Melt–Rock Interaction Experiments Reveal Rapid Microstructural and Chemical Changes at Lower Crustal Conditions

The reactive flow of melt through the mantle or crust triggers chemical disequilibrium, driving reactions that significantly alter the mineral assemblages and physical properties of host rocks. However, the degrees of chemical difference required to initiate these reactions and their timescale remain poorly understood. In this study, we present piston–cylinder reaction experiments simulating lower crustal conditions, where largely anhydrous lower crustal granoblastic dioritic gneiss interacts with a hydrous mafic melt, created from the same gneiss but modified by the addition of ~6-wt.% H₂O. Remarkably, reactions occurred within just 12 h, producing microstructures that closely resemble those observed in natural, melt-fluxed rocks from the lower arc crust in Fiordland, New Zealand. Melt–rock interactions led to the formation of epitaxial, multilayer symplectic coronae of pargasite + plagioclase or quartz partially replacing pre-existing pyroxene grains. The protolith plagioclase and amphibole are either completely dissolved into the melt or replaced by a modified composition of the same mineral. The melt exhibits compositional variations that correlate with distance from the melt–rock reaction front. Quenched melt chemistry data demonstrate the potential for melt compositions to continuously evolve in response to both crystallisation and melt–rock interactions during reactive flow. Importantly, our findings reveal that melt–rock reactions, initiated by melt not drastically different from the solid rock (protolith), can induce significant changes in rock composition and thus physical properties in a short time. Our findings have broad implications for understanding the compositional evolution of migrating melts and the chemical and mechanical evolution of the Earth's mantle and lower crust in general.