TESTING THE REFLUXING BRINE HYPOTHESIS WITH XRD-DETERMINED DOLOMITE STOICHIOMETRY: A CASE STUDY FROM THE SILURIAN REEFS OF THE MICHIGAN BASIN
To test this hypothesis a total of 129 samples were acquired from the Kalkaska 1-21 core (Kalkaska County, Michigan) between 6500 ft and 6756 ft. Samples were collected at a two foot interval using a Dremmel power drill. Loose material was then powered by hand using a mortar and pestle. The relative amounts of dolomite and calcite and the stoichiometry of the dolomite in each sample were determined by X-ray diffraction (Bruker D2 Phaser) employing an internal CaF2 standard.
Dolomite stoichiometry increases linearly from ~48 to 49 mole% MgCO3 between 6500 ft and 6600 ft. From 6600 ft to 6700 ft dolomite stoichiometry remains constant at ~49 mole% MgCO3. From 6700 ft to 6756 ft (the base of the core), dolomite stoichiometry decreases linearly to ~48 mole% MgCO3. Although the predicted decreasing MgCO3 trend was not observed, the results suggest that dolomite stoichiometry as a proxy for fluid flow may be possible. Furthermore, the results indicate that the dolomitization history of the Niagaran-Salina reef complexes in the Michigan Basin is more complicated than suggested by previous studies.