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

Shepherd, K; Namur, O; Neave DA; Zhang, Y; van Gerve, T; Van Helden, T; Van Acker, T; Klemme, S; Berndt, J; Charlier, B; Vander Auwera, J;

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.