its significance?

Story is in the Upper crust Story is in the Upper crust

Its composition is constrained fromIts composition is constrained fromsurface samplingsurface sampling (e.g., Canadian Shield) (e.g., Canadian Shield)

•Major elementsMajor elements•Soluble elementsSoluble elements

Eade & Fahrig, 1971, 1973; Eade & Fahrig, 1971, 1973; Shaw et al. 1967, 1976, 1986; Shaw et al. 1967, 1976, 1986;

Gao et al., 1998Gao et al., 1998

log Klog Kswsw-10.0-10.0 -8.0-8.0 -6.0-6.0 -4.0-4.0 -2.0-2.0 0.00.0

log

log

-2.0-2.0

0.00.0

2.02.0

4.04.0

6.06.0

8.08.0

10.010.0SolubleSolubleModerately solubleModerately solubleInsoluble Insoluble

CuCu

AuAu

MoMo

CaCaLiLi

ReReSrSr

KKMgMg

BB

NaNa

WWSbSb

SeSeRbRbUUCsCs

BiBiCdCd

AsAs

SiSiVV

AgAg

NiNiBaBa

TlTl

FeFe

MnMn

HfHfTaTa

GaGaInIn

GeGe

ZnZn

CrCr

ThTh AlAlScSc CoCo

TiTiYY

SnSnZrZr

NbNb

PbPbBeBe

REEREE

yy From Taylor & McLennan, 1985From Taylor & McLennan, 1985

Insoluble elements from clastic sedimentsInsoluble elements from clastic sediments

increasing increasing solubilitysolubility

Shale composites Shale composites and Loessand Loess

LaLa CeCe PrPr NdNd SmSm EuEu GdGd TbTb DyDy HoHo ErEr TmTm YbYb LuLu11

1010

100100

10001000

Australia Australia N. America N. America Europe Europe Eastern China Eastern China

loess

Chondrite Chondrite NormalizedNormalized

2

4

6

8

10

12

14

10 15 20 25 30 35 40

Th

r2 = 0.82

La (ppm)

Loess -- insoluble elementsLoess -- insoluble elements

Taylor & McLennanTaylor & McLennan

Gao et al.Gao et al.

Rudnick & GaoRudnick & Gao

(ppm)

r2 = 0.15

K2O

0.0

1.0

2.0

3.0

4.0

10 15 20 25 30 35 40

La (ppm)

Loess -- soluble element (K)Loess -- soluble element (K)

Taylor & McLennanTaylor & McLennan

Gao et al.Gao et al.Rudnick & GaoRudnick & Gao

2.0

4.0

6.0

8.0

10.0

12.0

14.0

0.50 1.0 1.5 2.0 2.5 3.0 3.5 4.0

U(ppm)

Th(ppm)

55 4466

33

Th/U =Th/U =

Taylor & McLennanTaylor & McLennanGao et al.Gao et al.

Rudnick & GaoRudnick & Gao

Loess -- soluble element (U)Loess -- soluble element (U)

Deep crust composition from:Deep crust composition from:

1)1) Analyses of deep crustal rocks:Analyses of deep crustal rocks:• Crustal cross sectionsCrustal cross sections• Metamorphic terrainsMetamorphic terrains• XenolithsXenoliths

2)2) Seismic velocitiesSeismic velocities

3)3) Surface heat flowSurface heat flow

Granulite Facies Terrains

Granulite FaciesXenoliths

The beauty of xenoliths

• Direct sampling of deep lithosphere:Direct sampling of deep lithosphere:

compositioncompositionageagetemperaturetemperaturethicknessthicknessdeformationdeformationfluidsfluids

““The poor man’s drill hole”The poor man’s drill hole”

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#Lower crustal Lower crustal

xenolithsxenoliths

SiOSiO22 (wt. %) (wt. %)

Granulite Facies Granulite Facies Terrains Terrains ArcheanArchean Post-ArcheanPost-Archean

