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
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Allegre et al (1995) EPSL
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McDonough & Sun (1995) Chem G
the volatile budget?
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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
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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
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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!
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Earth’s volatiles from chondrites?
Let’s hear from what Sujoy has to say!…
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Mantle Siderophileelements
Lithophileelements
Fe, Ni, P, Os
Core
Atmophilie elements
N2 , O
2 , Ar
“my Earth”
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First observations -- got it right at the 1-sigma level
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SCIENCEAccepted as the fundamental reference and set the bar at
K/U = 104
Th/U = 3.5 to 4.0
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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…
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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
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Volatility trend@ 1AU from Sun
Th & U
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Core
Mantle
Siderophileelements
Lithophileelements
Ca, Al, REE, K, Th & U
Fe, Ni, P, Os
Atmophilie elements
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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
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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%
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-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)
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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
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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
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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?
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Model 1 Model 2
Core compositional models
others
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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
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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