  The stars of the middle main sequence have relatively quiescent outer layers,
and unusual chemical abundance patterns may develop in their atmospheres. The
presence of chemical peculiarities reveal the action of such subsurface
phenomena as gravitational settling and radiatively driven levitation of trace
elements, and their competition with mixing processes such as turbulent
diffusion.
  We want to establish whether abundance peculiarities change as stars evolve
on the main sequence, and provide observational constraints to diffusion
theory.
  We have performed spectral analysis of 15 magnetic Bp stars that are members
of open clusters (and thus have well-known ages), with masses between about 3
and 4 M_sun. For each star, we measured the abundances of He, O, Mg, Si, Ti,
Cr, Fe, Pr and Nd.
  We have discovered the systematic time evolution of trace elements through
the main-sequence lifetime of magnetic chemically peculiar stars as their
atmospheres cool and evolve toward lower gravity. During the main sequence
lifetime, we observe clear and systematic variations in the atmospheric
abundances of He, Ti, Cr, Fe, Pr and Nd. For all these elements, except He, the
atmospheric abundances decrease with age. The abundances of Fe-peak elements
converge toward solar values, while the rare-earth elements converge toward
values at least 100 times more abundant than in the Sun. Helium is always
underabundant compared to the Sun, evolving from about 1% up to 10% of the
solar He abundance. We have attempted to interpret the observed abundance
variations in the context of radiatively driven diffusion theory, which appears
to provide a framework to understand some, but not all, of the observed
anomalous abundance levels and variations.
