Evidence for 92gNb in the Early Solar System and Evaluation of a New p-Process Cosmochronometer from 92gNb/92Mo
Initial abundances of extinct radionuclides in the solar system constrain both the history of nucleosynthesis in the Galaxy and the age of the solar system's parental molecular cloud complex (PMCC). 92gNb is a p-process radionuclide with a half-life of ˜36 Myr. Evidence for the presence of 92gN in the early solar system is based upon a well-resolved 92Zr excess observed in Zr separated from a 110 μg sample of rare high-Nb/Zr rutile from the Toluca iron meteorite. The initial 92gN/93gN ratio in the rutile was (1.6±0.3) x 10-5, no later than ˜10 Myr after the formation of the solar system. 92gN is indexed to stable p-only 92Mo to infer the extent of its decay during presolar Galactic history: 92gN/92Mo = (2.9±0.6) x 10-5 in the solar abundance distribution. This is 0.7% of the theoretically estimated nucleosynthetic production ratio (˜4 x 10-3) for the p-process in both Type Ia and Type II supernova models, indicating a 9-29 Gyr model age range for the p-process in the Galactic disk at the solar Galactocentric radius. A best estimate of 15 Gyr is closely consistent with a 12±2 Gyr disk age determined independently from nuclear cosmochronology, photometry-isochrone stellar ages, and the white dwarf luminosity function. Alternatively, if the age of the disk is known, then 92gN/92Mo can be used to estimate the age of the PMCC. The results suggest that the Sun formed 25±15 Myr after the formation of its parental complex and therefore likely in a highly evolved cloud in the vicinity of an OB association. Other shorter lived extinct radionuclide abundances are consistent with self-contamination of the cloud by one or more massive star supernovae and provide further independent support for the OB association model for the origin of the solar system.