How stars in globular clusters reveal the depletion of the Spite plateau
  of lithium
  I summarize the results of 15 years of research on the surface abundances of
stars in globular clusters. Globular-cluster stars afford a unique view of the
physical processes that shape the surface abundances of stars on and beyond the
Spite plateau, as one can study stars in different evolutionary phases with the
same composition at birth. The main result is the finding of observational
trends in surface abundances of heavy elements (Mg, Ca, Ti, Cr, Fe, Ba) that
range from ~0.1 dex (M4 at [Fe/H]=-1.1) to ~0.3 dex (M30 at [Fe/H]=-2.3) with
turn-off and subgiant stars generally showing lower abundances than giant
stars. Any model that tries to resolve the cosmological lithium discrepancy by
means of stellar physics must simultaneously describe these systematic
heavy-element trends between dwarfs and giants. Models of atomic diffusion
moderated by some additional mixing can indeed explain the observed abundance
trends and make quantitative predictions for lithium depletion. The inferred
birth-cloud lithium abundances of Spite-plateau stars are higher by ~0.3 dex
leaving a small (~0.15 dex) nominal offset to the primordial value.
