Chronology of the oldest CC planetesimals inferred from Cr isotopes in group IVB irons

Magmatic iron meteorites sample the metal cores of differentiated planetesimals formed through segregation and crystallization of metallic melts. Group IVB irons are the most depleted in volatile elements and have the youngest Hf–W ages among magmatic irons [1]. Variations in their metallographic cooling rates suggest that the IVB core cooled without an insulating mantle, likely removed during a collisional disruption of the parent body [2]. To better understand the chronology of the IVB parent body, and to search for isotopic signatures that may record post-core formation re-mixing of metal and silicates, we applied the ⁵³Mn-⁵³Cr system to chromites from four IVB irons. Chromites were chosen because they are unaffected by cosmic ray-induced Cr isotope shifts [3]. All four samples exhibit indistinguishable nucleosynthetic ⁵⁴Cr, similar to CV chondrites, supporting a genetic link to the carbonaceous chondrite (CC) reservoir. However, the samples show distinct radiogenic ⁵³Cr, corresponding to an apparent ⁵³Mn–⁵³Cr model age spread of ~3 Ma. This spread likely does not reflect differences in metal segregation timing, as all samples yield identical Hf–W ages [1]. Instead, the ⁵³Cr variations likely result from the addition of mantle-derived radiogenic ⁵³Cr during partial metal–silicate re-equilibration caused by impact disruption. This interpretation is supported by metallographic cooling rates for outward crystallization of the IVB core [2] and our observation that later-crystallized IVBs display more radiogenic ⁵³Cr.

Ref. [1] Kruijer et al. (2014) Science, 344, 1150-1154. [2] Yang et al. (2010) GCA, 74, 4493-4506. [3] Anand et al. (2021) GPL, 20, 6-10.