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

Abstract

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 ^(230)Th ages of 219 ± 3 and 224 ± 6 ky, and have high and variable initial δ^(234)U relative to values for modern seawater. These high δ^(234)U 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 δ^(234)U than the unreplaced aragonite, and (^(230)Th/^(238)U)_(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^(234)U values close to secular equilibrium, and low (^(230)Th/^(238)U)_(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 ^(230)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 elevatedδ234U values of the corals. The mechanism by which apparently well-preserved corals become enriched in ^(234)U remains to be identified. Low δ^(234)U for the dolomites and the groundwaters records the transport of dissolved U from portions of the aquifer that are significantly older (e.g., > 10^6 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 ^(234)U enrichment in the waters from dissolution or alpha-recoil, has been extremely limited.

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