¹Manfred Vogt, ¹Daniel Mahr, ²Andreas Pack, ¹Mario Trieloff
Geochimica et Cosmochimica Acta, (in Press), (in Press)
Open Source Link to Article [DOI: 10.1016/j.gca.2026.11.010]

¹Institut für Geowissenschaften, Ruprecht-Karls-Universität Heidelberg, INF 234-236, 69120 Heidelberg, Germany
²Geowissenschaftliches Zentrum, Abteilung Geochemie und Isotopengeologie, Georg-August-Universität Göttingen, Goldschmidtstraße 1, 37073 Göttingen, Germany
Copyright Elsevier

The ungrouped iron meteorite Washington County, so far unique for hosting solar-wind derived He and Ne within its metal, contains previously undocumented µm-sized chromite inclusions, extremely enriched in Mn and with highly variable oxygen isotopic compositions. The chromites of ≤ 13 µm in diameter occur as isolated, angular to subangular grains, which are erratically but frequently distributed within the metal of the iron meteorite. 76 inclusions were characterized by SEM–EDS, micro-Raman spectroscopy and electron microprobe analyses as Mn-bearing chromites (FeCr₂O₄), containing several wt% MnO, and manganochromites ((Mn,Fe)(Cr,V)₂O₄) with up to ∼ 27 wt% MnO, representing the Mn-richest natural chromite ever reported. δ¹⁸O, δ¹⁷O and Δ¹⁷O values of ten Mn-bearing chromites and five manganochromites were determined by Secondary Ion Mass Spectrometry (SIMS) to reveal potential genetic associations with other meteorites.

The δ¹⁸O values of individual chromite inclusions, with spatial distances of only a few millimeters, range from −3.52‰ to + 20.38‰, defining a huge measured isotopic spread – unprecedented among differentiated meteorites – while the mean Δ¹⁷O value of −0.07 ± 0.13‰ (2σ) is indistinguishable from the Terrestrial Fractionation Line (TFL). The measured oxygen isotopic spread shows no systematic relationship with chemical composition of the inclusions as both low- and high-Mn chromites show δ¹⁸O variations of ∼ 22‰ and ∼ 24‰, respectively. However, because potential matrix-dependent and other analytical effects cannot be fully quantified, the magnitude of the intrinsic isotopic variability remains uncertain. If a substantial component is intrinsic, the δ¹⁸O and δ¹⁷O variations are consistent with kinetic isotope fractionation during oxygen diffusion when chromites formed by solid-state exsolution within the FeNi metal upon cooling. Δ¹⁷O values overlap with those of enstatite chondrites, aubrites, and other non-carbonaceous (NC) meteorites but differ from most carbonaceous (CC) meteorites. A comparative analysis of oxygen isotope trends in NC versus CC meteorites shows that Δ¹⁷O values support classification of Washington County as NC-iron, suggesting a common formation region in relation to the NC population. Further literature review reveals additional NC-irons (i.e., irons of groups IIAB, IIG, IIIAB, IVA and IAB) that might, like Washington County, contain volume-correlated solar-type noble gases, potentially consistent with more intense exposure to solar irradiation in the inner solar system and incorporation of solar signatures preferentially into NC parent bodies.