Zirconium-hafnium and rare earth element signatures discriminating the effect of atmospheric fallout from hydrothermal input in volcanic lake water

摘要

Geochemical behaviour of rare earth elements (REE), Zr, and Hf was investigated in CO2-rich waters circulating in Pantelleria Island also including 'Specchio di Venere' Lake within a calderic depression. A wide range of total dissolved REE concentrations was found (2.77-12.07 nmol L-1), with the highest contents in the lake. The main REE complexes in the CO2-rich waters are [REE(CO3)(2)](-) and [REECO3](+), showing changeable proportions as a function of pH. The REE normalized to post-Archean Australian Shale (PAAS) showed similar features with heavy REE (HREE) enrichments in CO2-rich waters collected from springs and wells, whereas a different REE pattern was found in the 'Specchio di Venere' Lake water with middle REE (MREE) enrichments. The PAAS normalized concentration ratios (LREE/HREE)(N) and (MREE/HREE)(N) in waters are <1, except for the lake water in which (MREE/HREE)(N) > 1. Positive Eu anomalies were found in the investigated waters owing to water-rock interactions with less evolved host rocks. Ce anomalies as a function of Eh values were recognized, with the highest Ce anomaly occurring in the lake water with respect to the CO2-rich waters. The Y/Ho and Zr/Hf molar ratios are higher in the investigated waters (except for lake water) than that in the local rocks, with values ranging from 35.4 to 77.9 and from 76.3 to 299, respectively. The precipitation of authigenic phases was considered to be responsible for the increase in the Y/Ho and Zr/Hf ratios owing to enhanced Hf and Ho removal with respect to Zr and Y.