Estudio goquimico de la sedimentacion salina actual en un sistema playa-lake estacional de tipo sulfatado sodico: La Salada de Mediana (Zaragoza, Espana) (Spain, Spanish text).

摘要

Salada Mediana is an active playa lake saline system placed in the Central Ebro Basin (NE Spain). This system is located within a little basin without surface drainage and lies on Tertiary sedimentary materials with high evaporitic content. Dilute surficial waters accumulate in the bottom of the depression, evolving under semi-arid climatic conditions to become brines in the spring season and reaching total dryness in summertime. After dessication, the bottom of the basin is covered by a thin saline crust mainly composed by thenardite, bloedite, mirabilite, and gypsum. Brines sampled were of neutral type and rich in sulphate, sodium and magnesium.; Natural brine has been evaporated in laboratory, in order to obtain the precise evolving path until reaching the higher concentration stages. Experimental evaporation has been carried out at 25°C to total dryness. Mineral saturation indexes have been calculated through the PHRQPITZ geochemical code using the chemical data of the analysed samples. The mineral equilibrium sequence obtained by thermodynamic calculations is: calcium carbonates, gypsum, mirabilite, bloedite, and thenardite. Moreover, mineralogical analyses realized on the solid samples indicate that the saline deposit obtained after total dessication consists mainly of bloedite and thenardite, with gypsum as accessory. The evolution of saturation indexes calculated by the PHRQPITZ geochemical code are in good agreement with the mineral determinations carried out on the solid experimental samples.; The evolution of saline system has been described starting from the main active processes identified. As in the model of Lowenstein & Hardie (1985), three main stages can be established: Recharge of system, evolution under evaporation and, finally, total dessication. These stages take place in an annual cyclic evolution.; The treatment of results obtained in this work have us allowed to describe the geochemical behaviour of an active saline system, which correspond to a little known chemical type (sulphate sodic).