  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.
