Composition of the Earth: a more volatile elements perspective
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Cider 2010
Bill McDonough
Geology, University of Maryland
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Th & U
Volatility trend
@ 1AU from Sun

Allegre et al (1995) EPSL

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McDonough & Sun (1995) Chem G the volatile budget?

• 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

Last CIDER report on volatiles in the Earth - Saal et al 2009
Progress Report Conclusions:
Approximate concentrations
Depleted Mantle H
2
O 50 ppm; CO
2
20 ppm; Cl 1 ppm; F 7 ppm
Enriched Mantle H
2
O 500 ppm; CO
2
420 ppm; Cl 10 ppm; F 18 ppm
Total Mantle H
2
O 366 ppm; CO
2
301 ppm; Cl 7 ppm; F 15 ppm
• Earth: 6 
10 24 kg Oceans: 1.4

10 21 kg
• Ordinary chondritic planet -- 4 oceans
• Carbonaceous chondritic planet -- 600 oceans
• Enstatite chondritic planet -- ~2-4 oceans

Volatile Budget!
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)

Earth’s volatiles from chondrites?
Let’s hear from what Sujoy has to say!…

Atmophilie elements
Fe, Ni,
P, Os
Core
Mantle
Siderophile elements
“my Earth”

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

SCIENCE
Accepted as the fundamental reference and set the bar at
K/U = 10 4
Th/U = 3.5 to 4.0

MORB (i.e., the Depleted Mantle ~ Upper Mantle)
K/U ~ 10 4 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 X i
-- 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

Th & U
Volatility trend
@ 1AU from Sun

Lithophile elements
Ca, Al, REE, K, Th & U
Atmophilie elements
Mantle
Core
Fe, Ni,
P, Os
Siderophile elements

~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
Chromatographic separation
Mantle melting & crust formation

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This translates to 11 ppb U
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Silicate Earth
REFRACTORY ELEMENTS VOLATILE ELEMENTS
Allegre et al (1995), McD & Sun (’95)
Palme & O’Neill (2003)
?
Lyubetskaya & Korenaga (2007)
Potassium in the core
Half-mass Condensation Temperature

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All peridotites are 2-component mixtures!
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Melt-depletion
Melt”re-enrichment”
(aka - metasomatism)
From McDonough (1994)

Initial results from:
McDonough & Sun ‘95
- trends not pretty, but robust
- trends cross chondritic pt
-trends are melting products
-important not to use highlyITE
Lyubetskaya & Korenaga
(2007) made this mistake
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Log concentrations
(in ppm)
Shaded symbols denote samples with MgO 40.5%
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degree of melting

2.0
Log normal trend for peridotites
1.0
Sc/Yb
0.0
-1.0
Y/Yb
-0.8
-0.6
-0.4
chondritic trends
-0.2
0.0
-1.0
Lu/Yb xenoliths & massifs
-2.0
Based on mantle samples: MgO 35-41 wt% (n =330)

Mantle is depleted in some elements (e.g., Th & U) that are enriched in the continents.
-models of mantle convection and element distribution
Th & U poor
Th & U rich

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

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?

others
Model 1 Model 2

(wt%)
Fe 88.3
O 3
Ni 5.4
S 1.9
Cr 0.9
P 0.2
C 0.2
% in core rel. Earth
87
3
93
96
60
93
91
V
(ug/g)
150
% in core rel. Earth
50
Mn 300 10
Cu
Pd
125
3.1
65
>98
Re 0.23
>98
Os
Au
2.8
0.5
>98
>98

REFRACTORY ELEMENTS
b decay
Nature 436 , 499-503 (28 July 2005)

Detecting Electron Antineutrinos from inverse beta -decay
 e
 p  n  e 
2 flashes close in space and time
Rejects most backgrounds

Geo-neutrinos at KamLAND
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Silicate Earth has
~20 ng/g U