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.
Keywords: Niobium-92; Zr isotopes; mesosiderite; p-process nucleosynthesis; short-lived radionuclide.