Fluorine in the rocks and sediments of volcanic areas in central Italy: total content, enrichment and leaching processes and a hypothesis on the vulnerability of the related aquifers

摘要

Rock, sediment and water samples from areas characterised by hydrothermal alterations in the Sabatini and Vico Volcanic Districts, near Rome and the large city of Viterbo, respectively, were collected and analysed to determine the total fluorine (F) content and to understand the F geochemical background level in the volcanic districts of central Italy. Leaching and alteration processes controlling the high concentration of F in water were also investigated. Fluorine concentrations were directly determined (potentiometrically) by an F selective electrode in water samples, while the procedure for rock samples included preliminary F dissolution through alkaline fusion. F concentrations higher than 800 mg kg~(-1) were commonly found in the analysed rocks and sediments; the concentration depended on the lithol-ogy and on the distance from the alteration areas. A specific successive sampling campaign was conducted in three areas where the F content in sediments was particularly high; in the same areas, measurements of CO_2 flux were also performed to investigate the possible deep origin of F. To verify the relationships among the high F contents in rocks and sediments, the leaching processes involved and the presence of F in the aquifer, we also collected water samples in the western sector of the Sabatini Volcanic District, where hydrothermal manifestations and mineral springs are common. The data were processed using a GIS system in which the F distribution was combined with morphological and geological observations. The main results of our study are that (1) F concentrations are higher in volcanic and recently formed travertine (especially in hydrothermally altered sediments) than in sedimentary rocks and decrease with distance from hydrothermal alteration areas, (2) F is more easily leached from hydrothermally altered rocks and from travertine and (3) sediments enriched with F may indicate the presence of deep regional fractures that represent direct pathways of hydrothermal fluids from the crust to the surface.