Earth's accretion inferred from iron isotopic anomalies of supernova nuclear statistical equilibrium origin

Abstract

Nucleosynthetic Fe isotopic anomalies in meteorites may be used to learn about the early evolution of the solar system and to identify the origin and nature of the material that built the terrestrial planets. Using high-precision iron isotopic data of 23 iron meteorites from nine major chemical groups we show that all iron meteorites define the same dichotomy between non-carbonaceous (NC) and a carbonaceous (CC) meteorites previously observed for other elements. The Fe isotopic anomalies are predominantly produced by variations in ⁵⁴Fe, where all CC iron meteorites are characterized by an excess in ⁵⁴Fe relative to NC iron meteorites. This excess in ⁵⁴Fe is accompanied by an excess in ⁵⁸Ni observed in the same CC meteorite groups. Together, these overabundances of ⁵⁴Fe and ⁵⁸Ni are explained by nuclear statistical equilibrium either in type Ia supernovae or in the Si/S shell of core-collapse supernovae. The Fe isotopic composition of Earth's mantle plots on or close to correlations defined by Fe, Mo, and Ru isotopic anomalies in iron meteorites, indicating that throughout Earth's accretion, the isotopic composition of its building blocks did not drastically change. While Earth's mantle has a similar Fe isotopic composition to CI chondrites, the latter are clearly distinct from Earth's mantle for other elements (e.g., Cr and Ni) whose delivery to Earth coincided with Fe. The fact that CI chondrites exhibit large Cr and Ni isotopic anomalies relative to Earth's mantle, therefore, demonstrates that CI chondrites are unlikely to have contributed significant Fe to Earth and are not its main building blocks.