A new Mg-in-plagioclase geospeedometer for the determination of cooling rates of mafic rocks

Abstract We present a new Mg-in-plagioclase geospeedometer based on the diffusive exchange of Mg between plagioclase and clinopyroxene that allows cooling rates of a wide range of mafic rocks to be determined. The major element composition of plagioclase is shown to play an important role in driving the flux of Mg as well as the partitioning of Mg between plagioclase and clinopyroxene. Therefore, commonly used analytical solutions to the diffusion equation are not applicable in this system and an alternate approach is developed. Practical aspects of the method such as the role of anorthite zoning patterns and influence of shapes of cooling paths and grain geometries in controlling the Mg concentration profile shapes are discussed. Propagation of uncertainties in the knowledge of diffusion and partition coefficients shows that the method is capable of resolving small differences in cooling rates (e.g., log[d T /d t  °C yr −1 ] = −3 ± 0.3 in one example). The approach is illustrated by application to two samples from the lower oceanic crust.