The oxygen isotope compositions of magnetite grains, hosted in a calcite marble from the Bancroft terrane of the Ontario Grenville province, vary systematically with grain size. The δ18O values of magnetite and corresponding closure temperatures (Tc) based on Δcalcite-magnetite range from δ18O = 2.6‰, Tc = 505 °C for a grain radius of 0.075 mm, to δ18O = 5.5‰ Tc = 660 °C for a grain radius of 1.15 mm. The δ18O of the calcite is constant within a scale of 100 μm at a value of 12.3‰. The observed isotopic variations can be fit to the diffusion model of Dodson by the method of least squares (r = 0.98) to yield an activation energy (Q) = 211 (±20) kJ/mole and a pre-exponential factor (Do) = 4.3 (+3.3, -1.9) x 10-7 cm2/s for a cooling rate of 4 °C/m.y. The activation energy estimate is independent of the assumed cooling rate, but the calculated pre-exponential factor varies as follows: Do (cm2/s) = (dT/dt) x (-1.08 x 10-7). (Note: dT/dt is in °C/m.y.) The activation energy is identical to an experimental determination by Giletti and Hess, but the pre-exponential factor is 100 times lower. The difference is attributed to the water-rich conditions in the experiments and the absence of fluid in the slowly cooled marbles investigated in this study. The strong dependence of diffusion rate on water presence, or fH2O, may be used as a sensor for water-rich fluids during cooling in natural systems.