Download The Composition of the Continental 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