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Models of Crust Composition Roberta L. Rudnick Geochemistry Laboratory Department of Geology University of Maryland Appolo image of Earth Apollo 17 view.

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Presentation on theme: "Models of Crust Composition Roberta L. Rudnick Geochemistry Laboratory Department of Geology University of Maryland Appolo image of Earth Apollo 17 view."— Presentation transcript:

1 Models of Crust Composition Roberta L. Rudnick Geochemistry Laboratory Department of Geology University of Maryland Appolo image of Earth Apollo 17 view of Earth

2 Plate tectonics gives rise to two types of crust: oceanic and continental

3 Oceanic Crust: Young (on average 80 Ma, <200 Ma) ~7 km thick High density: ~3.0 g/cm 3 Low standing (-4000 m) Composition: Basalt (SiO 2 ~50 wt.%)

4 Oceanic Generation of the Earths Crust Intrusion and differentiation of mantle-derived basalt From Press & Siever

5 Continental Crust: Ancient (on average 2 Ga, <4 Ga) ~40 km thick Low density: ~2.7 g/cm 3 High standing (+800 m) Compositionally stratified Diverse rock types Composition: Andesite (SiO 2 ~60 wt.%)

6 Generation of the Earths Crust ? Convergent margin processes? Intraplate processes? Continental

7 Upper Crust Lower Crust

8 Shuttle view of granite intruding metamorphic basement, northern Chile. Continental crust: Lots of heterogeneity! Every rock type known on Earth occurs in continental crust

9 How is crust composition determined?

10 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

11 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

12 Upper crust: grid sampling & sedimentary rocks Deep crust: determined from seismic velocities, heat flow Empirical Models

13 Upper crust major elements: Grid sampling Space shuttle view of Thunder Bay, Ontario

14 Upper continental crust is granitic (67 wt.% SiO 2 )

15 Trace elements: analyses of sedimentary rocks Quantitative transport of insoluble elements from site of weathering to deposition.

16 log Soluble Moderately soluble Insoluble Cu Au Mo Ca Li Re Sr K Mg B Na W Sb Se Rb U Cs Bi Cd As Si V Ag Ni Ba Tl Fe Mn Hf Ta Ga In Ge Zn Cr Th Al Sc Co Ti Y Sn Zr Nb Pb Be REE log K sw y Insoluble elements: Transferred from source of weathering to sediments After Taylor & McLennan, 1985 (residence time) (sea water partition coefficient) La (REE)

17 r 2 = 0.15 K2OK2O La (ppm) Th r 2 = U r 2 = 0.48 Rudnick & Gao, 2003 Taylor & McLennan, 1985 Gao et al., 1998 Loess: samples of averaged upper crust? La (ppm)

18 Upper crustal estimates: Major elements Shaw et al. Eade & Fahrig Taylor & McLennan N o r m a l i z e d t o U C R & G SiAlFeMgCaNa K Borodin Condie Gao et al. Ronov & Yaroshevsky N o r m a l i z e d t o U C R & G Wt. % K 2 O: 2.7 to 3.4% Rudnick & Gao: 2.8 wt.%

19 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 = TlPbBi ThU Actinides & heavy metals Shaw Gao et al. Taylor & McLennan Eade & Fahrig Condie

20 Granulite Facies Terrains Granulite Facies Xenoliths Deep Crustal Samples Ross Taylor, KSZ, Ontario, 1983

21 Shukrani Manya, Univ. Dar es Salaam, Tanzania The great xenolith hunt Profs. Gao and Wu, Shanxi, China Bill McDonough, Queensland, Australia

22 Mg# Mg# Granulite Facies Terranes Archean Archean Post-Archean Post-Archean Lower crustal xenoliths SiO 2 (wt. %)

23 Rifted Margin ContractionalShield & Platform Paleozoic Orogen Rift Extensional Arc Forearc Km VpVp Middle and Lower Crust -- Seismic evidence From Rudnick & Fountain, 1995

24 Mafic rocks Felsic rocks Eclogites m=21 m=22 Ultramafic rocks V p (m/s) Density (g/cm 3 ) Upper Mantle Basalt Granite

25 N o r m a l i z e d t o R & G Weaver & Tarney Shaw et al. Gao et al. Rudnick & Fountain SiAlFeMgCaNa K Comparison of middle crustal models: Major elements Wt. % K 2 O: 2.1 to 3.4% Rudnick & Gao: 2.3 wt.%

26 ThU LiRbCsSrBaPb LiRbCsSrBaPb 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

27 N o r m a l i z e d t o R & F SiAlFeMgCaNa K Weaver & Tarney Shaw et al. Gao et al. Wedepohl Taylor & McLennan Terrains and models Comparison of lower crustal models: Major elements Wt. % K 2 O: 2.1 to 3.4% Rudnick & Gao: 2.3 wt.%

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29 Composition of the Continental Crust ChristensenRudnick & WedepohlTaylor & Rudnick & & MooneyFountain1995McLennanGao, , 1995 SiO Al 2 O FeO T MgO CaO Na 2 O K 2 O *1.8 Mg# *Updated by McLennan and Taylor, 1996

30 Rudnick &Clarke* Gao, SiO TiO Al 2 O FeO T MnO0.10 MgO CaO Na 2 O K 2 O P 2 O Mg# *Clarke, Frank Wigglesworth, for whom the Clarke medal is named F.W. Clarke, Composition of the Continental Crust

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