Evolution of the Earth’s mantle and crust - 2013 (Geol445Crust).pdf
-
Upload
laura-johnson -
Category
Documents
-
view
11 -
download
1
Transcript of Evolution of the Earth’s mantle and crust - 2013 (Geol445Crust).pdf
-
Evolution of the Earths mantle and crust
Geol445 High Temperature Geochemistry Lecture
Huan Cui
Department of Geology University of Maryland
2013 fall semester
GodangHighlight
-
Continents ride high and ocean basins ride low. Why?
Continents are buoyed up by thick, low-density felsic (Si-rich and Mg-poor) rocks, such as granodiorites and granites, whereas ocean basins are underlain by thinner- and higher-density mafic (Si-poor and Mg-rich) rocks like basalt.
(Figure from Taylor and McLennan, 2005, Scientific American) ( Global relief, figure from http://www.smate.wwu.edu/teched/geology/globe.html )
Bimodal topography of the Earth
-
Issues related to the
Continental Crust
When did continental crust form (e.g., mass vs.
time)?
What is the composition of the continental crust
(and how is it determined)?
What can we infer about formation and
modification of continental crust from its composition?
-
Pursue the composition
of the mantle and crust
-
Pursuing the composition of the crust
Ross Taylor (ANU) and Scott McLennan(StonyBrook) 2010 1985
-
Pursuing the composition of the crust
Roberta Rudnick (UMD) Shan Gao (CUG-Wuhan)
Nature 1995
ToG 2014
ToG
-
Nature 1997
ToG 2014
Albrecht W. Hofmann (Max Planck Institute for Chemistry)
Pursuing the composition of the mantle
-
How to approach the
Earths mantle?
Direct sampling: orogenic massif, ophiolite, mantle xenolith carried by volcanic lava
Mantle-derived volcanic lava (such as MORB, OIB)
Geophysical approaches: heat flow, density and seismic velocity, as well as high-P and T experiments
Analogue composition of chondrite presumably making the bulk Earth
-
Peridotite xenolith trapped in basalt
-
Samples that can
presumably represent the
upper continental crust
(diamictite, loess, shale)
-
Erosion and Chemical Weathering
-
Glacial Sediments
Figure from Grotzinger and Jordan, 2010, Understanding Earth 6th edition
-
Photo by Kaufman
Prof. Jay Kaufman exhausted after a climb to the Blaskranz diamictite in the Naukluft nappes of central Namibia. 2013
Science 1998
Neoproterozoic Ghuab diamictite and Maieberg cap carbonate in Namibia
Diamictite and Global Glaciation
-
Loess Plateau
Photo from internet
Loess is an aeolian sediment formed by the accumulation of wind-blown silt.
-
Devonian Marcellus Formation in Pennsylvania
-
Photo by Jay Kaufman
Shale drilling core samples
-
resi
den
ce t
ime
seawater upper crust partition coefficient from Rudnick and Gao, 2014, ToG, 2nd edition.
Insoluble elements:
Transferred from
source of
weathering to
sediments
Insoluable elements in the sediments
-
Composition of
the crust
-
Upper crust major 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
-
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)
-
REE partition
coefficients for
mafic magmas
REE is incompatible ,
LREE is even more incompatible than HREE.
Compare LREE and HREE Compare the D values
between different minerals
Note the Eu in Plag Note HREE in garnet (Figure from Whites textbook Geochemistry 2013)
-
Comparison of REE patterns between (a) average post-Archean shales and loess and (b) various estimates of the upper continental crust composition. PAAS = post-Archean Australian Shale (Taylor and McLennan, 1985); NASC = North American shale composite (Haskin et al., 1966); ES = European shale composite (Haskin and Haskin, 1966); ECPAS = Eastern China post-Archean shale (Gao et al., 1998a). The loess range includes samples from China, Spitsbergen, Argentina, and France (Gallet et al., 1998; Jahn et al., 2001). Chondrite values are from Taylor and McLennan (1985). Figure from Rudnick and Gao, 2014, ToG 2nd edition
-
Figure from Rudnick and Gao, 2014, ToG 2nd edition
-
Composition of
the mantle
-
Comparison of the abundances of trace and (some) major elements in average continental crust and average MORB. Abundances are normalized to the primitive-mantle values (McDonough and Sun, 1995). Figure from Hofmann 2014.
