The Composition of the Continental Crust The Composition of the ...

24 downloads 1062 Views 2MB Size Report
Fractional geo-neutrino flux at SNO+*. Most of the geo-neutrino signal in continental- based detectors originates in the continental crust. Oceanic Crust ...
The Composition of the Continental Crust

Roberta L. Rudnick Geochemistry Laboratory Department of Geology University of Maryland Apollo 17 view of Earth

Rationale: Why is studying crust composition important?

Fractional geo-neutrino flux at SNO+*

Most of the geo-neutrino signal in continentalbased detectors originates in the continental crust

Total

Crust

Mantle *Calculated assuming seismological and geochemical reference models From Chen, 2006, Earth, Moon & Planets

Oceanic Crust

How is Earth’s Crust Made? Continental

? Convergent processes? Return flow margin via Intraplate processes? Subduction (sediments, subduct. erosion)? Density foundering?

What do we “know” about the continental crust?

Continental Crust, physical: Ancient (on average 2 Ga, ≤4 Ga) ~40 km thick (20-80+ km) Low density: ~2.7 g/cm3 High standing (+800 m)

Continental Crust, chemical: Compositionally stratified Diverse rock types Composition: “Andesite” (SiO2 ~60 wt.%)

Upper Crust

Lower velocities Lower density “granitic”

Lower Crust

Higher velocities Higher density “basaltic”

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 & McLennan, 1985)

1. Empirical models (Christensen & Mooney, 1995; Wedepohl, 1995, Rudnick & Fountain, 1995; Rudnick & Gao, 2003, and others)

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

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

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

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

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

Upper crustal estimates: Major elements

Normalized to UCR&G

Normalized to UCR&G

1.4 1.2

Shaw et al. Eade & Fahrig Taylor & McLennan

1 0.8

Wt. % K2O:

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 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

Analyses of sedimentary rocks

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

Loess: samples of averaged upper crust? 14

Th

12 10 8 6

r2 = 0.82

4 2 10

15

20

25

30

35

40

4.0

3.5

U

K2O

3.0

3.0

2.5 2.0 2.0

Rudnick & Gao, 2003 Taylor & McLennan, 1985 Gao et al., 1998

1.0

r2 = 0.15

r2 = 0.48

0.0 10

15

20

25

30

La (ppm)

35

40

10

15

20

25

30

La (ppm)

35

40

1.5 1.0 45

Gamma-ray spectroscopy

Equivalent U (µg/g) Data courtesy of Canadian Geological Survey

Upper crustal estimates: U & Th Actinides & heavy metals

1.5

Weathering? 1.0

Th ppm: 8.6 to 10.8 (10.5) U ppm: 1.5 to 2.8 (2.7)

U6+ Th/U = 3.8-7.2 (3.9)

0.5

Tl

Pb

Shaw Eade & Fahrig Condie

Bi

Th

U

Taylor & McLennan Gao et al.

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

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 McDonough, 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 60

Mg#

Lower crustal xenoliths

50 40 30 20 10

30

40

50

60

70

SiO2 (wt. %)

80

90

m=21

8.5

Ultramafic rocks

8.0

Vp (m/s)

7.5

Eclogites

Mafic rocks

Basalt

7.0 6.5

Upper Mantle

Metapelites (meta-shales)

Granite Felsic rocks

6.0

m=22

2.6

2.8

3.0

Density

3.2

(g/cm3)

3.4

3.6

Middle and Lower Crust -- Seismic evidence Paleozoic Rifted Margin Rift Orogen 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

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. % K2O: 2.1 to 3.4% Rudnick & Gao: 2.3 wt.%

K

Comparison of middle crustal models: Alkali, alkaline Earth & Actinides N or m al i zed to R & G

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 & McLennan

0.0 Si

Al

Fe

Mg

Ca

Na

Wt. % K2O: 0.6 to 1.8% Rudnick & Gao: 0.6 wt.%

K

Comparison of lower crustal models: Trace elements 4.0 N or m al i zed to R & F

3.5 3.0 2.5 2.0 1.5 1.0 0.5

Weaver & Tarney Shaw et al. Gao et al. Wedepohl Taylor & McLennan Median xenolith

Li

Rb

Cs

Sr

Th ppm: 0.4 to 6.6 (1.2) U ppm: 0.05 to 0.9 (0.2) Th/U = 6.0

Ba

Pb

Th

U

Surface Heat Flow Data

Local heat production of upper crust @ 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

Surface Heat Flow Data & Xenolith Thermobarometry U-bearing accessory minerals lose the daughterproduct (Pb*) when they reside above their “closure temperature” (Tc)

Titanite Tc ~550oC

Apatite Tc ~420oC

Determining the amount of Pb* in such minerals from deep-seated xenoliths allows the temperature of the Moho to be determined

Surface Heat Flow Data & Xenolith Thermobarometry Example from Tanzanian lower crustal xenoliths Apatite (Tc ~ 420oC) has no Pb*

Surface Heat Flow Data & Xenolith Thermobarometry Example from Tanzanian lower crustal xenoliths Titanite (Tc < 580oC) has retained Pb* since >300 Ma

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

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)

Composition of the Continental Crust Christensen Rudnick & Wedepohl Taylor & Rudnick & & Mooney Fountain 1995 McLennan Gao, 2003 1995 1995 1985, 1995 SiO2 Al2O3 FeOT MgO CaO Na2O K2O

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

54.3

50.9

55.3

5.6 1.4

8.5 1.7

3.5 0.9

5.6 1.3

Th U

*Updated by McLennan and Taylor, 1996

Composition of the Continental Crust Rudnick & Gao, 2003

Clarke* 1889

SiO 2 TiO 2 Al 2O 3 FeO T MnO MgO CaO Na 2O K 2O P 2O 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

8.5

Rudnick & Fountain

8.0

Average Vp for lower crustal rock types (0oC, 600 MPa)

Eclogite

7.5 Mafic granulite Anorthosite

7.0

6.5

Mafic gt granulite Amphibolite

Felsic granulite Metapelite - Amphbolite facies Felsic amphibolite

6.0 6.0

6.5

7.0 7.5 Christensen & Mooney

8.0

8.5