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

The short-lived radionuclide, niobium-92 (Nb-92), 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 Nb-92/Nb-93 ratio of (2.72 +/- 0.25) x 10(-5) at the NWA 6704 formation, 4562.76 +/- 0.30 million years ago. This corresponds to a Nb-92/Nb-93 ratio of (2.96 +/- 0.27) x 10(-5) at the time of solar system formation, which is similar to 80% higher than the values obtained from meteorites formed in the inner disk. The results suggest that a significant proportion of the solar Nb-92 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 Al-26 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.

Automated summary

In this study, the niobium-zirconium (Nb-Zr) isochron dating of Northwest Africa (NWA) 6704 is presented, which formed about 4562.76 million years ago with an initial Nb-92/Nb-93 ratio of (2.72 +/- 0.25) x 10(-5). This ratio is approximately 80% higher than values obtained from meteorites formed in the inner disk, suggesting that a significant proportion of solar Nb-92 was produced by a nearby core-collapse supernova (CCSN) and that the outer disk was more enriched in CCSN ejecta. The findings highlight the importance of NWA 6704 in mapping relative Nb-Zr ages of objects in the outer solar system onto the absolute timescale.