Role of Membrane Hyperfiltration on Origin of Thermal Brines, Imperial Valley, California: ABSTRACT

Unique saturated Na-Ca-K Cl thermal brines (380°C.) are recovered from geothermal wells near the Salton Sea, Imperial Valley, California. The reservoir chamber is fractured metamorphic rock (mainly greenschist facies) at 3,900-8,000 feet. The shallow waters are relatively dilute NaHCO3-Cl, high in B; NH4, I, and F are present; the Na/K ratio is less than in the brines; CO2 gas is abundant. The waters of these brines are dominantly meteoric, as evidenced by End_Page 454------------------------------ the previously reported deuterium content of the brines and local surficial waters. The most critical geochemical questions concern the mechanism for brine concentration, the origin of the Cl ion, and the surprisingly high Ca/Mg, K/Na, and Ca/K ratios of the brine. The arkosic sedimentary fill of the Imperial Valley graben contains ample material to provide by solution every chemical in the brines except for the Cl ion. Models of the brine column versus original material contained in the encompassing rock column suggest a deficiency of Cl ion. A high degree of interchange between the host rock and the thermal waters exists, as evidence by previously reported data (18O and B content) and the similarity of the Rb/K ratios of the brines to those of arkosic materials. The Cl ion must be either juvenile in origin, or a result of the concentration of meteoric interstitial water of the sedimentary fill, or derived from Cl absorbed onto silicate surfaces. The similarity of the Br/Cl ratio of the brines to local meteoric surface and ground waters, and its complete dissimilarity to those of Cl evaporites, suggest that the brines are dominantly interstitial meteoric water concentrated manyfold. Doubtless some of these brine halogens also were obtained by desorption of absorbed material at high temperatures. Hyperfiltration of relatively dilute hydrothermal solutions through electrostatic semi-permeable membranes composed of abundant montmorillonitic and illitic clays in the sedimentary fill provides the best mechanism for concentrating the brines within the proposed thermal convection cell and of affecting the relative composition of the brine and the relatively dilute waters underlying the thermal anomaly. In particular, this mechanism best explains the Ca/Na ratios of the brine; the relative abundance of Sr, Ca, and Mg within the brines possibly may be a result of this mechanism; the increase of HCO3/Cl, F/Cl, and B/Cl ratios in the dilute overlying waters, which would be effluent to the proposed membrane system, probably is a result of such hyperfiltration. High-temperature metastable equilibria between the thermal brine and its enclosing rocks strongly affect the specific composition of the brine. Such reactions probably control completely the trace-element metal content of the brines. The relative abundance of the alkali-metals appears to be strongly influenced by such rock-water reactions as well as by relative hyperfiltration. Experimental investigations are needed to understand further the origin of these waters. End_of_Article - Last_Page 455------------