The excess of highly siderophile elements in the Earth’s mantle is thought to reflect the addition of primitive meteoritic material after core formation ceased1,2,3,4. This ‘late veneer’ either comprises material remaining in the terrestrial planet region after the main stages of the Earth’s accretion5,6, or derives from more distant asteroidal7 or cometary8 sources. Distinguishing between these disparate origins is important because a late veneer consisting of carbonaceous chondrite-like asteroids7 or comets8 could be the principal source of the Earth’s volatiles and water. Until now, however, a ‘genetic’ link between the late veneer and such volatile-rich materials has not been established or ruled out. Such genetic links can be determined using ruthenium (Ru) isotopes, because the Ru in the Earth’s mantle predominantly derives from the late veneer9, and because meteorites exhibit Ru isotope variations arising from the heterogeneous distribution of stellar-derived dust10,11. Although Ru isotopic data and the correlation of Ru and molybdenum (Mo) isotope anomalies in meteorites were previously used to argue that the late veneer derives from the same type of inner Solar System material as do Earth’s main building blocks6, the Ru isotopic composition of carbonaceous chondrites has not been determined sufficiently well to rule them out as a source of the late veneer. Here we show that all chondrites, including carbonaceous chondrites, have Ru isotopic compositions distinct from that of the Earth’s mantle. The Ru isotope anomalies increase from enstatite to ordinary to carbonaceous chondrites, demonstrating that material formed at greater heliocentric distance contains larger Ru isotope anomalies. Therefore, these data refute an outer Solar System origin for the late veneer and imply that the late veneer was not the primary source of volatiles and water on the Earth.
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We thank the Meteorite Working Group at NASA, A. Bischoff and A. Greshake for providing the meteorite samples for this study, and K. Bermingham and B. O’Driscoll for providing the Shetland chromitite sample. We thank U. Heitmann, T. Kruijer and C. Proksche for their assistance, and A. Brandon, C. Brennecka and D. Papanastassiou for comments on the paper. This work was supported by the Deutsche Forschungsgemeinschaft (SFB-TRR 170, subproject B3-1). This is TRR 170 publication no. 11.
About this article
The Astronomy and Astrophysics Review (2018)