  Context: The stable element hafnium (Hf) and the radioactive element thorium
(Th) were recently suggested as a suitable pair for radioactive dating of
stars. The applicability of this elemental pair needs to be established for
stellar spectroscopy. Aims: We aim at a spectroscopic determination of the
abundance of Hf and Th in the solar photosphere based on a \cobold 3D
hydrodynamical model atmosphere. We put this into a wider context by
investigating 3D abundance corrections for a set of G- and F-type dwarfs.
Method: High-resolution, high signal-to-noise solar spectra were compared to
line synthesis calculations performed on a solar CO5BOLD model. For the other
atmospheres, we compared synthetic spectra of CO5BOLD 3D and associated 1D
models. Results: For Hf we find a photospheric abundance A(Hf)=0.87+-0.04, in
good agreement with a previous analysis, based on 1D model atmospheres. The
weak Th ii 401.9 nm line constitutes the only Th abundance indicator available
in the solar spectrum. It lies in the red wing of an Ni-Fe blend exhibiting a
non-negligible convective asymmetry. Accounting for the asymmetry-related
additional absorption, we obtain A(Th)=0.09+-0.03, consistent with the
meteoritic abundance, and about 0.1 dex lower than obtained in previous
photospheric abundance determinations. Conclusions: Only for the second time,
to our knowledge, has am non-negligible effect of convective line asymmetries
on an abundance derivation been highlighted. Three-dimensional hydrodynamical
simulations should be employed to measure Th abundances in dwarfs if similar
blending is present, as in the solar case. In contrast, 3D effects on Hf
abundances are small in G- to mid F-type dwarfs and sub-giants, and 1D model
atmospheres can be conveniently used.
