Abstract

Crystal zonations provide valuable snapshot of the dynamic changes within a magma reservoir. However, crystal zonations are often heterogeneous down to the hand-sample scale, such that deciphering their signatures becomes convoluted. Crystals are reactively precipitated and dissolving as a function of temperature, pressure, and composition. In this manuscript, we investigate what temperature histories crystals experience in a magma lens after its injection into a cooler magma reservoir. We simulate the cooling interface in either hot basaltic or dacitic magmas after their injection into a cooler magma reservoir. We couple fluid dynamics to thermodynamics by resolving flow at the crystalline-scale and allowing crystals with constant density and size to precipitate and dissolve based on ambient melt properties. We infer zonations in our simulated crystals by tracking the magma temperatures they sample over time. Our results show that when thermodynamics and fluid dynamics are coupled, a reactive, crystal-driven instability arises, because the negative buoyancy of crystals pulls along the cooler-than-ambient melt in which they precipitated. As crystals continue to precipitate along the cooling boundary, the instability develops into a sustained convective flow. Our results show that crystals record complex and unique zonations in this crystalline-scale domain, suggesting that zonations and their heterogeneity can be indicative of local instead of system scale processes. Also, our results show that many of the crystals in the instability dissolve and lose their thermal record of the instability. These results highlight the challenges of deciphering system-scale process from crystalline data.