THE Hf-W ISOTOPIC SYSTEM AND THE ORIGIN OF THE EARTH AND MOON

The Earth has a radiogenic W-isotopic composition compared to chondrites, demonstrating that it formed while ¹⁸²Hf (half-life 9 Myr) was extant in Earth and decaying to ¹⁸²W. This implies that Earth underwent early and rapid accretion and core formation, with most of the accumulation occurring in ∼10 Myr, and concluding approximately 30 Myr after the origin of the Solar System. The Hf-W data for lunar samples can be reconciled with a major Moon-forming impact that terminated the terrestrial accretion process ∼30 Myr after the origin of the Solar System. The suggestion that the proto-Earth to impactor mass ratio was 7:3 and occurred during accretion is inconsistent with the W isotope data. The W isotope data is satisfactorily modeled with a Mars-sized impactor on proto-Earth (proto-Earth to impactor ratio of 9:1) to form the Moon at ∼30 Myr.