Title:
Molybdenum isotopic evidence for late accretion of outer Solar System
materials

Abstract:
Many refractory elements in chondritic meteorites exhibit
nucleosynthetic isotopic
anomalies characterized by non-terrestrial stable isotopic compositions.
However, the multi-elemental isotopes in enstatite chondrites are similar
to those of Earth, indicating that the Earth formed predominantly by
accretion of enstatite chondrite-like materials. Here we show that the
proto-Earth accreted from inner Solar System materials with Mo isotopic
compositions that were possibly distinct from those in the modern
terrestrial mantle and enstatite chondrites. The s-process Mo isotopes in
the inner Solar System materials gradually deplete with increasing
heliocentric distance, and exhibit a small, uniform r-process deficit (~10
ppm for 95Mo) relative to the modern mantle. Consequently, the proto-Earth
developed a bulk composition that was probably enriched and depleted in
s-process and r-process Mo isotopes, respectively, relative to the modern
mantle. The Mo isotope offset in the modern mantle (relative to the
proto-Earth) was compensated by two-stage late accretion events occurring
after the cessation of core formation. These events, which first supplied
the inner Solar System materials, were followed by the formation of
carbonaceous
chondrite-like outer Solar System materials that were depleted and enriched
in s- and r-process Mo isotopes, respectively. A signature of the
isotopically distinct proto-Earth possibly exists in the lunar mantle,
which was less influenced by late accretion than the Earth, and in Archean
rocks that record the heterogeneous ancient terrestrial mantle.