Tracking Crystals Evolution in Episodes of Magma Mixing

Petrology and geochemistry reconstruct from plutons and eruptive products the underground chemical and thermodynamic conditions of magma at the time of crystallization. Accretionary layers in crystals record the composition of the surrounding melt, as well as the confining pressure and temperature. Such a backward reconstruction should be paired with a forward computation of the solidifying crystals during their transport in convective motions inside a refilled magmatic reservoir. This work develops a framework for the solution of magma fluid-dynamics and for the related Lagrangian trajectories of suspended crystals. Episodes of magma mixing due to injection of fresh magma into a shallow chamber are simulated at first in a Eulerian reference system. Afterwards, the Lagrangian trajectories of passive tracers are computed, tracking the magma composition, pressure and temperature through which these particles move. On the base of the compositional, pressure and temperature conditions, the crystallizing phases are computed with the MELTS code. The history of accretionary layers is thus obtained by interface-controlled growth and solid-state diffusion. Our results show that crystals residing in different parts of the underground system acquire a distinctive signature and are well mixed together. A small population will register the successive refilling episodes, while a substantial one will record each fresh injection.