  Photometric investigations have revealed that Galactic globular clusters
exhibit internal metallicity variations amongst the so-called first-population
stars, until now considered to have a homogeneous initial chemical composition.
This is not fully supported by the sparse spectroscopic evidence, which so far
gives conflicting results. Here, we present a high-resolution re-analysis of
five stars in the Galactic globular cluster NGC 2808 taken from the literature.
Target stars are bright red giants with nearly identical atmospheric parameters
belonging to the first population according to their identification in the
chromosome map of the cluster, and we have measured precise differential
abundances for Fe, Si, Ca, Ti, and Ni to the ~0.03 dex level.
  Thanks to the very small uncertainties associated to the differential
atmospheric parameters and abundance measurements, we find that target stars
span a range of iron abundance equal to 0.25 +/- 0.06 dex. The individual
elemental abundances are highly correlated with the position of the star along
the extended sequence described by first population objects in the cluster
chromosome map: bluer stars have a lower iron content. This agrees with
inferences from the photometric analysis.
  The differential abundances for all other elements also show statistically
significant ranges that point to intrinsic abundance spreads. The Si, Ca, Ti,
and Ni variations are highly correlated with iron variations and the total
abundance spreads for all elements are consistent within the error bars. This
suggests a scenario in which short-lived massive stars exploding as supernovae
contributed to the self-enrichment of the gas in the natal cloud while star
formation was still ongoing.
