title: Geochemical arguments for an Earth-like Moon-forming impactor.
abstract: Geochemical evidence suggests that the material accreted by the Earth did not change in nature during Earth's accretion, presumably because the inner protoplanetary disc had uniform isotopic composition similar to enstatite chondrites, aubrites and ungrouped achondrite NWA 5363/5400. Enstatite meteorites and the Earth were derived from the same nebular reservoir but diverged in their chemical evolutions, so no chondrite sample in meteorite collections is representative of the Earth's building blocks. The similarity in isotopic composition (Δ(17)O, ε(50)Ti and ε(54)Cr) between lunar and terrestrial rocks is explained by the fact that the Moon-forming impactor came from the same region of the disc as other Earth-forming embryos, and therefore was similar in isotopic composition to the Earth. The heavy δ(30)Si values of the silicate Earth and the Moon relative to known chondrites may be due to fractionation in the solar nebula/protoplanetary disc rather than partitioning of silicon in Earth's core. An inversion method is presented to calculate the Hf/W ratios and ε(182)W values of the proto-Earth and impactor mantles for a given Moon-forming impact scenario. The similarity in tungsten isotopic composition between lunar and terrestrial rocks is a coincidence that can be explained in a canonical giant impact scenario if an early formed embryo (two-stage model age of 10-20 Myr) collided with the proto-Earth formed over a more protracted accretion history (two-stage model age of 30-40 Myr).
authors.affiliation: Origins Laboratory, Department of the Geophysical Sciences and Enrico Fermi Institute, University of Chicago, 5734 South Ellis Avenue, Chicago, IL 60637, USA dauphas@uchicago.edu.
authors.forename: Nicolas
authors.initials: N
authors.lastname: Dauphas
authors.affiliation: Origins Laboratory, Department of the Geophysical Sciences and Enrico Fermi Institute, University of Chicago, 5734 South Ellis Avenue, Chicago, IL 60637, USA.
authors.forename: Christoph
authors.initials: C
authors.lastname: Burkhardt
authors.affiliation: Institute of Geophysics and Planetary Physics, University of California, Los Angeles, CA 90095-1567, USA.
authors.forename: Paul H
authors.initials: PH
authors.lastname: Warren
authors.affiliation: Isotope Laboratory, Department of Earth and Space Sciences, University of Washington, Seattle, WA 98195, USA.
authors.forename: Teng
authors.initials: T
authors.lastname: Fang-Zhen
keywords: Moon; impact; isotopes; origin
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