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
