Composition of the Earth: a more volatile elements perspective Cider 2010 Bill McDonough Geology, University of Maryland Support from:

Composition of the Earth:a more volatile elements perspective

Cider 2010Bill McDonoughGeology, University of Maryland

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Volatility trend@ 1AU from Sun

Th & U

Allegre et al (1995) EPSL



McDonough & Sun (1995) Chem G

the volatile budget?

Earth’s D/H ratio

• Do we really know

comets

• D/H ratio of the oceans

• What do chondrites tell

us?

• Source of water and

other volatiles vs the

sources of noble

gases?Ref: Owen and Bar-Nun, in R. M. Canup and K. Righter, eds., Origin of the Earth and Moon (2000), p. 463

Progress Report Conclusions:

Approximate concentrations

Depleted Mantle H2O 50 ppm; CO2 20 ppm; Cl 1 ppm; F 7 ppm

Enriched Mantle H2O 500 ppm; CO2 420 ppm; Cl 10 ppm; F 18 ppm

Total Mantle H2O 366 ppm; CO2 301 ppm; Cl 7 ppm; F 15 ppm

Last CIDER report on volatiles in the Earth - Saal et al 2009

• Earth: 61024 kg Oceans: 1.41021 kg• Ordinary chondritic planet -- 4 oceans• Carbonaceous chondritic planet -- 600 oceans• Enstatite chondritic planet -- ~2-4 oceans

H/C ratio of the bulk silicate Earth is superchondritic, owing chiefly to the high H/C ratio of the exosphere.

H/C ratio of the mantle is lower than that of the exosphere, requiring significant H/C fractionation during ingassing or outgassing at some point in Earth history.

Hirschmann and Dasgupta (2009)

Volatile Budget!

Earth’s volatiles from chondrites?

Let’s hear from what Sujoy has to say!…

Mantle Siderophileelements

Lithophileelements

Fe, Ni, P, Os

Core

Atmophilie elements

N2 , O

2 , Ar

“my Earth”

First observations -- got it right at the 1-sigma level

SCIENCEAccepted as the fundamental reference and set the bar at

K/U = 104

Th/U = 3.5 to 4.0

MORB (i.e., the Depleted Mantle ~ Upper Mantle)

K/U ~ 104 and slightly sub-chondritic Th/U

DM & Continental Crust – complementary reservoirs

DM + Cc = BSE

ahh, but the assumptions and samples…

Earth is “like” an Enstatite Chondrite!

1) Mg/Si -- is very different

2) shared isotopic Xi -- O, Cr, Mo,Ru, Nd,

3) shared origins -- unlikely

4) core composition -- no K, U in core.. S+

5) “Chondritic Earth” -- lost meaning…

6) Javoy’s model? -- needs to be modified

Volatility trend@ 1AU from Sun

Th & U

Core

Mantle

Siderophileelements

Lithophileelements

Ca, Al, REE, K, Th & U

Fe, Ni, P, Os

Atmophilie elements

U in the Earth: ~13 ng/g U in the Earth

Metallic sphere (core) <<<1 ng/g U

Silicate sphere 20* ng/g U

*Javoy et al (2010) predicts 11 ng/g

Continental Crust 1000 ng/g U

Mantle ~12 ng/g U

“Differentiation”

Chromatographic separationMantle melting & crust formation



This translates to 11 ppb U


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Allegre et al (1995), McD & Sun (’95)Palme & O’Neill (2003)

Lyubetskaya & Korenaga (2007)

No

rmal

ized

co

nc

entr

ati

on

REFRACTORY ELEMENTS VOLATILE ELEMENTS

Half-mass Condensation Temperature

Potassiumin the core

Silicate Earth

?





All peridotites are 2-component mixtures!

From McDonough (1994)

Melt-depletion

Melt-”re-enrichment”(aka - metasomatism)



Initial results from: McDonough & Sun ‘95

- trends not pretty, but robust

- trends cross chondritic pt

-trends are melting products

-important not to use highly-ITE

Lyubetskaya & Korenaga (2007) made this mistake





Log concentrations (in ppm)

degree of melting

Shaded symbols denote samples with MgO 40.5%

-2.0

-1.0

0.0

1.0

2.0

-1.0 -0.8 -0.6 -0.4 -0.2 0.0

Sc/Yb

Y/Yb

Lu/Yb

Log normal trend for peridotites

xenoliths & massifs

chondritic trends

Based on mantle samples: MgO 35-41 wt% (n =330)

Mantle is depleted in some elements (e.g., Th & U) Mantle is depleted in some elements (e.g., Th & U)

that are enriched in the continents.that are enriched in the continents. -- models of mantle convection and element distribution

Th & Urich

Th & Upoor

4 most abundant elements in the Earth:Fe, O, Si and Mg

6 most abundance elements in the Primitive Mantle: - O, Si, Mg, and – Fe, Al, Ca

This result and 1st order physical data for the core yield a precise estimate for the planet’s Fe/Al ratio : 20 ± 2

What’s in the core?

What would you like?

Constraints: density profile, magnetic field, abundances of the elements,

Insights from: cosmochemistry, geochemistry, thermodynamics, mineral physics, petrology, Hf-W isotopes (formation age)

How well do we know some elements?

Model 1 Model 2

Core compositional models

others

Model Core composition

(wt%) % in core rel. Earth (ug/g) % in core

rel. Earth

Fe 88.3 87 V 150 50

O 3 3 Mn 300 10

Ni 5.4 93 Cu 125 65

S 1.9 96 Pd 3.1 >98

Cr 0.9 60 Re 0.23 >98

P 0.2 93 Os 2.8 >98

C 0.2 91 Au 0.5 >98

REFRACTORY ELEMENTS

Nature 436, 499-503 (28 July 2005)

Detecting Geoneutrinoin the Earth

Detecting Electron Antineutrinos from inverse beta -decay

€

ν e + p→ n + e+

2 flashes close in space and 2 flashes close in space and timetime

Rejects most backgroundsRejects most backgrounds

- decay



Geo-neutrinos at KamLAND

Silicate Earth has ~20 ng/g U

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Composition of the Earth: a more volatile elements perspective Cider 2010 Bill McDonough Geology, University of Maryland Support from: