Computer Science
Scientific paper
Oct 1991
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1991e%26psl.107..129b&link_type=abstract
Earth and Planetary Science Letters, Volume 107, Issue 1, p. 129-137.
Computer Science
10
Scientific paper
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 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 δ234U values close to secular equilibrium, and low (230Th/238U)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 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.
Address for correspondence
Banner Jay L.
Chen James H.
Humphrey John D.
Wasserburg Gerald J.
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