Polybaric fractional crystallisation of arc magmas: an experimental study simulating trans-crustal magmatic systems

Abstract Crystallisation-driven differentiation is one fundamental mechanism proposed to control the compositional evolution of magmas. In this experimental study, we simulated polybaric fractional crystallisation mantle-derived arc Various pressure–temperature trajectories were explored cover a range potential magma ascent paths and investigate role decompression on phase equilibria liquid lines descent (LLD). Fractional was approached in step-wise manner by repetitively synthesising new starting materials chemically corresponding liquids formed previous runs. Experiments performed at temperatures ranging from 1140 870 °C with 30 steps, pressure varied between 0.8 0.2 GPa steps. For most fractionation paths, oxygen fugacity (fO 2 ) buffered close Ni-NiO equilibrium (NNO). An additional series conducted fO Re-ReO buffer (RRO ≈ NNO+2). High-pressure experiments (0.4–0.8 GPa) run piston cylinder apparatus while runs externally heated vessels. Resulting follow calc-alkaline trends where onset pronounced silica enrichment coincides saturation amphibole and/or Fe–Ti–oxide. Both exert crucial stability fields olivine, pyroxene, amphibole, plagioclase, Fe–Ti–oxide phases behaviour Key observations are shift olivine–clinopyroxene cotectic towards more clinopyroxene-rich composition, an expansion plagioclase field decrease decreasing pressure. Decompression-dominated result approaching metaluminous observed for typical volcanic rocks, obtained cooling-dominated evolve peraluminous compositions, similar isobaric elevated pressures. RRO provide closer match natural compared conditions NNO. We conclude that decompression-dominated oxidising represents possible scenario differentiation.