X-RAY-MICROANALYSIS OF FLUID INCLUSIONS AND ITS APPLICATION TO THE GEOCHEMICAL MODELING OF EVAPORITE BASINS

Direct access to brines trapped in halite is possible by freezing and breaking halite crystals on a cooling stage inside a scanning electron microscope. Quantitative analyses of major solutes (Na, Mg, K, Ca, Cl, and SO4) in natural brines were obtained by means of X-ray microanalysis of the frozen fluid inclusions using frozen droplets of brines with known composition as standards. Precision errors were estimated to be lower than 10%, and detection limits vary from below 0.02 mol/L for Ca, K, and SO4 to 0.55 mol/L for Na. Analyses of several inclusions of sizes larger than 20 mum were performed for each cross section of halite. The composition of primary fluid inclusions in halites from a drill core in the Messinian basin of Lorca, SE Spain, were used to constrain the parameters of a geochemical model of the evolution of the basin. A computer program to simulate evaporation paths was developed using thermodynamic equilibrium and mass balance as constraints. The halite sequence was interpreted as resulting from the evaporation of initial seawater in a hydrologically open basin, with seawater replacing the evaporated water and a small fraction of basinal brine leaking out. In the middle of the sampled sequence, the basin was closed to the sea, and only continental water flowed into the basin, reworking part of the halite deposited in the margins. In contrast with these results, only very poor information would have been obtained from the mineralogical record alone, consisting mainly of halite and minor gypsum.