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 1

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

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 26 Al 2. energy supplied with collisions. Chondrites Achondrites Stony - iron meteorites Iron meteorites mantle + crust core 3 differentiation Small planetary bodies

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

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?

Decay systems used for Earth and Planetary Sciences ParentDaughter half life decay constant 87 Rb 87 Sr 48.8 Byr 1.42 E-11yr Sm 143 Nd Byr6.54 E-12yr U 206 Pb 4.47 Byr1.5521E-10yr U 207 Pb Byr9.885E-10yr Th 208 Pb Byr E-10yr -1 (Byr 10 9 yr) 6 Sm-Nd decay is unique in that its isotopic evolution of the Earth’s mantle can be estimated by that in chondrites.

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

Two important events in the early Earth Condensation and accretion Core formation 8 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.

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. 9

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

Rb/SrU/PbSm/Nd Raw material for the Earth smaller than chondritic higher than chondritic chondritic Condensation and Accretion decreaseincreaseno change Core Formation Silicate Earth How did two events change parent daughter ratios of decay systems in silicate Earth? 11 Chondrite parent bodies were located more distant from the Sun compared to the Earth.

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 12

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

Rb/SrU/PbSm/Nd Raw material for the Earth smaller than chondritic higher than chondritic chondritic Condensation and Accretion decreaseincreaseno change Core Formationno changeincreaseno change Silicate Earthnot chondritic chondritic How did two events change parent daughter ratios of decay systems in silicate Earth? 14 Sm-Nd evolution of the Earth’s mantle can be estimated from that of chondrites.

Evolution of Nd isotope ratio Parent(Daughter)Decay constantHalf life ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 147 Sm( 143 Nd)6.54E-12(yr -1 )106.0ByrLugmair and Marti (1978) The average 143 Nd/ 144 Nd of chondrites at present is The average 147 Sm/ 144 Nd of chondrites at present is 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)”. 15

Variation of Sm/Nd inchondrites

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

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 ( 143 Nd/ 144 Nd)=( 143 Nd/ 144 Nd) 0 + ( 147 Sm/ 144 Nd)×(e 147Sm ×t -1) Faster evolution in residual solid (mantle) 18

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

20 Variation of 143 Nd/ 144 Nd MORB = present depleted mantle high Sm/Nd old Continental crustlow Sm/Nd Chondritic Uniform Reservoir = Bulk Silicate Earth >

Hofmann (1997) Continental crust 143 Nd/ 144 Nd: ~ Sr/ 86 Sr: ~ Average of chondrites

Isotopes of Nd and Sm Sm → 143 Nd 146 Sm → 142 Nd Two decay systems 22

Extinct nuclides Parent nuclides Half life （ Myr) Daughter nuclides Abundances at the beginning of the solar system 26 Al Mg 26 Al/ 27 Al=5× Mn Cr 53 Mn/ 55 Mn=6× I Xe 129 I/ 127 I=1× Hf W 182 Hf/ 180 Hf=1× Sm Nd 146 Sm/ 144 Sm ＝ Pu82Fission Xe 244 Pu/ 238 U ＝ 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.

Decay of radioactive nuclides Half life 146 Sm:6.8 × 10 7 year 147 Sm:1.03×10 11 year Sm decay away after 500 million years since the beginning of the Solar system.

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

Crust formed before 4.3Ga < chondrites < Residual mantle complementary to the crust 26

Thermal Ionization Mass Spectrometer (TIMS) 27 Thermo Fisher

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

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

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

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

Possible reasons that caused higher terrestrial 142 Nd/ 144 Nd than 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. 32

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

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. Formation of hidden reservoir enriched in incompatible elements 2

Possible reasons that caused higher terrestrial 142 Nd/ 144 Nd than 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. 35

ablasion Enriched reservoir has been lost to the space. 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 142 Nd/ 144 Nd. 36

37 Campbell and O’Neill (2012) 6% higher Sm/Nd ration can explain higher 142 Nd/ 144 Nd 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.

Tighter constraint from geo-neutrino analyses could 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 39

U and Th abundances estimated from geo-nutrino observations. TW scales with U concentration; 10, 20, 30 TW ≈ 10, 20, 30 ppb U in BSE 40 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