Insights into the magma plumbing system architecture beneath an off-ridge ocean island (Terceira, Azores) from crystal zoning

Abstract

Understanding the architecture and dynamics of crustal magmatic plumbing systems is essential for interpreting volcanic processes and eruption styles in ocean island settings. We present a comprehensive investigation of the magmatic plumbing system beneath Terceira Island (Azores), integrating petrographic observations, whole-rock geochemistry, high-resolution elemental mapping of clinopyroxene, thermobarometry, and diffusion chronometry to elucidate magma storage and evolution processes. Our results reveal a vertically extensive, multi-level magmatic system comprising interconnected melt pockets and mush zones spanning from the Moho (~400–500 MPa), ~18–20 km depth) to shallow upper crustal levels (~100 MPa), ~3 km depth). High-resolution mapping of major and trace elements in clinopyroxene uncovers zoning (notably in Cr and Ni) providing new constraints on magma recharge and mixing processes. Primitive, Mg-, Cr-, and Ni-enriched basalts ascend from the mantle and pond at deep crustal levels, where interaction with a partially crystallized mush promotes the growth of olivine crystals and complex clinopyroxene zoning. Multiple mafic recharge events at mid-crustal depths (~200–300 MPa) induce dynamic disequilibrium textures and oscillatory zoning in clinopyroxene and plagioclase, reflecting variable pressure-temperature-composition conditions. Shallower reservoirs crystallize evolved, Cr-depleted melts that feed trachytic and rhyolitic eruptions. Timescale modelling suggests pre-eruptive storage durations averaging ~10 months, with maxima reaching up to 8 years. These findings highlight the critical role of crustal architecture and tectonic setting in controlling magma differentiation pathways and eruption styles within ocean islands