  We study preheating in plateau inflation in the Palatini formulation of
general relativity, in a special case that resembles Higgs inflation. It was
previously shown that the oscillating inflaton field returns to the plateau
repeatedly in this model, and this leads to tachyonic production of inflaton
particles. We show that a minimally coupled spectator scalar field can be
produced even more efficiently by a similar mechanism. The mechanism is purely
gravitational, and the scalar field mass can be of order $10^{13}$ GeV, larger
than the Hubble scale by many orders of magnitude, making this a candidate for
superheavy dark matter.
