A natron source at Pikrolimni Lake in Greece? Geochemical evidence

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140 E. Dotsika et al. / Journal of Geochemical Exploration 103 (2009) 133–143 carbonate loss is probably related to calcium and sodium carbonate precipitation. An additional mechanism is also associated with efflorescent crust (Eugster, 1966). These crusts are the products of complete desiccation of lake water and consist predominantly of soluble salts. When rain or dilute runoff comes in contact with these crusts only the most soluble salts, such chlorides, carbonates and sulfates are resolved into solution and increasing the solute load, while the less soluble phases like the alkaline earth carbonates and silica remain in the crust. The dissolution of these crusts leads to high alkaline water with a specifically lower HCO 3 /CO 3 ratio. The high pH (9.6) of some waters of Lake Pikrolimni (like PL-1/2003) can be attributed to recycling of residual brines (PL-8/2002) from the fresh water. The finding of abundant calcite precipitation in lake beds that are covered by crusts, consisting of trona, burkeite, thenardite and halite minerals, as shown by the XRD analysis (Table 2) of Pikrolimni crusts, shows that the above play a competitive role in the geochemistry processes of Pikrolimni brines. In fact, salt analyses from different parts of the lake showed that they consist of carbonates (calcite, dolomite), trona, burckeite and halite. Salts from Wadi Natrun show similar mineral composition (Shortland et al., 2006). In the graph of SO 4 2− versus Cl − (Fig. 4d), the molar SO 4 2− /Cl − ratio indicates generally neither loss nor gain of SO 4 2− between diluted and brine waters. The relations SO 4 2− versus Cl − suggest a possible incipient precipitation of a sulphate mineral only for PL-8/2002. All the other lake waters indicate an excellent conservative behaviour of sulphate relative to chloride. This also suggests that sulfate reduction is not a major process at Lake Pikrolimni. 6. Geochemical model 6.1. Thermodynamic calculation model for groundwaters and lake water An ion-specific interaction model based on Pitzer's equations has been used (EQL/EVP by Risacher and Clement, 2001) to calculate the saturation index versus different solid phases, of the Pikrolimni waters and brines. The lake brine samples are put in ionic strength diagram versus the saturation index (Fig. 5a, b, c) and in Table 3 the minerals that are supersaturated in the lakes Pikrolimni, Magadi, Natron and Deep Springs are given. The calculation of saturation index versus different carbonate and/or bicarbonate sodium minerals shows that the borehole waters are saturated in calcium minerals. The lake waters are saturated in Na/Ca carbonate minerals and only the lake brines are saturated also in Na–carbonate and bicarbonate, Na–sulfate and Na–chloride minerals. Indeed, the brines of PL-8/2002 and PL-9/2006 are saturated in sodium carbonate–bicarbonate minerals like nahcolite, trona and natron. Τhe calculation of ionic strengths versus different sodium–sulfate and sodium–chloride minerals show that the brines with Ι>6 are saturated in halite (NaCl), thenardite (Na 2 SO 4 ) and mirabilite (Na 2- SO 4 ⁎10H 2 O) confirming that in such lakes, the Ca–Na–HCO 3 salts precipitate first and the SO 4 –HCO 3 salts follow. The results of measurement of Na + and Cl − show indeed that only the saturation in halite, thenardite and mirabilite is achieved. 