  NGC1851 possibly shows a spread in [Fe/H], but the relation between this
spread and the division in the SGB is unknown. We obtained blue (3950-4600 A)
intermediate resolution (R~8,000) spectra for 47 stars on the bright and 30 on
the faint SGB of NGC 1851 (b-SGB and f-SGB, respectively). The determination of
the atmospheric parameters to extremely high internal accuracy leads to small
errors when comparing different stars in the cluster. We found that the b-SGB
is slightly more metal-poor than the f-SGB, with [Fe/H]=-1.227+/-0.009 and
[Fe/H]=-1.162+/- 0.012, respectively. This implies that the f-SGB is only
slightly older by ~0.6 Gyr than the b-SGB if the total CNO abundance is
constant. There are more C-normal stars in the b-SGB than in the f-SGB. This is
consistent with what is found for HB stars, if b-SGB are the progenitors of red
HB stars, and f-SGB those of blue HB ones. The abundances of the n-capture
elements Sr and Ba have a bimodal distribution, reflecting the separation
between f-SGB (Sr and Ba-rich) and b-SGB stars (Sr and Ba-poor). In both
groups, there is a clear correlation between [Sr/Fe] and [Ba/Fe], suggesting
that there is a real spread in the abundances of n-capture elements. There is
some correlation between C and Ba abundances, while the same correlation for Sr
is much more dubious. We identified six C-rich stars, which have a moderate
overabundance of Sr and Ba and rather low N abundances. This group of stars
might be the progenitors of these on the anomalous RGB in the (v, v-y) diagram.
These results are discussed within different scenarios for the formation of
NGC1851. It is possible that the two populations originated in different
regions of an inhomogeneous parent object. However, the striking similarity
with M22 calls for a similar evolution for these two clusters. Deriving
reliable CNO abundances for the two sequences would be crucial.
