Evidence for Enrichment of Niobium-92 in the Outer Protosolar Disk

The short-lived radionuclide, niobium-92 (92Nb), has been used to estimate the site of nucleosynthesis for p-nuclei and the timing of planetary differentiation, assuming that it was uniformly distributed in the early solar system. Here, we present the internal niobium–zirconium (Nb–Zr) isochron dating of Northwest Africa (NWA) 6704, an achondrite thought to form in the outer protosolar disk due to nucleosynthetic isotope similarities with carbonaceous chondrites. The isochron defines an initial 92Nb/93Nb ratio of (2.72 ± 0.25) × 10−5 at the NWA 6704 formation, 4562.76 ± 0.30 million years ago. This corresponds to a 92Nb/93Nb ratio of (2.96 ± 0.27) × 10−5 at the time of solar system formation, which is ∼80% higher than the values obtained from meteorites formed in the inner disk. The results suggest that a significant proportion of the solar 92Nb was produced by a nearby core-collapse supernova (CCSN) and that the outer disk was more enriched in CCSN ejecta, which could account for the heterogeneity of short-lived 26Al and nucleosynthetic stable-isotope anomalies across the disk. We propose that NWA 6704 serves as the best anchor for mapping relative Nb–Zr ages of objects in the outer solar system onto the absolute timescale.