Planetary accretion and core formation inferred from Ni isotopes in enstatite meteorites

Abstract

Nickel is a siderophile and near-refractory element, making its isotopes a potential tool for tracing planetary accretion and differentiation. However, the origin of the Ni stable isotope difference between bulk silicate Earth (BSE) and chondrites remains enigmatic. To address this problem, we report high precision Ni stable isotope data of enstatite chondrites and achondrites that possess similar mass independent O and Ni isotope compositions like the Earth-Moon system. Bulk enstatite chondrites have δ 60/58 Ni values of 0.24 ± 0.08 ‰ (2 s.d., n = 13). Enstatite achondrites, including main-group aubrites, Shallowater and Itqiy, show relatively large δ 60/58 Ni variations, ranging from 0.03 ± 0.02 ‰ to 0.57 ± 0.04 ‰. This could reflect fractionations between sulfide and metal phases, as is evidenced by correlation between their S/Ni ratios and δ 60/58 Ni values. In enstatite achondrites, Ni is mainly hosted in metal and to a lesser extent in sulfides, so δ 60/58 Ni values in enstatite achondrites may represent the Ni isotopic values of the cores of their parent bodies. The overlapping δ 60/58 Ni values between bulk enstatite achondrites and enstatite chondrites indicate limited Ni stable isotope fractionation during core formation processes on reduced, sulfur-rich parent bodies. The Ni stable isotope gap between chondrites and the BSE could be possibly explained by chondrule-rich accretion model.

Ni isotopes provide a glimpse of Earth’s pre-late-veneer mantle

“…2). This offset from chondrite (Δ 60 Ni chondrites-NUB ≈ 0.07 ‰) is smaller than estimates from previous studies ( 10 , 11 , 31 ), suggesting Δ 60 Ni chondrites-BSE ≈ 0.13 ‰, enabling us to relax the previous constraint on the Moon-forming giant impactor ( 10 ). On the basis of mass balance, if using the older estimates of pre–late accretion mantle δ 60 Ni, the materials accreted to the proto-Earth mantle would need to have δ 60 Ni values as low as −0.35‰.…”

“…The δ 60 Ni of the ~3.8-Ga NUB peridotites are clearly slightly heavier than that (δ 60 Ni = 0.11 ± 0.01‰, 95% CI) of the BSE value estimated from unmetasomatized post-Archean peridotites (Fig. 2) (10-12) but obviously lighter than the average (δ 60 Ni = 0.23 ± 0.02‰, 95% CI) of bulk chondrites (10,15,31,32) (Fig. 2).…”

“…Blue, purple, and gray shaded fields represent the 95% CIs for the NUB sample group (δ 60 Ni = 0.16 ± 0.01‰; altered rocks are excluded from the mean), BSE estimate [δ 60 Ni = 0.11 ± 0.01‰; recalculated from 40 unmetasomatized post-Archean mantle peridotites; data from ( 10 – 12 )] and different groups of chondrites (δ 60 Ni = 0.23 ± 0.02‰), respectively. The δ 60 Ni values of ordinary (OC), carbonaceous (CC), and enstatite (EC) chondrites; ureilites; iron meteorites; and lunar meteorites ( 10 , 15 , 31 , 32 ) are also shown for comparison.…”

“…In a further extension of these results, it was proposed that an aubrite-like differentiated planetary body (Theia) whose mantle was enriched in light Ni isotopes, collided with, and merged into the proto-Earth during the Moon-forming giant impact ( 10 , 32 ), leading to a lowering of Earth’s mantle from an approximately chondritic composition (δ 60 Ni = 0.23 ± 0.02‰) to the modern estimate (δ 60 Ni = 0.11 ± 0.01‰) for the BSE. Some researchers ( 31 ) have cast doubt on whether the proto-Earth was chondritic in Ni isotopes, introducing other possible scenarios such as chondrule-rich accretion ( 15 ) and nebular fractionation ( 52 ), but these models are difficult to reconcile with isotopic evidence from lithophile to siderophile systems that support a chondritic character to Earth’s building blocks ( 1 , 3 , 4 ). Although still controversial, on the basis of the similarities of nucleosynthetic (mass-independent) isotopic compositions of both lithophile and siderophile elements (e.g., O, Ti, Nd, Cr, Ni, Mo, and Ru), enstatite chondrite or enstatite chondrite-like differentiated planetary bodies were considered as the most likely building blocks for Earth ( 4 , 19 , 53 ).…”

“…Assuming that the positive nucleosynthetic Ru isotopic anomaly (ɛ 100 Ru = 0.22 ± 0.04) ( 19 ) and light mass-dependent Ni isotopic composition (δ 60 Ni = 0.16 ± 0.01‰) of the NUB mantle source were both imparted by the Moon-forming impactor, this signature is not represented by any known chondritic meteorites because they all exhibit negative nucleosynthetic Ru isotopic anomalies ( 55 ) and heavy mass-dependent Ni isotopic compositions (Fig. 2) ( 10 , 15 , 31 , 32 ). Nevertheless, both the s-process Ru excess and isotopically light Ni exhibited by Earth’s pre-late-veneer mantle require that the building materials (i.e., moon-forming impactor) that contributed to the early stages of Earth’s growth came from the innermost region of the Solar System characterized by a reduced, noncarbonaceous component ( 10 , 19 ).…”