Source processes of the thermal waters from the Phlegraean Fields (Naples, Italy) by means of the study of selected minor and trace elements distribution

Abstract

The geochemical characteristics of hydrothermal waters from the Phlegraean Fields (P.F.) (Naples, Italy) were analysed for minor and trace elements, selectively mobilised in hydrothermal systems such as B, F, Hg, As, Pb and Tl. The water samples, collected from a shallow aquifer likely to be fed by deeper fluids, showed various geochemical features, resulting from the mixing of three components: (1) surface waters of meteoric origin; (2) hot deep waters deriving from water–rock interaction and including deep waters of marine origin; (3) magmatic fluids rising from the local magma chamber, lying a few kilometres below the town of Pozzuoli. This setting, although very complex, provides a reliable means of studying the distribution of the investigated trace elements. In particular, within the Phlegraean area, high contents of B (0.1–48 mg/l), F (0.5–8 mg/l), As (16–6050 μg/l) and Hg (0.7–232 μg/l) were observed. The levels of thallium in the springs close to Solfatara (about 7 μg/l) were in line with those normally recorded in hydrothermal areas, whereas high levels of this element (up to 23.3 μg/l) were identified in other wells of the study area. Lead (1.3 to 29.1 μg/l) appears to be anomalous with respect to its normal content in groundwater (about 1 μg/l), owing to the presence of high-density brines at depth, which enhance the solubility of Pb in volcanic rocks under hydrothermal conditions. The distribution of the investigated trace elements in the Phlegraean Fields thermal area is probably related to the different ascent pathways of the fluids. Clearly, apart from the influence exerted by anomalous thermal conditions, each element shows a different behaviour, depending on its geochemical affinity with mineral phases and as a consequence of the different enthalpy values, which determine ion partitioning in gaseous phases. Based on geochemical evidences and on the distribution of minor and trace elements, the source processes of the investigated hydrothermal waters were defined. Five main groups were identified: (1) acid sulphate waters, resulting from mixing of meteoric water with magmatic gases (mainly H 2S); (2) high sulphate–chloride waters, from a deep reservoir located in the major upflow zone; (3) waters associated with significant degassing of magmatic CO 2; (4) waters from a deep geothermal neutral chloride reservoir, resulting from heating of marine water modified by water–rock interaction processes; (5) cold waters from the inner area, influenced by low-temperature, water–rock interaction processes.