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

Banner, Jay L. and Wasserburg, G. J. and Chen, James H. and Humphrey, John D. (1991) Uranium-series evidence on diagenesis and hydrology in Pleistocene carbonates of Barbados, West Indies. Earth and Planetary Science Letters, 107 (1). pp. 129-137. ISSN 0012-821X. https://resolver.caltech.edu/CaltechAUTHORS:20131030-155459799

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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.


Item Type:Article
Related URLs:
URLURL TypeDescription
http://dx.doi.org/10.1016/0012-821X(91)90049-NDOIArticle
http://www.sciencedirect.com/science/article/pii/0012821X9190049NPublisherArticle
Additional Information:© 1991 Elsevier Science Publishers B.V. Received December 12, 1990; revision accepted July 31, 1991. We wish to thank L. Barker, M. Denham and S. Thieben for technical assistance in the field and laboratory. Constructive comments were provided by two anonymous reviewers. We also thank H. Cander, R. Stoessel and D. Papanastassiou for helpful scientific discussion. This research was supported by grants from the donors to the Petroleum Research Fund of the American Chemical Society (21213-AC2 and 22483-G8), the Department of Energy (DE-FG03-88ER13851), the National Science Foundation (EAR-8720376), Texaco, Inc., and the Geology Foundation of the University of Texas. This is contribution 710.
Funders:
Funding AgencyGrant Number
American Chemical Society Petroleum Research Fund21213-AC2
American Chemical Society Petroleum Research Fund22483-G8
Department of Energy (DOE)DE-FG03-88ER13851
NSFEAR-8720376
Texaco Inc.UNSPECIFIED
University of Texas Geology FoundationUNSPECIFIED
Issue or Number:1
Record Number:CaltechAUTHORS:20131030-155459799
Persistent URL:https://resolver.caltech.edu/CaltechAUTHORS:20131030-155459799
Official Citation:Jay L. Banner, G.J. Wasserburg, James H. Chen, John D. Humphrey, Uranium-series evidence on diagenesis and hydrology in Pleistocene carbonates of Barbados, West Indies, Earth and Planetary Science Letters, Volume 107, Issue 1, October 1991, Pages 129-137, ISSN 0012-821X, http://dx.doi.org/10.1016/0012-821X(91)90049-N. (http://www.sciencedirect.com/science/article/pii/0012821X9190049N)
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:42146
Collection:CaltechAUTHORS
Deposited By: Ruth Sustaita
Deposited On:31 Oct 2013 16:39
Last Modified:03 Oct 2019 05:55

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