--> Constraints on the Composition of Diagenetic Fluids that Formed Burlington-Keokuk Regional Dolomites: Boron, Sodium, Chloride and Sulfate as Fluid Tracers, by W. J. Staudt, G. Hemming, W. J. Meyers, and M. A. A. Schoonen; #90986 (1994).

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Abstract: Constraints on the Composition of Diagenetic Fluids that Formed Burlington-Keokuk Regional Dolomites: Boron, Sodium, Chloride and Sulfate as Fluid Tracers

Wilfried J. Staudt, Gary Hemming, William J. Meyers, Martin A. A. Schoonen

Based on detailed field and petrographic studies the diagenesis and relative timing of two dolomitization events (dolomite I & II) in the Mississippian Burlington-Keokuk Formation (Iowa, Illinois, Missouri) has been established. The composition and nature of the solutions that have dolomitized lime mud to form dolomite I, and partly replaced dolomite to form dolomite II have been previously constrained using various trace elements and isotopes. The use of boron isotopic compositions and boron, sodium, chloride and sulfate as fluid tracers permits us not only to distinctly differentiate between the two dolomite generations but also to further constrain the compositions of the dolomitizing solutions.

Sodium concentrations (fluid-inclusion corrected) range from 124-584 ppm (_x =289 ppm) for dolomite I and from 0-186 ppm (_x =58 ppm) for dolomite II. Sulfate concentration range from 1354-3521 ppm (_x =2237 ppm) for dolomite I and from 150-384 ppm (_x =276 ppm) for dolomite II. Dolomites I & II also have distinctly different boron concentrations which are about a factor of two higher in dolomite I.

The low sulfate and boron concentrations in dolomite II are consistent with a subsurface fluid model. The high sulfate concentrations in dolomite I, however, require near surface dolomitization under oxidizing conditions. Dolomite I samples show a distinct trend of increasing sodium and sulfate concentrations from the paleo basin towards the paleo-shoreline. These data indicate increasing fluid Na/Ca and SO4/CO3 ratios towards the paleo shore, and are consistent with moderately concentrated dolomitizing fluids of seawater-like origin. Observed low boron concentrations and light boron isotopic compositions compared to pristine Mississippian components may have resulted from the interaction of this evapo-concentrated marine fluid with the marine carbonate mud precu sor at a somewhat lower solution pH than expected from normal marine waters.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994