Rifted MarginContractional Shield & Platform

Paleozoic OrogenRift

ExtensionalArc

Forearc0

20

40

60

KmVp

6.4 6.6 6.8 7.0 7.2

Rudnick & Fountain, 1995

Seismic Constraints

1

10

100

1000

Rb Th K La Pb Sr Zr Sm Ti Ho

Cs Ba U Nb Ce Pr Nd Hf Eu Y Yb

Weaver & TarneyRudnick & Fountain

WedepohlGao et al.Taylor & McLennan

Rudnick & Gao

mantle normalized

Models of the Bulk Continental CrustModels of the Bulk Continental Crust

heat producing elements

Heat Flow Data …

• Qs Tmoho

SNO+

Perry et al (2006) JGR Crustal heat production … Canadian Shield

Lithosphere (strong layer) Lithosphere (strong layer)

and Asthenoshpere (weak layer)and Asthenoshpere (weak layer)

Lithosphere: crust + mechanically couple mantle

Lithosphere: sits on the asthenoshpere

“Moho”

Heat production

100

200

300

150

Depth(km)

Temperature (oC)

500 1000 1500 2000

50

250

350

1

234

Mantleadiabat

Moho

5

Surface heat flow = 40 mW/m2

all crust

no HPE inlithopshere

Where are the HPE?

lithosphericthickness

How thick is the lithospheric lid?

Kalihari Slave

Pre

ssu

re (

GP

a)

Jericho

Lac de Gras

Torrie

Grizzly

Dep

th (km

)

Best Fit(44 mWm-2)

Kaliharigeotherm

50

100

150

200

250

300

0

2

4

6

8

10200 600 1000 1400 200 600 1000 1400

Temperature (oC)Temperature (oC)

Lesotho

Kimberley

Letlhakane

Archean lithosphere is thick & cold

From Rudnick & Nyblade, 1999

Africa Canada

Heat flow constraintsHeat flow constraintsCrustal ModelCrustal Model A A

(µWm(µWm-3-3))

Shaw et al. (1986)Shaw et al. (1986) 1.311.31Wedepohl (1995)Wedepohl (1995) 1.251.25

Rudnick & Fountain (1995)Rudnick & Fountain (1995) 0.930.93Gao et al. (1998)Gao et al. (1998) 0.930.93Weaver & Tarney (1984) Weaver & Tarney (1984) 0.920.92Rudnick & Gao (2003)Rudnick & Gao (2003) 0.890.89

McLennan & Taylor (1996)McLennan & Taylor (1996) 0.700.70Taylor & McLennan (1985)Taylor & McLennan (1985) 0.580.58

Total Cont.Total Cont. 0.79-0.990.79-0.99

Heat flow constraintsHeat flow constraints

Crustal AgeCrustal Age A*A* % Area% Area (µWm(µWm-3-3))

ArcheanArchean 0.56-0.73 0.56-0.73 9 9ProterozoicProterozoic 0.73-0.90 0.73-0.90 5656PhanerozoicPhanerozoic 0.95-1.21 0.95-1.21 3535

Total Cont.Total Cont. 0.79-0.99 0.79-0.99*heat production*heat production

Jaupart & Mareschal, 2003Jaupart & Mareschal, 2003

Bulk Crust K, Th & U Bulk Crust K, Th & U from heat flowfrom heat flow

KK22OO 1.3-2.1 wt.% 1.3-2.1 wt.%

Th Th 4.7-6.8 4.7-6.8 ppm ppm UU 1.05-1.55 ppm1.05-1.55 ppm

Assuming: Th/U = 3.8 to 5.0 Assuming: Th/U = 3.8 to 5.0 K/U = 10,000 to 13,000K/U = 10,000 to 13,000

KK22OO

ThTh

UU

K, Th, U in Upper crustK, Th, U in Upper crust

% Total Crust Budget% Total Crust Budget

min.min.

0 20 40 60 80 100 0 20 40 60 80 100

max.max.

Summary:Summary:Deep crust compositionDeep crust composition

• Uncertainties are greatUncertainties are great

• Increasingly more mafic with Increasingly more mafic with depthdepth

• Incompatible element depleted Incompatible element depleted relative to upper crustrelative to upper crust

Conclusions: crust compositionConclusions: crust composition

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

Deep crust is more poorly knownDeep crust is more poorly known

Incompatible elements are mainly Incompatible elements are mainly concentrated in the upper crust concentrated in the upper crust

Therefore uncertainties in bulk crust reflect Therefore uncertainties in bulk crust reflect upper crustal uncertaintiesupper crustal uncertainties

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

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

decay “wish list”

(g/g) % in crust rel. Earth

K 1.5 50

Rb 3 60

La 5.4 20

Lu 1.9 5

core

Re 0.23 99

Distribution of emitters