--> ABSTRACT: Uranium-Thorium Isotope Geochemistry of Saline Ground Waters from Central Missouri, by Jay L. Banner, J. H. Chen, and G. J. Wasserburg; #91022 (1989)

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Uranium-Thorium Isotope Geochemistry of Saline Ground Waters from Central Missouri

Jay L. Banner, J. H. Chen, G. J. Wasserburg

The isotopic and elemental distributions of uranium and thorium were examined in a suite of saline ground waters from central Missouri using mass spectrometric techniques. The waters were sampled from natural springs and artesian wells in Mississippian and Ordovician aquifers and have a wide range in salinity (5 to 26^pmil), ^dgrD (-108 to -45^pmil), and ^dgr18O (-14.7 to -6.5^pmil) values.

The suite of samples has a large range in 238U (50 to 200 × 10-12g/g) and 232Th (0.3 to 9.1 × 10-12g/g) concentrations and extremely high 234U/238U activity ratios ranging from 2.15 to 16.0. These isotopic compositions represent pronounced uranium-series disequilibrium compared with the value of modern seawater (1.15) or the equilibrium value (1.00). For such 234U-enriched waters, 234U/238U isotope ratios can be determined with a precision of ± 10^pmil (2^sgr) on 10 mL of sample and less than ± 5^pmil on 100 mL. In contrast to the large 234U enrichments, 230Th/238U activity ratios in the ground waters are significantly lower than the e uilibrium value. The more saline samples have markedly higher 234U/238U activity ratios and lower ^dgrD and ^dgr18O values. Unfiltered and filtered (< 0.1 µm) aliquots of a saline sample have the same isotopic composition and concentration of uranium, indicating uranium essentially occurs entirely as a dissolved species. The filtered/unfiltered concentration ratio for thorium in this sample is 0.29, demonstrating the predominant association of thorium with particulates.

These results are consistent with a hydrologic model involving relatively old far-traveled saline water and leaching into solution of 234U from ^agr-recoil-damaged sites in aquifer minerals. The 230Th appears to be efficiently removed from solution by particulate scavenging on a short time scale (~ 500 years).

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.