earth’s unique continentscomposition of continental crust? upper crust ~10 to 15 km thick major...
TRANSCRIPT
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Earth’s unique Continents
Apollo 17 view of Earth
Roberta L. Rudnick Department of Geology University of Maryland
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Plate tectonics gives rise to two types of crust: oceanic and continental
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Generation of the Oceanic Crust
• Melt ascends from mantle and freezes as basaltic crust
• Young (<200 million years old)
From Press & Siever
http://www.geol.ucsb.edu/faculty/hacker/geo102C/lectures/sheetedDikes.jpg
pillow basalts
sheeted dikes
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• Compositionally diverse • Ancient (up to 4 billion years old)
Generation of the Continental Crust
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? Subduction zone genesis?
(e.g., Mt. St. Helens) Intraplate genesis?
(e.g., Yellowstone)
Growth of Continental Crust via magmatic addition
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Questions: • How does
continental crust form?
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• Earth is the only planet with continents. Why?
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What is continental crust?
Vertical: surface to Moho Horizontal: slope break on continental shelves ~34± 4 km thick* (15 to 80 km) Low density: ~2.8 g/cm3
High standing (average elevation 800 m)
*Huang et al., 2013
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How do we determine the composition of continental
crust?
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Upper crust ~10 to 15 km thick major (abundant) elements: grid sampling
Space shuttle view of Thunder Bay, Ontario
Eade & Fahrig (1973): >14,000 grid samples in outcrop-weighted composites, analyzed for
major & a few trace elements
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Upper continental crust is “granitic” (67 wt.% SiO2)
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Sedimentary rocks: natural average
• Rocks weather • Soluble elements washed into oceans (salty) • Insoluble elements deposited in shales/loess
Mancos Shale, Utah, photo USGS
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Upper Crust
Lower Crust
http://www.ub.es/ggac/research/piris
Seismic cross section through Central Pyrenees
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Lower Crust Rocks
• Metamorphic (“transformed”) • Minerals record high pressure (0.5 to
1.2 GPa ~ 20 to 40 km depth) and high temperature (500-1000oC): granulites – windows into the lower crust
http://www.geol.ucsb.edu/faculty/hacker/geo102C/lectures/part2.html
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Granulite Terrains
Rocks formed at 20-40 km depth,
tectonically uplifted during continent-continent collision
Granulite Xenoliths “foreign” rock fragments
carried by magmas “poor man’s drill core”
Two sources of granulites
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Animations courtesy T. Atwater, UCSB http://emvc.geol.ucsb.edu/
Continents Colliding
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Formation of granulite terranes during mountain building
Continent 2
crust mantle
granulites
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Mauricio Mazzucchelli with granulite facies metapelites, Ivrea Zone, Italy
Granulites brought to surface through uplift, erosion and tectonic transport (along faults)
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Prof. Shukrani Manya, Univ. Dar es Salaam, Tanzania
The great xenolith hunt
Profs. Gao and Wu, Shanxi, China
Bill McDonough, Queensland, Australia
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n = 489
n = 495
n = 1098
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Rifted Margin Contractional Shield & Platform
Paleozoic Orogen Rift
Extensional Arc
Forearc 0
20
40
60
Km Vp
6.4 6.6 6.8 7.0 7.2
Middle and Lower Crust -- Seismic evidence
From Rudnick & Fountain, 1995
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Remote sensing of lower crust from seismic waves
Huang et al., 2013
Room T ultrasonic velocities
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Composition of the Continental Crust Wedepohl
1995 Christensen & Mooney
1995
Rudnick & Fountain
1995
Taylor & McLennan 1985, 1995
SiO 2 TiO 2 Al 2 O 3 FeOT MnO MgO CaO Na 2 O K 2 O P 2 O 5
Mg#
62.4 0.9
14.9 6.9 0.1 3.1 5.8 3.6 2.1
0.20
44.8
60.1 0.7
16.1 6.7 0.1 4.5 6.5 3.3 1.9
0.20
54.3
62.8 0.7
15.4 5.7 0.1 3.8 5.6 3.3 2.7
54.3
57.1 0.9
15.9 9.1 0.2 5.3 7.4 3.1 1.3*
50.9
60.6 0.7
15.9 6.7 0.1 4.7 6.4 3.1 1.8
0.13
55.3
*Updated by McLennan and Taylor, 1996
65.2 0.7
15.0 5.8 0.1 2.5 3.4 3.0 1.9
0.10
44.0
Hacker et al., 2011
Rudnick & Gao, 2003
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Rudnick & Clarke Gao, 2003 1889*
SiO 2 60.6 60.2 TiO 2 0.7 0.57 Al 2 O 3 15.9 15.27 FeO T 6.7 7.26 MnO 0.10 0.10 MgO 4.7 4.59 CaO 6.4 5.45 Na 2 O 3.1 3.29 K 2 O 1.8 2.99 P 2 O 5 0.13 0.23
Mg# 55.3 53.0
*Frank Wigglesworth Clarke, Phil. Soc. Washington Bull. Vol. XI pp. 131-142
Composition of the Continental Crust
F.W. Clarke, 1847-1931
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Crust is andesitic, Crust grows by addition of
“andesitic” island arcs
Andesite Model S.R. Taylor, 1967
Mt. St. Helens, April, 1980
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Animations courtesy T. Atwater, UCSB http://emvc.geol.ucsb.edu/
Island Arc
2. Hydrated basaltic crust dewaters, causing melting in overlying mantle
1. Basaltic oceanic crust reacts with seawater, creating hyrdous alteration minerals
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1
10
100
1000
Rb Th K La Pb Sr Zr Sm Ti Ho
Cs Ba U Nb Ce Pr Nd Hf Eu Y Yb
Weaver & Tarney Rudnick & Fountain
Wedepohl Gao et al. Taylor & McLennan
This Study
mantle normalized
‘DNA’ elements reveal where crust forms
Average Continental Crust
Rudnick & GaoNb depletion and Pb enrichment: just like arc lavas!
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The problem with andesite
• Can’t melt mantle and make andesite • Basalt forms by mantle melting
How did basalt become andesite?