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

Plate tectonics gives rise to two types of crust: oceanic and continental Start by review of plate tectonics.

Young (on average 80 Ma, <200 Ma) ~7 km thick 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.%) Properties of oceanic crust -- young, thin, dense basalt.

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

Ancient (on average 2 Ga, <4 Ga) ~40 km thick 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.%) Properties of continental crust -- old, thick, buoyant andesite.

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

Upper Crust Lower Crust http://www.ub.es/ggac/research/piris Crust is vertically stratified. Upper and lower crust (sometimes “middle” crust) defined on the basis of physical properties (i.e. seismic wave speeds).

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

How is crust composition determined?

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

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

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

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

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

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

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

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

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

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 Eade & Fahrig Gao et al. Condie

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

The great xenolith hunt Shukrani Manya, Univ. Dar es Salaam, Tanzania Profs. Gao and Wu, Shanxi, China Bill McDonough, Queensland, Australia Xenoliths -- carried by volcanoes from great depths and tossed out onto the Earth’s surface.

Granulite Facies Terranes Archean Post-Archean Lower crustal xenoliths 10 20 30 40 50 60 70 80 90 Granulite Facies Terranes Archean Post-Archean Mg# 30 40 50 60 70 80 90 10 20 30 40 50 60 70 80 90 Lower crustal xenoliths Mg# 30 40 50 60 70 80 90 SiO2 (wt. %)

Middle and Lower Crust -- Seismic evidence Paleozoic Orogen Rifted Margin Rift Arc Contractional Shield & Platform Extensional Forearc 20 40 Vp 60 Km 6.4 6.6 6.8 7.0 7.2 From Rudnick & Fountain, 1995

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

Comparison of middle crustal models: Major elements 2.0 G 1.5 & R o t d 1.0 e z i l a m r Weaver & Tarney 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.%

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

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

Composition of the Continental Crust Christensen Rudnick & Wedepohl Taylor & Rudnick & & 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

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 2 F.W. Clarke, 1847-1931 P O 0.13 0.23 2 5 Mg# 55.3 53.0 *Clarke, Frank Wigglesworth, for whom the Clarke medal is named