Oxygen isotope and chemical composition of chromites in the unique iron meteorite Washington County constrain derivation from the inner Solar System non-carbonaceous (NC) reservoir

Iron meteorites provide key constraints on early Solar System differentiation and core formation processes [1, and refs. therein]. They furthermore convey insights into genetic relationships and accretionary environments of their parent bodies inferred from oxygen [2] and nucleosynthetic isotope anomalies [e.g., 3,4]. The hitherto ungrouped iron meteorite Washington County is particularly unique, as it is, so far, the only known specimen hosting volume-correlated, solar-wind derived He and Ne within its metal [5], implying an intense solar irradiation history preceding its formation. Despite the significance of Washington County, a genetic link with other meteorite groups as well as an association with non-carbonaceous (NC) or carbonaceous (CC) meteorites remain ambiguous.

Here, we report the discovery of previously undocumented chromite inclusions of ≤13 µm in size, occurring as isolated, angular to subangular grains, which are erratically distributed throughout the metal of Washington County, and use their chemical and oxygen isotopic signatures to trace its origin. Combined SEM–EDS, micro-Raman spectroscopy and electron microprobe analyses identify the mineral inclusions as Mn-bearing chromites (FeCr₂O₄) and manganochromites ((Mn,Fe)(Cr,V)₂O₄), with MnO contents reaching up to ~27 wt%, representing the most Mn-rich natural chromites reported to date. Secondary Ion Mass Spectrometry (SIMS) of 15 chromite inclusions, which are separated by only a few millimeters, reveal a huge δ¹⁸O spread with values ranging from –3.52‰ to +20.38‰, defining an isotopic variability unprecedented among differentiated meteorites. Despite this extreme mass-dependent isotope fractionation, the mean Δ¹⁷O of -0.07 ± 0.13‰ (2σ) tallies with the terrestrial fractionation line (TFL) and overlaps with enstatite chondrites, aubrites and other NC meteorites. The oxygen isotopic data appear uncorrelated with the chromite chemistry ruling out analytical artifacts and instead indicate kinetic isotope fractionation during oxygen diffusion associated with solid-state exsolution of chromite from cooling Fe–Ni metal.

Comparative oxygen isotope systematics of NC and CC iron meteorites [2,3,4] strongly support classification of Washington County as non-carbonaceous iron and infer a common formation region in the inner Solar System. Together with literature evidence for solar-type noble gases in several other NC iron meteorites [5], our results suggest that incorporation of solar signatures into metallic cores preferentially occurred in NC parent bodies, placing new constraints on early Solar System accretion and core segregation.

[1] Scott, E.R.D. (2020), Iron Meteorites: Composition, Age, and Origin, Oxford University Press, 75 pp. [2] Clayton, R.N. and Mayeda, T.K. (1996), Geochim. Cosmochim. Ac. 60, 1999-2017. [3] Worsham, E.A. et al. (2019), Earth Planet. Sci. Lett. 521, 103-112. [4] Kleine, T. et al. (2020), Space Sci. Rev. 216, 55. [5] Vogt, M. et al. (2021), Commun. Earth Environ. 2, 92.