Chromium isotopes in CR chondrites reveal two distinct chondrule reservoirs

Abstract

Chondrules, the dominant components of chondrites, provide crucial insights into early solar system formation and evolution. Due to the variability of Cr isotopes in the solar system, they can serve as powerful tracers of chondrule genealogy and the transport of precursor materials in the evolving solar disk. High-precision mass-independent Cr isotope data for individual chondrules from the CR chondrite Shişr 033 reveal a bimodal distribution of ε⁵⁴Cr that defines two isotopically distinct chondrule populations. The major population exhibits ε⁵⁴Cr values (∼1.3) consistent with prior analyses of CR chondrules, while a minor population has significantly lower ε⁵⁴Cr (∼0.8), overlapping with CV-CO-CM chondrules. This suggests chondrule migration across spatial gaps in the protoplanetary disk, likely from the CV-CO-CM reservoir to the CR sub-reservoir. Further, ⁵³Mn–⁵³Cr systematics indicate that Mn–Cr fractionation in CR chondrule precursors occurred ∼0.7 Ma after solar system formation, predating the primary CR chondrule formation event at ∼3.8 Ma, inferred from ²⁶Al–²⁶Mg, and ¹⁸²Hf–¹⁸²W chronometry of individual chondrules. These findings support a model in which secondary disk substructures, likely linked to early Jupiter growth, facilitated isotopic heterogeneities and material transport in the outer solar system. The presence of CV-CO-CM chondrules in CR chondrites further suggests outward material transport in the protoplanetary disk before CR parent body accretion. This study provides new constraints on chondrule formation, precursor evolution, and planetary disk dynamics.