Ascent-driven differentiation: a mechanism to keep arc magmas metaluminous?

Abstract Arc magmatism is fundamental to the generation of new continental or island arc crust. However, mechanisms that add chemical complexity natural calc-alkaline magmas ranging from basaltic rhyolitic compositions are debated. Differentiation currently discussed include magma mixing, assimilation, crustal melting, (fractional) crystallisation. In this contribution, differentiation by decompression-driven crystallisation investigated. We performed a set equilibrium experiments at variable pressures (200–800 MPa) on hydrous high-Al basalt (3.5 wt.% H 2 O in starting material) with run temperatures varying near-liquidus conditions (1110 °C) 900 °C. Oxygen fugacity was buffered moderately oxidising close NNO equilibrium. Combining these novel previous polybaric fractional (Marxer et al., Contrib Mineral Petrol 177:3, 2022) we demonstrate effects pressure behaviour respect liquid and cumulate lines descent, mineral chemistry, phase proportions. Decompression shifts olivine-clinopyroxene cotectic curve towards melt higher normative clinopyroxene enlarges stability field plagioclase. This exerts key control alumina saturation index residual liquids. argue near-adiabatic (or near-isothermal) decompression accompanied dissolution entrained during extraction lower crust keeps metaluminous crystallisation-driven thereby closely reproducing compositional spread observed for rocks.