Geochemistry of lanthanides in thermal waters of Issyk-Kul Lake Basin

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Introduction
Low enthalpy thermal waters are widely presented within the Issyk-Kull intermountain basin of the Tien Shan.Locals have long known about the waters' balneological benefits.In the 20th century, a lot of hydrogeological research was done in this area [1,2,3].Unfortunately, they are still not well-represented in English-language publications [4,5] and do not include chemical analyses of rare earth elements.The Issyk-Kull Lake basin is not being investigated for REE in groundwater While similar types of thermal waters have been well studied in granite massifs across the globe [6,7,8,9].The purpose of this work is to establish the levels of REE and their fractionation in thermal waters of the Issyk-Kull Lake, explain the anomalies from geological point of view and to check if these elements may be useful tracers of the water-rock interaction.This study reports REE data for 12 thermal manifestations taken from boreholes and springs located within water basin of Lake Issyk-Kul (Kirgizstan) (Figure 1).Concentrations and geochemical features of lanthanides are considered for the first time.

Geological conditions
The Issyk-Kul Basin is one of Tien-Shan's biggest intermountain basins (Figure 1).It was formed on the Late Precambrian-Paleozoic basement of the Northern Tien Shan [10].There are three major units that present the basin [4]: 1) water-bearing sediments (boulders, pebbles) of Quaternary and Pliocene-Quaternary age up to 450 m thick which compose alluvial pebble beds of foothill plains and stream valleys; 2) Mesozoic to Upper Pliocene terrigenous stratified sediments up to 4.5 km thick represented by fine-grained clay, silt, sand, and 3) Proterozoic to Upper Paleozoic bedrocks represented by metasedimentary and igneous rocks with patterns exposed in the ridges around the lake.
The geochemistry of groundwaters of these units depends on lithology and circulation time [5].Upper units are characterized by the active water exchange, it is predominantly of freshwater.The elevated temperatures and TDS of waters are common for the second unit.Waters with temperatures higher than 40 o C and mineralization from 0.35 to 45 g/l have been disclosed in the depth interval from 800 to 1600 m [4].The geothermal gradients here reach 2.0-3.5 o C/100 m in boreholes to depths below 2000 m, and 2.3-3.5 °C/100 m in wells attaining a depth of 4900 m.The Tarim Plate subduction is likely to play an important role in generating heat flux, reflecting the temperature of the North Tian Shan Hot Springs [5].

Sampling and analytical procedures
Water samples from 12 thermal springs located on the shores of Issyk-Kul Lake were collected during the field works in August of 2019.A Mettler Toledo probe was used to detect the water's conductivity, alkalinity, pH, and other parameters.Membrane filters with 0.45 µm pore sizes were used to filter the fluids.The ion chromatography method was used to determine major cations and anions.Lanthanides concentrations were determined using an inductively coupled plasma mass spectrometer (ICP-MS, Agilent 7500s) in the Far East Geological Institute of RAS, Vladivostok, Russia.The REE had an analytical precision of 5% RSD or less, which is close to the standard deviation (RSD; 2).The Ce, Eu and Pr anomalies were calculated as: Ce/Ce*=2Ce n /(La n +Pr n ), Eu/Eu* = 2Eu n /(Sm n +Gd n ), and Pr/Pr*=2Pr n /Ce n +Nd n ), where n -normalized abundance [6,7].
Rare earth elements were analyzed for the first time.It was found that: 1) concentrations of REE changes from 0.07 to 0.81 ppb; 2) positive correlation observed for REE and TDS (Figure 2A).The highest concentrations of total REE contained waters from the Djety Oguz wells (#5-8) with high TDS (Table 1, Figure 2A) and minimal concentration obtain in the well of Ak-Su spa resort; 3) minimal REE concentrations observed in waters with high pH (8.1-9.7),whereas maximal correlate with pH interval 7.3-7.9(Figure 2B); 4) La, Ce and Nd are characterized by the highest concentrations, this is also the reason why the light REE (LREE N from 57 to 64) dominate over the heavy REE (HREE N from 65 to 71) in all water types.The observed differences can be related both to different water circulation paths and circulation times, as well as to different REE contents in the host rocks.Also, for example, one of the potential reasons for the low concentration of HREE may be a lack of CO2 [11].Many trace elements demonstrate positive correlations with REE indicating the possible sources of elements in water.There is a strong correlation between REE and zirconium, hafnium, thorium and phosphorus in waters.Obviously, that zircon and monazite have widespread distribution in present geological environment.From one hand it could be evidence that zircon (ZrSiO 4 ) and Monazite destructions are the sources of REE in water, this also implies that the REE are associated with colloidal particles, most likely in the form of adsorbed organic complexes.Many studies show that insoluble tetravalent elements, such as Th, and Zr and Hf, are dominated by hydrolyzed species present in large Fe-rich organic colloids.Thermal water with high REE content are demonstrate highest statistically significant correlation with Fe (r=0.7).The correlations REE with Al and Mn are moderate.Thus we can expect that one dominant source control on REE in thermal waters is dissolution of Fe-Mn oxyhydroxides.Colloidal transport in these waters also supports by high pH.These relationships may simply reflect the fact that large colloidal fractions cannot be removed by filtering at 0.45 m rather than offering information about the sources of REE.
Negative Ce anomalies are formed as a result of the oxidation of Ce 3+ to Ce 4+ , which is subsequently removed from solution [14,15].In our case, this rule has exceptions because measured Eh indicates reducing conditions.An indicator of reducing conditions is the positive Eu-anomalies [13,16], which reflects the active water-rock interaction processes, but may also indicate unsteady conditions of hydrogeological systems [14].There can be sevеral reasons for such bеhavior: the first one is related to the mineralogical and chemical composition of the host rocks (for example, Ce depleted minerals); the second one is dissоlution of previоusly deposited cerium dioxide (CeO 2 ) resulting in Ce 4+ reduction to Ce 3+ [15] and the last reason could be different time of groundwater circulation [17].Аnomalous abundancеs of Cе, may be additionally arise via abnormal La.Bau and Dulski [18] propose a calculation of the Pr/Pr* ratio, the results of which show that for Pr/Pr* > 1 there is a negative Сe anomaly and for Pr/Pr*<1 there is a positive Сe anomaly.The combination of Ce < 1 and Pr~ 1, however, indicates a positive La anomaly.Only half of samples indicating a genuine negative Ce anomaly, whereas others indicate slightly overestimated Ce anomaly values due to a positive La anomaly (Figure 4A).This in a good agreement with Eh-pH stability diagram (Figure 4B).

Conclusion
This study provides, for the first time, a REE data in thermal waters of the northern Tien Shan (Issyk-Kul lake basin).According to the geochemical and geological data, the bedrock mineralogy and lithology are the main sources of REE for the thermal waters.The positive Eu anomaly might be a reflection of both a host-rock signal and the higher mobility of Eu at pH > 7. Contrarily, a typical strong negative Ce anomaly in the waters indicates that Ce is not dissolved from the source rocks at a particular pH level and is likely also trapped by clay minerals or Fe-oxyhydroxides.Present data of the lanthanides behaviour in the Issyk-Kul thermal waters demonstrate that normalised REE patterns have use as geochemical tracers, and that sources of REE can potentially be Project No. 23-27-00119 of the Russian Science Foundation provided funding for this work.

Fig. 2 .
Fig. 2. Total REE content in thermal waters vs TDS and pH.

Table 1 .
Represented chemical analyses of the studied nitric thermal waters.