Continental crust
MORB
Residual mantle
-
Crust-mantle differentiation patterns for the decay systems Rb-Sr, Sm-Nd, Lu-Hf, and Re-Os. The diagram illustrates the depletion-enrichment relationships of the parent-daughter pairs, which lead to the isotopic differences between continental crust and the residual mantle. Figure from Hofmann 2014.
Crust-mantle differentiation patterns for the decay systems
-
Figures from Hofmann 2014.
Sr and Nd isotopes of the Earths mantle
-
Trace element abundances of 250 MORB between 40S and 55Salong the Mid-Atlantic Ridge. Each sample is represented by one line. The data are normalized to primitive-mantle abundances of (McDonough and Sun, 1995) and shown in the order of mantle compatibility. This type of diagram is popularly known as spidergram. The data have been filtered to remove the most highly fractionated samples containing less than 5% MgO.Figure from Hofmann 2014.
Trace element of MORB
-
Figures from Hofmann 2014.
recycled continental lithosphere, lower continental crust, ancient pelagic sediment?
recycled sediment?
recycled oceanic crust which lost alkali and Pb during alteration/subduction)
A heterogeneous mantle:
Mantle Zoo
FOZO (FOcal ZOne)
-
The growth of the
continental crust
-
Early Earth. Image credit: Peter Sawyer / Smithsonian Institution.
Early Earth
-
BIF, 3.8Ga, Isukasia region, West Greenland
Gobble, 3.8Ga, Isukasia region, West Greenland
Graywacke sandstone, 3.8Ga, Isukasia region, West Greenland
The Oldest Rocks on Earth
Photo taken in Smithsonian natural history museum by Huan Cui
-
Stromatolite, 3.5 Ga, Western Australia
Photo taken in Smithsonian natural history museum by Huan Cui From Book: Origin and Evolution of Earth 2008
The Oldest Sedimentary Rocks and fossils on Earth
-
(From Van Kranendonk, The Geologic Time Scale 2012)
-
When did continental
crust form?
Crystallization age - Age of present
continental crust (U-Pb zircon)
Model ages crustal extraction (whole rock Nd, zircon Hf isotopes)
-
Radioactive decay Half life Decay constant 238U 206Pb + 8a t1/2 = 4.47 x 10
9 y l1 = 1.551 x 10-10 yr-1
235U 207Pb + 7a t1/2 = 7.04 x 108 y l2 = 9.849 x 10
-10 yr-1 232Th 208Pb + 6a t1/2 = 1.40 x 10
10 y l3 = 4.948 x 10-11 yr-1
206Pb/204Pb = (206Pb/204Pb)i + 238U/204Pb (e1t 1)
207Pb/204Pb = (207Pb/204Pb)i + 235U/204Pb (e2t 1)
208Pb/204Pb = (208Pb/204Pb)i + 232Th/204Pb (e3t 1)
U-Pb dating of zircons
Zircon has very high U/Pb ratio with almost no
non-radiogenic lead, making it a perfect
candidate for U-Pb dating.
This age is crystallization age ( model age).
-
(Figure from Whites textbook Geochemistry, 2013)
143Nd/144Nd is extrapolated backward (slope depending on Sm/Nd) until it intersects
a mantle or chondritic growth curve. ( model age crystallization age )
Assumptions: Sm/Nd does not change during crustal differentiation; and Rocks do
not form as mixtures between crustal and mantle melts.
SmNd model ages
-
Nature 2001
Scale bars are 50 m. Analyzed by SHRIMP II ion microprobe at Curtin University
-
Prof. Bill Compston at ANU
Ph
oto
co
urt
esy
of
Au
stra
lian
Nat
ion
al U
niv
ersi
ty
Sensitive High Resolution Ion Micro Probe (SHRIMP)
CL image of the 200 m diameter, approximately 4.4 Ga Jack Hills zircon. This zircon is the oldest-known part of Earth. Ion microprobe analytical sites are indicated by black ellipses with ages in billions of years. Qt denotes quartz inclusions in the zircon crystal. Image by John Valley.
