The behaviour of zirconium, hafnium and rare earth elements during the crystallisation of halite and other salt minerals

CHEMGE-18245; No of Pages 12 Chemical Geology xxx (2017) xxx–xxx

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The behaviour of zirconium, hafnium and rare earth elements during the crystallisation of halite and other salt minerals P. Censi a,⁎, C. Inguaggiato a,b, S. Chiavetta c, C. Schembri d, F. Sposito a, V. Censi e, P. Zuddas b a

Department of Earth and Marine Sciences, University of Palermo, Via Archirafi, 36, 90123 Palermo, Italy Université Pierre et Marie Curie, Paris 6 Institut des Sciences de la Terre 4, place Jussieu, F75005 Paris, France SIDERCEM S.R.L., Via L. Grassi, 7, 93100 Caltanissetta, Italy d Italkali S.P.A., Miniera di Realmonte, Via Scavuzzo, Realmonte (AG), Italy e Politecnico di Bari, DICATECH Department, Via E. Orabona 4, 80100 Bari, Italy b c

a r t i c l e

i n f o

Article history: Received 16 June 2016 Received in revised form 30 January 2017 Accepted 1 February 2017 Available online xxxx Keywords: Zr/Hf REE Halite Evaporites

a b s t r a c t Halite crystals from Messinian and Tortonian evaporites from Sicily and Spain and current precipitated halite crystals and the relative parent brines (active evaporation systems) were investigated in order to evaluate the Zr, Hf and Rare Earth Element (REE) behaviour. Halite crystallisation from evaporating brines fractionates Zr, Hf and REE through a two-step process. During the first step, dissolved complexes of studied elements are scavenged onto the surfaces of crystallising halite. During the second step, elements are co-precipitated into the crystal lattice as it grows. The first step mechanism is determined by the dissolved REE speciation. In saltworks where carbonate-REE complexes occur, surface complexation of REE onto halite crystals does not occur. On the contrary, surface REE-complexes onto halite crystals are forming in the Dead Sea water where aqueous REE speciation is dominated by chloride-complexes. Under the latter conditions, halite crystallises with cubic and cubic-octahedral composite habitus. Octahedral planes involve the formation of strong coulombic interactions, mainly with [Hf(H2O)3(OH)5]− rather than with [Zr(H2O)4(OH)4]0 complexes. As a consequence, newly formed halite in the Dead Sea shows strong subchondritic Zr/Hf ratios. Based on these indications, analyses carried out on salt minerals from Messinian and Tortonian evaporites in Sicily and Spain show that their overall REE content can be considered a discriminating parameter between authigenic minerals and diagenetic modified materials. However, features of shale-normalised REE patterns are driven by the mineralogical composition of evaporites rather than their authigenic or secondary nature. On the contrary, the Zr/Hf signature of salt minerals is influenced by their origin. Indeed, subchondritic Zr/Hf values are found in primary salt minerals, whereas larger Zr/Hf values are recognised in those diagenetically modified. Calculated distribution coefficients of Zr, Hf and REE are employed for modelling the REE distribution in halite equilibrated with the deep-sea brines from Typo, Medee and Thetis basins (Eastern Mediterranean). The obtained indications allow us to discriminate brines formed by dissolution of evaporites relative to those representing relics of fossil evaporated seawater. © 2017 Elsevier B.V. All rights reserved.

1. Introduction The elemental fractionation between authigenic minerals and parent solutions drives elemental distributions in dissolved phase (Wood and Blundy, 2014 and references therein). This phenomenon occurs both through surface interactions between newly formed crystals and the remaining aqueous phase, and by the crystal-chemical substitution for major metal ions according to Goldschmidt's Rules. The former process is driven by the external electronic configuration of elements and

⁎ Corresponding author. E-mail address: [email protected] (P. Censi).

then related to their aqueous speciation (Koschinsky and Hein, 2003), whereas the latter process is controlled by the charge and radius of involved elements (CHARAC process) (Bau, 1996). Bau (1996) first pointed out the different behaviour of lanthanides and yttrium (hereafter defined as Rare Earth Elements, REE), Zr and Hf during CHARAC and non-CHARAC processes. Several studies investigate the REE, Zr and Hf fractionations driven by CHARAC mechanisms during magmatic crystallisation (see Wood and Blundy, 2014 for a comprehensive review), yet only few studies pointed out REE, Zr and Hf behaviour during the crystallisation of authigenic minerals from an aqueous phase (Guichard et al., 1979; Möller and Dulski, 1983; Humphris and Bach, 2005; Ogawa et al., 2007; Karakaya, 2009; Inguaggiato et al., 2015).

http://dx.doi.org/10.1016/j.chemgeo.2017.02.003 0009-2541/© 2017 Elsevier B.V. All rights reserved.

Please cite this article as: Censi, P., et al., The behaviour of zirconium, hafnium and rare earth elements during the crystallisation of halite and other salt minerals, Chem. Geol. (2017), http://dx.doi.org/10.1016/j.chemgeo.2017.02.003

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Although knowledge of REE, Zr and Hf distributions between marine minerals and seawater represents a useful approach for reconstructing ancient ocean compositions (Bąbel and Schreiber, 2014 and references therein) these studies are rarely carried out (Terakado and Masuda, 1988; Kagi et al., 1993; Zhong and Mucci, 1995). This knowledge would be of wide interest if focused on evaporite minerals as it permits the reconstruction of the nature and origin of the ancient brines (Vengosh et al., 2000; Boschetti et al., 2011). To achieve this goal, knowledge of distribution coefficients of the elements under investigation is required. In particular, these distribution coefficients should take in account REE, Zr and Hf fractionations induced both by surface complexation and by the substitution for the major ions in the crystal lattice of salt minerals. Therefore, REE distributions in crystallising halite and in parent solutions must be investigated in current evaporating systems where halite deposition occurs. In this study, halite samples and coexisting parent brines were studied both from saltworks and from the Dead Sea. These sites were chosen due to their different chemicalphysical conditions. In saltworks the man-induced regulation of the evolution path of evaporating seawater inflow relative to the evaporation rate induces a high crystallisation rate and halite deposition. In the Dead Sea the brine evaporation occurs in a terminal lake under natural conditions. Therefore, these systems can be considered two very different features of the same natural process: the halite crystallisation from evaporating waters. Ancient halite crystals were collected along Tortonian and Messinian evaporitic sequences from Spain and Sicily, respectively. Along the latter sequences both primary evaporites and diagenetic modified samples were studied. 2. Materials and methods During this research suspended halite microcrystals in saltwork brines and current halite crystals precipitating during production periods were collected from “Saline di Trapani” saltworks in western Sicily. Sample collection was carried out in November 2014 and July 2015. In July Pool 1 was sampled, which represented the first stage of seawater evaporation. Pools 5, 6 and 7, which represented the progressively evolving stages of brine evaporation up to halite deposition, were also sampled. Suspended halite crystals and the coexisting parent solution were collected from the Dead Sea close to Ein Gedi Beach in March, April or May between 2013 and 2015 in both dry and rainy periods. Evaporite minerals from the different sampling sites consist of a mixture of kainite [MgSO4KCl*3H2O], Ca-sulphates (anhydrite and/or gypsum) and halite (NaCl) as reported in Table S1 (on-line supplementary information). They were collected along the Messinian salt sequence exposed in the Realmonte Mine (ITALKALI S.P.A.), Petralia Mine (ITALKALI S.P.A.) and outside the old Raineri mine in a subaerial

