Magmatic reservoirs feed active volcanoes and contribute to Earth’s crust formation. Among the processes operating in these systems, those controlling the early chemical evolution stages of magma are the most difficult to identify. This paper reports the first multi-mineral crystal archive that offers insights on the magmatic dynamics governing the production of basalt to andesite lavas at an oceanic back-arc spreading centre. As a result of these dynamics, a mush-dominated transcrustal system developed in the last 0.7 million years. The micro-scale crystal view identifies physicochemical magmatic environments that pass unobserved using whole-rock chemistry alone. It also supplies an interpretative formation model for plumbing system components, functional for the interpretation of geophysical data.