The Composition of the Continental Crust

Transcription

1 The Composition of the Continental Crust Roberta L. Rudnick Geochemistry Laboratory Department of Geology University of Maryland Apollo 17 view of Earth

2 Rationale: Why is studying crust composition important?

3 Most of the geo-neutrino signal in continentalbased detectors originates in the continental crust Fractional geo-neutrino flux at SNO+* Total Crust Mantle *Calculated assuming seismological and geochemical reference models Oceanic Crust From Chen, 2006, Earth, Moon & Planets

4 How is Earth s Crust Made? Continental? Convergent margin processes? Return flow via Subduction Intraplate (sediments, processes? subduct. erosion)? Density foundering?

5 What do we know about the continental crust?

6 Continental Crust, physical: Ancient (on average 2 Ga, 4 Ga) ~40 km thick ( km) Low density: ~2.7 g/cm 3 High standing (+800 m)

7 Continental Crust, chemical: Compositionally stratified Diverse rock types Composition: Andesite (SiO 2 ~60 wt.%)

8 Upper Crust Lower Crust Lower velocities Lower density granitic Higher velocities Higher density basaltic

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

10 How is crust composition determined?

11 Models of Crust Composition 1. Crustal growth scenarios (Taylor & McLennan, 1985) 1. Empirical models (Christensen & Mooney, 1995; Wedepohl, 1995, Rudnick & Fountain, 1995; Rudnick & Gao, 2003, and others)

12 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: 66% basalt, 33% granite *A special type of granite called tonalite, with relatively low K, Th and U

13 Empirical Models Upper crust: grid sampling, sedimentary rocks, γ-ray spectroscopy Deep crust: determined from seismic velocities, geochemistry of high-grade metamorphic rocks, surface heat flow data

14 Upper crust major elements: grid sampling Eade & Fahrig (1973): >14,000 grid samples in outcrop-weighted composites, analyzed for major & a few trace elements Space shuttle view of Thunder Bay, Ontario

15 Upper crust major elements: Geological sampling Gao et al. (1998): >11,000 samples from major geological units in eastern China, analyzed for major and many trace elements

16 Upper continental crust is granitic (~67 wt.% SiO 2 )

17 Upper crustal estimates: Major elements Normalized to UC R&G Normalized to UC R&G 1.4 Shaw et al. Eade & Fahrig Taylor & McLennan Borodin Condie Gao et al. Ronov & Yaroshevsky Wt. % K 2 O: 2.7 to 3.4% Rudnick & Gao: 2.8 wt.% Si Al Fe Mg Ca Na K

18 Upper crust trace elements (e.g., Th, U) Inherently difficult to estimate due to Orders of magnitude variation Non-modal distributions Global averages from fine-grained terrigenous sediments (e.g., shales, loess, glacial till) Regional averages from γ-ray spectroscopy

19 Analyses of sedimentary rocks Quantitative transport of insoluble elements from site of weathering to deposition. Th: insoluble K, U: soluble

20 Loess: samples of averaged upper crust? Th r 2 = K 2 O 3.0 U Rudnick & Gao, r 2 = 0.15 r 2 = Taylor & McLennan, 1985 Gao et al., La (ppm) La (ppm)

21 Gamma-ray spectroscopy Equivalent U (µg/g) Data courtesy of Canadian Geological Survey

22 Upper crustal estimates: U & Th Actinides & heavy metals Weathering? U 6+ Th ppm: 8.6 to 10.8 (10.5) U ppm: 1.5 to 2.8 (2.7) Th/U = (3.9) Tl Pb Bi Th U Shaw Eade & Fahrig Condie Taylor & McLennan Gao et al.