outcrop. Halite samples from Tortonian evaporite sequences in Spain were collected from the Granada Basin from a 200 m drill located in the central part of the Granada basin. The sampling sites are reported in Fig. 1. Petralia mine exploits a deformed salt body where recrystallized halite occurs. Only scarce evidences of primary halite are reported in the salt deposit (Speranza et al., 2013). Sample collection was carried out from recrystallized levels where halite occurs in ialine crystals, similar in shape and usually free from impurity (Fig. 2A, B, C). As shown, these crystals are free from growth bands and shapes like chevrons and hoppers chevron features that usually indicate primary crystallisation (Lowenstein and Hardie, 1985). Realmonte mines works in an about 600 m thick salt deposit consisting in different units (Decima and Wezel, 1971; Lugli et al., 1999). From the bed to the top, the salt body is formed by halite-bearing cumulates where halite shows classical chevron-like features and cloudy crystals characteristic of primary crystallisation. This sequence is followed by a further second unit contains halite levels embedded in kainite-rich levels and grey mud micro-levels (Garcia-Veigas et al., 1995). As follows the sequence shows another unit mainly consisting of more massive halite. The top of sequence consists of skeletal halite with anhydrite (Speranza et al., 2013). The collected samples in the Realmonte mine come from the second unit (Fig. 2D, E). Only two “secondary salt samples were collected from stalactites in a cave in the mine (Fig. 2F). The salt deposit of Raineri mine (currently closed) can be recognised outside the original site in a subaerial outcrop where a sequence of kainite and halite levels is exposed. The Upper Tortonian evaporites in the Granada basin were formed during an early stage of the Messinian salinity Crisis that attained its zenith during the Upper Miocene (Garcia-Veigas et al., 1995and references therein), consistently with the closure of the Thetys produced by Africa-Europe convergence. In the Granada basin, the evaporitic sequence consists of selenitic gypsum, sometimes transformed in secondary gypsum, halite and stromatolitic carbonates often replaced by celestine (Martin et al., 1984). Halite samples from the Granada basin come from a 200 m drill located in the central part of the basin along the evaporite sequence close to anhydrite (Fig. 2G, H, I, J). The treatment of studied salts was carried out as follows. A 1 g aliquot of unwashed salt from the evaporite sequences under study was dissolved in 1 l of 5% HNO3 acid solution and the obtained solution stored in a previously cleaned polyethylene bottle. A 10 ml sample of this solution was removed and stored in a plastic vial for the analysis of the major components. A 1 g aliquot of halite crystallising from the saltworks and from the Dead Sea was collected during the filtration of evaporating water (Millipore™ manifold filter diameter 47 mm, pore size 0.45 μm). Filters

Fig. 1. Schematic map of sampling areas. The map was drawn from an Ocean Data View file (Schlitzer, 2015).

Please cite this article as: Censi, P., et al., The behaviour of zirconium, hafnium and rare earth elements during the crystallisation of halite and other salt minerals, Chem. Geol. (2017), http://dx.doi.org/10.1016/j.chemgeo.2017.02.003

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Fig. 2. A, B: Recrystallized halite crystals from Petralia mine with characteristic ialine feature. C: Primary halite sample from the evaporite sequence in Realmonte mine. D: kainite mass in Realmonte mine. E: Cave in Realmonte mine where RE-6 and RE-7 samples were collected from stalactites. F: Pure halite in “cloudy” crystals (sample SP04). G: Anhydrite nodules and associated halite (sample SP-01). H: Banded halite crystals and associated anhydrite nodules (sample SP-02). I: The salt deposit of Raineri mine exposed in subaerial outcrop where sample BF was collected.

were immediately stored in plastic vials for further manipulations. After filtration, the collected water samples were acidified with HNO3 ultrapure solution (BAKER ULTREX-II) to attain pH ≈ 2, diluted 1:2 to avoid salt crystallisation and then stored in previously cleaned polyethylene bottles. The vials containing halite crystals were transferred to the laboratory, the crystals removed from membranes and thoroughly washed in ultrapure water and dried at 50 °C according to procedures carried out by Herut et al. (1998). Then, 1 g aliquot of each sample was dissolved in 1 l of 5% HNO3 acid solution as previously carried out for salt minerals from evaporite sequences and then stored in a previous cleaned polyethylene bottle. Also for halite crystallised in saltworks and in the Dead Sea, 10 ml of solution were collected the analysis of the major components. Crystals from saltworks were washed before the dissolution whereas those collected in the Dead Sea were studied both washed and unwashed in order to assess the extent of the removal of studied elements both in the sorbed fraction and incorporated in the halite crystal.

