  A phenomenological model for chemical enrichment in early-type galaxies is
presented, in which the process of star formation is reduced to a set of four
parameters: star formation efficiency (Ceff), fraction of ejected gas in
outflows (Bout), formation redshift (zF) and infall timescale (Tf). Out of
these four parameters, only variations of Bout or Ceff can account for the
color-magnitude relation. A range of outflows results in a metallicity
sequence, whereas a range of star formation efficiencies will yield a mixed age
+ metallicity sequence. The age-metallicity degeneracy complicates the issue of
determining which mechanism contributes the most (i.e. outflows versus
efficiency). However, the determination of the slope of the correlation between
mass-to-light ratio and mass in clusters at moderate or high redshift will
allow us to disentangle age and metallicity.
