Geoquímica de las lagunas saladas de los Monegros (Zaragoza). l. determinación experimental de los efectos del reequilibrio mirabilita-solucion con la temperatura en un sistema natural

Abstract

Geochemical evolution of Los Monegros playa-lakes is affected by temperature fluctuations in the brine body at different time scales. Temperature shifts promote seasonally, daily and even during minor cycles mineralogical and compositional changes. Seasonal impacts of temperature change the brine composition and the crystallized mineral sequences (Pueyo, 1978-79). So, during summer cycle minerals crystallize according to the sequence: carbonates-gypsum-halite; and during winter the sequence of precipitation changes to carbonates-gypsum-mirabilite. Daily cycles also exist during winter brine evolution: mirabilite crystallization occurs during spring nights by lowering temperature, whereas diurnal temperatures promote its dissolution. And when high saturation levels are reached by evaporative concentration and the amount of precipitated mirabilite is important in the system (mirabilite stage, at spring), diurnal temperature fluctuations induce quick mirabilite-solution reequilibrium processes. Sampling of brines during this stage and thermodynamic calculations through the extended HMW model (Harvie, Moller and Weare, 1984) enclosed in the PHRQPITZ code (Plummer et al., 1988) indicate that mirabilite equilibrium holds in spite of variations in the concentration degree and temperature of samples. Field observations confirm that the reequilibrium process is efective in a few minutes when temperature varies only sorne degrees. Experimental determination of mirabilite solubility between Oand 30oC, using a natural brine sampled in advanced concentration stage, allows to isolate temperature effects on solution composition from those of evaporative concentration. Results indicate that modifications of mirabilite solubility produce their maximum effects between 20-30oC, fluctuation common in the natural system during spring: several hundreds of grams/kg water of mirabilite are mobilized, brine ionic strength changes from 4 to 8 molal and water activity varies from 0.943 to 0.896. Thenardite, bloedite and glauberite saturation states are affected by that reequilibrium process but those of gypsum and halite are almost insensitive to it. Predicted brine evolution by means of classical chemical divide or generalized residual alkalinity rules fails because of the special characters of these systems (high concentration solutions and evolution paths under variable temperature conditions). This non-isothermal evolution must be taken into account in more elaborated physicochemical approaches to brine evolution: the normal simplification using isothermal conditions (25oC) in thermodynamic calculations leads to important discrepancies in the predicted timing of mirabilite precipitation refered to field observations. Partial validation of the extended HMW model by comparison of laboratory and field solubility data appear to be confirmed.