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Major elements dissolved in the waters were analysed by two Dionex ionic chromatographs: one equipped with a CS-12A column to determine major cations (Na, K, Mg, and Ca) and another equipped with an AS-14A column to analyse major anions (Cl, SO4). Alkalinity was determined by titration with HCl 0.1 N. Zr, Hf and REE both in aqueous samples and in solutions representative of halite crystals and salt minerals were analysed following the method of Raso et al. (2013). An excess of FeCl3 (1%) solution was added to each sample (1 l) and then a suitable volume of NH4OH (25%) solution was added in order to attain a pH of 8 to induce the precipitation of solid Fe(OH)3. During this process, Zr, Hf and REE were scavenged onto the surface of the crystallising solid and then separated from the remaining liquid by filtration. In order to be sure that the crystallisation of Fe(OH)3 was complete, the solution was left in a closed flask for 48 h in a stirrer manifold. Therefore, the iron concentration was measured for assessing the recovery of the process. It was always better than 95%. Precipitated Fe(OH)3 was collected onto a membrane filter (Millipore™ manifold filter diameter 47 mm, pore size 0.45 μm). Then, the solid filtrate was dissolved in HCl 6 M and the obtained solution diluted to 1 M and analysed in Quadrupole-ICP-MS (Agilent 7500 series) with an external calibration procedure. The overall procedure and a detailed evaluation of the analytical error are reported in Raso et al. (2013). All chemicals used during lab manipulations were of ultrapure grade. Ultra-pure water (resistivity of 18.2 MΩ cm−1) was obtained by an EASY pure II purification system (Thermo, Italy). Nitric acid 65% (w/w), ammonia solution and hydrochloric acid were purchased from Baker chemicals. Working standard solutions for studied elements were prepared on a daily basis by stepwise dilution of the multi-element stock standard solution DBH, Merck or CPI International (1000 ± 5 mg l−1) in a HCl 1 M medium. All labwares were polyethylene, polypropylene or Teflon and the calibration of all volumetric equipment was verified. The pH measurements were carried out using a portable Thermo Scientific Orion Star meter equipped with a conventional glass electrode. The pH meter was calibrated with three buffers: pH 4.01, 7.00, and 9.95 at 25 °C. The ionic strength of the water does not influence the pH measurement over 0.2 pH unit even if the solution can reach ionic strength ~ 10 M (Golan et al., 2014). Moreover, the effect of the ionic strength on the pH was evaluated from Golan et al. (2014) in the Dead Sea waters and its dilution. The assessment of the analytical precision of Zr, Hf and REE was carried out by analysing three aliquots (one litre each) of NASS-6 (Seawater certified reference material for trace metals) distributed by National Research Council of Canada. These were treated as water samples according to the above mentioned procedure and the obtained concentrations compared with those previously reported by Jochum et al. (2007). Simulation with PHREEQC software using the LLNL database (version 3.0.6; Parkhurst and Appelo, 2010) was carried out to calculate the aqueous REE speciation at the chemical-physical conditions measured on the field. Recognition of the dissolved speciation of these elements is also needed to interpret the geochemical behaviour of REE on a simple qualitative basis. To achieve to this goal, the very high salt contents of aqueous phase samples from the saltworks and the Dead Sea would require an approach based on Pitzer equations using the PHREEQC code. However, the pitzer.dat database includes only a limited number of elements and REE are not included. Therefore, the treatment of the aqueous REE speciation carried out according to the llnl.dat database was preferred in order to achieve to a qualitative assessment of REE speciation related to the limitations imposed by assessment of thermodynamic calculations carried out according to the Debye-Huckel approach. These calculations were performed using compilations of stability constants of REE complexes occurring in the LLNL database integrated with more recent thermodynamic values (Millero, 1992; Lee and Byrne, 1992; Klungness and Byrne, 2000; Luo and Byrne, 2000, 2001, 2004; Schijf and Byrne, 2004) according to the same approach

Please cite this article as: Censi, P., et al., The behaviour of zirconium, hafnium and rare earth elements during the crystallisation of halite and other salt minerals, Chem. Geol. (2017), http://dx.doi.org/10.1016/j.chemgeo.2017.02.003

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previously carried out in Johannesson et al. (2004), Tang and Johannesson (2006) and; Leybourne et al. (2006). Aqueous Zr and Hf speciation assessed with the PHREEQC software is driven by hydroxyl complexes in the studied waters, in agreement with previous researches (Aja et al., 1995; Byrne, 2002). Scanning Electron Microscopic (SEM) observations and Energy Dispersive X-ray Spectra (EDS) were carried out on suspended particulate matter (SPM) collected in studied waters after filtration. These were carried out at the SIDERCEM SRL laboratory (Caltanissetta, Italy). SPM were gently dried under vacuum, mounted on aluminium stubs and gold coated. SEM observations were carried out using a LEO 440 SEM equipped with an EDS system OXFORD ISIS Link and Si (Li) PENTAFET detector that was used to obtain EDS spectra. The mineralogy of the studied salts was assessed by norm calculations from bulk chemical analyses and checked using X-ray diffraction (XRD) spectra. These were carried out using a Philips PW14 1373 X-ray spectrometer using monochromatic CuKα radiation. The apparent trace element distribution between crystallising halite and the coexisting parent brine can be assessed following the equation:
DREE ¼



REEi Na
halite REEi Na sol

ð1Þ

(Nishri, 1984; Herut et al., 1998). Eq. (1) provides the amplitude of REE contents relative to Na+ in halite with respect to the same ratio in the coexisting solution. DREE values are reported in Table S2 (on-line supplementary information). The brine composition used for their calculation in the saltworks is sample TS-7 (Table S3 and S4 of the online supplementary information). The composition of the brine collected in 2013 (Table S3 and S4 of the online supplementary information) was used to calculate DREE values in the Dead Sea. 3. Results 3.1. Saltworks The major element concentrations in the dissolved phase from the saltworks samples and the physical-chemical parameters directly measured in the field are reported in Table S3 (on-line supplementary information). The evolution of major ion compositions in aqueous phase was investigated during the hot period by measuring the concentration of 7 pools in the saltworks. Total dissolved solids (TDS) values range from 155 to 327 g l−1, pH values fall between 7.3 and 8 and Eh is maintained between −110 and 25 mV. The aqueous speciation analyses carried out with PHREEQC software shows that the most abundant REE species in the studied brines occur as Cl- and CO3-complexes for La and Ce, as CO3-complexes for other REE during the summer when the ionic strength is N5 and as CO3-complexes for all the REE during the autumn when the ionic strength ranges from 2.9 to 3.9. Zr, Hf and REE concentrations in dissolved phase from the saltworks are reported in Table S4 (on-line supplementary information). The overall REE contents range from 107 to 1397 ppt whereas Zr and Hf range from 5.7 to 227 and 0.2 to 5.7 ppt, respectively. Y/Ho weight ratios in dissolved phase show slight superchondritic values (30.2 ≤ Y/Ho ≤ 37.4) whereas Zr/Hf weight ratios range between 28.3 and 62.7. The largest REE, Zr and Hf concentrations are found in brine collected during summer. Shale-normalised REE patterns assessed relatively to Post Archean Australian Shale, PAAS, (Taylor and McLennan, 1995) in aqueous phase show progressively ascending features from La to Gd referenced to PAAS average values, followed by a slight depletion from Tb to Lu that allow to a moderate Middle Rare Earth Elements (MREE) enrichment relative to Light Rare Earth Elements (LREE) and Heavy Rare Earth Elements (HREE) (Fig. 3).

Fig. 3. Shale-normalised REE patterns of halite crystals and coexisting brines from Trapani saltworks.

