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

Abstract

Recent unrest events on São Jorge Island (Azores, Portugal) may signal impending volcanic eruptions, highlighting the urgent need to better understand the island’s magma system. Some of the analyzed basaltic and hawaiitic S. Jorge lavas show evidence of magma mixing and rapid assembly of crystals shortly preceding the eruptions. A subset of the studied olivine crystals displays reverse zoning, with rimward increase of forsterite (Fo) content, while others contain distinctly different Fo values within the same rock sample. Diffusion chronometry reveal variable mixing-to-eruption timescales (years to days), but the fastest timescales occur in olivine from the two most recent eruptions on the island and possibly provide constrains for the tempo of future volcanic eruptions. Volatile analyses emphasize the role of CO2 in the magmatic system of the island. Crystallized MIs in olivine and clinopyroxene commonly contain gas bubbles, with CO2 as the sole fluid phase. Calculated CO2 concentrations in the MIs reach up to 1.5 wt%. H2O is absent in the bubbles, even if the system was probably water rich, as hydrous minerals are present in the MIs and occasionally as phenocrysts. The high CO2 and likely H2O budgets increase magma mobility and explosive potential. Findings of this study show that São Jorge mafic magmas ascend rapidly through a transcrustal system, with eruptions potentially occurring after short warning times.