The relationship between apparent equilibrium temperature and closure temperature with application to oxygen isotope geospeedometry

Abstract

Oxygen isotope fractionation between coexisting minerals in slowly cooled rocks conveys information about their cooling history. By using the fast grain boundary (FGB) model to simulate closed-system diffusive exchange of oxygen isotopes between coexisting minerals, I show that the apparent equilibrium temperatures (T ae) by the mineral pair with the largest isotopic fractionation (PLIF) always lies between the closure temperatures (T c) of those two minerals. Therefore, when the rate of oxygen diffusion and hence T c for the PLIF chance to be comparable (such as in the case of quartz and magnetite), T ae will serve as a good approximation of T c regardless of variation in mineral proportions. The specialty of the PLIF in constraining T ae within their T c range can be generalized to other stable isotope systems and element partitioning. By approximating T c with T ae and inverting Dodson’s equation, the cooling rate of plutonic or metamorphic rocks can be inferred.