DOLOMITE STOICHIOMETRY AS A PROXY FOR TRACKING FLUID CHEMISTRY, TEMPERATURE, AND FLOW DIRECTION IN FAULT-RELATED HYDROTHERMAL DOLOMITES: FIELD OBSERVATIONS FROM THE EARLY SILURIAN SUCCESSION OF THE MICHIGAN BASIN

Various petrographical and geochemical proxy datasets harvested from dolomite (CaMg(CO₃)₂) are routinely used to infer past diagenetic conditions. More recently, detailed mineralogical data have been touted as a valuable proxy resource from which to assess dolomitization models. High-temperature experiments show that dolomite stoichiometry - the abundance of Mg²⁺ in the dolomite crystal lattice relative to the Ca²⁺ - is influenced by a host of common environmental factors, including the Ca/Mg, temperature, ionic strength, and pCO₂ of the dolomitizing fluids. Recently, field-based studies have demonstrated the utility of using dolomite stoichiometry to track vertical and temporal changes in the dolomitizing conditions in near-surface diagenetic settings.

The current study aims to extend the application of dolomite stoichiometry (mole% MgCO₃) proxy to dolomites in a deep basin setting. The dolomites of the early Silurian Burnt Bluff Group (BBG) in the central Michigan Basin are investigated using core analysis, petrographic thin sections, scanning electron microscopy, x-ray diffraction, x-ray fluorescence, carbon (δ¹³C) and oxygen (δ¹⁸O) isotopes, and clumped isotope (Δ47) data. Based on the core description and petrographic analyses, the BBG in the studied area is characterized by homogeneous burrowed lime mudstone that is dolomitized near the fault plane. Fluid inclusion and δ¹⁸O_carb data suggest that dolomite formed in elevated temperatures (~160 °C) compared to the temperature predicted by the geothermal gradient (~66 °C). Within these hydrothermal dolomites, dolomite stoichiometry shows an inverse relationship with the distance from the fault. That is, highly stoichiometric dolomites are observed closest to the fault plane, whereas less stoichiometric dolomites are observed farther from the fault. These observations are consistent with predictions from high-temperature dolomitization experiments showing higher temperatures result in more stoichiometric dolomites. Collectively, these data further support the applicability of the dolomite stoichiometry proxy to understand dolomitization.