23 Deep crustal estimates Challenging due to heterogeneity orders of magnitude variation in concentrations non-modal distributions inaccessibility Global averages from integrating seismic velocities, lithologies and geochemistry Regional averages from surface heat flow data

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

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

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

27 Ultramafic rocks Upper Mantle m=21 V p (m/s) Mafic rocks Basalt Eclogites Granite m=22 Felsic rocks Metapelites (meta-shales) Density (g/cm 3 )

28 Middle and Lower Crust -- Seismic evidence 0 Rifted Margin Rift Paleozoic Orogen Arc Contractional Shield & Platform Extensional Forearc Km V p From Rudnick & Fountain, 1995

29 Comparison of middle crustal models: Major elements 2.0 N orm al i zed to R & G Weaver & Tarney Shaw et al. Gao et al. Rudnick & Fountain Si Al Fe Mg Ca Na K Wt. % K 2 O: 2.1 to 3.4% Rudnick & Gao: 2.3 wt.%

30 Comparison of middle crustal models: Alkali, alkaline Earth & Actinides 2.0 N orm al i zed to R & G Weaver & Tarney Shaw et al. Gao et al. Rudnick & Fountain 2.6 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

31 Comparison of lower crustal models: Major elements 2.0 Terrains and models N ormalized to R& F Weaver & Tarney Shaw et al. Gao et al. Wedepohl Taylor & McLennan Si Al Fe Mg Ca Na K Wt. % K 2 O: 0.6 to 1.8% Rudnick & Gao: 0.6 wt.%

32 Comparison of lower crustal models: Trace elements N orm al i zed to R & F Weaver & Tarney Shaw et al. Gao et al. Wedepohl Taylor & McLennan Median xenolith Li Rb Cs Sr Ba Pb Th U Th ppm: 0.4 to 6.6 (1.2) U ppm: 0.05 to 0.9 (0.2) Th/U = 6.0

33 Surface Heat Flow Data Local heat production of upper SNO+ is ~12 mwm -2 higher than Superior Province average*, doubling the flux of upper crustal geo-neutrinos *local heat production ~ global continental crust Perry et al., 2006; 2009

34 Surface Heat Flow Data & Xenolith Thermobarometry U-bearing accessory minerals lose the daughterproduct (Pb*) when they reside above their closure temperature (T c ) Titanite T c ~550 o C Apatite T c ~420 o C Determining the amount of Pb* in such minerals from deep-seated xenoliths allows the temperature of the Moho to be determined

35 Surface Heat Flow Data & Xenolith Thermobarometry Example from Tanzanian lower crustal xenoliths Apatite (T c ~ 420 o C) has no Pb*

36 Surface Heat Flow Data & Xenolith Thermobarometry Example from Tanzanian lower crustal xenoliths Titanite (T c < 580 o C) has retained Pb* since >300 Ma

37 Surface Heat Flow Data & Xenolith Thermobarometry Example from Tanzanian lower crustal xenoliths Present-day Moho temperature = 420 to 580 o C Crustal heat production 0.5 µwm -3 cf. ~0.9 µwm -3 in average continental crust

38 Conclusions Global models converge for K, but not Th and U Largest uncertainties are for deep crust Geo-neutrino community needs regional models based on a variety of methods γ-ray spectroscopy (surface) Seismic + geochemistry (whole crust) Heat-flow + thermochronology (whole crust)

39 Composition of the Continental Crust Christensen Rudnick & Wedepohl Taylor & Rudnick & & Mooney Fountain 1995 McLennan Gao, , 1995 SiO Al 2 O FeO T MgO CaO Na 2 O K 2 O * 1.8 Mg# Th U *Updated by McLennan and Taylor, 1996

40 Composition of the Continental Crust Rudnick & Clarke* Gao, SiO TiO Al 2 O FeO T MnO MgO CaO Na 2 O K 2 O P 2 O F.W. Clarke, Mg# *Clarke, Frank Wigglesworth, for whom the Clarke medal is named

41 Rudnick & Fountain Average Vp for lower crustal rock types (0 o C, 600 MPa) Eclogite Mafic granulite Anorthosite Amphibolite Felsic granulite Metapelite - Amphbolite facies Felsic amphibolite Mafic gt granulite Christensen & Mooney