The initial abundance and distribution of 92Nb in the Solar System

Abstract

Niobium-92 is an extinct proton-rich nuclide, which decays to 92Zr with a half-life of 37 Ma. This radionuclide potentially offers a unique opportunity to determine the timescales of early Solar System processes and the site(s) of nucleosynthesis for p-nuclei, once its initial abundance and distribution in the Solar System are well established. Here we present internal Nb-Zr isochrons for three basaltic achondrites with known U-Pb ages: the angrite NWA 4590, the eucrite Agoult, and the ungrouped achondrite Ibitira. Our results show that the relative Nb-Zr isochron ages of the three meteorites are consistent with the time intervals obtained from the Pb-Pb chronometer for pyroxene and plagioclase, indicating that 92Nb was homogeneously distributed among their source regions. The Nb-Zr and Pb-Pb data for NWA 4590 yield the most reliable and precise reference point for anchoring the Nb-Zr chronometer to the absolute timescale: an initial 92Nb/93Nb ratio of $(1.4\pm 0.5)\times 10^{-5}$ at $4557.93\pm 0.36$ Ma, which corresponds to a 92Nb/93Nb ratio of $(1.7\pm 0.6)\times 10^{-5}$ at the time of the Solar System formation. On the basis of this new initial ratio, we demonstrate the capability of the Nb-Zr chronometer to date early Solar System objects including troilite and rutile, such as iron and stony-iron meteorites. Furthermore, we estimate a nucleosynthetic production ratio of 92Nb to the p-nucleus 92Mo between 0.0015 and 0.035. This production ratio, together with the solar abundances of other p-nuclei with similar masses, can be best explained if these light p-nuclei were primarily synthesized by photodisintegration reactions in Type Ia supernovae.

Accepted to Earth and Planetary Science Letters

References in corpus (5)

• Constraining the astrophysical origin of the p-nuclei through nuclear physics and meteoritic data
• The rp-Process in Neutrino-driven Winds
• Stellar origin of the 182Hf cosmochronometer and the presolar history of solar system matter
• Testing the role of SNe Ia for Galactic chemical evolution of p-nuclei with 2D models and with s-process seeds at different metallicities
• Radiogenic p-isotopes from SNIa, nuclear physics uncertainties and Galactic chemical evolution compared with values in primitive meteorites

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