Chemical and 87Sr/86Sr signatures of rainwaters at two active central volcanoes in São Miguel (Azores) – first survey

Rainwater samples were collected during final of February and early March of 2022, at Furnas and Fogo volcanoes, São Miguel (Azores), both of which display secondary manifestations of volcanism. The major-ion composition reveals that marine aerosols contribute significantly to the chemistry of these rainwaters, showed by the proximity of some samples to the Na/Cl ratio of the seawater. The 87Sr/86Sr ratios (0.70849–0.71027) presented by the rainwater samples are similar to the seawater, indicating that marine sprays are possibly the major source of dissolved Sr, with values within the range of values presented by rainwater on the Portuguese mainland. However, other sources, such as volcanic emissions and mineral dust components showed to have also an impact. The 87Sr/86Sr ratios depicted by one sample (CC) deviates from the above mentioned values, exhibiting a lower strontium isotopic ratio (0.70701), which suggests the influence of the fumarolic fluids. Some trace elements (Al, Fe, Mn, Ni, Cu, Zn, Rb, Sr, Ba and REE) content also confirms the influence of the fumarolic emissions, at least in samples collected near Furnas Village fumarolic field (Furnas volcano), and the influence of mineral dust from the surrounding continents (positive anomalies in Eu). Overall, the proximity to an active fumarolic field had a significant impact on the chemistry of meteoric waters, as seen not only in the major-ions, but also in trace elements and in the Sr isotopic ratios.