--> Abstract: Determination of Trace Anion Concentrations Cl-, F-, SO4(2-) in Dolomites: A New Analytical Procedure to Constrain the Composition of Dolomitizing Fluids, by W. Staudt, M. A. A. Shoonen, and E. J. Oswald; #91012 (1992).

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ABSTRACT: Determination of Trace Anion Concentrations Cl-, F-, SO4(2-) in Dolomites: A New Analytical Procedure to Constrain the Composition of Dolomitizing Fluids

STAUDT, WILFRIED, and MARTIN A.A. SCHOONEN, S.U.N.Y. at Stony Brook, Stony Brook, NY, and ERIK J. OSWALD, Exxon Production Research Co., Houston, TX

It is generally accepted that the formation of massive dolomite occurs through a reaction between an Mg-bearing solution and CaCO3. To constrain the composition of Mg-bearing fluids responsible for the formation of a given ancient dolomite, we have developed a new analytical procedure that allows for determination of bulk Cl-, F-, and SO4(2-) concentrations in dolomites. To complement determination of bulk anion concentrations, the abundances of Na, K, Sr, Al, and Ca were measured using standard DCP (Direct Coupled Plasma) methods.

Twenty milligrams of pure dolomite were dissolved in 1 mL of 3% ultrapure nitric acid and subsequently diluted with deionized water by a factor of 4000. Anion concentrations in the solution were determined using a DIONEX 2010 Ion Chromatograph with an AS4A column and an NaHCO3/Na2CO3 eluant. Detection limits of our method for chloride, fluoride, and sulfate concentrations in dolomites are 80 ppm, 90 ppm, and 300 ppm, respectively. The analytical uncertainty is +/- 15 ppm for Cl- and F-, and +/- 85 ppm for SO4(2-). Cl- concentrations range from <80 ppm to 800 ppm, F- ranges from <90 ppm to 1460 ppm, and SO4(2-) from <300 ppm to 7000 ppm for a variety of dolomites from different diagenetic origins (i.e., burial dolomite, mixing zone dolomite, and evaporitic dolomite). To purify natural dolomites, a powdered sample is leached with 4% acetic acid and thoroughly rinsed with deionized water. Successive leaching removes associated CaCO3 phases, gypsum, halite, and other soluble salts occurring in natural dolomites. After each leaching the mineralogy of the sample is determined using a SCINTAG powder diffractometer.

For dolomites that are interpreted to have formed in various environments, the bulk Cl-, F-, SO4(2-) and Na+ concentrations are

distinctly different. The individual contributions of fluid inclusions, solid inclusions, lattice-incorporated trace ions, as well as interstitial trace ions to the bulk concentrations cannot be determined. However, the measured Cl-, F-, SO4(2-) and Na+ abundances of the dolomite samples are consistent with the proposed dolomitizing solutions. Therefore, this new procedure is potentially a valuable geochemical tool to constrain the composition of dolomitizing fluids, complementing other geochemical techniques, fluid inclusion studies, and petrographic evidence.

 

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)