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Shun’ichi Nakai ERI, The University of Tokyo

Understand Earth's composition using meteorite data. Explore isotopic evolution and decay systems for precise estimations.

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Shun’ichi Nakai ERI, The University of Tokyo

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  1. Importance of tighter constraints on U and Th abundances of the whole Earth by Geo-neutrino determinations Shun’ichi Nakai ERI, The University of Tokyo

  2. How can we estimate the composition of the Earth? The Earth is a differentiated planet. No sample records its average composition. Building blocks of the Earth can be used for the purpose.

  3. Evolution of parent bodies of meteorites Dust of the solar nebula Condensation and accretion melting by 1. heat produced by short-lived radioactive isotopes such as 26Al 2. energy supplied with collisions. Chondrites Achondrites Stony - iron meteorites Iron meteorites mantle + crust core Small planetary bodies differentiation

  4. Chemical compositions of the Sun and CI chondrites Solar photosphere CI chondrite

  5. Geochemists have believed in the chondrite model that the composition of the Earth can be estimated from that of chondrites. Recent Nd isotope studies challenge this model. What is the problem?

  6. Decay systems used for Earth and Planetary Sciences Parent Daughter half life decay constant 87Rb 87Sr 48.8 Byr 1.42 E-11yr-1 147Sm 143Nd 106.0 Byr 6.54 E-12yr-1 238U 206Pb 4.47 Byr 1.5521E-10yr-1 235U 207Pb 0.704 Byr 9.885E-10yr-1 232Th 208Pb 14.01 Byr 0.49475E-10yr-1 (Byr 109yr) Sm-Nd decay is unique in that its isotopic evolution of the Earth’s mantle can be estimated by that in chondrites.

  7. Evolution of Ndisotope ratio Parent(Daughter) Decay constant Half life ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 147Sm(143Nd) 6.54E-12 (yr-1) 106.0Byr Lugmair and Marti (1978) The evolution of 143Nd/144Nd depends on (143Nd/144Nd)0 and 147Sm/144Nd.

  8. Two important events in the early Earth • Condensation and accretion • Core formation These two events could have changed parent / daughter (eg. Sm/Nd) abundance ratios, which resulted in different evolution of the Earth from that of chondrites.

  9. unique property of Sm-Nd decay system • Both parent and daughter elements are refractory. No elemental fractionation occurred during condensation of solid from solar nebula gas.

  10. Condensation temperature U, Th, Sm, Nd volatile refractory Rb Pb Sr behaves similarly to Ca.

  11. How did two events change parent daughter ratiosof decay systemsin silicate Earth? Chondrite parent bodies were located more distant from the Sun compared to the Earth.

  12. unique property of Sm-Nd decay system • Both parent and daughter elements are refractory. No elemental fractionation occurred during condensation of solid from solar nebula gas. • Both parent and daughter elements are lithophile elements. No elemental fractionation occurred during core formation in the Earth

  13. Elemental fractionation during core formation Sm, Nd, Rb, Sr, U and Th are lithophile. They prefer to stay in silicate part. Pb is chalcophile and enters into the core.

  14. How did two events change parent daughter ratiosof decay systemsin silicate Earth? Sm-Nd evolution of the Earth’s mantle can be estimated from that of chondrites.

  15. Evolution of Ndisotope ratio Parent(Daughter) Decay constant Half life ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 147Sm(143Nd) 6.54E-12(yr-1) 106.0Byr Lugmair and Marti (1978) The average 143Nd/144Nd of chondrites at present is 0.512638. The average 147Sm/144Ndof chondrites at present is 0.1967. The two parameters can describe the Nd isotope evolution of whole silicate Earth. A reservoir which has the two parameters is called as “Chondritic Uniform Reservoir (CHUR)”.

  16. Variation of Sm/Ndinchondrites

  17. Differentiation within silicate Earth- Partial melting changes Sm/Nd - Partial melting of solid rock to form a felsic melt mad mafic residue

  18. Fractionations of elements during partial melting of silicate rocks • Incompatibility Nd>Sm • Changes of parent/daughter ratio (Sm/Nd)melt< (Sm/Nd)source material< (Sm/Nd)residual solid (143Nd/144Nd)=(143Nd/144Nd)0+ (147Sm/144Nd)×(el147Sm×t -1) Faster evolution in residual solid(mantle)

