Identification of the giant impactor Theia in lunar rocks

Abstract

The Moon was probably formed by a catastrophic collision of the proto-Earth with a planetesimal named Theia. Most numerical models of this collision imply a higher portion of Theia in the Moon than in Earth. Because of the isotope heterogeneity among solar system bodies, the isotopic composition of Earth and the Moon should thus be distinct. So far, however, all attempts to identify the isotopic component of Theia in lunar rocks have failed. Our triple oxygen isotope data reveal a 12 ± 3 parts per million difference in Δ(17)O between Earth and the Moon, which supports the giant impact hypothesis of Moon formation. We also show that enstatite chondrites and Earth have different Δ(17)O values, and we speculate on an enstatite chondrite-like composition of Theia. The observed small compositional difference could alternatively be explained by a carbonaceous chondrite-dominated late veneer.

The origin of aubrites: Evidence from lithophile trace element abundances and oxygen isotope compositions

“…The first possibility is unlikely because enstatite chondrites, which are the best analogs for the parent materials to aubrites, have much more heterogeneous ∆ 17 O values [from -0.32 to +0.16 ‰ for twelve EH3-6 chondrites, and from -0.11 to +0.07 ‰ for sixteen EL3-6 chondrites taken into account only falls and HCl treated finds (Newton et al, 2000)]. Moreover, the level of ∆ 17 O homogeneity shown by the aubrite falls (± 0.010 ‰ (2σ)) is equivalent to that of other bodies which show evidence for an early global stage of melting i.e., 4-Vesta [± 0.014 ‰ (2σ), Greenwood et al, 2005Greenwood et al, , 2014Scott et al, 2009], the angrite-parent body [± 0.014 ‰ (2σ), Greenwood et al, 2005], the main-group pallasites parent body [± 0.016 ‰ (2σ), Greenwood et al, 2006Greenwood et al, , 2015, the Moon [< ± 0.021 ‰ (2σ), Spicuzza et al, 2007;Hallis et al, 2010;Herwartz et al, 2014;Young et al, 2016] and Mars [± 0.026 ‰ (2σ), Franchi et al, 1999]. Thus, the narrow range of oxygen isotope compositions displayed by the aubrites indicates that some form of isotopic homogenization took place on their parent body.…”

“…O plot for aubrites (Clayton and Mayeda, 1996;Newton et al, 2000;Miura et al, 2006;Keil et al, 2011, and this work). Chondritic fields are drawn with data obtained by Clayton et al (1991), Newton et al (2000), and Herwartz et al (2014). (2010).…”

Copper isotope evidence for large-scale sulphide fractionation during Earth’s differentiation

“…If this is the case, then our data would seem to support their model; our estimate for BSE is almost identical to the Cu isotope composition of CI chondrites. However, more recent isotope data seems to rule out a CI-like impactor; in particular, precise lunar O isotope data suggests that the impactor had an enstatite chondrite isotope signature (Herwartz et al, 2014). In this instance, again, the enstatite chondrite model would seem most representative of bulk Earth.…”

The feeding zones of terrestrial planets and insights into Moon formation

“…In contrast, meteorites from Mars and Vesta are distinctly different from the Earth, with ∆ 17 O val-ues of 0.32 and -0.28 , respectively (Franchi et al 1999;Clayton & Mayeda 1996). More recent work hints at a slight difference between the Moon and the Earth of ∆ 17 O 0.012 , but the fact remains that the Moon is much more isotopically similar to Earth than samples from any other large object in the solar system (Herwartz et al 2014).…”