Amphibole reaction rims as 4D petrological recorders of pre-eruptive magma transport

Amphibole reaction rims are routinely treated as “thermometers and barometers” for magmas, used to back-calculate storage and ascent conditions from changes in pressure, temperature, and melt chemistry. But ascent is dynamic: crystals are transported, rotated, and strained, and those mechanical effects can modify reaction textures in ways that are easily overlooked if rims are interpreted purely in P–T–X space. We demonstrate that amphibole breakdown responds to deformation as well as to thermodynamic forcing. We integrate EBSD orientation mapping from time-resolved experiments and natural rim-bearing samples (Unzen, Soufrière Hills Volcano, Bezymianny, and El Misti) with numerical models that predict how newly formed crystals rotate during shear. The data indicate an initial topotactic relationship in which pyroxene forms in a crystallographically controlled way, replacing the parent amphibole during rim growth. Crucially, that early orientation signal can be progressively overprinted, where pyroxene grains rotate and develop systematic misorientations as strain accumulates. Two endmembers illustrate this behaviour: for amphibole reaction rims formed in petrological experiments, which are designed to be mechanically quiet, simple gravitational settling produces detectable, evolving misorientation patterns. In contrast, natural samples display stronger, more systematic orientation changes consistent with externally imposed shear during transport. Across both settings, the shape of misorientation distributions reflects not only the magnitude of strain but also the relative timing of rim crystallisation versus deformation. These results expand what amphibole reaction rims can record. Rather than archives of conditions alone, rim fabrics measured by EBSD provide a coupled record of chemical–thermal evolution and mechanical history, motivating a P–T–X–ε interpretation framework for tracking magma ascent paths and the dynamics of pre-eruptive transport.