  19. Evolution of Nd isotopic ratios in parts of silicate Earth 143Nd/144Nd depleted mantle > CHUR > crust time

  20. Variation of 143Nd/144Nd 0.5132 MORB = present depleted mantle high Sm/Nd Chondritic Uniform Reservoir = Bulk Silicate Earth 0.512638 > 0.511 old Continental crust low Sm/Nd

  21. Average of chondrites 0.512638 Continental crust 143Nd/144Nd: ~0.511 87Sr/86Sr: ~ 0.73 Hofmann (1997)

  22. Isotopes of Nd and Sm 147Sm → 143Nd 146Sm → 142Nd Two decay systems 140 145 150 155

  23. Extinct nuclides These isotopes were present in the beginning of the Solar system. Because of their short lives, the decayed away and now extinct. The major input was limited at the time of the formation of the Solar system.

  24. Decay of radioactive nuclides Half life 146Sm :6.8×107year 147Sm :1.03×1011year 146Sm decay away after 500 million years since the beginning of the Solar system. 24

  25. 142Nd/144Nd 146Sm-142Nd decay system The slope of the line indicates the (146Sm/144Sm) at the time of meteorite formation. 1.14187 1.14180

  26. 142Nd/144Nd Crust formed before 4.3Ga< chondrites< Residual mantle complementary to the crust

  27. Thermal Ionization Mass Spectrometer (TIMS) Thermo Fisher

  28. Thermo Fisher Repeated analyses on 142Nd/144Nd of a standard reagent show typical variations of 0.00001. Errors are on five decimal place.

  29. Different 142Nd/144Ndof terrestrial samples from chondrites Boyet and Carlson, 2005

  30. Rizo et al. (2012) 142Nd/144Nd 1.14186 chondrites 1.14184 20ppm difference between modern terrestrial samples (1.14186)and chondrites (1.14184). Limited variation of modern samples suggests the Earth has been homogenized by mantle convection through its history.

  31. 142Nd/144Nd Crust formed before 4.3Ga< chondrites< Residual mantle complementary to the crust

  32. Possible reasons that caused higher terrestrial 142Nd/144Ndthan chondrites • The building block of the Earth has different composition from chondrites. - It is difficult to test the hypothesis. • We have not obtained samples from the part with lower Sm/Nd than chondrites. Material with lower Sm/Nd stayed around the core-manlte boundary and no sample has risen to the surface.

  33. Formation of hidden reservoir enriched in incompatible elements 1 Labrosse et al. 2007

  34. Formation of hidden reservoir enriched in incompatible elements 2 • Boyet and Carlson (2005) proposed that an incompatible-element reservoir with low Sm/Nd formed early in Earth’s history, sunk to the core – mantle boundary. • No sample has been derived from the enriched reservoir after its formation.

  35. Possible reasons that caused higher terrestrial 142Nd/144Ndthan chondrites • The building block of the Earth has different composition from chondrites.- It is difficult to test the hypothesis. • We have not obtained samples from the part with lower Sm/Nd than chondrites. Material with lower Sm/Nd stayed around the core-manlte boundary and no sample has risen to the surface. • The Earth lost a part with lower Sm/Nd than chondrites A part with lower Sm/Nd stayed at the surface of the Earth in the beginning, however it has ablated by heavy bombardment of meteorites.

  36. Enriched reservoir has been lost to the space. ablasion It was likely that incompatible elements were enriched in melt covering the surface of the Earth. The enriched layer with low Sm/Nd could have been ablated by heavy bombardments of meteorites resulting in high 142Nd/144Nd.

  37. Campbell and O’Neill (2012) 6% higher Sm/Nd ration can explain higher 142Nd/144Nd of terrestrial samples than chondrites. In this case, U and Th depletion factors reach half of the chondirite value.

  38. Collisional erosion on the Mercury Density of a planet usually depends on its mass. Mercury has abnormally high density for its mass. It is considered that rock part of the planet was ablated by heavy collisions.

  39. Tighter constraint from geo-neutrino analysescould solve the problem. 1. Inaccessible deep part of the Earth may by enriched in incompatible elements and has low Sm/Nd. U and Th chondritic 2. The earth may have lost a part enriched in incompatible elements in its early history. U and Th non condritic

  40. U and Th abundances estimated from geo-nutrino observations. U and Th abundances estimated from geo-nutrino have still large uncertainties. Center value of the Kamland, however, is consistent with the erosion model. The KamLAND collaboration, 2011 TW scales with U concentration; 10, 20, 30 TW ≈ 10, 20, 30 ppb U in BSE

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