ABSTRACT

Regionally extensive, fine-grained dolostones from the Mississippian Burlington-Keokuk Fm. comprise two major generations of dolomite that can be distinguished by their cathodoluminescence, chemical compositions, and times of formation. The petrography and geochemistry of glauconite, the predominant clay mineral in the dolostones, indicates that it is an authigenic phase, some of which formed episodically during various stages of postmarine diagenesis. This study evaluates techniques for the determination of the Sr isotopic compositions of the dolomite generations through the analysis of nearly pure whole-rock samples ( 95% soluble) and mineral separates, each composed predominantly of one cathodoluminescent type of dolomite.

Sr isotopic compositions and Rb and Sr concentrations were measured on samples prepared by three separate methods: 1) leaching with 1.25N HCl, 2) leaching with 1.25N HCl and concentrated HF, and 3) complete dissolution with concentrated HF and HNO₃, and 6N HCl. Leaching with HCl removes 24-60 percent of the Rb in the samples. Compared to acid leaching, complete sample dissolution yields higher ⁸⁷Sr/⁸⁶Sr ratios and Rb concentrations and more reproducible results. This indicates that most of the Rb and radiogenic ⁸⁷Sr is associated with minor clay fractions in the samples and that HCl leaches Rb and Sr from clays to a variable extent.

The relatively high Rb/Sr ratios of the dolostones necessitate an age correction of each sample's ⁸⁷Sr/⁸⁶Sr ratio. The determination of such initial ⁸⁷Sr/⁸⁶Sr ratios requires contemporaneous formation of carbonate and clay, closure of the Rb-Sr system after diagenesis, and constraints on the timing of diagenesis. Calculated initial ⁸⁷Sr/⁸⁶Sr ratios for the dolostones by method 3 differ considerably from the uncorrected measured ⁸⁷Sr/⁸⁶Sr ratios by any of the dissolution methods. These results demonstrate that even minor clay components can make significant contributions to the analyses, and that analysis for Rb and Sr, and constraints on the timing and diagenetic history of both the clay and carbon te phases can be critical for the determination of the Sr isotopic compositions of even nearly pure dolostones. An evaluation of the uncertainties associated with the timing and type of diagenesis indicates that the initial ⁸⁷Sr/⁸⁶Sr ratios of the dolostones are representative of the isotopic composition of the dolomite generations at the time of their formation.

Two major cathodoluminescent types of Burlington-Keokuk dolomites preserve a record of two isotopically distinct diagenetic environments. Samples composed of the earliest dolomite (dolomite I) have initial ⁸⁷Sr/⁸⁶Sr ratios ranging from 0.70757 to 0.70808, while samples composed of dolomites II and II range from 0.70885 to 0.70942.