Partitioning of Uranium in High-Temperature, Replacement Dolomitization Experiments: Implications for Isotopic Studies of Ancient Carbonate Rocks

The concentration of uranium ([U]) in natural fluids is controlled by its oxidation state, whereby the oxidized U(VI) is soluble and the reduced U(IV) is insoluble. Consequently, [U] in carbonate minerals has been used to track marine paleoredox conditions, but our confidence in such interpretations is limited by a lack of understanding of the partitioning behavior of uranium. During dolomitization, for example, uranium partitions between the dissolved CaCO3, the aqueous solution, and the precipitated CaMg(CO3)2. Thus, it is unclear whether [U] in dolomite reflects the geochemistry of the precursor aragonite/calcite, the dolomitizing fluid, or an intermediate composition between these endmembers.

This study evaluates the partitioning behaviour of U(VI) under a range of diagenetic conditions using high-temperature, replacement dolomitization experiments. In the experiments, either aragonite or calcite were dolomitized in Ca–Mg–Cl solutions at temperatures that range from 150 to 200°C. To investigate the extent that [U] reflects that of the precursor sediment, [U] was tracked throughout the reaction in uranium-free solutions. Next, a series of reactions at higher [U] were conducted by spiking the dolomitizing fluid with a uranium standard ([U] = 0.1 μM, 1.0 μM, and 10 μM). Reaction progress and the incorporation of uranium in the products was evaluated by XRD and ICP-MS, respectively. The results of this study have critical implications for the interpretation of [U] in ancient carbonate rocks, with future work that will investigate the fractionation of the stable isotopes of uranium in dolomite (238U/235U).