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).

Earth's moderately volatile element composition may not be chondritic: Evidence from In, Cd and Zn

“…Isotopic compositions of Mo, Cr and Ti display nucleosynthetic anomalies in meteorites that, with the exception of enstatite chondrites and aubrites, do not match the isotopic composition of the BSE (e.g., Dauphas et al, 2014;Javoy et al, 2010). The highly depleted abundances of moderately volatile elements such as Zn and Rb in the bulk Earth argue against a simple enstatite chondrite model for the volatile composition of bulk Earth (Fig.…”

“…6) (Kong et al, 1997;Wang and Lipschutz, 2005). Isotopic similarities of refractory elements between Earth's building materials and enstatite chondrites probably indicate a common reservoir for their refractory components, perhaps in the terrestrial planet formation region (Dauphas et al, 2014). However, the chemical fractionation history of refractory elements in these materials may be decoupled from the volatile elements as is the case in chondrites (e.g., Albarède, 2009;Kadlag and Becker, 2015).…”

“…1 and 6). Thus, the abundances of moderately volatile elements in Earth's building materials most likely reflect pre-accretionary volatile element depletion, suggesting that the predominant fraction of bulk Earth cannot be reconstructed by a mixture of known primitive chondrites (e.g., Campbell and O'Neill, 2012;Dauphas et al, 2014;Drake and Righter, 2002).…”

In search of late-stage planetary building blocks

“…This is because it is unlikely that two, very different size bodies simultaneously segregated cores in ways that resulted in mantles with nearly identical Hf/W, which is required to generate identical W isotopic compositions. One possible explanation for the W isotopic similarity between the Earth and Moon at the time of formation is that through happenstance, the impactor and Earth formed from genetically similar materials, and had roughly similar average core segregation ages (e.g., Dauphas et al, 2014). Another option is that the Earth"s mantle and the proto-lunar disk somehow chemically and isotopically equilibrated prior to lunar coalescence (Pahlevan and Stevenson, 2007), as has been suggested by the similarity of isotopic compositions for other elements, such as O, Ca, Ti, and Si (e.g., Wiechert et al, 2001;Trinquier et al, 2009;Simon et al, 2010;Zhang et al, 2012;Fitoussi and Bourdon, 2012).…”

The feeding zones of terrestrial planets and insights into Moon formation

“…If so, an isotopically identical Earth and Theia may not conflict with an isotopically distinct Mars in this case. This could also potentially allow for an isotopically uniform inner solar nebula (Javoy et al 2010;Dauphas et al 2014), with Mars being polluted by more distant planetesimals. However, this scenario is untested and should be the subject of future studies.…”