Abstract

Oxygen isotope studies of fluid-rock interactions and metamorphic and igneous thermal histories commonly consider volume diffusion to be the rate-limiting step for oxygen exchange between minerals. Recrystallization processes, such as exsolution, however, can also control exchange rates. Here, we assess the effect of oxyexsolution on retrograde oxygen exchange in magnetite grains from the Laramie anorthosite complex, Wyoming. Oxygen closure temperatures for magnetite were directly determined and compared with temperatures for exsolution reactions resulting from oxidation (oxyexsolution) and those predicted by volume diffusional constraints. We conclude that oxyexsolution promotes oxygen exchange to temperatures below diffusional closure temperatures. The controlling variable appears to be the rate of oxyexsolution and not domain size or the absolute amount of exsolution. An interesting consequence of this hypothesis is the prediction that exsolved minerals in quickly cooled rocks may record lower oxygen isotope closure temperatures than those in slowly cooled rocks. Further application of these methods suggests that other exsolution processes also promote retrograde oxygen exchange.

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