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

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Presentation transcript:

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

Oceanic crust <<200 million years old

Continents up to 3500 million years old < >2.6 ages (Ga)

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

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

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

How is crust composition determined? What is its significance?

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

log K sw 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 y From Taylor & McLennan, 1985 Insoluble elements from clastic sediments increasingsolubility

Shale composites and Loess LaCePrNdSmEuGdTbDyHoErTmYbLu Australia N. America Europe Eastern China loess ChondriteNormalized

Th r 2 = 0.82 La (ppm) Loess -- insoluble elements Taylor & McLennan Gao et al. Rudnick & Gao (ppm)

r 2 = 0.15 K2OK2O La (ppm) Loess -- soluble element (K) Taylor & McLennan Gao et al. Rudnick & Gao

U(ppm) Th (ppm) Th/U = Taylor & McLennan Gao et al. Rudnick & Gao Loess -- soluble element (U)

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

Granulite Facies Terrains Granulite Facies Xenoliths

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”

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

Rifted Margin ContractionalShield & Platform Paleozoic Orogen Rift Extensional Arc Forearc Km VpVp Rudnick & Fountain, 1995 Seismic Constraints

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

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

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

Depth (km) Temperature ( o C) 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?

KalihariSlave Pressure (GPa) Jericho Lac de Gras Torrie Grizzly Depth (km) Best Fit (44 mWm -2 ) Kalihari geotherm Temperature ( o C) Lesotho Kimberley Letlhakane Archean lithosphere is thick & cold From Rudnick & Nyblade, 1999 AfricaCanada

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

Heat flow constraints Crustal AgeA*% Area (µWm -3 ) (µWm -3 ) Archean Proterozoic Phanerozoic Total Cont *heat production Jaupart & Mareschal, 2003

Bulk Crust K, Th & U from heat flow K 2 O wt.% Th ppm U ppm Assuming: Th/U = 3.8 to 5.0 K/U = 10,000 to 13,000 K/U = 10,000 to 13,000

K2OK2OK2OK2O Th U K, Th, U in Upper crust % Total Crust Budget min max.

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

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%

Isotope Emax (MeV)natural abundancehalf life 40 K % 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 Lu % 3.8E10 y 187 Re % 4.4E10 y   decay “wish list”

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