Zr, Hf and REE concentrations of washed halite crystals precipitating from saltwork brines are reported in Table S3 (online supplementary information). REE concentration spans from 34.0 to 170 ppb. Zr and Hf contents range between 2.8 and 26.9 ppb and from 0.1 and 0.7 ppb, respectively. The related Zr/Hf weight ratios range between 33.1 and 44.2, whereas Y/Ho weight ratios fall between 24.2 and 60.1. Shale-normalised REE patterns in halite crystals show a strong MREE enrichment referenced to PAAS average values that is often symmetrical in shape relative to Eu. Only in the sample TS-7 a slightly different pattern was observed with progressively ascending features from La to Gd followed by Tb, Dy and Ho depletion referenced to PAAS average values (Fig. 3). 3.2. Dead Sea Major element concentrations in the dissolved phase from Dead Sea samples and physical-chemical parameters directly measured in the field are reported in Table S3 (on-line supplementary information). Dead Sea water sampled from Ein Gedi has TDS values ranging from 282 to 373 g l−1 in the dry and rainy collection period, respectively. In contrast to the saltwork brine, the pH values of Dead Sea are constant close to 6 whereas Eh values range from −120 to 19 mV. Speciation calculations carried out on the Dead Sea water suggest that the REE distribution is always determined by Cl-complexes ranging the ionic strength between 6.3 and 7.8. Zr, Hf and REE concentrations in the dissolved phase from the Dead Sea are reported in Table S4 (on-line supplementary information). The overall REE contents range between 86.1 and 202 ppt, with the largest value recorded during 2014 rainy period. At the same time, Zr and Hf change in the ranges 11.6 to 24.0 and 0.2 to 0.7 ppt, respectively. Y/Ho weight ratios in dissolved phase are always superchondritic, falling between 63.0 and 73.0. On the contrary, Zr/Hf molar ratios change from subchondritic to superchondritic values (23.6 to 56.9). Shalenormalised REE patterns in Dead Sea brines are very similar in both 2013 and 2015 and are characterised by an upward-concave shape with positive Ce and Gd anomalies. In 2015 the features of REE patterns

Please cite this article as: Censi, P., et al., The behaviour of zirconium, hafnium and rare earth elements during the crystallisation of halite and other salt minerals, Chem. Geol. (2017), http://dx.doi.org/10.1016/j.chemgeo.2017.02.003

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are slightly different with La and Eu concentrations increasing relative to the other brines (Fig. 4). Zr, Hf and REE concentrations in halite crystals from the Dead Sea are reported in Table S5 (on-line supplementary information). REE, Zr and Hf concentrations were investigated both in washed and unwashed crystals from the Dead Sea in order to assess the contribution of surface processes and crystal-chemical substitution in lattice position of studied elements. The washed halite crystals are characterised by a very narrow range of total REE amounts, around 15.9 ppb, whereas Zr and Hf range from 1.1 to 1.6 and from 0.03 to 0.05 ppb, respectively. Zr/Hf weight ratios are clustered around 32.0 and Y/Ho weight ratios around 33.8, showing that Zr/Hf values are similar to those found in the washed halite crystals from the saltworks. The total REE concentrations in the unwashed halite crystals range from 353 to 477 ppb. These values are approximately one order of magnitude higher with respect to the washed crystals. Zr and Hf range from 49.3 to 67.2 and from 10.0 to 14.8 ppb, respectively. Zr/Hf and Y/Ho molar ratios fall within narrow ranges corresponding to 4.7 ± 0.3 and 38.7 ± 3.9, respectively. 3.3. Salt minerals from evaporites XRD analyses carried out on studied samples collected from the Realmonte Mine indicate the presence of kainite (KClMgSO4*3H2O),

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sometimes with carnallite (KMgCl3*6H2O), and halite (NaCl). Features of the salt deposit in the Realmonte Mine show that it consists of thick halite strata with K\\Mg salt-bearing levels interbedded. These features allowed us to collect both prevailing kainite samples and materials where the most abundant mineral was halite with kainite occurring in lesser amounts. Previous geological and petrographic research explained this mineralogical composition of the Realmonte deposit by the direct crystallisation of the above-mentioned salts under extreme hypersaline conditions (Yoshimura et al., 2016 and references therein). On the contrary, in Petralia Mine, halite samples come from an area where textural evidence characterises halite crystals as secondary in nature (Speranza et al., 2013). BF sample collected outside the Raineri mine consists of microcrystalline halite with clear dissolution features on the exposed surfaces. In light of XRD data providing a qualitative recognition of occurring minerals, the composition of major components of studied salt minerals was used for the construction of the CIPW-norm (Cross et al., 1902), according to the approach recently used by Kackstaetter (2014). Results of CIPW calculations are reported in Table S1 (on-line supplementary information). Zr, Hf and REE concentrations measured in salt minerals from evaporites are reported in Table S6 (on-line supplementary information). The overall REE contents in salt minerals from studied evaporitic sequences are very different from one place to another: • The total REE concentration measured in halite crystals from the Petralia Mine (Miocene age), ranges from 61.45 to 673.81 ppb. Zr and Hf contents range from 12.00 to 150.62 and 0.77 to 5.39 ppb, respectively. Related Zr/Hf values fall within the range 13.8 to 28.0, whereas Y/Ho values range from 16.0 to 33.8. Similar values are found in two secondary growing stalactites from the Realmonte Mine showing REE contents of 72.44 and 221.71 ppb and Zr-Hf values of 23.21 and 78.07 and of 1.29 and 2.63 ppb, respectively. The same values are found for Zr/Hf and Y/Ho ratios measured in the latter samples (18.0–29.7 and 18.2–29.7, respectively). These values are very similar to those found in secondary halite from the Petralia Mine. • In the Tortonian evaporite sequence from Spain, the total REE concentration ranges from 133.50 to 779.10 ppb. Zr and Hf contents range from 15.74 to 25.71 and 1.08 to 1.84 ppb, respectively. Related Zr/Hf values fall within the 10.88 to 14.53 range, whereas Y/Ho values range from 29.15 to 30.68. • In primary halite-kainite deposits from Realmonte mine (Miocene age), the total REE concentration ranges from 207.50 to 1654.93 ppb. The corresponding Zr and Hf values range between 10.25 and 342.62 ppb and 0.86 and 100.87 ppb, respectively. The corresponding Zr/Hf values range between 2.6 and 12.3, whereas Y/Ho values range from 28.1 to 46.1. • The total REE concentration in weathered halite sample from Raineri mine is 415.55 ppb. The corresponding Zr and Hf concentrations are 95.89 and 2.63 ppb, respectively. Therefore, Zr/Hf ratio corresponds to 36.44, whereas Y/Ho ratio is 20.79. The shale-normalised REE patterns of studied samples show two distinct features:

Fig. 4. Shale-normalised REE patterns of halite crystals and coexisting brines collected between 2013 and 2015 in the Dead Sea. The behaviour of formation constants of [REECl]2+ complexes (Luo and Byrne, 2001) are reported for reference.

• Progressively “ascending” patterns referenced to PAAS average values along the REE series in Messinian evaporites with prevailing kainite content, • MREE enriched patterns referenced to PAAS average values in prevailing halite samples, often associated with gypsum, coming from Tortonian and Miocene evaporites from Spain and Sicily, respectively. In these samples variable La enrichments are observed (Fig. 5). Secondary halite samples coming from Messinian deposits in Petralia and Realmonte mines show shale-normalised patterns similar to those found in prevailing halite samples (Fig. 5).

