Models of Crust Composition

1. Models of Crust Composition Roberta L. Rudnick Geochemistry Laboratory Department of Geology University of Maryland Apollo 17 view of Earth Appolo image 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/cm3 Low standing (-4000 m) Composition: Basalt (SiO2 ~50 wt.%)

4. From Press & Siever Generation of the Earth’s Crust Oceanic Intrusion and differentiation of mantle-derived basalt

5. Continental Crust: Ancient (on average 2 Ga, <4 Ga) ~40 km thick Low density: ~2.7 g/cm3 High standing (+800 m) Compositionally stratified Diverse rock types Composition: Andesite (SiO2 ~60 wt.%)

6. Generation of the Earth’s Crust Continental ? Convergent margin processes? Intraplate processes?

7. Upper Crust Lower Crust http://www.ub.es/ggac/research/piris

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

9. How is crust composition determined?

10. Models of Crust Composition Crustal growth scenarios (Taylor & McLennan, 1985) Empirical models (Christensen & Mooney, 1995; Wedepohl, 1995, Rudnick & Fountain, 1995; Rudnick & Gao, 2003)

11. Taylor & McLennan Recipe 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 *A special type of granite called tonalite, with relatively low K, Th and U

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

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

14. Upper continental crust is granitic (67 wt.% SiO2)

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

16. t log sw log K y 10.0 Soluble Na Moderately soluble 8.0 K Mg Insoluble Insoluble elements: Transferred from source of weathering to sediments B U Re Sr Au Li 6.0 Ca Se Rb Mo Sb W As Cs Cd Bi 4.0 Ag Si V Tl Ba Ge Ta Cr Hf Ni (residence time) Zn In Cu Ga Nb Zr Ti 2.0 Sn Y Be Pb Mn Sc Th Al Co REE La (REE) 0.0 Fe -2.0 -10.0 -8.0 -6.0 -4.0 -2.0 0.0 (sea water partition coefficient) After Taylor & McLennan, 1985

17. Th 14 12 10 8 6 K2O r2 = 0.82 4 2 4.0 10 15 20 25 30 35 40 3.0 2.0 r2 = 0.15 1.0 0.0 10 15 20 25 30 35 40 Loess: samples of averaged upper crust? 3.5 U Rudnick & Gao, 2003 Taylor & McLennan, 1985 Gao et al., 1998 3.0 2.5 2.0 1.5 r2 = 0.48 1.0 10 15 20 25 30 35 40 45 La (ppm) La (ppm)

18. Upper crustal estimates: Major elements Shaw et al. 1.4 Eade & Fahrig G & R Taylor & McLennan C 1.2 U o t d 1 e z i l a Wt. % K2O: 2.7 to 3.4% Rudnick & Gao: 2.8 wt.% m 0.8 r o N 0.6 1.4 G & R C 1.2 U o t d 1.0 e z i l a Borodin m 0.8 r Condie o Gao et al. N Ronov & Yaroshevsky 0.6 K Si Al Fe Mg Ca Na

19. Upper crustal estimates: U & Th Actinides & heavy metals 1.5 Th ppm: 8.6 to 10.8 (10.5) U ppm: 1.5 to 2.8 (2.7) Th/U = 3.9 1.0 0.5 Tl Pb Bi Th U Shaw Taylor & McLennan Gao et al. Eade & Fahrig Condie

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

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

22. 90 80 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 Granulite Facies Terranes Archean Post-Archean Mg# Lower crustal xenoliths Mg# SiO2 (wt. %)

23. Middle and Lower Crust -- Seismic evidence 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 From Rudnick & Fountain, 1995

24. m=21 8 . 5 Ultramafic rocks Upper Mantle 8 . 0 Eclogites 7 . 5 Mafic rocks Vp (m/s) Basalt 7 . 0 6 . 5 Granite Felsic rocks 6 . 0 m=22 3 . 0 2 . 6 2 . 8 3 . 2 3 . 4 3 . 6 Density (g/cm3)

25. Comparison of middle crustal models: Major elements 2.0 G 1.5 & R o t d 1.0 e z i l a m Weaver & Tarney r 0.5 o Shaw et al. N Gao et al. Rudnick & Fountain 0.0 K Si Al Fe Mg Ca Na Wt. % K2O: 2.1 to 3.4% Rudnick & Gao: 2.3 wt.%

26. Comparison of middle crustal models: Alkali, alkaline Earth & Actinides 2.0 2.6 1.5 1.0 0.5 Weaver & Tarney Shaw et al. Gao et al. Rudnick & Fountain Li Li Rb Rb Cs Cs Sr Sr Ba Ba Pb Pb Th U Th ppm: 6.1 to 8.4 (6.5) U ppm: 0.9 to 2.2 (1.3) Th/U = 5.0

27. Comparison of lower crustal models: Major elements 2.0 Terrains and models F & 1.5 R o t d e 1.0 z i l a m Weaver & Tarney r o Shaw et al. 0.5 N Gao et al. Wedepohl Taylor & McLennan 0.0 K Si Al Fe Mg Ca Na Wt. % K2O: 2.1 to 3.4% Rudnick & Gao: 2.3 wt.%

29. Composition of the Continental Crust Christensen Rudnick & Rudnick & Wedepohl Taylor & & Mooney Fountain 1995 McLennan Gao, 2003 1995 1995 1985, 1995 SiO 62.4 60.1 62.8 57.1 60.6 2 Al O 14.9 16.1 15.4 15.9 15.9 2 3 FeOT 6.9 6.7 5.7 9.1 6.7 MgO 3.1 4.5 3.8 5.3 4.7 CaO 5.8 6.5 5.6 7.4 6.4 Na O 3.6 3.3 3.3 3.1 3.1 2 K O 2.1 1.9 2.7 1.3* 1.8 2 Mg# 44.8 54.3 54.3 50.9 55.3 *Updated by McLennan and Taylor, 1996

30. Composition of the Continental Crust Rudnick & Clarke* Gao, 2003 1889 SiO 60.6 60.2 2 TiO 0.7 0.6 2 Al O 15.9 15.3 2 3 FeO 6.7 7.3 T MnO 0.10 0.10 MgO 4.7 4.6 CaO 6.4 5.5 Na O 3.1 3.3 2 K O 1.8 3.0 F.W. Clarke, 1847-1931 2 P O 0.13 0.23 2 5 Mg# 55.3 53.0 *Clarke, Frank Wigglesworth, for whom the Clarke medal is named