  The recent derivation of a lower limit for the $^{14}$N/$^{15}$N ratio in
Saturn's ammonia, which is found to be consistent with the Jovian value,
prompted us to revise models of Saturn's formation using as constraints the
supersolar abundances of heavy elements measured in its atmosphere. Here we
find that it is possible to account for both Saturn's chemical and isotopic
compositions if one assumes the formation of its building blocks at $\sim$45 K
in the protosolar nebula, provided that the O abundance was $\sim$2.6 times
protosolar in its feeding zone. To do so, we used a statistical thermodynamic
model to investigate the composition of the clathrate phase that formed during
the cooling of the protosolar nebula and from which the building blocks of
Saturn were agglomerated. We find that Saturn's O/H is at least $\sim$34.9
times protosolar and that the corresponding mass of heavy elements ($\sim$43.1
\Mearth) is within the range predicted by semi-convective interior models.
