Determination of Initial Sr-Isotope Compositions of Dolostones from the Burlington-Keokuk Formations (Mississippian) of Iowa, Illinois, and Missouri

J. L. Banner, G. N. Hanson, W. J. Meyers

Sr-isotope compositions, and Rb and Sr concentrations were measured on fine-grained, nearly pure dolostones, each consisting predominantly of one cathodoluminescent generation of dolomite. The results differ significantly, depending on the dissolution method used. For example, one sample gives replicate ⁸⁷Sr/⁸⁶Sr analyses by leaching with 1.25N HCl of 0.71032 and 0.71062. Adding HF to the leaching process gave a value of 0.71216, whereas complete dissolution with concentrated HF, HNO₃, and HCl yields a value of 0.71219. Using the time of dolomitization with Rb (12.4 ppm) and Sr (59.7 ppm) concentrations for the completely dissolved sample gives an initial ⁸⁷Sr/⁸⁶Sr ratio of 0.70943. Similar results for other samples i dicate that most Rb and radiogenic Sr are associated with minor clay fractions (predominantly glauconite) in the samples, and that HCl leaches Rb and Sr from clays to a variable extent. Morphologic relationships between dolomite, calcite, and nonpelloidal glauconite suggest that some clay formed episodically during various stages of nonmarine diagenesis. Sr-isotopic exchange may have occurred between carbonate and clay during major influxes of diagenetic fluids. To obtain initial ⁸⁷Sr/⁸⁶Sr ratios on samples that are completely dissolved, the following corrections are needed: isotopic equilibration between carbonate and clay, closure of the Rb-Sr system after diagenesis, and constraints on the timing of diagenesis. Such initial ⁸⁷Sr/⁸⁶Sr ratios ive the isotopic composition of the carbonate phase at the time of its crystallization, whereas leaching with weak acid is not always carbonate selective and can produce ambiguous results.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.