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