Geochemical Modeling of the Precipitation Process in SO4-Mg/Na Microbialites / ÓSCAR CABESTRERO (1*), PABLO DEL BUEY (1), M
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macla nº 21. 2016 revista de la sociedad española de mineralogía 20 Geochemical Modeling of the Precipitation Process in SO4-Mg/Na Microbialites / ÓSCAR CABESTRERO (1*), PABLO DEL BUEY (1), M. ESTHER SANZ MONTERO (1) (1) Petrology and Geochemistry Department, Geological Sciences Faculty (UCM). 12th, Jose Antonio Novais St. 28040, Madrid (Spain). INTRODUCTION MATERIALS AND METHODS RESULTS Longar is an endorheic mesosaline to Fieldwork was conducted in November As a result of the intense evaporation, hypersaline lake (> 10 g·L-1), with 2016. Water samples taken were the summer and autumn in 2016 year sulphate being the dominant anion over filtered (using 0.45 μm pure cellulose left a thinner water layer (< 10 cm). The chloride (Cabestrero and Sanz-Montero, acetate (CA) membrane filters). The water collected in Longar Lake 2016). It is located in Lillo (La Mancha), main cations and anions were analyzed watershed at 30 ºC was, with a salinity a region with a continental semi-arid by ion chromatography, using Dionex DX surpassing 400 g·L-1, the most climate and is characterized by high 500 ion and METROHM 940 concentrated in the last six years. evaporation (1300–1700 mm·yr-1) and Professional IC Vario chromatographs in Furthermore, pH values were the lowest low precipitation (300–500 mm·yr-1). the CAI for geological techniques in the ever measured, ranging from 7.1 to 7.3. The annual mean temperature is 14 ºC, Geological Sciences Faculty, In contrast, ionic composition did not and extreme values of -7 ºC and 40 ºC Complutense University of Madrid. The show a significant variation compared to are registered in January and July, 2- carbonate (CO3 ) and bicarbonate all other values found before (Mg2+-SO42- respectively (Sanz-Montero et al., - (HCO3 ) ion concentration in the water Cl- brine type). The absence of dissolved 2015a). The most abundant mineral in was determined by titration. oxygen and ORP values ranging from - the lake is lenticular gypsum and a suite Hydrochemical parameters such as 112.90 to -90.20 mV suggests reduction of hydrated sulphates, such as salinity (S), temperature (T), dissolved processes. hexahydrite, epsomite, pentahydrite, oxygen (DO), oxidation reduction starkeyite, konyaite, bloedite, and potential (ORP), and pH values were The simulation model of mineral thenardite. The major authigenic measured in situ using a precipitation during the day showed that carbonates are calcite and aragonite, multiparameter meter. only carbonates were supersaturated although dolomite, hydromagnesite, (Fig. 1). Dolomite and calcite had monohidrocalcite, magnesite and Geochemical modeling was carried out positive values of SI in the original brine nesquehonite can also be found in using the PHREEQC program (Parkhurst solutions at noon temperatures (25-30 smaller amounts (Cabestrero and Sanz- and Appelo, 1999) in order to calculate ºC). Glauberite, aragonite and gypsum Montero, 2016). ion activities and saturation indices of were very close to the saturation with minerals commonly found evaporative values of -0.01, -0.12 and -0.16 After an extremely dry summer and environments and included in the Llnl respectively (Table 1). All other phases autumn, in November 2016, the water database. In addition, a natural brine were clearly undersaturated. Night lamina ponded in the lake was very thin evolution during day and night was model (decreasing temperature in steps and the concentration of the brine was performed according the instructions of 1 ºC from 25 ºC) showed that the maximum recorded. The high salinity provided in the software manual glauberite oversaturated at favored the subaqueous crystallization “Evaporation and Homogeneous Redox temperatures lower than 25 ºC (Fig. 1). of a hard crust of evaporites on the bed. Reactions” of the PHREEQC program. At temperatures lower than 13 ºC, The up to 0.5 cm thick curst consisted of The saturation indices (Table 1) for the mirabilite also oversaturated. bloedite, epsomite, gypsum and day were calculated using the Decreasing temperature, gypsum got mirabilite that occur within a microbial temperature registered in the field, but even closer to saturation but was never mat matrix as