Evolution of the magma plumbing system at São Jorge Island, Azores

Recent unrest events on São Jorge Island (Azores) may signal impending volcanic eruptions, highlighting the urgent need to better understand the island’s magma plumbing system. São Jorge lies on a thickened segment of oceanic crust, with the Moho estimated at approximately 15–18 km depth. Subaerial volcanic activity began around 1.8 million years ago, with the most recent eruption occurring in 1808 CE. The erupted magmas are primarily basaltic and basanitic in composition. Olivine phenocrysts reveal evidence of open-system differentiation processes, including magma mixing and replenishment. Some olivine crystals display reverse zoning, with lower forsterite (Fo) content in the cores compared to their rims, while others contain distinctly different Fo values within the same sample. These zoning patterns are used to estimate the residence times of olivine phenocrysts before eruption, providing valuable insights for volcanic hazard assessment. The exsolution of volcanic gases—particularly CO₂—from deep-seated magmas is likely a major trigger for eruptive activity. Olivine phenocrysts host commonly melt inclusions containing gas bubbles and exceptionally fluid inclusions. Raman microspectroscopy revealed that the fluid phase of these inclusions consists solely of CO₂, likely formed at high pressures. Collectively, these observations suggest that São Jorge’s mafic magmas ascend rapidly from the deep crust or mantle to the surface.