Reequilibration Processes Occurring in Plagioclase‐Hosted Melt Inclusions From Plagioclase Ultraphyric Basalts

Abstract Plagioclase‐hosted melt inclusions are infrequently used to investigate magmatic processes owing to the perception that they are less robust than olivine‐hosted inclusions. Based on a set of time series experiments ranging from 30 min to 4 days, we demonstrate that plagioclase‐hosted melt inclusions can preserve the original (primitive) magmatic signal if steps are taken to correct for post entrapment crystallization. Diffusion within plagioclase‐hosted melt inclusions is sufficiently fast that the melt inclusions can be homogenized within 30 min through heating experiments. As heating time increases, the composition of melt inclusions drifts. We attribute this longer‐term phenomenon to plastic deformation of the host plagioclase (crystal relaxation) inducing a decrease in the internal pressure in the melt inclusion combined with diffusion of elements across the host/melt inclusion interface. The rate of chemical reequilibration within melt inclusions is limited by the much slower rate of diffusion within the solid host. Indeed, the host/melt inclusion partition coefficient for MgO decreases by 25% from the 30‐min to the 4‐day experiments. Our results suggest that the primary character of plagioclase‐hosted melt inclusions can be recovered if one recognizes and corrects for the effects of the complex physical and chemical processes that occur during melt inclusion homogenization.