Please cite this article as: Censi, P., et al., The behaviour of zirconium, hafnium and rare earth elements during the crystallisation of halite and other salt minerals, Chem. Geol. (2017), http://dx.doi.org/10.1016/j.chemgeo.2017.02.003

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Fig. 5. Shale-normalised REE patterns of salt minerals collected along evaporite sequences from studied sites in Italy (Messinian age) and Spain (Tortonian age). A: Prevailing kainite samples. B: Prevailing halite samples. The dashed green area represents the calculated ranges of the composition for halite crystals equilibrated with deep-sea brines from Tyro, Thetis and Medee basins (Eastern Mediterranean). C: Secondary salt minerals. D: The dashed grey area indicates the REE values recognised in halite samples collected from Stassfurt mine (Yui et al., 1998). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

4. Discussion 4.1. REE behaviour during the halite crystallisation Yui et al. (1998) investigated the REE distribution in halite samples from Stassfurt in Germany in order to identify the extent of trace element contaminations in salts. In studied samples the authors found a total REE content ranging from 0.8 to about 3.0 ppb. Although these are lower concentrations than those found in our samples, observed shale-normalised patterns show similar MREE enrichments referenced to PAAS average values (Fig. 5). Further analyses of the REE content carried out on authigenic carbonates and sulphates free from postdepositional changes in sediments and soils, always indicate MREE enrichments in shale-normalised patterns referenced to PAAS average values (Toulkeridis et al., 1998; Playà et al., 2007; Huang et al., 2015). This REE behaviour is explained both as a consequence of gypsum

crystallisation and as inherited by the REE distribution in coexisting aqueous phase. In hydrothermal minerals the same MREE enrichment observed in shale-normalised patterns was often explained by their preferential incorporation in these minerals through coprecipitation (Morgan and Wandless, 1980; Bau and Möller, 1992; Mills and Elderfield, 1995). On the other hand, Humphris and Bach (2005) suggest that the REE complexation can play an important role in driving some of the REE features observed in shale-normalised patterns of hydrothermal sulphates. Recently, MREE enrichments in shale-normalised patterns were observed in saline sediments collected from anoxic hypersaline deep-sea basins in the Eastern Mediterranean Sea and attributed to the halite contents of these sediments (Censi et al., 2014). In general, the REE incorporation into an authigenic mineral crystallising from an aqueous phase can be divided into two steps: the REE adsorption onto the newly-forming mineral surface and their incorporation into the crystal lattice as a consequence of the crystal growth (Morse et al., 2007). The REE distribution in the crystal-solution system should reach a dynamic balance during these steps depending from the precipitation rate (Qu et al., 2009). The first step of the overall process is a combination of coulombic electrostatic interactions and specific surface complexation of dissolved metal species onto forming crystal surfaces. The coulombic interactions play a major role if crystal surface has a residual charge, depending on pH (Kosmulski, 2012) or if those considered are ionic crystals where polar surfaces can occur. The uptake of dissolved species on a mineral surface during the surface complexation requires the formation of covalent bonds (Curti, 1997) and allows the binding of uptaken metal with ligands from the mineral surface (Koschinsky and Hein, 2003). The results of REE aqueous speciation suggest that the uptake of REE via surface complexation is probably inhibited during halite crystallisation in saltworks where the REE-carbonate complexes are dominant from Pr to Lu and Cl-complexes would occur as more abundant species only for La and Ce. Hence, the surface interactions between dissolved REE and halite surfaces should occur according an electrostatic mechanism in the saltworks, apart from La and Ce. This process would be simplified by the ionic character of halite crystals (Bruno, 2013) and should not fractionate REE complexes having the same charge along the series. In the Dead Sea, shale-normalised REE concentrations increase slightly from La to Lu in unwashed halite crystal referenced to PAAS average values (Fig. 4). Their features are similar to those characterising both the Dead Sea water in the 2013 to 2015 period and the sequence of the formation constants for REECl2 + complexes that are the most abundant species suggested by speciation calculations in the Dead Sea (Luo and Byrne, 2001). These similarities in shapes of the normalised REE features among unwashed crystals, parent brines and formation constants of the [REECl]2 + complexes resemble analogous REE features observed in chemical sediments very often reflecting those occurring in precipitating fluids (Rollinson, 1993; Roy and Smykatz-Kloss, 2007). On the contrary, the MREE-enriched shale-normalised pattern of washed crystals referenced to PAAS average values is similar to that observed in unwashed halite from the saltworks. As the studied crystals were carefully washed according to procedures reported in Herut et al. (1998), this MREE enrichment in washed halite crystals from the Dead Sea requires the preferential MREE sorption onto halite surfaces and a mechanism avoiding their removal, despite the cleaning procedures implemented. Otherwise, preferential MREE incorporation in halite crystals would be expected. According to Curti (1997), the sorption of dissolved chemical species onto pre-existing surfaces can involve coulombic interactions or surface complexation with the formation of covalent bonds between aqueous species and a specific surface-binding site. If the process is driven by coulombic interactions, REE fractionations would not occur since the sorbed aqueous species are always the same from La to Lu (i.e., [REECl]2+, based on qualitative results of speciation calculations). In

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this case, the washing procedures should easily remove any sorbed REE residual from halite surfaces. If aqueous REE are surface-complexed onto halite crystals, this process requires surface–REE binding with an inner-sphere mechanism. This hypothesis agrees with the indication of strong interactions occurring between aqueous Cl-complexes of 154 Eu3+ and 241Am3+ and halite (Carlsen and Platz, 1986). The formation of an inner-sphere surface REE complex onto halite crystals requires that Cl− occurs in the first coordination sphere of the related aqueous complex and this evidence is disputed. Mayanovic et al. (2002, 2009) report the outer-sphere nature of Cl-REE complexes at room temperature under Cl− concentrations b 0.02 M. On the other hand, the Cl presence in the inner coordination sphere of REE complexes is demonstrated as Cl− concentration is larger than 5 M and is favoured by the decreasing H2O activity (Allen et al., 2000). This indication was recently confirmed by ab-initio calculations showing that two water molecules are replaced by Cl− in the inner coordination sphere of Lachloride at 5 M Cl− concentration (Beuchat et al., 2010). If Cl− occurs in the inner coordination sphere of dissolved REE species, the formation of surface-REE complexes onto halite has to involve a change of the inner REE coordination and evidences of this phenomenon have to be highlighted by the features of the sequence of distribution coefficients along the REE series between REE contents in halite and coexisting solution. 4.2. REE distribution coefficients between halite and parent brine D(REE) DREE ¼

ð

REEi Na Þhalite REE ð Nai Þsol

DREE values assess the amplitude of REE contents in

halite both enclosed in the solid and scavenged onto crystals surfaces. The latter contribution should be negligible in washed crystals but will represent a significant fraction of REE contents in halite from unwashed crystals. On the other hand, the incorporation of trivalent elements in halite, although unconvincing, was evidenced in some studies for trivalent (Tanji et al., 1994) and divalent trace elements (Nishri, 1984; Stiller and Sigg, 1990; Herut et al., 1998) and justified by the incorporation of these traces in structural defects, vacancies or dislocations in halite crystals. The occurrence of these defects is related to their abundance in the crystal structure and increases with the crystallisation rate of halite (Gornitz and Schreiber, 1981; Bąbel and Schreiber, 2014 and references therein). Fig. 6 shows that: • DREE values in unwashed halite crystals from the Dead Sea are ten to thirty times larger than in washed halite crystals form the Dead Sea. This evidence suggests that REE are retained more strongly on halite surfaces rather than incorporated in the solid.

