Istituto di Geoscienze e Georisorse     
Cortecci G., Noto P., Panichi C. Environmental isotopic study of the Campi Flegrei (Naples, Italy) geothermal field. In: Journal of Hydrology, vol. 36 (1-2) pp. 143 - 159. Elsevier, 1978.
The H- and O-isotopic analyses show δ values ranging from −30 + 12 and from −6.7 to 1.4, respectively, relative to SMOW. These values and the positive relationships between the isotopic values and the chemical parameters, such as, for example, the Cl content, suggest that the analyzed spring waters are formed by a mixing, in different proportions, of two components: a water of marine origin filling the volcanic cover and coming from the Tyrrhenian Sea and a meteoric water. This mixing model is confirmed by the T contents, as the springs fed by marine water and meteoric water have 0.5 0.7 and 48 TU, respectively. The S-isotopic compositions range from δ = + 2.4 to + 20.1 relative to the meteoritic troilite, as the sulfate in the water from the thermal spring formed by practically undiluted marine water shows O- and S-isotopic ratios of +9.1 and +19.4, respectively; these values are close to those measured for the Tyrrhenian Sea sulfate (δ 12O = +9.5 and δ 34S = + 20.0. On the contrary, the thermal spring fed by meteoric water has a dissolved sulfate with a δ 12O = +9.0 and a δ 34S = +3.4, these being the mean values for those measured in 1971 and 1975. The 34S content of the dissolved sulfate very probably reflects the mean S-isotopic composition of the volcanic sulfates widespread in the volcanites. In order to investigate the 34S/32S ratios of the sulfates widely distributed in the volcanites, analyses were made of several sulfatic incrustations from Pozzuoli active solfatara, located on the eastern side of the study area. These analyses gave a mean δ 34S = +2.3, in agreement with the δ 34S = +3.4 measured for the sulfate dissolved in the hot spring fed by local meteoric waters and with the mean δ 34S measured for sulfate incrustation samples from other active volcanic areas such as in Japan, Iceland and Kenya. The mean δ 34S value for all the analyzed fumarolic sulfates is +2.7 0.5, as this value may represent the mean S-isotopic composition of the sulfate formed by oxidation of sulfur and/or H2 S in volcanic environments. Large chemical and isotopic variations, with regard to sampling times, have been observed in some manifestations as more negative isotopic contents of the dissolved sulfate and lower chloride and sulfate concentrations correspond to more negative water isotopic values, and vice versa. The chemical and sulfate-water isotopic geothermometers in general do not reflect the real situation in the system. Only the spring fed by marine water and unaffected by mixing phenomena shows consistent equilibrium temperatures at depth.
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Subject isotopic composition

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