Dating the oldest mineral on Earth
-
Taylor and McLennan, 2005, Scientific American
-
Cawood et al. (2012). GSA Bull.
Different growth rate models for the
continental crust
-
Taylor and McLennan, 2005, Scientific American
Continental growth rate models
by Taylor and McLennan
-
Cawood et al., 2013, GSAB
Peaks correspond to supercontinent formation. Represent preferential preservation, rather than
crustal growth episodes?
Peaks in Zircon U-Pb ages
Age (Ga)
0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5
-
(Halverson et al., 2007)
Strontium isotope 87Sr/86Sr evolution
More chemical weathering on land
-
4.4 billion years of crustal maturation:
oxygen isotope ratios of magmatic zircon
(Valley et al., 2005 CMP)
-
Crust Composition
Conundrum
-
Basalt lava Peridotite
Mantle melting and the production of oceanic crust
Photo from internet
-
Crust Composition Paradox Crust is andesitic,
Crust grows by addition of basalt, Basalt is primary melt of peridotite mantle.
How did this evolved crust come about?
Basalt Andesite
Photo from internet
How?
The building is andesitic in composition, but the building blocks are basalt!!!???
-
(Fro
m U
nd
erst
and
ing
Eart
h, 4
th E
dit
ion
)
Mantle Oceanic crust Bulk Continental crust Upper continental crust
-
Possible solutions to crust composition paradox
-
Recycling of lower crust
Evolved melt addition: growth by silicic slab melts
Weathering & seafloor alteration: preferential return of Mg to mantle
Sub-Moho cumulates
Possible solutions to crust composition paradox
-
Recycling of lower crust
Evolved melt addition: growth by silicic slab melts
Weathering & seafloor alteration: preferential return of Mg to mantle
Sub-Moho cumulates
Possible solutions to crust composition paradox
-
Whats requried*? thickening of mafic
lower crust
granulite eclogite low viscosity
*Kay and Kay, 1991; Jull and Kelemen, 2001
From Laubcher
Eclogite Lithospheric mantle
Lower Crustal Recycling (delamination, density foundering)
-
Recycling of lower crust
Evolved melt addition: growth by silicic slab melts
Weathering & seafloor alteration: preferential return of Mg to mantle
Sub-Moho cumulates
Possible Solutions to the Crust
Composition Paradox
-
Archean Subduction (after Martin, 1986)
From Rudnicks slides
-
Recycling of lower crust
Evolved melt addition: growth by silicic slab melts
Weathering & seafloor alteration: preferential return of Mg to mantle
Sub-Moho cumulates
Possible Solutions to the Crust
Composition Paradox
-
From Rudnicks slides
The weathering solution
-
Using a mass balance model for lithium inputs and outputs from the continental crust, we find that the mass of continental crust that has been lost due to chemical weathering is at least 15% of the original mass of the juvenile continental crust, and may be as high as 60%, with a best estimate of approximately 45%. Our results suggest that chemical weathering and subsequent subduction of soluble elements have major impacts on both the mass and the compositional evolution of the continental crust.
-
Recycling of lower crust
Evolved melt addition: growth by silicic slab melts
Weathering & seafloor alteration: preferential return of Mg to mantle
Sub-Moho cumulates
Possible Solutions to the Crust
Composition Paradox
-
Sub-Moho "crust"
high density cumulates:pyroxenites, dunitesVp 7.8 km/s
Continental Crust
-
Earths Crust in a Planetary Perspective
-
Earth is the only terrestrial planet with continents.
Earth is the only planet with
liquid water. Coincidence?
Apollo 17 view of Earth
-
No Water, No Granites
No Oceans, No Continents
-
No Water, No Granites
No Oceans, No Continents
-
Big Questions
Relative contributions of weathering, lower
crustal recycling, slab melting to crustal
signature?
Nature and volume of pre 4.0 Ga crust?
Secular change in crust composition?
Relationship between continental lithospheric mantle and overlying crust?
-
Credit: S. Selkirk after L. Kump.
Earths Oxygenation
-
Welcome to visit the GEOL445 High Temperature Geochemistry Lab website:
http://www.geol.umd.edu/~hcui/HighTemp.html