  The iron mass in galaxy clusters is about 6 times larger than could have been
produced by core-collapse SNe, assuming the stars in cluster galaxies formed
with a standard IMF. Type-Ia SNe have been proposed as the alternative dominant
iron source. We use our HST measurements of the cluster SN-Ia rate at high
redshift to study the cluster iron enrichment scenario. The measurements can
constrain the star-formation epoch and the SN-Ia progenitor models via the mean
delay time between the formation of a stellar population and the explosion of
some of its members as SNe-Ia. The low observed rate of cluster SNe-Ia at z~1
pushes back the star-formation epoch in clusters to z>2, and implies a short
delay time. We also show a related analysis for high-z field SNe which implies,
under some conditions, a long SN-Ia delay time. Thus, cluster enrichment by
core-collapse SNe from a top-heavy IMF may remain the only viable option.
