  By means of chemo-photometric models for galaxies of different morhological
types, we have carried out a detailed study of the history of element
production by spheroidal and dwarf irregular galaxies. Spheroidal galaxies
suffer a strong and intense star formation episode at early times. In dwarf
irregulars, the SFR proceeds at a low regime but continuously. Both galactic
types enrich the IGM with metals, by means of galactic winds. We have assumed
that the galaxy number density is fixed and normalized to the value of the
optical luminosity function observed in the local universe. Our models allow us
to investigate in detail how the metal fractions locked up in spheroid and
dwarf irregular stars, in the ISM and ejected into the IGM have changed with
cosmic time. By relaxing the instantaneous recycling approximation and taking
into account stellar lifetimes, for the first time we have studied the
evolution of the chemical abundance ratios in the IGM and compared our
predictions with a set of observations by various authors. Our results indicate
that the bulk of the IGM enrichment is due to spheroids, with dwarf irregular
galaxies playing a negligible role. Our predictions grossly account for the
[O/H] observed in the IGM at high redshift, but overestimate the [C/H].
Furthermore, it appears hard to reproduce the abundance ratios observed in the
high-redshift IGM. Some possible explanations are discussed in the text. This
is the first attempt to study the abundance ratios in the IGM by means of
detailed chemical evolution models which take into account the stellar
lifetimes. Numerical simulations adopting our chemical evolution prescriptions
could be useful to improve our understanding of the IGM chemical enrichment.
