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

TL;DR

This study establishes the initial abundance and distribution of extinct 92Nb in the early Solar System using meteorite data, enabling precise dating of early Solar System events and insights into nucleosynthesis processes.

Contribution

It provides the first well-constrained initial 92Nb/93Nb ratio and demonstrates the use of Nb-Zr chronometry for dating early Solar System objects.

Findings

Initial 92Nb/93Nb ratio at Solar System formation: (1.7 ± 0.6) × 10^{-5}

Homogeneous distribution of 92Nb among meteorite source regions

Nucleosynthetic production ratio of 92Nb to 92Mo suggests origin in Type Ia supernovae

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.