Nickel isotopic evidence for late-stage accretion of Mercury-like differentiated planetary embryos

Abstract

Earth’s habitability is closely tied to its late-stage accretion, during which impactors delivered the majority of life-essential volatiles. However, the nature of these final building blocks remains poorly constrained. Nickel (Ni) can be a useful tracer in characterizing this accretion as most Ni in the bulk silicate Earth (BSE) comes from the late-stage impactors. Here, we apply Ni stable isotope analysis to a large number of meteorites and terrestrial rocks, and find that the BSE has a lighter Ni isotopic composition compared to chondrites. Using first-principles calculations based on density functional theory, we show that core-mantle differentiation cannot produce the observed light Ni isotopic composition of the BSE. Rather, the sub-chondritic Ni isotopic signature was established during Earth’s late-stage accretion, probably through the Moon-forming giant impact. We propose that a highly reduced sulfide-rich, Mercury-like body, whose mantle is characterized by light Ni isotopic composition, collided with and merged into the proto-Earth during the Moon-forming giant impact, producing the sub-chondritic Ni isotopic signature of the BSE, while delivering sulfur and probably other volatiles to the Earth.

Nickel and Chromium Stable Isotopic Composition of Ureilites: Implications for the Earth's Core Formation and Differentiation of the Ureilite Parent Body

“…Recently, Wang et al. (2021) proposed that the light Ni isotopic composition of the Earth was a consequence of the Moon forming a giant impact with a highly reduced planet with low‐δ 60/58 Ni value in the sulfur‐rich mantle. Based on the siderophile element patterns (Warren et al., 2006), this hypothesis is also consistent with that UPB partitioned sulfur dominantly into the core, leaving a sulfur‐poor mantle that has chondritic Ni isotope compositions.…”

“…All the supporting data can be found in the cited references (Barrat et al, 2015;Bischoff et al, 2014;Bonnand, Parkinson, & Anand, 2016;Bonnand, Williams, et al, 2016;Cameron et al, 2009;Chernonozhkin et al, 2016;Gall et al, 2017;Jerram et al, 2020;Klaver et al, 2020;Schoenberg et al, 2008Schoenberg et al, , 2016P. Sossi et al, 2018;Steele et al, 2012;Wang et al, 2021;Zhu et al, 2019, ;Zhu, Moynier, Schiller, Wielandt, et al, 2020;Zhu, Moynier, Schiller, Alexander, Barrat, et al, 2021;. one anonymous reviewer for insightful comments.…”

“…The Ni stable isotope composition of UPB mantle can be estimated by the average δ 60/58 Ni values of all the main-group ureilites: 0.26 ± 0.13‰ (2SD), ±0.03‰ (2SE, N = 22), which is indistinguishable from that of chondrites δ 60/58 Ni = 0.23 ± 0.14‰ (2SD) ± 0.02‰ (2SE, N = 37; Table S1 in Supporting Information S1) (Cameron et al, 2009;Chernonozhkin et al, 2016;Gall et al, 2017;Klaver et al, 2020;Moynier et al, 2007;Wang et al, 2021), including ordinary chondrites (OCs, δ 60/58 Ni = 0.25 ± 0.18‰, 2SD, N = 16) that have close nucleosynthetic anomaly signatures (e.g., Ca 48 , Ti 50 , Cr 54 and Ni 62 ) to ureilites (Quitté et al, 2010;Schiller et al, 2018;Trinquier et al, 2009;Yamakawa et al, 2010;Zhu, Moynier, Schiller, Wielandt, et al, 2020) (Figure 2). Thus, the UPB mantle has the same Ni stable isotope composition as UPB core, and core formation process for UPB did not fractionate Ni stable isotopes.…”

“…Hence, Ni stable isotopes may be robust to trace planetary core formation by comparison between the compositions of planetary silicate fractions and chondrites that are taken as chemical proxies for bulk Earth and other terrestrial planets (Allègre et al., 1995), considering that the actual samples from the core are not accessible. Previous studies have comprehensively and systematically investigated the Ni stable isotope composition (expressed as δ 60/58 Ni, mass‐dependent per mil deviation of Ni 60 /Ni 58 ratios relative to NIST SRM 986) of chondrites with an average δ 60/58 Ni value of 0.23 ± 0.14‰ (2SD; Figure S1 in Supporting Information ) (Cameron et al., 2009; Chernonozhkin et al., 2016; Gall et al., 2017; Klaver et al., 2020; Moynier et al., 2007; Wang et al., 2021) and of the bulk silicate Earth (BSE) with an average δ 60/58 Ni value of 0.10 ± 0.07‰ (2SD) (Gall et al., 2017; Gueguen & Rouxel, 2021; Klaver et al., 2020; Saunders et al., 2020; Wang et al., 2021). Klaver et al.…”

“…Klaver et al (2020) first observed the Ni isotope difference between chondrites and BSE, that is, Δ 60/58 Ni Chondrites-BSE = ∼0.13‰, and interpreted it by terrestrial core formation. However, both ab-initio calculation (Guignard et al, 2020;Wang et al, 2021) and high-pressure experiments (Guignard et al, 2020;Lazar et al, 2012) show that core segregation may not effectively induce Ni stable isotope fractionation. Comparison of Ni stable isotope composition of chondrites and the silicate fractions of other planets in the Solar System is another way to test whether core formation fractionates Ni stable isotopes, and the key to unravel the Ni isotope paradox between chondrites and BSE.…”