EVIDENCE FOR NB-92G IN-THE EARLY SOLAR-SYSTEM AND EVALUATION OF A NEWP-PROCESS COSMOCHRONOMETER FROM NB-92G MO-92/

Initial abundances of extinct radionuclides in the solar system constr ain both the history of nucleosynthesis in the Galaxy and the age of t he solar system's parental molecular cloud complex (PMCC). Nb-92g i, a p-process radionuclide with a half-life of similar to 36 Myr. Evidenc e for the presence of Nb-92g in the early solar system is based upon a well-resolved Zr-92 excess observed in Zr separated from a 110 mu g s ample of rare high-Nb/Zr rutile from the Toluca iron meteorite. The in itial Nb-92g/Nb-93 ratio in the rutile was (1.6 +/- 0.3) x 10(-5), no later than similar to 10 Myr after the formation of the solar system. Nb-92g is indexed to stable p-only Mo-92 to infer the extent of its de cay during presolar Galactic history: Nb-92g/ Mo-92 = (2.9 +/- 0.6) x 10(-5) in the solar abundance distribution. This is 0.7% of the theore tically estimated nucleosynthetic production ratio (similar to 4 x 10( -3)) for the p-process in both Type Ia and Type II supernova models, i ndicating 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 independen tly from nuclear cosmochronology, photometry-isochrone stellar ages, a nd the white dwarf luminosity function. Alternatively, if the age of t he disk is known, then Nb-92g/Mo-92 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 highl y evolved cloud in the vicinity of an OB association. Other shorter li ved extinct radionuclide abundances are consistent with self-contamina tion of the cloud by one or more massive star supernovae and provide f urther independent support for the OB association model for the origin of the solar system.