  Recent measurements by the Pierre Auger Observatory suggest that the
composition of ultra-high energy cosmic rays (UHECRs) becomes dominated by
heavy nuclei at high energies. However, until now there has been no
astrophysical motivation for considering a source highly enriched in heavy
elements. Here we demonstrate that the outflows from Gamma-Ray Bursts (GRBs)
may indeed be composed primarily of nuclei with masses A ~ 40-200, which are
synthesized as hot material expands away from the central engine. In
particular, if the jet is magnetically-dominated (rather than a
thermally-driven fireball) its low entropy enables heavy elements to form
efficiently. Adopting the millisecond proto-magnetar model for the GRB central
engine, we show that heavy nuclei are both synthesized in proto-magnetar winds
and can in principle be accelerated to energies >1e20 eV in the shocks or
regions of magnetic reconnection that are responsible for powering the GRB.
Similar results may apply to accretion-powered GRB models if the jet originates
from a magnetized disk wind. Depending on the precise distribution of nuclei
synthesized, we predict that the average primary mass may continue to increase
beyond Fe group elements at the highest energies, possibly reaching the A ~ 90
(Zirconium), A ~ 130 (Tellurium), or even A ~ 195 (Platinum) peaks. Future
measurements of the UHECR composition at energies >~ 1e20 eV can thus confirm
or constrain our model and, potentially, probe the nature of GRB outflows. The
longer attenuation length of ultra-heavy nuclei through the extragalactic
background light greatly expands the volume of accesible sources and alleviates
the energetic constraints on GRBs as the source of UHECRs.
