Convection of saline brines in enclosed lacustrine basins: A mechanism for potassium metasomatism

Abstract

Metasomatic potassium enrichment is found in a number of different geologic environments. One type of metasomatism that is often spatially and temporally associated with volcanism and sedimentation is found in ancient closed lacustrine basins. We propose that metasomatizing fluids may have been plumes driven by instabilities inherent in evaporation-induced density stratification found beneath saline lakes. For instabilities to occur, the hydraulic conductivity of the stratigraphic section should be greater than ∼10 −7 m/s. Metasomatized Tertiary volcanic and sedimentary rocks near Socorro, New Mexico, and in the Harcuvar Mountains, Arizona, show potassium enrichment by a factor of two or more. In both areas, the stratigraphic section is estimated to have an overall vertical hydraulic conductivity larger than ∼10 −7 m/s, sufficient for ground-water convection and potassium transport. In contrast, data from Searles Lake and Lake Tecopa in eastern California show little or no net addition of potassium. Lithology of these sediments suggests initial vertical hydraulic conductivities to range from 10 −8 to 10 −10 m/s, insufficient to sustain salinity-driven convection. Vertical convective transport of potassium was therefore unlikely at Searles Lake and Lake Tecopa.