ABSTRACT

The concentration of uranium has been measured using the fission track technique in vadose and freshwater phreatic cements from South Joulters Cay, in The Bahamas. These analyses reveal dramatic differences in the U concentration of vadose (< 0.28 ± 0.11 ppm) and freshwater phreatic (1.80 ± .75 ppm) cements. This distinctive change in concentration is proposed to result from 1) variations in the CO₂ of the waters of the phreatic and vadose environments, and 2) selective exclusion of U during the precipitation of calcite. The pCO₂ is important because it controls the pH and hence the concentration of CO₃^2- ions in the pore fluids as well as the amount of dissolution of the U rich aragonitic precursors. For example, an increas in pCO₂ will lower the pH and decrease the concentration of CO₃^2- present in the porewaters. As the U, present as the uranyl ion (UO₂²⁺), is mainly complexed with CO₃^2-, a decrease in the concentration of CO₃^2- will not only alter the speciation of U but also increase the UO₂(CO₃)₂^2-/ CO₃^2- ratio of the fluid and hence the concentration of U in the precipitated carbonate cements.

The second mechanism for explaining the increase of U in phreatic cements relies on the low distribution coefficient for the incorporation of U into low-Mg calcite (LMC). The precipitation of LMC therefore leads to an increase in the concentration of U in the residual solution. As fluids do not remain in the vadose zone, but rather accumulate in the freshwater phreatic portion of the lens, phreatic waters become relatively enriched in U compared to vadose fluids. The clear difference in the U concentrations of cements in these two zones may enable the U concentration of cements to be used as an indicator of paleo-water tables in ancient rocks even where other geochemical and petrographic evidences are equivocal.