6.2. Simulation of evaporation of groundwaters Using geochemical program (EQL/EVP by Risacher and Clement, 2001), it is possible to investigate the mineralogical composition of the minerals that precipitate from the water, with different initial chemical compositions, as they evaporate. All sampled waters from boreholes and the spring were evaporated in order to check if sodium carbonates could precipitate by evaporation of these waters theoretically. As shown in Table 4, most waters can give nahcolite (NaHCO 3 ) and thenardite (Na 2 SO 4 ) precipitates in the last phase of evaporation. Ιn Table 4, the evaporated samples that can give trona and natron precipitates are shown. In the same table, a diluted water from Lake Pikrolimni (PL-5/2003) is also shown. It is observed that three samples, S/1/2002; b/2/2002; PL-5/2003, give Na–carbonate–bicarbonate minerals [gaylussite Na 2 Ca⁎(CO 3 ) 2 ⁎5H 2 O; pirssonite Na 2- Ca⁎ (CO 3 ) 2 ⁎ 2H 2 O; trona Na 2 CO 3 ⁎ NaHCO 3 ⁎ 2H 2 O and natron Na 2 CO 3 ⁎10H 2 O], Na–sulfate minerals (burkeite Na 2 CO 3 ⁎2Na 2 SO 4 ; thenardite Na 2 SO 4 ) and Na–chloride minerals [halite NaCl]. Water samples S/1/2002, b/2/2002 and PL-5/2003 give similar minerals with those found by XRD analysis in collected salt samples but also similar minerals referred in literature (Shortland et al., 2006; Taher, 1999; Gueddari, 1984). However, only the water sample coded S/1/ 2002 and b/2/2002, Na–ΗCO 3 type, could give natron and trona. Fig. 6 shows the results of S/1/2002 sample when the initial chemical composition of this water was modeled. As it can be seen, as the water is concentrated about 250 times of the initial chemical composition, the first Na–carbonate mineral, gaylussite, deigns to precipitate. After a concentration of 395 times of the initial chemical composition, natron, pirssonite, and mirabilite precipitate. The gaylussite and the pirssonite are among the first salts that precipitateand their formation is related to calcite. The gaylussite [Na 2 Ca⁎(CO 3 ) 2 ⁎5H 2 O] generally forms by the reaction of primary calcite (CaCO 3 )andthesodic carbonate solution, according to the reaction CaCO 3 þ 2Na þ CO 3 þ 5H 2 O↔Na 2 Ca⁎ðCO 3 Þ 2 ⁎5H 2 O: The pirssonite is usually found in the presence of gaylussite or of calcite crystals. The direct precipitation of pirssonite takes place according to the reaction Na 2 Ca⁎ðCO 3 Þ 2 ⁎2H 2 O↔2Na þ Ca þ 2CO 3 þ 2H 2 O: Secondarily, pirssonite forms either by the dehydration of gaylussite or by the reaction of Ca–Na solutions with calcite according to the following equations Na 2 Ca⁎ðCO 3 Þ 2 ⁎5H 2 O↔Na 2 Ca⁎ðCO 3 Þ 2 ⁎2H 2 O þ 3H 2 O Table 2 XRD results on evaporites from different lakes. Lake Pikrolimni 2002 Wadi Natrun a Lake Natron, Tanzania b Lake Magadi, Kenya b The Deep Springs Lake, California b Calcite (CaCO 3 ) Dolomite c Calcite (CaCO 3 ) Calcite (CaCO 3 ) Dolomite [Ca(Mg)CO 3 ) Pirsonnite Na 2 Ca⁎(CO 3 ) 2 ⁎2H 2 O Pirsonnite Na 2 Ca⁎(CO 3 ) 2 ⁎2H 2 O Nahcolite (NaHCO 3 ) Trona (Na 2 CO 3 ⁎NaHCO 3 ⁎2H 2 O) Trona (Na 2 CO 3 ⁎NaHCO 3 ⁎2H 2 O) Trona (Na 2 CO 3 ⁎NaHCO 3 ⁎2H 2 O) Trona (Na 2 CO 3 ⁎NaHCO 3 ⁎2H 2 O) Trona (Na 2 CO 3 ⁎NaHCO 3 ⁎2H 2 O) Thermonatrite (Na 2 CO 3 ⁎H 2 O), Thermonatrite (Na 2 CO 3 ⁎H 2 O), Burkeite (Na 2 CO 3 ⁎2Na 2 SO 4 ) Burkeite (Na 2 CO 3 ⁎2Na 2 SO 4 Burkeite (Na 2 CO 3 ⁎2Na 2 SO 4 ) Kogarkoite (Na 2 SO 4 ⁎NaF) Thenardite (Na 2 SO 4 ) not detected Thenardite (Na 2 SO 4 ) Thenardite (Na 2 SO 4 in 2006 salt Halite (NaCl) Halite (NaCl) Halite (NaCl) Halite (NaCl) Halite (NaCl) a Shortland A.J. (2004). b Gueddari M. (1984). c Wenigswieser (1992).
