Models of Crust Composition Roberta L Rudnick Geochemistry
Models of Crust Composition Roberta L. Rudnick Geochemistry Laboratory Department of Geology University of Maryland Apollo 17 view of Earth
Plate tectonics gives rise to two types of crust: oceanic and continental
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 (Si. O 2 ~50 wt. %)
Generation of the Earth’s Crust Oceanic From Press & Siever Intrusion and differentiation of mantle-derived basalt
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 (Si. O 2 ~60 wt. %)
Generation of the Earth’s Crust Continental ? Convergent margin processes? Intraplate processes?
Upper Crust Lower Crust http: //www. ub. es/ggac/research/piris
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 1. Crustal growth scenarios (Taylor & Mc. Lennan, 1985) 2. Empirical models (Christensen & Mooney, 1995; Wedepohl, 1995, Rudnick & Fountain, 1995; Rudnick & Gao, 2003)
Taylor & Mc. Lennan Recipe 25% “Andesite model” 75% Archean crust: Mixture of Archean basalt & Archean granite* Assume 50% of 40 m. Wm-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. % Si. O 2)
Trace elements: analyses of sedimentary rocks Quantitative transport of insoluble elements from site of weathering to deposition.
10. 0 Soluble Moderately soluble 8. 0 K Insoluble U 6. 0 Sb log t 4. 0 Si Ta Hf (residence time) Zr Sn Be 2. 0 Th 0. 0 -2. 0 -10. 0 Sc Al -8. 0 Ag V Ba In Zn Cu Ga Nb Au Re Se Sr Li Ca Insoluble elements: Transferred from source of weathering to sediments Mo As Cd Bi Ni Ti Y Pb Mn Co REE Fe Ge Cr. Tl Cs Mg B Rb W Na La (REE) -6. 0 -4. 0 -2. 0 0. 0 log K sw y (sea water partition coefficient) After Taylor & Mc. Lennan, 1985
Loess: samples of averaged upper crust? 14 Th 12 10 8 6 4 2 r 2 = 0. 82 10 15 20 25 30 35 40 4. 0 3. 5 U K 2 O 3. 0 2. 5 2. 0 Rudnick & Gao, 2003 Taylor & Mc. Lennan, 1985 Gao et al. , 1998 1. 0 r 2 = 0. 15 r 2 = 0. 48 0. 0 10 15 20 25 30 La (ppm) 35 40 1. 5 1. 0 45
Upper crustal estimates: Major elements Normalized to UCR&G 1. 4 1. 2 Shaw et al. Eade & Fahrig Taylor & Mc. Lennan 1 0. 8 Wt. % K 2 O: 0. 6 2. 7 to 3. 4% 1. 4 Rudnick & Gao: 2. 8 wt. % 1. 2 1. 0 0. 8 0. 6 Borodin Condie Gao et al. Ronov & Yaroshevsky Si Al Fe Mg Ca Na K
Upper crustal estimates: U & Th Actinides & heavy metals Th ppm: 8. 6 to 10. 8 (10. 5) U ppm: 1. 5 to 2. 8 (2. 7) 1. 5 1. 0 0. 5 Th/U = 3. 9 Tl Pb Shaw Eade & Fahrig Condie Bi Th U Taylor & Mc. Lennan Gao et al.
Deep Crustal Samples 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 Mc. Donough, Queensland, Australia
90 80 70 Granulite Facies Terranes Archean Post-Archean 60 Mg# 50 40 30 20 10 30 40 50 60 70 80 90 90 80 70 Lower crustal xenoliths 60 Mg# 50 40 30 20 10 30 40 50 60 70 Si. O 2 (wt. %) 80 90
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
m=21 8. 5 Ultramafic rocks 8. 0 Vp (m/s) 7. 5 Eclogites Mafic rocks Basalt 7. 0 6. 5 Upper Mantle Granite 6. 0 m=22 2. 6 Felsic rocks 2. 8 3. 0 3. 2 Density (g/cm 3) 3. 4 3. 6
Comparison of middle crustal models: Major elements N or m al i zed to R & G 2. 0 1. 5 1. 0 0. 5 Weaver & Tarney Shaw et al. Gao et al. Rudnick & Fountain 0. 0 Si Al Fe Mg Ca Na Wt. % K 2 O: 2. 1 to 3. 4% Rudnick & Gao: 2. 3 wt. % K
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 Rb Cs Sr Ba Pb Th ppm: 6. 1 to 8. 4 (6. 5) U ppm: 0. 9 to 2. 2 (1. 3) Th/U = 5. 0 Th U
Comparison of lower crustal models: Major elements 2. 0 N orm al i zed to R& F Terrains and models 1. 5 1. 0 Weaver & Tarney 0. 5 Shaw et al. Gao et al. Wedepohl Taylor & Mc. Lennan 0. 0 Si Al Fe Mg Ca Na Wt. % K 2 O: 2. 1 to 3. 4% Rudnick & Gao: 2. 3 wt. % K
Composition of the Continental Crust Christensen Rudnick & Wedepohl Taylor & Rudnick & & Mooney Fountain 1995 Mc. Lennan Gao, 2003 1995 1985, 1995 Si. O 2 Al 2 O 3 Fe. OT Mg. O Ca. O Na 2 O K 2 O 62. 4 14. 9 6. 9 3. 1 5. 8 3. 6 2. 1 60. 1 16. 1 6. 7 4. 5 6. 5 3. 3 1. 9 62. 8 15. 4 5. 7 3. 8 5. 6 3. 3 2. 7 57. 1 15. 9 9. 1 5. 3 7. 4 3. 1 1. 3* 60. 6 15. 9 6. 7 4. 7 6. 4 3. 1 1. 8 Mg# 44. 8 54. 3 50. 9 55. 3 *Updated by Mc. Lennan and Taylor, 1996
Composition of the Continental Crust Rudnick & Gao, 2003 Clarke* 1889 Si. O 2 Ti. O 2 Al 2 O 3 Fe. O T Mn. O Mg. O Ca. O Na 2 O K 2 O P 2 O 5 60. 6 0. 7 15. 9 6. 7 0. 10 4. 7 6. 4 3. 1 1. 8 0. 13 60. 2 0. 6 15. 3 7. 3 0. 10 4. 6 5. 5 3. 3 3. 0 0. 23 Mg# 55. 3 53. 0 F. W. Clarke, 1847 -1931 *Clarke, Frank Wigglesworth, for whom the Clarke medal is named
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