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CIDER 2008

Composition of the Earth

Stan HartWoods Hole Oceanographic Institution

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Courtesy of NASA/JPL-Caltech

4.56 Billion Years AgoLets begin at the beginning^

7

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How do we determinethe composition of the Earth??

Best Way:- grind up the Earth.- take a representative sample.- analyze in the lab for everything.

Or we can take a desperate guess (sometimescalled the chondrite model).

The problem:

- direct sampling to only ~15 km.- eruptive entrainment sampling to 200 km,and possibly to 500 km.

- mantle plume advection from the base of themantle (2900 km). If plumes exist.

- no bona fide samples yet from the core.

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The Solar Connection:

Palme and Jones 2005

Chondrites ~ Solar Nebula, to within 20%

Why do we think meteorites have anythingto do with the Earth?

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The Solar Connection:

C1 Chondrites ~ Solar Nebula, to within the uncertaintiesof the solar spectroscopic measurements.

Allegre, Hart and Shimizu, 2008

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The Chondritic Earth model

All classes of Chondrites have the same Sm/Nd ratio (1%!)- maybe the Earth is also the same?

(note: Sm/Nd weight ratio is directly proportional to 147Sm/144Nd)

1%

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Hutchinson, 2004

Chondrites have variable Ca/Si and Al/Si but allclasses of chondrites have the same Ca/Al ratio -

Maybe the Earth also has the same Ca/Al?


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Hutchinson, 2004

Why is Ca/Si and Al/Si variable betweenchondrite classes?

Because Si has a lower condensation temperature than Ca and Al.

Then what is the Earths Ca/Si and Al/Si?


Condensationtemperatures ofthe elements, K:

Al - 1655Ca - 1520Mg - 1340Fe - 1335Si - 1310

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Albarede 2003

The first fuzzy step -

The chondritic Earth model

Peridotites represent residuesof partial melting.

Chondrites represent differingcondensation temperatures.

Intersection defines thecomposition of the primitiveupper mantle (PUM) andsuggests Earth had a highercondensation temperaturethan chondrites.

QED - we know the relativeAl, Mg and Si contents ofthe Earth.

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The more the data, the fuzzier it gets!

Mg/Si

A

l/Si

0.81.01.21.4

Canil 2008 Line is Canils best fit to the

off-craton xenoliths.

Blue pentagon is PUM fromMcDonough and Sun 1995.

Green star is PUM from Hartand Zindler 1986 (aka HaZi).

(PUM = primitive upper mantle)

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Chondrite model can also be used for trace elements:

Hart and Zindler 1986

Like Sm/Nd, Sm/Ca appears constant in chondrites (exceptingsome cooked carbonaceous chondrites).

Ignore the open squares (metasomatized upper mantle peridotites).

Tic marks on melting curve are % increments of melt removal.

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McDonough, 2005

Refractory Element

Condensation Temps

Re - 1820KW - 1790Zr - 1740Th - 1660REE -1660 - 1490 (Yb)Al - 1655U - 1610

Ti - 1580Ca - 1520

Semi-refractory

Mg - 1340Fe - 1335Si - 1310

8001000120014001600

Estimated Earth Composition relative to C1 chondrites

Good match for Refractory Elements (Tcond. >1500K)

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Abyssal Peridotites = Simple Residues of DMMMeltingLinearized relationshipbetween two elements, A & B, ina residue of fractional melting:

Where slope, R

ln Cs

A R ln CsB ln CoA

Co

B R

R DB(1 D

A)

DA(1 D

B)

-9

-8

-7

-6

-5

-4

-3

-2

-1

-9 -8 -7 -6 -5 -4 -3 -2 -1 0

ln(Sm)

Primitive Upper Mantle (PUM)McDonough & Sun (1995)

Workman and Hart, 2005

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PUM PUM

Some other trace element trends in abyssal peridotites:

We know the composition of DMM is somewhere onthe regression between PUM and the least depletedabyssal peridotite - but where?


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We work backward from the average143Nd/144Nd of melts from thedepleted upper mantle (=0.51317).

Given the Sm/Nd of PUM, we canmodel the evolution of a continuouslydepleting reservoir that ends at thispresent day 143Nd/144Nd of N-MORB.

From this model, we can estimate thepresent day Sm/Nd of DMM (=0.411).

The intersection of this line with theabyssal peridotite trend defines theSm and Nd concentration of DMM.


