Pathways and fate of REE in the shallow hydrothermal aquifer of Vulcano island (Italy)

Abstract

We investigated the geochemical behaviour of major and Rare Earth Elements (REE), together with oxygen and deuterium isotopic composition in the aquifer of Vulcano, the southernmost island of the Aeolian archipelago (Italy). Studied wells, located at different distances from the crater, are characterised by different contributions of the rising volcanic fluids. In particular, those located in the proximity of La Fossa crater are affected by a strong interaction with volcanic-hydrothermal fluids and show REE behaviour similar to that of fresh rocks, suggesting a congruent dissolution of the solid matrix. Samples from the other wells, located in an area where the volcanic deposits are hydrothermally altered as an “advanced argillic facies”, are enriched in HREE and mirror the corresponding depletion observed in the altered rocks. Moreover, the different grade of interaction with hydrothermal fluids determines the main ligand that complexes the REE. The main ligand is CO3 2– in the wells that are more directly affected by hydrothermal circulation, whereas SO4 2− dominates in those located at greater distances from La Fossa crater. This information provides further clues to the complex groundwater circulation model of Vulcano Island, which is regulated by the variable mixing and interacting of rising volcano-hydrothermal fluids, meteoric infiltration and seawater, differently interacting with fresh and altered rocks.