Precise initial abundance of Niobium-92 in the Solar System and implications for p-process nucleosynthesis

The niobium-92-zirconium-92 (⁹²Nb-⁹²Zr) decay system with a half-life of 37 Ma has great potential to date the evolution of planetary materials in the early Solar System. Moreover, the initial abundance of the p-process isotope ⁹²Nb in the Solar System is important for quantifying the contribution of p-process nucleosynthesis in astrophysical models. Current estimates of the initial ⁹²Nb/⁹³Nb ratios have large uncertainties compromising the use of the ⁹²Nb-⁹²Zr cosmochronometer and leaving nucleosynthetic models poorly constrained. Here, the initial ⁹²Nb abundance is determined to high precision by combining the ⁹²Nb-⁹²Zr systematics of cogenetic rutiles and zircons from mesosiderites with U-Pb dating of the same zircons. The mineral pair indicates that the ⁹²Nb/⁹³Nb ratio of the Solar System started with (1.66 ± 0.10) × 10^-5, and their ⁹²Zr/⁹⁰Zr ratios can be explained by a three-stage Nb-Zr evolution on the mesosiderite parent body. Because of the improvement by a factor of 6 of the precision of the initial Solar System ⁹²Nb/⁹³Nb, we can show that the presence of ⁹²Nb in the early Solar System provides further evidence that both type Ia supernovae and core-collapse supernovae contributed to the light p-process nuclei.