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Composing Trace Element Composition of DMM

Abyssal Peridotite Constraints

0.01

0.10

1.00

Rb Ba Th U Nb Ta La Ce Pb Pr Nd Sr Zr Hf Sm Eu Ti Gd Tb Dy Ho Y Er Yb Lu

PUMN

ormalizedCon

centrations


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Parent/Daughter Constraints

0.01

0.10

1.00


PUMNormalizedCon

centrations


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Hofmann 2005

Negative slope means numerator

element is more compatible thandenominator element.

i.e mineral/melt partition coefficient Diis larger

Horizontal slope means both elements

have the same partition coefficient.

Canonical Ratios

DNb > DTh

DNb < DLa

DNb ~ DU

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Some trace elements dont fractionate from each other!

So ratio in melt equals ratio in residue

Canonical rat ios

Spreading Center Lavas

PETDB Database

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Cannonical Ratios Constraints

0.01

0.10

1.00


PUMN

ormalizedCon

centrations


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Connecting the Dots

0.01

0.10

1.00


PUMN

ormalizedCon

centrations


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MORB Generation from model DMM


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0.01

0.10

1.00

10.00

100.00

Rb Ba Th U K Nb Ta La Ce Pb Pr Nd Sr Zr Hf Sm Eu Ti Gd Tb Dy Ho Y Er Yb Lu

SumDMM

Cont. Crust

Crust-Mantle Mass Balance - I

How much DMM does it take to balance JUST Continental Crust?

CC mass = 0.6% of BSE

Bulk Continental Crust from Rudnick and Fountain (1995)

DMM mass = 33% of BSE

Sum of DMM + CC

PUMn

ormalizedconce

ntrations

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Crust-Mantle Mass Balance - II

Bulk Continental Crust from Rudnick and Fountain (1995)

0.01

0.10

1.00

10.00

100.00

Rb Ba Th U K Nb T a La Ce Pb Pr Nd Sr Zr Hf Sm Eu Ti Gd T b Dy Ho Y Er Yb Lu

Sum

DMM

Cont. Crust

N-MORB

0.6% CC

Adding Oceanic Crust into the Balance

Most element fit to within 8%

43 3% DMM

2 0.3% MORB

Sum of MORB + DMM + CC

PUM

normalizedconcentrations

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Physical properties calculated with model of Stixrude and Lithgow-Bertelloni 2005

- produces a huge effect on isotopes, heat production and some traceelements but an insignificant effect on density and shear wave velocity!

Deplete the primitive upper mantle to make a depleted MORB mantle:

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So were done, right?

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Boyet and Carlson 2006

hmmmmmm

142Nd is the daughter of146Sm, an extinct parent.

142Nd in the accessible Earth is20 ppm higher than inchondrites.

So is the chondritic modelfor the Earth wrong?

Maybe!

Only two simple choices:

- the earth is not chondritic.- there is a hidden terrestriallow Sm/Nd reservoir wevenot yet seen.

All consequences are drastic!

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Hutchinson 2004

PUM

Terrestrial fractionation line

Oxygen isotope compositions of Earth, Ordinary chondrites (H, L LL),Enstatite chondrites (EH, EL), and Carbonaceous chondrites (C1, CM, etc).

Earth is similar onlyto the EnstatiteChondrites.

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Hutchinson 2004

PUM

Terrestrial fractionation line

Oxygen isotope compositions of Earth, Moon, Mars,Iron meteorites and differentiated meteorites

Earth is similar onlyto the Moon, andAubrites (Enstatiteachondrites).

This oxygen DNA testsuggests the deep Earthmay be richer in enstatitethan olivine (higherperovskite/periclase ratio).

Seismologists andmineral physiciststo the rescue??

Im done!

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Staytuned -

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0.01

0.1

1

10

Rb Ba T h U Nb Ta La Ce Pb Pr Nd Sr Zr Hf Sm Eu Ti Gd Tb Dy Ho Y Er Yb L u

PUMN

ormalizedConcentrations

Average Upper Mantle

Observed Oceanic Crust

ElementConcentrations

NormalizedtoBulkSilicateEarth

Generation of Oceanic Crust using our Upper Mantle Composition

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0.01

0.1

1

10


PUMN

ormalizedConcentrations

DMM

Observed Oceanic Crust

Model Oceanic Crust

Average Upper Mantle

ElementConcentrations

NormalizedtoBulkSilicateEarth

Generation of Oceanic Crust using our Upper Mantle Composition

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-8

-7

-6

-5

-4

-3

-2

-1

0

1

-7 -6 -5 -4 -3 -2 -1 0

ln(Sm)

PUM

Defining a unique position on the mantle depletion trends

DMM

earth08-hart1

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