Cyclic variations of uranium concentrations and oxygen isotopes in tufa from the middle Miocene Barstow Formation, Mojave Desert, California

Uranium concentrations of 42 to 169 ppm in fibrous calcite from spring-depo sited tufa in the middle Miocene Barstow Formation, California, are among t he highest reported for calcite. Fission-track maps of multiple bands of un iformly dull-luminescent fibrous calcite show that the concentration of U i ncreases in the outward growth direction of the calcite of each individual band. Homogeneous dull luminescence in the fibrous calcite indicates no cha nge in redox conditions of the fluid from which the calcite was precipitate d. It is proposed that the cyclic pattern of increase in U concentration re flects a cyclical change in the U/Ca ratio in the fluid. Episodic mixing be tween Ca-rich spring water with a low U/Ca ratio and Ca-poor saline alkalin e lake water with a high U/Ca ratio could produce the episodes of formation of fibrous calcite with increasing U concentrations. The spring water supp lies the Ca for calcite precipitation, and the U concentrations increase wi th the decreasing fraction of spring water and increasing fraction of lake water. These cycles reflect a variable recharge of groundwater into the lak e by springs. The large variation in U concentrations suggests mixing of 0% to 45% spring water with the lake, while the narrow range in delta O-18 of -6.37 parts per thousand to -6.87 parts per thousand limits the variation of the proportions of spring and lake water to 5%.