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.