  Studies of the broad emission-line regions (BLRs) in quasars have revealed
solar or higher enrichment levels up to the highest redshifts. In combination
with the presence of large amounts of dust in QSOs at $z$ about 6, this implies
that substantial amounts of star formation and nucleosynthesis took place at
significantly earlier epochs. Here, we examine whether a top-heavy stellar
initial mass function (IMF) is indicated by current data, by modelling the
contributions from different regions of the IMF, including Type Ia/II and pair
instability supernovae, to the metal synthesis in BLRs. We find that, in order
to reproduce the observations of roughly solar values of N/C and Fe/Mg in these
objects, (i) stars with a present-day IMF are sufficient, regardless of their
metallicity, (ii) zero-metallicity stars with a top-heavy IMF severely
underproduce N/C, and (iii) the contribution of Type Ia SNe is not strongly
required by the data. Therefore, stars of mass about 1-40 $M_\odot$ must have
existed at $z$ about 10-20, possibly coeval with any hypothesized stars of
masses exceeding about 100 $M_\odot$ at these epochs. This is in agreement with
the nucleosynthetic abundance pattern detected in extremely metal-poor stars in
the galactic halo.
