Abstract

Marine carbonates of the Burlington-Keokuk Fm. (Miss.) of the midcontinent region have been subject to two major episodes of regionally extensive (100,000 km 2 ) dolomitization. Previous studies of the cathodoluminescent petrography, Sr, C, and O isotopic compositions, and Sr, Fe, Mn, and Ca concentrations of Burlington-Keokuk dolostones outline a diagenetic history in which lime mud was pervasively dolomitized by a seawater-dominated fluid. At many localities, this early dolomite generation (dolomite I) was recrystallized by nonmarine diagenetic fluids at shallow-burial depths to produce dolomite II. Dolomites I and II have similar rare earth element (REE) patterns and abundances and a narrow range of initial 143 Nd/ 144 Nd isotope ratios of 0.511906 + or - 0.000025 (1 sigma ). This consistency in the Nd isotope ratios and REE patterns, along with the presence of anomalously low abundances of cerium in the dolomites, cherts, glauconite pellets, fish apatite, and crinoids, suggests that 1) seawater-derived REE dominated the system, and 2) the extensive water-rock interaction that produced profound textural and compositional changes during the formation of dolomite II did not significantly alter the REE patterns or Nd isotopic signatures of the dolomites. The results of quantitative modeling of isotopic and trace element exchange during water-rock interaction in carbonate systems indicate that extremely large water: rock ratios are required to alter the REE signatures of diagenetic carbonates. Such large water: rock ratios can reflect either 1) large extents of fluid flow during carbonate recrystallization, or 2) minimal interaction between diagenetic fluids and host rocks during carbonate cement crystallization. Late vug-filling calcite and dolomite have low Sm/Nd and high (Ce/Nd) n ratios relative to the dolostones. Such distinct REE signatures probably reflect limited interaction between late diagenetic fluids and Burlington-Keokuk host rocks. Penecontemporaneous clastic sediments have similar initial 143 Nd/ 144 Nd isotope ratios (0.511857-0.511877) and lower Sm/Nd ratios compared to the dolomites, suggesting 1) a common crustal source of REE for the clastics and carbonates, and 2) REE fractionation during the formation of carbonate, apatite, glauconite, and other authigenic phases in the marine environment.

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