Tracing the origin of volatiles on Earth using nitrogen isotope ratios in iron meteorites

Abstract

Understanding the relationships between the nitrogen (N) isotope ratios of early solar system planetesimals and terrestrial reservoirs is crucial for tracing the origin of volatiles on Earth. The Earth primarily grew from planetesimals and planetary embryos that accreted rapidly (within ∼1–2 Ma after CAIs) in the inner solar system, also known as the non-carbonaceous (NC) reservoir. Magmatic iron meteorites, which sample the metallic cores of the earliest solar system planetesimals, have emerged as a promising proxy in this exercise. NC irons are distinctly ¹⁵N-poor compared to their CC (carbonaceous or outer solar system) counterparts. However, the utility of this proxy is limited by the lack of knowledge of N isotope fractionation during core crystallization. Using high pressure-high temperature experiments, we show that equilibrium N isotopic fractionation between solid and liquid metal (Δ¹⁵N^{solid–liquid} = δ¹⁵N^solid − δ¹⁵N^liquid) is limited (≤1.2 ‰) under conditions relevant for core crystallization. This, combined with the siderophile character of N and limited equilibrium N isotope fractionation during core-mantle differentiation, suggests that the δ¹⁵N values of iron meteorites accurately represent the N isotopic composition of their parent bodies. Unlike the variation in the N isotope ratios of NC and CC chondrites, which can be attributed to the effects of parent-body processes acting on organic precursors, the ¹⁵N-poor nature of NC irons relative to CC irons likely offers the most definitive evidence for the distinct N isotopic compositions of the earliest inner and outer solar system planetesimals. The N isotopic composition of Earth’s primordial mantle (δ¹⁵N = <−40 ‰) suggests that it retains the memory of the early accretion of ¹⁵N-poor NC iron meteorite parent body-like planetesimals. The early accreted ¹⁵N-poor nitrogen may be stored in the deep mantle, segregated into the core, or lost to space during atmospheric loss caused by impacts. This signature was overprinted by the subsequent accretion and admixing of CC materials, which is reflected in the relatively ¹⁵N-rich nature of Earth’s atmosphere (δ¹⁵N = 0) and convecting mantle (δ¹⁵N = −5 ‰).