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 U-Th isotopes during water-rock interaction in carbonate systems. Well-preserved aragonitic corals from the Golden Grove terrace yield precise 23°Th ages of 219 _+ 3 and 224 ± 6 ky, and have high and variable initial 3234U relative to values for modern seawater. These high 3234U 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 ~234U than the unreplaced aragonite, and (23°Th/23SU)AcT (activity ratios) in excess of values for secular equilibrium. Groundwaters sampled from 125-460 ky old portions of the Pleistocene carbonate aquifer have 3234U values close to secular equilibrium, and low (23°Th/Z38u)AcT of less than 4 × 10 -3. The pronounced differences in the U-Th 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 U-Th isotopic composition of marine carbonates and therefore the accuracy of high-precision 23°Th 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 3234U values of the corals. The mechanism by which apparently well-preserved corals become enriched in 234U remains to be identified. Low 6234U 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.