Geochemistry of Zr, Hf and REE in extreme water environments: hyperacid, hyper saline and lake waters in hydrothermal systems

This PhD thesis concerns the geochemistry of Zr, Hf and REE in extreme water environments characterized by a wide spectrum of chemical physical-parameters and compositions. The investigations were carried out in hypersaline waters covering a wide range of Eh values along Dead Sea Fault (Israel), in hyperacid waters circulating in Nevado del Ruiz volcano-hydrothermal system (Colombia) and in CO2-rich waters belonging to the Pantelleria volcano-hydrothermal system (Italy), including the alkaline lake “Specchio di Venere” formed within a calderic depression. The wide spectrum of chemical-physical conditions and various water chemical compositions interacting with different solid phases allowed to depict a scenario where Zr, Hf and REE are ruled by different processes, filling the still missing geochemical aspects. The important role of the pH (from 1 to 8.8) and the water chemical composition in regard to the distribution of Zr, Hf and REE was mainly investigated in the Nevado del Ruiz volcano-hydrothermal system. The pH rules the precipitation of authigenic Fe-, Al-oxyhydroxides producing changes in Zr, Hf and REE abundances and strong cerium anomaly. Significant LREE (Light Rare Earth elements) depletion was found in acidic sulphate waters, where the formation of alunite and jarosite was recognized. Sub-chondritic Zr/Hf ratios (lower than magmatic local rocks) and chondritic Y/Ho ratios (close to the local magmatic rocks) are shown in acidic sulphate waters (1<pH<3.6). Zr/Hf ratio increases as Cl/SO4 ratio increases, suggesting a different behaviour of Zr and Hf as function of complexing ligands in solution. Differently to the acid waters, the same fractionation of Y-Ho and Zr-Hf was found in near neutral waters where Al-, Fe-oxyhydroxides were found. The twin pairs show Y/Ho and Zr/Hf ratios increasing towards super-chondritic values due to the preferential removal of Ho and Hf compared to Y and Zr by Al-, Fe-oxyhydroxides. Pantelleria hydrothermal waters are dominated by Na and Cl ions with variable HCO3- enrichments due to the interaction with deep-seated CO2. Different behaviour of Zr, Hf and REE was found in the alkaline lake “Specchio di Venere” with respect to the CO2-rich thermal waters circulating in the Pantelleria volcano-hydrothermal system. Shale-normalised REE (relative to Post Archean Australian Shale, PAAS) in CO2-rich waters showed the same pattern increasing along the REE series, with the elemental speciation dominated by carbonate ligands. Zr, Hf and REE show higher concentrations in lake “Specchio di Venere” with intermediale REE (MREE) enrichments and positive Ce anomaly. Similar features (MREE enriched and positive Ce anomaly) were found in the settling dust and in the Desert Varnish, mainly constituted by Fe-, Mn-oxyhydroxides and clay minerals. Moreover, Y/Ho and Zr/Hf molar ratios in “Specchio di Venere” lake (35.37 and 76.30, respectively) show also a Desert Varnish signature. These latter data, coupled with the MREE enrichments and the presence of Fe-oxyhydroxides and phyllosilicates in the shallowest water layer of “Specchio di Venere”, testify for an aeolian input from the nearby Sahara desert demonstrating that Zr, Hf and REE are useful tracers to identify the contribution of atmospheric particle in open water bodies. The key role of the Eh values and the water composition towards the distribution of Zr, Hf and REE was mainly evaluated in waters along the Dead Sea Fault. Here, both cold and hot waters fall within a wide range of salt contents (from 0.3 to 193.5 g l-1) and Eh values (from -400 to 256 mV). These waters are mainly NaCl dominated with variable enrichments in SO4, HCO3 and Ca due to water rock-interactions. The investigated waters are oversaturated with respect to carbonate minerals, Fe-, Mn-oxyhydroxides and pyrite, and always undesaturated in gypsum and halite. The REE distribution shows MREE enrichments, due to the dissolution of evaporitic minerals characterized by MREE enrichments. The redox conditions influence the amplitude of Ce and Eu anomalies. Oxidized waters show negative Ce anomalies related to the oxidative Ce scavenging, whereas positive Eu anomalies are found in waters characterised by Eh values < -100 mV consistently with the Eu occurrence as the dissolved Eu2+. This condition enhances the Eu stability in dissolved phase relatively to its trivalent neighbours along the REE series. Since dissolved Zr/Hf molar ratio is sensitive to the occurrence of solid Fe-oxyhydroxide surfaces where Hf is preferentially scavenged, redox conditions influence the Zr/Hf signature in these waters despite neither Zr nor Hf are redox sensitive elements. Therefore, the Zr and Hf in waters oversaturated with respect to Fe-oxyhydroxides show superchondritic Zr/Hf values due to the preferential Hf scavenging onto solid surfaces whereas the waters oversaturated relative to pyrite show chondritic Zr/Hf signatures.