  Context. The intensity profiles of iron and oxygen in Solar Energetic
Particle (SEP) events often display differences that result in a decreasing
Fe/O ratio over time. The physical mechanisms behind this behaviour are not
fully understood, but these observational signatures provide important tests of
physical modelling efforts. Aims. In this paper we study the propagation of
iron and oxygen SEP ions using a 3D model of propagation which includes the
effect of guiding centre drift in a Parker spiral magnetic field. We derive
time intensity profiles for a variety of observer locations and study the
temporal evolution of the Fe/O ratio. Methods. We use a 3D full orbit test
particle model which includes scattering. The configuration of the
interplanetary magnetic field is a unipolar Parker spiral. Particles are
released instantaneously from a compact region at 2 solar radii and allowed to
propagate in 3D. Results. Both Fe and O experience significant transport across
the magnetic field due to gradient and curvature drifts. We find that Fe ions
drift more than O ions due to their larger mass-to-charge ratio, so that an
observer that is not magnetically well connected to the source region will
observe Fe arriving before O, for particles within the same range in energy per
nucleon. As a result, for the majority of observer locations, the Fe/O ratio
displays a decrease in time. Conclusions. We conclude that propagation effects
associated with drifts produce a decay over time of the Fe/O ratio,
qualitatively reproducing that observed in SEP event profiles.
