Proposed Origin of Subsurface Thermal Brines, Imperial Valley, California: ABSTRACT

depth) of unique chemistry (reported by D. E. White) are recovered by geothermal wells near the Salton Sea in the Imperial Valley--a tectonically active graben area of high heat flow at the north end of the Gulf of California. The reservoir chamber consists of alternating fractured greenschist and zeolitic facies metamorphic rocks at depths of 3,900-8,000 feet. The shallow waters adjacent to and overlying this and many other thermal anomalies are dilute NaHCO3-Cl waters, high in B. NH4, I, and F are present; the Na/K ratio is less than in the brines. CO2 is abundant. The similarity of the deuterium content of these brines and various surficial waters of the Imperial Valley as determined by H. Craig and reported by White indicates that the waters of these brines are dominantly meteoric. The most critical geochemical questions concern the mechanism by which the brines are concentrated to such a high degree, the origin of the Cl ion within the brine, and the surprisingly high Ca/Na, K/Na, and Cs/K ratios of the brine. The arkosic sedimentary fill of the graben contains ample material to provide by solution every chemical found within these thermal brines with the exception of the Cl ion. The high 18O of the brines and its impoverishment End_Page 644------------------------------ in the reservoir rocks (Craig via White), the impoverishment of B in the reservoir rocks (White), and the similarity of the Rb/K ratios of the brines to those of arkosic materials indicate the high degree of interchange between the host rock and the thermal waters. The Cl ion either must be introduced at depth as juvenile Cl transported solely by diffusion in the gaseous phase from a magmatic source or must result simply from the concentration of meteoric interstitial water of the sedimentary fill. Strong evidence suggests that no Cl-evaporites are present at depth in the graben. The similarity of the Br/Cl ratio of the thermal brines to all of the meteoric surface and ground waters of the Imperial Valley area (Chevron Research data) and its complete dissimilarity to ratios found within Cl-evaporites suggest that the brines are merely the meteoric water of the graben fill concentrated many-fold. No exotic source is needed. Hyperfiltration of relatively dilute hydrothermal solutions through electrostatic semi-permeable membranes, composed of abundant montmorillonitic and illitic clays in the sedimentary fill, and probable zeolites overlying and laterally bounding the thermal anomaly, provides the best mechanism for concentrating the brines as well as determining their relative composition and that of the surface effluent waters overlying the thermal anomaly. Such high concentrations could be achieved only by a very large volumetric transfer of dilute hydrothermal waters through the membrane material due to the progressive decrease of hyperfiltration efficiency of semi-permeable membranes with increasing concentrations. Relative hyperfiltration of Ca with respect to Na and the relative increase of B, NH4, F, I, and HCO3 in solutions effluent from membranes has been observed by White in subsurface waters at lower temperatures. The relative increase of K/Na and Cs/K by selective membrane transport in a hyperfiltrated solution is consistent with the known behavior of solutions through ion-exchange columns where the smaller hydrated ion is adsorbed preferentially in the double layer, thereby permitting preferential membra e transport for the larger and less hydrated ion. A steadily expanding dome-shaped zone of brittle, fractured rocks metamorphosed by the hydrothermal solutions ascending by convective transport from a high heat source at depth, presumably a silica melt, and surrounded by relatively unmetamorphosed membrane materials (zeolites and clays) is assumed as a model. Hyperfiltration would occur within the dome by passage of solutions through the bordering membrane materials. Brines whose composition would have increased steadily through time until reaching an equilibrium would be found in the dome within which a convection cell characterized by channel flow should exist. Relatively dilute effluent solutions of a particular chemistry would emerge continuously from the membrane material to form the dilute shallow waters of specific chemical co position that typically occur near the surface at the Salton Sea and other thermal anomalies. Occasional fractures would permit leakage of the concentrated brine outward from the dome where it would mix with effluent waters. Meteoric interstitial water of the sedimentary fill would mix with the membrane-effluent and leakage waters on the borders of this hydrochemical system. End_of_Article - Last_Page 645------------