  The aim of this paper is to provide a plausible explanation for the large
amplitude microlensing events observed in the cluster lensed quasar system SDSS
J1004+4112. The microlensed quasar images appear to lie well clear of the
stellar population of the cluster, raising the possibility that the cluster
dark matter is composed of compact bodies which are responsible for the
observed microlensing. In the first part of the paper we establish the exact
structure of the difference light curves attributed to microlensing from
photometric monitoring programmes in the literature. We then show from measures
of surface brightness that the probability of microlensing by stars in the
cluster is negligibly small. Finally we relax our assumption that the cluster
dark matter is in the form of smoothly distributed particles, but instead is
made up of compact bodies. We then use computer simulations of the resulting
magnification pattern to estimate the probability of microlensing. Our results
show that for a range of values for source size and lens mass the observed
large microlensing amplitude is consistent with the statistics from the
simulations. We conclude that providing the assumption of smoothly distributed
dark matter is relaxed, the observed large amplitude microlensing can be
accounted for by allowing the cluster dark matter to be in the form of solar
mass compact bodies. We further conclude that the most plausible identity for
these bodies is primordial black holes.
