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Open Questions in Geosciences

Open Questions in Geosciences. Mapping Elements & Structure in the Earth. Geophysics tells us where we are at today Geochemistry tells us how we got there…. Collaborators : - Ricardo Arévalo, Mario Luong : UMD - Kevin Wheeler, Dave Walker : Columbia Univ

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Open Questions in Geosciences

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  1. Open Questions in Geosciences Mapping Elements & Structure in the Earth Geophysics tells us where we are at today Geochemistry tells us how we got there… Collaborators: - Ricardo Arévalo, Mario Luong : UMD - Kevin Wheeler, Dave Walker : Columbia Univ - Corgne, Keshav & Fei : Geophysical Lab, CIW - John Learned : University of Hawaii - Steve Dye: Hawaii Pacific University

  2. 5 Big Questions: • What is the Planetary K/U ratio? • Radiogenic contribution to heat flow? • Distribution of reservoirs in mantle? • Radiogenic elements in the core?? • Nature of the Core-Mantle Boundary? planetary volatility curve secular cooling whole vs layered convection Earth energy budget hidden reservoirs

  3. How much Th, U and K is there in the Earth? Heat flow measurements Geochemical modeling Neutrino Geophysics

  4. Time Line 1st order Structure of Earth Rock surrounding metal 1897 Emil Wiechert 1915 CORE-MANTLE 1925 UPPER-LOWER MANTLE 1935 CORE-MANTLE PLATE TECTONICS 1970 1995

  5. Earth’s Total Surface Heat Flow • Conductive heat flow measured from bore-hole temperature gradient and conductivity Data sources Total heat flow Conventional view 463 TW Challenged recently 311 TW

  6. Urey Ratio and Mantle Convection Models • Mantle convection models typically assume: mantleUrey ratio: 0.4 to 1.0, generally ~0.7 • Geochemical models predict: mantleUrey ratio 0.3 to 0.5 radioactive heat production Urey ratio = heat loss

  7. Discrepancy? • Est. total heat flow, 46 or 31TW est. radiogenic heat production 20TW or 31TW give Urey ratio ~0.3 to ~1 • Where are the problems? • Mantle convection models? • Total heat flow estimates? • Estimates of radiogenic heat production rate? • Geoneutrino measurements can constrain the planetary radiogenic heat production.

  8. “Standard” Planetary Model • Chondrites, primitive meteorites, are key • So too, the composition of the solar photosphere • Refractory elements (RE) in chondritic proportions • Absolute abundances of RE – model dependent • Mg, Fe & Si are non-refractory elements • Chemical gradient in solar system • Non-refractory elements – model dependent • U & Th are RE, whereas K is moderately volatile

  9. U and Th (and K) Distribution in the Earth • U and Th (K?) are thought to be absent from the core and present in the mantle and crust. • Core: Fe-Ni metal alloy • Crust and mantle: silicates • U and Th (and K) concentrations are the highest in the continental crust. • Continents formed by melting of the mantle. • K, U and Th prefer to enter the melt phase • Continental crust: insignificant in terms of mass but major reservoir for U, Th, K.

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

  11. 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

  12. sub-title: What is the K/U Th/U ratios for the Earth and modern mantle? Implications and History -- implications: K, Th and U are the radioactive elements that provide the sum of the internal radiogenic heat for the planet -- history: Urey 1950s Wasserburg 1960s Jochum 1980s

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

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

  15. 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…

  16. Two types of crust: Oceanic & Continental Oceanic crust: single stage melting of the mantle Continental crust: multi-stage melting processes Compositionally distinct

  17. Continental Crust Not simple to interpret, but K/U ~104 Th/U ~ 4

  18. Oeanic Crust K/U ~ 104 Th/U ~ 4 Crust above = basalt (melt, enriched in K, Th U) Mantle beneath = Refractory peridotite (residue, deplete in K, Th U)

  19. MORB Sample Locations

  20. MORBs: a compositional spectrum

  21. *BABB not included in above diagram,nor in the MORB average

  22. Urey Ratio and Mantle Convection Models • Mantle convection models typically assume: mantleUrey ratio: 0.4 to 1.0, generally ~0.7 • Geochemical models predict: Urey ratio 0.3 to 0.5. radioactive heat production Urey ratio = heat loss

  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. 5 Big Questions: • What is the Planetary K/U ratio? • Radiogenic contribution to heat flow? • Distribution of reservoirs in mantle? • Radiogenic elements in the core?? • Nature of the Core-Mantle Boundary? planetary volatility curve secular cooling whole vs layered convection Earth energy budget hidden reservoirs

  25. Long-lived chronometer: 147Sm 143Nd (T1/2 = 106 Ga) 147Sm abundance decreased by only 3% in 4.56 Ga Short-lived chronometer: 146Sm 142Nd (T1/2= 103 Ma) 146Sm exists only in the first 500 Myr of solar system history

  26. 19,000 ± 500 (2sMean)

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