Determination of oxygen diffusion rates in magnetite from natural isotopic variations

Abstract

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 δ 18 O values of magnetite and corresponding closure temperatures ( T c ) based on Δ calcite-magnetite range from δ 18 O = 2.6‰, T c = 505 °C for a grain radius of 0.075 mm, to δ 18 O = 5.5‰ T c = 660 °C for a grain radius of 1.15 mm. The δ 18 O 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 ( D o ) = 4.3 (+3.3, -1.9) x 10 -7 cm 2 /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: D o (cm 2 /s) = (d T /d t ) x (-1.08 x 10 -7 ). (Note: d T /d t 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 f H 2 O , may be used as a sensor for water-rich fluids during cooling in natural systems.