Although a large fraction of chemicals is provided from volcanic fluids to the environment (Oppenheimer et al., 2014) only a scarce literature was focused on the Rare Earth elements (REE) release from high temperature fumaroles (Möller et al., 2003; Gilbert & Williams-Jones, 2008; Zelenski et al., 2013). In order to fill this gap, this research was carried out mainly taking in account the REE partitioning during the emissions of fumarolic fluid between newly-forming sublimates and the remaining gas phase. The latter was collected as alkaline condensates according to Sortino et al. (2006). The investigated high temperature fumaroles (95-450°C) occur in several volcanic systems at Vulcano (Aeolian Islands) and Phlegrean Fields (Italy), Santorini (Cyclades Islands, Greece), Canary Islands (Spain) and Cape Verde where strong passive emission of fluids occur. Fumarolic sublimates show higher lanthanide contents, whereas alkaline condensates represent the most volatile fractions of these fluids. The analysis of geochemical behaviour of these elements was investigated studying the features observed in shale-normalised REE patterns (vs. PAAS, Taylor and McLennan, 1995). This analysis indicates that Gd is preferentially partitioned in the vapour phase during the sublimation of less volatile fluid fractions. Consequently, positive Gd anomalies occur in alkaline condensates with respect to coexisting solid sublimates. The amplitudes of these anomalies can be assessed according to the equation: (Moller et al., 2007) where n-suffix indicates normalised lanthanide concentrations and Gd* is the expected normalised Gd concentration according to Tb and Ho contents. Comparing the amplitudes of these anomalies with the temperatures of investigated fumaroles larger Gd/Gd* values are mainly found in higher temperature fumaroles (450-250°C) in Vulcano (1.5 ≤ Gd/Gd* ≤ 6.8) and less in lower temperature fumaroles (285-82°C) studied in Phlegrean Fields, Canary and Cape Verde islands (1.1 ≤ Gd/Gd* ≤ 2.7).Moreover, comparing the amplitudes of Gd/Gd* and related HCl/HF ratio values of alkaline condensates a significant positive relationship is observed only in alkaline condensates collected from higher temperature fumaroles (T > 200 °C). This evidence is consistent with a preferential REE complexation via Cl-complexes in vapour phase and can be considered a consequence of nephelauxetic effect. According to this suggestion the growth of positive Gd anomalies is a consequence of changes of the first Gd hydration sphere during the formation of Gd-chloride complexes at higher temperature and does not occur if complexes are formed at temperatures up to 200 °C (Mayanovic et al., 2007). Being REE-chloride complexes highly soluble species quickly removed from the atmosphere by water droplets, Gd coming from volcanic gas phase could represent a significant component of the geochemical Gd cycle in aqueous media.
|Numero di pagine||1|
|Stato di pubblicazione||Published - 2014|