1. Composition of the Continents Roberta Rudnick and Bill McDonough Geology, University of Maryland

2. Oceanic crust <<200 million years old

3. Continents up to 3500 million years old <0.6 06.-2.6 >2.6 ages (Ga)

4. Crustal model 5.1 – 5º x 5º grid Mooney, Lasker and Masters (1998)

5. Continental Heat Flow : example from Canadian Shield Perry et al (2006)

6. Continental Crust’s contribution... Mass % Earth’s K, Th & U 0.57% >40%... is insignificant in terms of mass, but is a major reservoir for incompatible elements

7. How is crust composition determined? What is its significance?

8. Story is in the Upper crust Its composition is constrained from surface sampling (e.g., Canadian Shield) Major elementsMajor elements Soluble elementsSoluble elements Eade & Fahrig, 1971, 1973; Shaw et al. 1967, 1976, 1986; Gao et al., 1998

9. log K sw -10.0-8.0-6.0-4.0-2.00.0 log  -2.0 0.0 2.0 4.0 6.0 8.0 10.0 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 y From Taylor & McLennan, 1985 Insoluble elements from clastic sediments increasingsolubility

10. Shale composites and Loess LaCePrNdSmEuGdTbDyHoErTmYbLu 1 10 100 1000 Australia N. America Europe Eastern China loess ChondriteNormalized

11. 2 4 6 8 10 12 14 10152025303540 Th r 2 = 0.82 La (ppm) Loess -- insoluble elements Taylor & McLennan Gao et al. Rudnick & Gao (ppm)

12. r 2 = 0.15 K2OK2O 0.0 1.0 2.0 3.0 4.0 10152025303540 La (ppm) Loess -- soluble element (K) Taylor & McLennan Gao et al. Rudnick & Gao

13. 2.0 4.0 6.0 8.0 10.0 12.0 14.0 0.501.01.52.02.53.03.54.0 U(ppm) Th (ppm) 546 3 Th/U = Taylor & McLennan Gao et al. Rudnick & Gao Loess -- soluble element (U)

14. Deep crust composition from: 1)Analyses of deep crustal rocks: Crustal cross sections Crustal cross sections Metamorphic terrains Metamorphic terrains Xenoliths Xenoliths 2)Seismic velocities 3)Surface heat flow

15. Granulite Facies Terrains Granulite Facies Xenoliths

16. The beauty of xenoliths Direct sampling of deep lithosphere: Direct sampling of deep lithosphere:  composition  age  temperature  thickness  deformation  fluids “The poor man’s drill hole”

17. 10 20 30 40 50 60 70 80 90 30405060708090 10 20 30 40 50 60 70 80 90 30405060708090Mg# Mg# Lower crustal xenoliths SiO 2 (wt. %) Granulite Facies Terrains Archean Archean Post-Archean Post-Archean

18. Rifted Margin ContractionalShield & Platform Paleozoic Orogen Rift Extensional Arc Forearc 0 20 40 60 Km VpVp 6.4 6.6 6.8 7.0 7.2 Rudnick & Fountain, 1995 Seismic Constraints

19. 1 10 100 1000 RbThKLaPbSrZrSmTiHo CsBaUNbCePrNdHfEuYYb Weaver & Tarney Rudnick & Fountain Wedepohl Gao et al. Taylor & McLennan Rudnick & Gao mantle normalized Models of the Bulk Continental Crust heat producing elements

21. Heat Flow Data … Q s T moho SNO+ Perry et al (2006) JGR Crustal heat production … Canadian Shield

22. Lithosphere (strong layer) and Asthenoshpere (weak layer) Lithosphere: crust + mechanically couple mantle Lithosphere: sits on the asthenoshpere “Moho” Heat production

23. 100 200 300 150 Depth (km) Temperature ( o C) 500100015002000 50 250 350 1 2 3 4 Mantle adiabat Moho 5 Surface heat flow = 40 mW/m 2 all crust no HPE in lithopshere Where are the HPE? lithospheric thickness How thick is the lithospheric lid?

24. KalihariSlave Pressure (GPa) Jericho Lac de Gras Torrie Grizzly Depth (km) Best Fit (44 mWm -2 ) Kalihari geotherm 50 100 150 200 250 300 0 2 4 6 8 10 20060010001400 20060010001400 Temperature ( o C) Lesotho Kimberley Letlhakane Archean lithosphere is thick & cold From Rudnick & Nyblade, 1999 AfricaCanada

25. Heat flow constraints Crustal Model A (µWm -3 )  Shaw et al. (1986)1.31  Wedepohl (1995) 1.25  Rudnick & Fountain (1995) 0.93  Gao et al. (1998) 0.93  Weaver & Tarney (1984) 0.92  Rudnick & Gao (2003)0.89  McLennan & Taylor (1996) 0.70  Taylor & McLennan (1985) 0.58 Total Cont. 0.79-0.99

26. Heat flow constraints Crustal AgeA*% Area (µWm -3 ) (µWm -3 ) Archean 0.56-0.73 9 Proterozoic 0.73-0.9056 Phanerozoic 0.95-1.2135 Total Cont. 0.79-0.99 *heat production Jaupart & Mareschal, 2003

27. Bulk Crust K, Th & U from heat flow K 2 O 1.3-2.1 wt.% Th 4.7-6.8ppm U1.05-1.55 ppm Assuming: Th/U = 3.8 to 5.0 K/U = 10,000 to 13,000 K/U = 10,000 to 13,000

28. K2OK2OK2OK2O Th U K, Th, U in Upper crust % Total Crust Budget min. 0 20 40 60 80 100 max.

29. Summary: Deep crust composition Uncertainties are great Uncertainties are great Increasingly more mafic with depth Increasingly more mafic with depth Incompatible element depleted relative to upper crust Incompatible element depleted relative to upper crust

30. Conclusions: crust composition Composition of the upper crust is known to ±20% for many elements Deep crust is more poorly known Incompatible elements are mainly concentrated in the upper crust Therefore uncertainties in bulk crust reflect upper crustal uncertainties Heat flow constrains bulk crust K, Th and U to ± 50%

31. Isotope Emax (MeV)natural abundancehalf life 40 K1.310.012% 1.3E9 y 1.51 electron capture- monoenergetic nue- BR 11% 87 Rb0.2828%4.9E10 y 138 La1.04 <0.1% 1.05E11 y 1.74 electron capture- monoenergetic nue- branching ratio? 176 Lu1.192.6% 3.8E10 y 187 Re0.003 63% 4.4E10 y   decay “wish list”

32. (  g/g) % in crust rel. Earth K1.550 Rb360 La5.420 Lu1.95 core Re0.2399 Distribution of   emitters