Partitioning of rare earth elements, yttrium, and some major elements among source rocks, liquid and vapor of Larderello-Travale Geothermal Field, Tuscany (Central Italy)

Rare earth elements (REE), yttrium and some major element concentrations have been measured in the high-enthalpy fluids (HEF) of several geothermal wells and the fluid's source rocks in the Larderello-Travale area (Tuscany/Central Italy). The REE and Y abundances in the HEF range from 0.1 to about 10 pmol/kg and are slightly higher in the HEF originating from evaporite/carbonate sequences (Calcare Cavernoso) than in those from phyllites. The resulting REY distribution factors between HEF and source rocks, ^appKd_source-rock ^HEF defined as the ratios of REY/Ca in both phases, range from <0.01 to 0.03 and 0.03 to 0.1 for phyllites and evaporite-limestone sequences (Calcare Cavernoso), respectively. REE + Y are more retained by the source rocks than Ca. HEF show no inherited and, with exception of a small Y anomaly, no acquired anomalies. This indicates a static equilibrium between HEF and the source rocks. The absence of any Eu anomalies points to temperatures less than 250°C in the source region. The small negative Y anomalies are the result of Y depletion in the rock fractions taking part in the water-rock interaction. Due to depressurization of the HEF to about 120°C and 2 bars, a liquid and a vapor phase is produced, which were sampled for the determination of the REE + Y partitioning between the two phases. The apparent partition factors between vapor and liquid ^appD _liquid ^vapor of REE + Y range between 0.05 to 0.2 and about 3 for HEF originating from the phyllites and evaporites/carbonates, respectively. Among all ionic species determined, only NH₄ ⁺ has an apparent partition factor ^appD_lIquId ^vapor above one. In general, REY partition more easily into the vapor phase than the earth alkaline and alkaline elements. No significant correlation of REE + Y in the vapor phase with any of the determined ionic species could be detected. This probably points to the dominant presence of ion pairing such as REY(OH)₃° or REYO(OH)°.