Volcanological implications inferred from chemical analyses of olivines collected at São Jorge Island (Azorean Archipelago, Portugal)

The Azorean Archipelago, situated in the North Atlantic Ocean, is composed of nine active volcanic islands, put in place during the Quaternary. They are a result of the interaction between the Azores hotspot and the Mid-Atlantic Ridge (MAR). The archipelago intersects the MAR, with the Western Islands situated on the ridge's western flanks, and the Central and Eastern Islands located at the east of it. This intricate setting is situated at the triple junction of the North American, Eurasian, and Nubian plates. In addition, the Terceira Rift, an exceptionally slow-spreading plate boundary, influence the archipelago's tectonics and seismicity. The Azorean plateau experiences active faults contributing to high-magnitude earthquakes, along with secondary manifestations of volcanism like fumaroles and thermal springs. Together with Madeira, the Canary Islands, and Cape Verde, the Azores constitute the Macaronesian island group. These islands are volcanic systems that result from mantle plume activity, featuring the characteristic composition of Ocean Island Basalt (OIB) magma. This study focuses on olivine phenocrysts from the Holocene Manadas Volcanic Complex basalts on São Jorge Island, Central Azores. These crystals provide insights into magma evolution and diffusion processes. Olivine, that is commonly found in effusive alkaline mafic volcanic rocks, reacts when the magma composition changes, developing chemical zoning. Examining this zoning, both with optical mineralogy techniques and with electron microprobe analyses, made it possible to reveal the evolution of magmatic reservoirs and conduits of this volcanic complex. In addition, chemical profiles of olivine crystals and chemical diffusion modelling were used to calculate an approximate time scale of the magma evolution and eruption. We found distinct cases of both close and open-system differentiation, like magma mixing, which occurred either well before or immediately preceding eruptions. Overall, the study improves the understanding of the complex geological processes shaping the Azores and contributes to the wider field of volcanic and magmatic studies.