documented by Del Buey for the night, the temperature et al., this volume. There is increasing considered was the recorded in SI Mineral Formula evidence that microbial mats proliferate Tembleque weather station (AEMET), Max in shallow lakes subjected to wet-dry during the five previous days to the Mirabilite Na2SO4·10H2O 0.47 cycles (Sanz-Montero et al., 2015b). It sampling. The program was constrained Epsomite MgSO4·7H2O -0.63 follows that the geochemistry of the to reduce the temperature of the water environment, the idealized precipitation mass of the brine considering night Bloedite Na2Mg(SO4)2·4H2O -0.42 sequences and the mineral temperatures. Night temperatures in the Gypsum CaSO4·2H2O -0.06 assemblages proposed by Eugster and area ranged between 1 and 7 ºC. Halite NaCl -0.19 Hardie (1978), are susceptible to change Considering a 4 ºC average temperature where microbes are present. and a temperature cushioning of 3 ºC, it Polyhalite K2Ca2Mg(SO4)4··2H2O 0.03 means that temperature of the water Glauberite Na2Ca(SO4)2 0.16 The purpose of this paper is the during the night could decrease up to a Thenardite Na2SO4 -0.01 geochemical modeling of the mineral minimum value of 7 ºC (equilibrium precipitation from the hyper- temperature). Table 1. Modeled minerals with their formula and concentrated brine. the maximum saturation indices (SI Max). palabras clave: Lagunas, Costras salinas, Tapices bacterianos, key words: Shallow lake, Saline crusts, Microbial biofilms, Sulphates. Sulfatos. Jornada SEM * corresponding author: [email protected] macla nº 21. 2016 revista de la sociedad española de mineralogía 21 supersaturated in carbonates and subsaturated in sulphates and chlorides, the precipitation of most of the minerals is only related with physicochemical conditions promoted by microbial activities. Organic matter can absorb or expel ions promoting hydrochemical changes. ACKNOWLEDGEMENTS The research has been financed through Fig 1. Saturation indices of the minerals while varying the temperature of the brine (Night and day). Only Project CGL2015-66455-R (MINECO- glauberite, mirabilite and Polyhalite are oversaturated. FEDER) and a grant given to O.C. BES- 2012-054282. It is part of the scientific oversaturated with a SI minimum value clearly lower than the temperature of activities of Research Group UCM- of -0.06. Dolomite stayed oversaturated, saturation, 12 ºC. Crust layering and the 910404. We wish to thank J.M. aragonite moved away from saturation absence of organic matter in the crystals Astilleros and D. Benavente for their point and calcite turned undersaturated described by Del Buey et al., this volume help. at 10 ºC. Thenardite, halite and bloedite is coherent with this type of were very close to the saturation point at precipitation. Glauberite was also REFERENCES 3 ºC, with values of -0.01, -0.19, and - oversaturated, although was not 0.42, respectively. Polyhalite which was detected by XRD. Thus, it may require Cabestrero Ó. & Sanz-Montero M.E. (2016): always undersaturated turned specific conditions of nucleation that will Brine evolution in two inland evaporative oversaturated under 5 ºC. All other be assessed in a near future. Polyhalite environments: influence of microbial mats sulphates were clearly still was oversaturated below 5 ºC, but this in mineral precipitation. 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(1999): User's their results, except Gypsum, most the crusts is lower than in the guide to PHREEQC (version 2). A computer sulphates and chlorides, cannot paragenesis commonly found in the program for speciation, batch-reaction, precipitate directly from the brine, lake. The ability of gypsum to nucleate one-dimensional transport, and inverse geochemical calculations: U.S. Geological because they are always in the organic matrix explains its Survey Water-Resources Investigations undersaturated, even when most of the precipitation (Cabestrero and Sanz- Report 99-4259. water is about to evaporate. Therefore, Montero, 2016). Halite was also Sanz-Montero M.E., Cabestrero Ó., Rodríguez- the authors attributed the increase of undersaturated. Aranda J.P. 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The results of the and sulphates at extreme hypersaline geochemical model show that bloedite conditions are of more importance than and epsomite are permanently previously supposed.