7

• DREE values in washed crystals are larger in halite from the Dead Sea relative to crystals formed in the saltworks. This highlights the lesser extent of REE scavenging onto halite crystals forming in saltworks with respect to the halite crystallising from the Dead Sea brine. Both these evidences suggest that the REE fraction scavenged onto the halite crystals from the saltworks is negligible and the related DREE values mainly represent the REE fraction incorporated in the halite crystals. DREE values assessed in halite from saltworks range between 10.1 (Er) and 25.8 (Nd), not far from those calculated by Stiller and Sigg (1990) and Herut et al. (1998) for Cd and Pb (47.3–11 and 9.3–3.5, respectively). REE can be incorporated in defects of halite crystals as was suggested for Cd and Pb (Herut et al., 1998). DREE values assessed for halite crystallised in the Dead Sea are related both to the REE fraction scavenged onto crystal surfaces and entrapped in the solid structure. Cerium negative anomalies in DREE patterns from the Dead Sea (Fig. 6) are related to the Ce enriched occurring in the brine, as recognised in our data and in those reported by Gavrieli and Halicz (2002). Cerium enrichment in the Dead Sea brine is probably determined by the dissolution of the desert varnish coating the atmospheric dust delivered to the Dead Sea surface from the surrounding deserts (Goldsmith et al., 2014). Fig. 6 also shows that DREE values from washed crystals are higher than those assessed in washed halite crystals from the saltworks. DREE values related to the Dead Sea crystals (washed and unwashed) show evident M-type tetrad effects in third (Gd-Tb-Dy-Ho) and fourth (Er-Tm-Yb-Lu) tetrads, whereas this feature does not affect the values calculated for halite from saltworks. This evidence agrees with similar features recognised in patterns of DREE values during the mineral precipitation from aqueous solutions (Kagi et al., 1993; Bau, 1999; Ohta et al., 2009). The amplitude of tetrad effects in third (Gd-Tb-Dy-Ho) and fourth (Er-Tm-Yb-Lu) tetrads can be calculated according the relations (Monecke et al., 2002): vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 32 2 32 u 2 u u1 4 ½Tb ½ Dy  i −15 þ 4h i −15 T3 ¼ t h 1=3 2 Ho1=3 Gd2=3 Ho2=3 Gd vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 32 2 32ffi u 2 u u1 4 ½Tm ½ Yb  i −15 þ 4h i −15 T4 ¼ t h 2 Lu1=3 Er2=3 Lu2=3 Er1=3

ð2Þ

ð3Þ

Several studies explained the occurrence of tetrad effects with the nephelauxetic effect following the changing electronic occupancy of the 4f orbital along the REE series and explained by changes of the inner-sphere of the REE coordination throughout the transition from dissolved to surface complexes (Reisfeld and Jørgensen, 1977; Jørgensen, 1979; Kawabe, 1992; Ohta and Kawabe, 2001). The occurrence of tetrad effects requires that at the solid-liquid interface, the Cl− ions from halite surface should be associated in the inner coordination sphere of dissolved REE complex. On the contrary, the sequence of DREE values for crystallising halite from the saltworks unaffected by tetrad effects can be explained with the different aqueous REE speciation that is driven by carbonate- rather than Cl-complexes. 4.3. Zr and Hf behaviour during the halite crystallisation

Fig. 6. Distribution coefficients of REE calculated for halite crystals from the Dead Sea and from the Trapani saltworks. The amplitude of tetrad effects calculated for the third and fourth tetrads are reported in red. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Similar to the Rare Earths Elements, Zr and Hf are also surface reactive elements and significant effects on their dissolved behaviour can be induced by halite crystallisation. This suggestion agrees with the recognised difference in DZr and DHf between halite crystallisation in the Dead Sea and the saltworks (Table S2 on-line supplementary information). These differences are summarised by the DZr/DHf ratio changing from 1.3 to 0.2 in washed and unwashed halite crystals, respectively, from the Dead Sea and by the DZr/DHf ratio of 0.9 in washed halite from the saltworks. The diverse behaviour of Zr and Hf during the

Please cite this article as: Censi, P., et al., The behaviour of zirconium, hafnium and rare earth elements during the crystallisation of halite and other salt minerals, Chem. Geol. (2017), http://dx.doi.org/10.1016/j.chemgeo.2017.02.003

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halite crystallisation in the saltworks and in the Dead Sea implies a different morphology of halite crystals in these sites. Fig. 7A shows that halite occurs in cubic form, often with hopper-shaped features, in saltworks, whereas both cubic and octahedral composite habitus are shown in samples formed in the Dead Sea (Fig. 7B–E). These different features are consistent with different crystallisation conditions occurring in the saltworks (rapid crystallisation under a high temperature gradient) and in the Dead Sea in the Ein Gedi area (slow crystallisation favoured by a low temperature gradient) as reported by Aquilano et al. (2009). The large Mn, Ni, Zn, Cd and Pb concentrations in halite crystals from the Dead Sea (Nishri, 1984; Herut et al., 1998) also explains their occurrence in octahedral forms (Gornitz and Schreiber, 1981 for a comprehensive review).

