Ghosts of phases past: The microstructural markers of completed mineral-melt reactions

Arc magmas commonly undergo many mineral-melt reactions during changes in P-T-X conditions (e.g. during ascent through the crust, magma mixing, or CO₂ flushing). Constraining these reactions can be key to unlocking details of the magmatic systems that underlie high-threat arc volcanoes, such as those of the Cascades Arc, USA. However, identifying “lost” phases remains a challenge. Clues can come from geochemical methods and phase-equilibria modelling, but direct textural evidence is often assumed to be eradicated upon completion of the mineral-melt reactions.

In this study, we demonstrate that even mineral phases that have been completely reacted out of the magma can still leave behind distinctive microstructures. Using electron backscatter diffraction (EBSD) and detailed petrographic and geochemical characterisation, we describe the reaction textures produced by the breakdown of olivine, amphibole and biotite in intermediate rocks from a variety of high-threat volcanoes in the Cascades. We trace how the microstructures evolve throughout the course of the reaction and continue to evolve during magma storage, even after the original phase is completely consumed. We outline key features that can distinguish reaction-generated glomerocrysts from other polycrystalline aggregates such as mush fragments.

We find that distinctive crystal lattice distortion occurs in the products of all of the reactions. We infer that this distortion arises due to the strain from volume changes and lattice mismatches. Reaction textures also commonly feature epitaxial relationships, and regions where neighbouring crystals share similar orientations. Other clues can include unusual mineral assemblages, and intergrowth textures such as symplectites.

Textural re-equilibration during magma storage can change originally unmistakeable reaction textures into much less distinctive textures that could be mistaken for mush fragments. Common changes include grain growth, progressive equilibration of grain shapes, the recrystallisation of metastable phase assemblages, and the consolidation of lattice strain into subgrain-like structures. However, some markers, including the distinctive orientation relationships between minerals, and the remnants of lattice distortion, remain relatively robust throughout re-equilibration. Application of such microstructural indicators helps to reveal the true diversity of phases present in these magmatic systems, enabling better reconstruction of the pre-eruptive histories of complex arc magmas.