  Evolutionary trends in the surface abundances of heavier elements have
recently been identified in the globular cluster NGC 6397 ([Fe/H]=-2),
indicating the operation of atomic diffusion in these stars. Such trends
constitute important constraints for the extent to which diffusion modifies the
internal structure and surface abundances of solar-type, metal-poor stars. We
perform an independent check of the reality and size of abundance variations
within this metal-poor globular cluster. Observational data covering a large
stellar sample, located between the cluster turn-off point and the base of the
red giant branch, are homogeneously analysed. The spectroscopic data were
obtained with the medium-high resolution spectrograph FLAMES/GIRAFFE on
VLT-UT2. We derive independent effective-temperature scales from profile
fitting of Balmer lines and by applying colour-temperature calibrations to
Str\"omgren uvby and broad-band BVI photometry. An automated spectral analysis
code is used together with a grid of MARCS model atmospheres to derive stellar
surface abundances of Mg, Ca, Ti, and Fe. We identify systematically higher
iron abundances for more evolved stars. The turn-off point stars are found to
have 0.13dex lower surface abundances of iron compared to the coolest, most
evolved stars in our sample. There is a strong indication of a similar trend in
magnesium, whereas calcium and titanium abundances are more homogeneous. Within
reasonable error limits, the obtained abundance trends are in agreement with
the predictions of stellar structure models including diffusive processes
(sedimentation, levitation), if additional turbulent mixing below the outer
convection zone is included.
