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Working depositional models |
Area de Petrologia y Geoquimica, Departamento de Ciencias de la Tierra, Universidad de Zaragoza, 50.009 Zaragoza, Spain (e-mail: pllopez{at}unizar.es)
Solutions coming from two natural playa–lake saline systems located in Central Ebro Basin (NE Spain) have been evaporated in the laboratory, in order to obtain the precise path of chemical evolution followed until high concentration stages. The lakes belong to two chemically different neutral brines: La Playa (with Na–Cl solutions) and La Salada (Na–Mg–SO4 type). Experimental evaporation has been carried out at 25 °C until total dryness, and samples collected along the experiment have been analysed for their major components. Application of geochemical modelling techniques allowed calculation of the saturation indexes for the main saline minerals using the PHRQPITZ program, which incorporates Pitzer's model. The mineral precipitation sequence for La Playa brines following the saturation data is: gypsum, halite, thenardite and epsomite. Brines reach saturation almost simultaneously with respect to both halite and thenardite, but halite precipitates more massively and hence the solid samples collected after total desiccation were composed mainly of halite. In the case of La Salada brines, the order of precipitation is somewhat different and is as follows: gypsum, mirabilite, thenardite and bloedite. Solid samples here consisted of bloedite and thenardite, this latter formed after subaquatic dehydration of mirabilite when brines attained a peritectic point. The evolution of saturation indexes is in good agreement with mineral determinations carried out on the solid experimental samples, and it allowed us to interpret the evaporative evolution of both La Playa and La Salada brines.
