composition of the earth: a more volatile elements perspective cider 2010 bill mcdonough geology,...
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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
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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
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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
?
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All peridotites are 2-component mixtures!
From McDonough (1994)
Melt-depletion
Melt-”re-enrichment”(aka - metasomatism)
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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
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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
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Geo-neutrinos at KamLAND
Silicate Earth has ~20 ng/g U