TESTING THE REFLUXING BRINE HYPOTHESIS WITH XRD-DETERMINED DOLOMITE STOICHIOMETRY: A CASE STUDY FROM THE SILURIAN REEFS OF THE MICHIGAN BASIN

Previous work suggests that the Silurian-age Niagaran-Salina reef complexes in the Michigan Basin were dolomitized by downward refluxing brines. From this diagenetic model it can be inferred that fluids became progressively Mg depleted down the flow path as dolomitization occurred. High-temperature experimental studies have shown that the Mg/Ca of the dolomitizing fluid is directly related to the stoichiometry (mole% MgCO₃) of the dolomite. Thus, a change in dolomite stoichiometry should be observable along the path of the dolomitizing fluids. Here we hypothesize that dolomite stoichiometry can be used as a proxy for fluid flow during dolomitization. More specifically, that dolomite nearer to the source of Mg-rich fluids (i.e. at shallower depths) is more Mg-rich and dolomite farther from the source (i.e. at deeper depths) is more Ca-rich.

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.