Collisional erosion and the non-chondritic composition of the terrestrial planets

Abstract

The compositional variations among the chondrites inform us about cosmochemical fractionation processes during condensation and aggregation of solid matter from the solar nebula. These fractionations include: (i) variable Mg-Si-RLE ratios (RLE: refractory lithophile element), (ii) depletions in elements more volatile than Mg, (iii) a cosmochemical metal-silicate fractionation, and (iv) variations in oxidation state. Moon-to Mars-sized planetary bodies, formed by rapid accretion of chondrite-like planetesimals in local feeding zones within 10 6 years, may exhibit some of these chemical variations. However, the next stage of planetary accretion is the growth of the terrestrial planets from approximately 10 2 embryos sourced across wide heliocentric distances, involving energetic collisions, in which material may be lost from a growing planet as well as gained. While this may result in averaging out of the 'chondritic' fractionations, it introduces two non-chondritic chemical fractionation processes: post-nebular volatilization and preferential collisional erosion. In the latter, geochemically enriched crust formed previously is preferentially lost. That post-nebular volatilization was widespread is demonstrated by the non-chondritic Mn/Na ratio in all the small, differentiated, rocky bodies for which we have basaltic samples, including the Moon and Mars. The bulk silicate Earth (BSE) has chondritic Mn/Na, but shows several other compositional features in its pattern of depletion of volatile elements suggestive of non-chondritic fractionation. The whole-Earth Fe/Mg ratio is 2.1G0.1, significantly greater than the solar ratio of 1.9G0.1, implying net collisional erosion of approximately 10 per cent silicate relative to metal during the Earth's accretion. If this collisional erosion preferentially removed differentiated crust, the assumption of chondritic ratios among all RLEs in the BSE would not be valid, with the BSE depleted in elements according to their geochemical incompatibility. In the extreme case, the Earth would only have half the chondritic abundances of the highly incompatible, heat-producing elements Th, U and K. Such an Earth model resolves several geochemical paradoxes: the depleted mantle occupies the whole mantle, is completely outgassed in 40 Ar and produces the observed 4 He flux through the ocean basins. But the lower radiogenic heat production exacerbates the discrepancy with heat loss.

Composition of the Oceanic Crust

“…It should be pointed out that Lyubetskaya and Korenaga's (2007) estimate of bulk silicate Earth composition, like the earlier ones such as that of McDonough and Sun (1995), is based partly on the assumption of chondritic relative abundances of refractory lithophile elements. However, the nonchondritic 142 Nd/ 144 Nd ratios of all modern terrestrial material (Boyet and Carlson, 2005) make this assumption questionable and raise the possibility that the Earth is depleted in highly incompatible elements relative to chondrites (Caro and Bourdon, 2010;O'Neill and Palme, 2008). If so, MORB and the oceanic crust may be less incompatible element-depleted relative to the bulk silicate Earth than this figure suggests.…”

Early differentiation of the Earth and the Moon

“…The second hypothesis involving impact erosion of a differentiated crust was also proposed by Caro et al (2008) and is discussed at greater length by O'Neill & Palme (2008). Numerous models (e.g.…”

Major and trace element composition of the high 3He/4He mantle: Implications for the composition of a nonchonditic Earth

“…[28] We note that, in contrast to our approach of anchoring the new BSE composition to Lu, O'Neill and Palme [2008] anchor their nonchondritic BSE composition to a K abundance that provides a possible solution to the missing Ar paradox. Their approach results in highly incompatible elements being $50% lower than MS95 values, while our approach yields values that are only $30% lower.…”