the composition of the continental...

The Composition of the Continental Crust

The The Composition of Composition of the Continental the Continental CrustCrust

Roberta L. RudnickGeochemistry LaboratoryDepartment of GeologyUniversity of Maryland

Apollo 17 view of Earth

Rationale: Why is studying crust composition

important?

Total

Crust

Mantle

*Calculated assuming seismological and geochemical reference models

From Chen, 2006, Earth, Moon & Planets

Fractional geo-neutrino flux at SNO+*

Most of the geo-neutrino signal in continental-based detectors originates in the continental crust

Oceanic Crust

How is Earth’s Crust Made?

?Convergent margin processes?

Intraplate processes?

Continental

Return flow viaSubduction (sediments, subduct. erosion)?Density foundering?

What do we “know” about the continental crust?

Continental Crust, physical:

Ancient (on average 2 Ga, ≤4 Ga) ~40 km thick (20-80+ km)Low density: ~2.7 g/cm3

High standing (+800 m)

Continental Crust, chemical:

Compositionally stratifiedDiverse rock types Composition: “Andesite”

(SiO2 ~60 wt.%)

Upper Crust

Lower Crust

http://www.ub.es/ggac/research/piris

Lower velocitiesLower density“granitic”

Higher velocitiesHigher density“basaltic”

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)

1. Empirical models (Christensen & Mooney, 1995; Wedepohl, 1995, Rudnick & Fountain, 1995; Rudnick & Gao, 2003, and others)

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: 66% basalt, 33% granite

Taylor & McLennan Recipe

*A special type of granite called tonalite, with relatively low K, Th and U

Upper crust: grid sampling, sedimentary rocks, γ-ray spectroscopy

Deep crust: determined from seismic velocities, geochemistry of high-grade metamorphic rocks, surface heat flow data

Empirical Models

Upper crust major elements: grid sampling

Space shuttle view of Thunder Bay, Ontario

Eade & Fahrig (1973): >14,000 grid samples in outcrop-weighted composites, analyzed for

major & a few trace elements

Upper crust major elements: Geological sampling

Gao et al. (1998): >11,000 samples from major geological units in eastern China, analyzed for

major and many trace elements

Upper continental crustis granitic (~67 wt.% SiO2)

Upper crustal estimates: Major elements

0.6

0.8

1

1.2

1.4 Shaw et al.Eade & Fahrig

Taylor & McLennan

Nor

mal

ized

toU

CR

&G

0.6

0.8

1.0

1.2

1.4

Si Al Fe Mg Ca Na K

BorodinCondieGao et al.Ronov & YaroshevskyN

orm

aliz

edto

UC

R&

G

Wt. % K2O:

2.7 to 3.4%

Rudnick & Gao: 2.8 wt.%

Upper crust trace elements (e.g., Th, U)

Inherently difficult to estimate due to•Orders of magnitude variation•Non-modal distributions

Global averages from fine-grained terrigenoussediments (e.g., shales, loess, glacial till)

Regional averages from γ-ray spectroscopy

Analyses of sedimentary rocks

Quantitative transport of insoluble elements from site of weathering to deposition.

Th: insolubleK, U: soluble

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)

Equivalent U (µg/g)Data courtesy of Canadian Geological Survey

Gamma-ray spectroscopy

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 = 3.8-7.2 (3.9) 0.5

1.0

1.5

Tl Pb Bi Th U

Actinides & heavy metals

ShawGao et al.Taylor & McLennan

Eade & FahrigCondie

Weathering?

U6+

Deep crustal estimates

Challenging due to•heterogeneity•orders of magnitude variation in concentrations•non-modal distributions•inaccessibility

Global averages from integrating seismic velocities, lithologies and geochemistry

Regional averages from surface heat flow data

Granulite FaciesTerrains

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

102030405060708090

30 40 50 60 70 80 90

102030405060708090

30 40 50 60 70 80 90

Mg#Mg#

Mg#Mg#

GranuliteGranulite FaciesFaciesTerranesTerranesArcheanArchean

PostPost--ArcheanArchean

Lower Lower crustalcrustalxenolithsxenoliths

SiOSiO22 (wt. %)(wt. %)

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/sm/s))

Density (g/cmDensity (g/cm33))

Upper Mantle

Basalt

Granite Metapelites(meta-shales)

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

Nor

mal

i zed

t oR

&G

Weaver & TarneyShaw 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%


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 & TarneyShaw 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

Nor

mal

i zed

t oR

&G

Nor

mal

i zed

toR

&F

0.0

0.5

1.0

1.5

2.0

Si Al Fe Mg Ca Na K

Weaver & TarneyShaw et al.Gao et al.WedepohlTaylor & McLennan

Terrains and models

Comparison of lower crustal models:Major elements

Wt. % K2O: 0.6 to 1.8%


4.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Li Rb Cs Sr Ba Pb Th UShaw et al.Gao et al.

Taylor & McLennanWedepohl

Weaver & Tarney

Median xenolith

Comparison of lower crustal models:Trace elements

Th ppm: 0.4 to 6.6 (1.2)U ppm: 0.05 to 0.9 (0.2)Th/U = 6.0

Nor

mal

i zed

t oR

&F

Surface Heat Flow Data

Perry et al., 2006; 2009

Local heat production of upper crust @ SNO+ is ~12 mWm-2 higher than Superior Province average*, doubling the flux of upper crustal geo-neutrinos

*local heat production ~ global continental crust

Surface Heat Flow Data & Xenolith Thermobarometry

U-bearing accessory minerals lose the daughter-product (Pb*) when they reside above their “closure temperature” (Tc)

Determining the amount of Pb* in such minerals from deep-seated xenoliths allows the temperature of the Moho to be determined

TitaniteTc ~550oC

ApatiteTc ~420oC


Example from Tanzanianlower crustal xenoliths

Apatite (Tc ~ 420oC) has no Pb*



Titanite (Tc < 580oC) has retained Pb* since >300 Ma



Present-day Moho temperature = 420 to 580oC

Crustal heat production≤ 0.5 µWm-3

cf. ~0.9 µWm-3 in average continental crust

Conclusions• Global models converge for K, but not Th

and U

• Largest uncertainties are for deep crust

• Geo-neutrino community needs regional models based on a variety of methods• γ-ray spectroscopy (surface)

• Seismic + geochemistry (whole crust)

• Heat-flow + thermochronology (whole crust)

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

Th 5.6 8.5 3.5 5.6U 1.4 1.7 0.9 1.3

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

6.0

6.5

7.0

7.5

8.0

8.5

6.0 6.5 7.0 7.5 8.0 8.5

Rud

nick

& F

ount

ain

Christensen & Mooney

Average Vp for lower crustal rock types

(0oC, 600 MPa) Eclogite

Mafic gt granulite

Felsic granulite

Felsic amphiboliteMetapelite - Amphbolite facies

Mafic granuliteAnorthosite

Amphibolite

the composition of the continental...

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