Only a limited dataset is known about the dissolved Zr and Hf behaviour in saline waters. It regards the composition of alkaline (pH N 10) and saline waters (TDS ≥ 55 g l− 1) from Lake Bogaria (Kenya) (Kerrich et al., 2002). Here, the aqueous speciation of Zr and Hf driven by [(Zr,Hf)(H2O)(8-n)(OH)n](+4-n) hydroxyl-complexes can be assumed considering the occurring alkaline conditions and the large stability of Zr, Hf-hydroxyl complexes under a wide range of physical-chemical conditions (Aja et al., 1995; Veyland et al., 1998; Byrne, 2002; Ekberg et al., 2004; Qiu et al., 2009). Increasing pH allows hydrolysis up to the formation of the anionic [(Zr,Hf)(H2O)3(OH)5]− species. These indications, from theoretical and laboratory studies, were corroborated by experimental research recognizing a larger surface affinity of Hf relative to Zr. As a consequence, the geochemical coherence of the Zr-Hf pair often

Fig. 7. Morphological Scanning Electron Microscope (SEM) image illustrating crystal features in halite crystals formed from brines in the Trapani saltworks and in the Dead Sea. A: Halite from the Trapani saltworks. B: Gypsum crystals (gy) associated to halite specimens (ha) showing both octahedral (red dashed lines) and cubic planes (white dashed lines). C: Particular of a halite crystal octahedral in shape. The arrow indicates a crystal defect of unknown nature. D: Halite crystal cubic in shape and showing both cubic and octahedral planes associated to gypsum prisms. E: Association of cubic halite crystals. The arrow indicates a well-depicted halite specimen showing both cubic (white dashed lines) and octahedral (red dashed lines) planes. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Please cite this article as: Censi, P., et al., The behaviour of zirconium, hafnium and rare earth elements during the crystallisation of halite and other salt minerals, Chem. Geol. (2017), http://dx.doi.org/10.1016/j.chemgeo.2017.02.003

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tends to disappear during aqueous processes (Firdaus et al., 2011; Censi et al., 2014, 2015; Inguaggiato et al., 2015, 2016) providing the precipitation of authigenic solids. This process occurs in Lake Bogoria where trona and nahcolite precipitate as water is concentrated by evaporation (De Cort et al., 2013) and strongly superchondritic Zr/Hf values ranging from 193 to 481 (weight ratios) were measured in dissolved phase (Kerrich et al., 2002). According to Godfrey et al. (1996) and Rickli et al. (2009) Zr-Hf decoupling is a consequence of the coulombic nature of the interactions between dissolved complexes and solid surfaces that favor the scavenging of aqueous Hf species relative to Zr complexes. Recently, a possible theoretical justification of the different Zr-Hf behaviour at the solid-liquid interface was given by Jahn et al. (2015) who recognised some small differences in the coordination environment of Zr and Hf aqueous species that may justify a larger stability of [Hf(H2O)3(OH)5]− and of [Zr(H2O)3(OH)5]− complexes. Under this hypothesis, the morphology of the halite crystals could drive the different Zr-Hf behaviour observed in the saltworks and in the Dead Sea. Considering the ionic halite crystals as a stack of planes, the cubic {100} and the octahedral {111} faces have different electric properties (Bruno, 2013). The former consists of uncharged planes since it is populated by an equal number of Na+ and Cl− ions. On the contrary, the latter is a stack of alternating equally spaced planes populated by opposite charged Na+ and Cl− ions (Bruno, 2013) producing a polar face (Radenovic et al., 2005). It follows that the larger Hf affinity relative to Zr towards solid surfaces could be explained by the extent of stability of [Hf(H2O)3(OH)5]− rather than of [Hf(H2O)4(OH)4] species in Dead Sea brine. Consequently, a larger affinity would be expected for anionic Hf complex species towards the octahedral surfaces relative to the neutral Zr species, thus explaining the different D values for Zr and Hf in halite from the Dead Sea.

9

kainite surfaces, suggesting a crystal structural explanation of this phenomenon similar to that occurring in halite. The kainite crystal lattice consists of four corner-shaped MgO6-octahedra and SO4-tetrahedra layered parallel to the {100} plane. In the interlayer volumes are located crystallisation water molecules, Cl− and K+ ions (Robinson et al., 1972). This framework provides a good cleavage along the {100} plane that is orthogonal to the hydrogen bridges between Cl− and H2O (Hoffmann et al., 2010). These hydrogen bonds located normally to {100} planes favor the adsorption of anionic species, as evidenced by flotation kainite experiments for industrial purposes (Hancer and Miller, 2000). Furthermore, kainite is usually defined a “water structure maker”, since it contains ions able to form strong hydration shells onto the crystal surfaces (Hancer and Miller, 2000; Du et al., 2014). Therefore, a larger capability of the kainite {100} plane to preferentially interact with [Hf(H2O)3(OH)5]− rather than [Zr(H2O)4(OH)4] would be expected, thus explaining the measured subchondritic Zr/Hf values in these salts. 4.6. Zr/Hf and Y/Ho distribution versus REE concentration in salt minerals The distribution of Y/Ho and Zr/Hf ratios towards the total REE concentration in studied materials confirms that primary evaporites are suitable REE collectors from parent brines and this process is coupled with Hf fractionation relative to Zr (Fig. 8). Y-Ho pair seems less sensitive than Zr-Hf towards the decoupling induced by interface fractionations. As a consequence, Y/Ho values in primary evaporites are only slightly the crustal signature characteristic of Y/Ho ratios in secondary

4.4. REE behaviour in salt minerals in evaporites In order to determine whether REE, Zr and Hf concentrations in authigenic primary salt minerals are related to the authigenic crystallisation or are representative of surface-complexation, we compare REE, Zr and Hf distributions in salt minerals from primary evaporite sequences and those found in secondary minerals. Evidence suggests that larger REE concentrations usually occur in primary salt minerals, both from Tortonian and Messinian evaporites, whereas lower concentrations are found in secondary diagenetic modified samples (Fig. 5). Both primary and secondary salt minerals and shale-normalised REE patterns show features that suggest an influence of the mineralogical composition on the REE behaviour. Samples with high halite contents show a significant MREE enrichment similar to that observed in authigenic halite from the saltworks. On the contrary, kainite-rich samples show REE distribution with “ascending” patterns from La to Lu. This feature suggests that a REE capability to substitute for Mg2+ in kainite progressively increases along the REE series, as occurs for REE behaviour in hydrothermal siderite where REE substitute for Fe2 + (Bau and Möller, 1992; Huang et al., 2015). 4.5. Zr and Hf behaviour in primary and secondary evaporites minerals Zr/Hf values measured in authigenic evaporites ranging between 2.6 and 14.5 are close to related values found in unwashed halite crystals from the Dead Sea (Zr/Hf = 4.4–5.0). These values are intermediate between Zr/Hf values measured in washed crystals from the Dead Sea (Zr/Hf = 27.0–34.9) and in washed crystals from the saltworks (Zr/Hf = 33.1–44.2). Also the Zr/Hf ratio measured in weathered halite from BF sample falls within this range (Zr/Hf = 36.4). These evidences suggest that strongly subchondritic Zr/Hf values can represent the geochemical signature of the authigenic primary deposition of unwashed halite from Dead Sea and Na-, K-rich salt minerals from Tortonian and Messinian evaporites. Strongly subchondritic Zr/Hf values measured in kainite-rich samples indicate a larger Hf affinity than Zr towards the

Fig. 8. Y/Ho (A) and Zr/Hf (B) vs REE concentrations in studied samples. Zr/Hf and Y/Ho values in Upper Continental Crust (Rudnick and Gao, 2003) and PAAS (Taylor and McLennan, 1985) are given as references.

