Zirconium, Hafnium and Rare Earths behaviour during the transport in volcanic fluids. Geochemical effects throughout the sublimation and after interactions with aqueous media

This research indicates the fate of Zr, Hf and Rare Earths during their sublimation from the high-temperature gas phase (100° - 420 °C), in volcanic systems associated with different geodynamic regimes, and processes associated with volcanic sublimation representing the last natural “inorganic” interface where a detailed investigation into the behaviour of Rare Earths had never been conducted. The research was carried out in active volcanic systems at Vulcano (Aeolian Islands, Italy), Santorini (Cyclades Islands, Greece), Phlegrean Fields (Italy), Tenerife (Canary Islands, Spain) and Fogo Island (Cape Verde) where both fumaroles and thermal waters from submarine springs and inland wells were collected and investigated. Both solid newly forming sublimates and the coexisting volcanic gas phase were studied in fumaroles in order to evaluate partitioning of the elements studied during the sublimation. Volatile geochemistry of Zr, Hf and Rare Earths (hereafter reported REE, being the sum of lanthanides and yttrium) data suggested a scenario where REE are transported in the volcanic gas phase as chloride complexes. This indication is apparently also confirmed for Zr and Hf by the Zr partitioning in the volcanic gas phase during sublimation relative to Hf. This evidence is attributed to the formation of [(Zr,Hf)Cl4(H2O)4] and other Cl-complexes that are more stable in the volatile phase when Zr is the coordinating metal relative to Hf. Features of shale-normalised REE patterns show positive Gd anomalies in volcanic gas during the sublimation whose amplitude progressively grows with increasing HCl contents in volcanic gas. These features are consistent with reference values of stability constants for chloride-REE complexes rather than for fluorine- and other REE complexes. This data may explain several instances of medium REE enrichments in natural waters during and immediately after large volcanic eruptions and suggests a particular Gd fate relative to other REE during the emission of volcanic soluble substances. Geochemical evidence coming from hydrothermal waters from Vulcano and Santorini Islands confirm the above-mentioned suggestion about the particular Gd behaviour in volcanic fluids since positive Gd anomalies are recognised in waters from submarine vents. Here Gd/Gd* ii up to 1.6 are associated with clear signs of W-type tetrad effects that allow us to identify these fluids as those leaching the authigenic solids that are immediately formed during the mixing between the hot reducing and acidic hydrothermal fluids with cold, oxidizing and basic seawater close to the vent. This and other geochemical evidence along with model calculations allow us to identify two different water groups in terms of physical-chemical characters, calculated saturation/oversaturation with respect to carbonates (group 1 waters) and Fe-oxyhydroxides (group 2 waters) and reciprocal Zr-Hf behaviour dissolved in natural pools. In both waters, the dissolved speciation of these elements is dominated by [Zr(OH)4]0 e [Hf(OH)5]- complexes. These species being differently charged, they fractionate during interactions with occurring solid surfaces. Our data confirms that Zr and Hf undergo a competition process between dissolved speciation and surface adsorption, but also suggests that the larger Hf surface reactivity, especially onto Fe-oxyhydroxides, can be related to the Hf surface complexation rather than to a simple interaction attributed to electrostatic attractions. Our findings prove that passive volcanic degassing can represent a suitable Gd source in a soluble and bioavailable form. Model calculations based of CO2 fluxes from the studied active volcanic areas indicate that about 1 kg Gd per year is released as a whole into the atmosphere. Comparing this value with reference data, this indication suggests that the volcanic Gd-flux is of the same order of magnitude as the yearly anthropogenic Gd delivered to the hydrosphere from hospital wastewaters in Germany. Therefore, Gd from volcanic source could represent a potential environmental risk under particular conditions.