  We present the abundance analysis of a sample of more than 120 red giants in
the globular cluster (GC) NGC 1851, based on FLAMES spectra. We find a small
but detectable metallicity spread. This spread is compatible with the presence
of two different groups of stars with a metallicity difference of 0.06-0.08
dex, in agreement with earlier photometric studies. If stars are divided into
these two groups according to their metallicity, both components show a Na-O
anticorrelation (signature of a genuine GC nature) of moderate extension. The
metal-poor stars are more concentrated than the metal-rich ones. We tentatively
propose the hypothesis that NGC 1851 formed from a merger of two individual GCs
with a slightly different Fe and alpha-element content, and possibly an age
difference up to 1 Gyr. This is supported also by number ratios of stars on the
split subgiant and on the bimodal horizontal branches. The distribution of
n-capture process elements in the two components also supports the idea that
the enrichment must have occurred in each of the structures separately, and not
as a continuum of events in a single GC. The most probable explanation is that
the proto-clusters formed into a (now dissolved) dwarf galaxy and later merged
to produce the present GC.
