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

The niobium-92-zirconium-92 (Nb-Zr) 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 -process isotope Nb in the Solar System is important for quantifying the contribution of -process nucleosynthesis in astrophysical models. Current estimates of the initial Nb/Nb ratios have large uncertainties compromising the use of the Nb-Zr cosmochronometer and leaving nucleosynthetic models poorly constrained. Here, the initial Nb abundance is determined to high precision by combining the Nb-Zr systematics of cogenetic rutiles and zircons from mesosiderites with U-Pb dating of the same zircons. The mineral pair indicates that the Nb/Nb ratio of the Solar System started with (1.66 ± 0.10) × 10, and their Zr/Zr 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 Nb/Nb, we can show that the presence of Nb in the early Solar System provides further evidence that both type Ia supernovae and core-collapse supernovae contributed to the light -process nuclei.