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Composition of the Earth: a more volatile elements perspective

Composition of the Earth: a more volatile elements perspective. Cider 2010 Bill McDonough Geology, University of Maryland. Support from:. Th & U. Volatility trend @ 1AU from Sun. Allegre et al (1995) EPSL. McDonough & Sun (1995) Chem G. the volatile budget?. Earth’s D/H ratio.

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Composition of the Earth: a more volatile elements perspective

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

  2. Th & U Volatility trend @ 1AU from Sun

  3. Allegre et al (1995) EPSL

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

  5. 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

  6. Last CIDER report on volatiles in the Earth - Saal et al 2009 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 • Earth: 61024 kg Oceans: 1.41021 kg • Ordinary chondritic planet -- 4 oceans • Carbonaceous chondritic planet -- 600 oceans • Enstatite chondritic planet -- ~2-4 oceans

  7. 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)

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

  9. Lithophile elements Atmophilie elements Ca, Al, REE, K, Th & U Fe, Ni, P, Os N2, O2, Ar Core Mantle Siderophile elements “my Earth”

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

  11. SCIENCE Accepted as the fundamental reference and set the bar at K/U = 104 Th/U = 3.5 to 4.0

  12. 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…

  13. Earth is “like” an Enstatite Chondrite! Mg/Si -- is very different shared isotopic Xi -- O, Cr, Mo,Ru, Nd, shared origins -- unlikely core composition -- no K, U in core.. S+ “Chondritic Earth” -- lost meaning… 6) Javoy’s model? -- needs to be modified

  14. Th & U Volatility trend @ 1AU from Sun

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

  16. 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 separation Mantle melting & crust formation

  17. This translates to 11 ppb U

  18. Silicate Earth REFRACTORY ELEMENTS VOLATILE ELEMENTS Allegre et al (1995), McD & Sun (’95) Palme & O’Neill (2003) ? Lyubetskaya & Korenaga (2007) Normalized concentration Potassium in the core Half-mass Condensation Temperature

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

  20. 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

  21. Shaded symbols denote samples with MgO 40.5% Log concentrations (in ppm) degree of melting

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

  23. 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

  24. 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

  25. 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?

  26. Core compositional models Model 1 Model 2 others

  27. Model Core composition

  28. Detecting Geoneutrino in the Earth b- decay REFRACTORY ELEMENTS Detecting Electron Antineutrinos from inverse beta -decay 2 flashes close in space and time Rejects most backgrounds Nature436, 499-503 (28 July 2005)

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

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