E. Dotsika et al. / Journal of Geochemical Exploration 103 (2009) 133–143 141 64% of all salts that precipitate, while halite only 10%. On the contrary, Fig. 6 shows the results of PL-5/2003 sample when the initial chemical composition of this water was modeled. It seems that when the model stops halite is more abundant in the precipitates than trona: trona represents only 0.5% of all salts that precipitate, while halite 60%. These two models show the change which may have happened in the water chemistry of the lake's springs and why trona is so hard to find in Lake Pikrolimni when the models reaches their lowest point: trona salts are hoarded by the deposition of large quantities of halite, as shown by Shortland et al. (2006) in their study about Wadi Natrun. The alimentation of the lake in the past had happened mainly from deep springs with similar chemistry while today is arisen mainly from meteoric waters and irrigation groundwaters (Pliny; Many public testimonies vouch that there were a lot of springs in the middle of the lake, but after the big earthquake of 1981 these springs have disappeared). The chemical composition of the residual water and of three samples (S/1/2002; b/2/2002; PL-5/2003) after the simulation of evaporation was similar with the chemical composition of the lake's water in conditions of intense evaporation (Table 5). Especially the chemical composition of the residual water, coded S/1/2002, after the evaporation simulation, is similar to that of the lake brine PL-9/2006 (Table 5). Furthermore, the model shows that natron starts to precipitate when Cl − content is 1417 mmol/L [when the concentration factor (CF) is about 395 times: 395⁎3.8 mmol/L (initial Cl of s/1/2002)=1500 mmol/L] while trona starts to precipitate when Cl − content is approximately 2500 mmol/L [when the concentration factor (CF) is about 720 times: 700⁎3.8 mmol/L (initial Cl of s/1/2002)=2651 mmol/L]. The lake water reaches similar concentrations only during the summer, August (but not every summer), as observed by sampling. According to Pliny, natron was collected in Macedonia only in the rising of the Dog Star for nine days. This timing places this event at the beginning of the hottest part of the summer, when the lake is found at the peak of evaporation and trona starts to precipitate. However, the lake does not reach such high concentrations every year, a fact that is due to the weather (temperature, humidity, precipitation). Therefore, the collection would have been taking place only during particularly hot and dry summers. 7. Conceptual model Fig. 5. Saturation Index (Log saturation ratio) versus ionic strength (logarithmic). White triangles: nahcolite; x: gaylussite; –: pirssonite; white diamonds: natron; black squares: trona; white circles: halite; crosses: mirabilite; white squares: thenardite; crossed x: thermonatrite. CaCO 3 þ 2Na þ CO 3 þ 2H 2 O↔Na 2 Ca⁎ðCO 3 Þ 2 ⁎2H 2 O: Also when the concentration factor (CF) is about 700 times of the initial chemical composition, burkeite and trona precipitation initiates. Much later, when the water is concentrated by about a factor of 1200, halite begins to precipitate. When the models stop, halite and trona are the more abundant precipitates. Trona is more abundant in the precipitates than halite: trona (50%) and natron (14%) represent The Na/Cl ratio in Lake Pikrolimni supports evaporative concentrations as the dominant process in the chemical evolution of these lake waters that have high proportions of Na + and HCO 3 − . The derived high Na + and HCO 3 − contents are due to silicate hydrolysis. The computing program has shown that the dilute spring inflow was concentrated (by the assumption of Cl − conservation in solution throughout evaporation) about 12000 times to obtain the most saline residual actual lake water, PL-8/2002; PL-9/2006). However, this evaporation budget calculation (Table 5), if compared with the actual analysis of the most saline Pikrolimni brine, shows that some constituents are not conservative, due to different removal mechanisms. Therefore, it is believed that in the past, the solute composition of such waters, which are controlled by silicate hydrolysis and evaporative concentration, gave the natron salts of Lake Pikrolimni. Namely, in the past, after acquisition of solutes from atmospheric precipitation, hydrolysis of silicate and possibly CO 2 of deep origin, the waters are subjected to evaporation (at the surface or by capillarity). Initially, the concentrations of all constituents increase until the mineral precipitation occurs. The precipitation may take the form of efflorescent crusts on top of the surface or of intergranural caliche-type films and cements. The efflorescent crusts are the product of complete desiccation and they are subjected to complete or differential solution byrain.ThisrecyclingofsaltsisobservedactuallyatLakePikrolimni. Thus, during summer when the water level of the lake is low or there is no water at all (total desiccation), the first rains dissolve the residual brines (or salts) and get charged by solutes. This recycling of salts is also
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A natron source at Pikrolimni Lake in Greece? Geochemical evidence

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