Uranium-series evidence on diagenesis and hydrology in Pleistocene carbonates of Barbados, West Indies

Uranium-series isotope measurements were made by mass spectrometry on aragonite, dolomite and groundwater samples from Pleistocene coral-reef terraces on Barbados in order to evaluate the behavior of UTh isotopes during water-rock interaction in carbonate systems. Well-preserved aragonitic corals from the Golden Grove terrace yield precise ²³⁰Th ages of 219 ± 3 and 224 ± 6 ky, and have high and variable initial δ²³⁴U relative to values for modern seawater. These high δ²³⁴U values are common to Pleistocene carbonates from many localities and indicate that the U-series has not followed closed-system behavior since the time of deposition. Pervasively dolomitized samples from the Golden Grove terrace have significantly lower δ²³⁴U than the unreplaced aragonite, and ( ²³⁰Th ²³⁸U)_ACT (activity ratios) in excess of values for secular equilibrium. Groundwaters sampled from 125-460 ky old portions of the Pleistocene carbonate aquifer have δ^234U values close to secular equilibrium, and low ( ²³⁰Th ²³⁸U)_ACT of less than 4 × 10^-3. The pronounced differences in the UTh isotope systematics between the aragonitic corals, dolomites and groundwaters reflect the enhanced mobility of U relative to Th during water-rock interaction. These results demonstrate that diagenesis can profoundly affect the UTh isotopic composition of marine carbonates and therefore the accuracy of high-precision ²³⁰Th ages determined on such samples. However, diagenesis involving fluids such as the present-day groundwaters on Barbados or those which formed the dolomites cannot account for the elevated δ²³⁴U values of the corals. The mechanism by which apparently well-preserved corals become enriched in ²³⁴U remains to be identified. Low δ²³⁴U for the dolomites and the groundwaters records the transport of dissolved U from portions of the aquifer that are significantly older (e.g., > 10⁶ yrs) than their present host carbonate rocks. The preservation of these distinct U-isotopic compositions during fluid migration through younger carbonates indicates that interaction between groundwaters and these carbonates, with accompanying ²³⁴U enrichment in the waters from dissolution or alpha-recoil, has been extremely limited.