Please cite this article as: Censi, P., et al., The behaviour of zirconium, hafnium and rare earth elements during the crystallisation of halite and other salt minerals, Chem. Geol. (2017), http://dx.doi.org/10.1016/j.chemgeo.2017.02.003

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evaporites (Fig. 8A). This evidence agrees with the previously discussed coulombic nature of the solute-halite surface interactions. So, larger decoupling is observed between elements with different dissolved speciation, as Zr and Hf, whereas lower geochemical effects arise geochemical pairs showing the same dissolved speciation, as Y and Ho. On the contrary, the dissolution of primary salts and their re-crystallisation leads to the deposition of secondary evaporites during a process highlighted by changes in Zr/Hf signature and REE concentrations in solids (Fig. 8B), similarly to those induced by washing halite crystals forming in current brines. On the other hand, salt minerals can also occur in subaerial outcrops where they interact with surface waters during the weathering. The composition of BF sample, collected from the subaerial part of the Raineri salt mine (Central Sicily), and then subjected to extensive weathering, indicates that this process determines Zr/Hf and REE signatures similar to those found in washed halite crystals collected from saltworks and the Dead Sea.

4.7. Calculated Zr, Hf and REE contents in halite based on the composition of deep-sea brines. Deep-sea hypersaline waters are observed underlying the Eastern Mediterranean water column (Jongsma et al., 1983; Corselli et al., 1996). Two different scenarios were suggested to explain their origin: (i) an origin related to the dissolution of old Messinian evaporites and (ii) the upward or lateral delivery of fossil brines representing the remnants of evaporated water bodies formed throughout the Messinian Salinity Crisis (Vengosh et al., 1999 and references therein). According to the first scenario, the REE distribution assessed for the hypothetical halite crystals equilibrated with these brines should resemble those found in secondary evaporites. On the contrary, the recognition of REE features similar to those found in primary evaporites would corroborate the second hypothesis. So, starting from the REE distribution in the deep-sea brines from the Typo, Medee and Thetis basins (Raso, 2012), the theoretical REE concentration in halite equilibrated within them was assessed based on D values calculated from washed halite crystals from the Dead Sea (see Eq. (1)). Obtained values normalised to PAAS fall within the range of compositions recognised in evaporites with prevailing halite mineralogy (Fig. 5). The calculated Zr/Hf weight ratios of halite equilibrated with different brine compositions range from 44.3 in halite equilibrated with Medee brine to 8.4 in crystals equilibrated with Thetis brine. The Zr/Hf value for halite equilibrated with Tyro brine (Zr/Hf = 19.2) is intermediate. If compared with analogous values measured in different studied samples, halite from Thetis falls between values found in Narich and K-rich primary evaporates; halite from Tyro falls within the values recognised in secondary Na-evaporites and halite from Medee shows Zr/Hf ratios slightly higher than those found in halite from the saltworks (Fig. 9). This evidence agrees Tyro brine originated from dissolution of halite (Vengosh and Starinsky, 1993) whereas the Zr/Hf signature of Thetis brine seems representative of brine precipitating primary evaporites. Hydrochemical data of Thetis brine (La Cono et al., 2011) confirms that the Br/Cl ratio is close to 1.13 · 10−3, which is close to 1.67 · 10−3 as measured by De Lange et al. (1990) and is considered to be representative of a remnant of evaporated seawater (Vengosh and Starinsky, 1993). The Zr/Hf values modelled for halite equilibrated with Medee brine are higher than those measured in other salt minerals (Fig. 9). The major ion composition of Medee brine shows very high Br/Cl ranging from 1.05 · 10−3 to 1.05 · 10−3 and a Na/Cl ratio close to 0.86 (Yakimov et al., 2013). This evidence agrees with that of evaporated seawater below halite saturation (Vengosh et al., 1994) that experienced the addition of Br, probably from organic matter (Vengosh et al., 1999). According to Zr and Hf D values, the superchondritic signature of halite equilibrated with Medee brine requires a chondritic or slightly subchondritic signature of the Zr/Hf ratio in the dissolved phase, consistent with the greater Zr affinity

Fig. 9. Zr/Hf values in different studied salt minerals compared with those calculated for halite crystals equilibrated with deep-sea brines from Tyro, Thetis and Medee basins (Eastern Mediterranean). Zr/Hf values in Upper Continental Crust (Rudnick and Gao, 2003) and PAAS (Taylor and McLennan, 1985) are given as references.

then Hf for amine-binding surface sites in a highly concentrated chloride medium (Poriel et al., 2006; Banda et al., 2012). 5. Conclusion The saltworks and the Dead Sea can be considered two extreme conditions for halite crystallisation and the deposition of salt minerals through seawater evaporation. In the saltworks, a strong oversaturation of aqueous phase is artificially reached and maintained, thus reducing the seawater inflow and increasing the thermal gradient during the production periods. In the Dead Sea, the negative water balance resulted in salinity growth since 1979, allowing the halite deposition. In both cases, during halite deposition the behaviour of REE is primarily driven by crystal-chemical processes. The dissolved behaviour of strongly surface sensitive elements, such as Zr, Hf and REE, is differently influenced by the latter mechanisms of halite crystallisation. In the saltworks, the crystal chemical control of Zr, Hf and REE incorporation in the halite crystal lattice overwhelms the effects of dissolved phase-surfaces, whereas the surface complexation of REE and coulombic interaction of Zr and Hf aqueous complexes strongly influence the concentration of these elements in halite from the Dead Sea. Comparing data obtained from current halite and salt minerals from primary, weathered and re-crystallised Messinian and Tortonian evaporites, we propose that Zr/Hf ratios and REE contents in salt minerals could be useful geochemical parameters for discriminating authigenic materials relative to diagenetically-modified or weathered salts. Acknowledgments This research was partially funded by the contract CORI 2012-ATE0364 and the 2015-COMM-0006 fund (University of Palermo). We are indebted with Drs. Ittai Gavrieli, Ludwik Halicz, Yehudit Harlavan and Yoseph Yechieli for their assistance and cooperation during sample collection and preliminary lab treatment of samples coming from the Dead Sea. We are also indebted towards two anonymous reviewers and the editor Professor Michael E. Boettcher for the manuscript handling and its critical review.

Please cite this article as: Censi, P., et al., The behaviour of zirconium, hafnium and rare earth elements during the crystallisation of halite and other salt minerals, Chem. Geol. (2017), http://dx.doi.org/10.1016/j.chemgeo.2017.02.003

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Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.chemgeo.2017.02.003.

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Please cite this article as: Censi, P., et al., The behaviour of zirconium, hafnium and rare earth elements during the crystallisation of halite and other salt minerals, Chem. Geol. (2017), http://dx.doi.org/10.1016/j.chemgeo.2017.02.003