Mechanisms of dolomitization in seawater and origin of the Middle Ordovician dolomites in the Illinois Basin

This investigation focuses on the chemistry of dolomitization and the geochemical and petrographic characteristics of the Middle Ordovician limestones and dolostones in the Illinois basin.The often cited dolomitization equation:$$\rm CaCO\sb3 + Mg\sp{2+} = CaMg(CO\sb3)\sb2 + Ca\sp{2+}$$cannot be applied to dolomite formation in seawater, because in seawater both calcite and dolomite are supersaturated. However, seawater (as a marine pore fluid) has the ability to dolomitize calcite and aragonite. In seawater, crystallization of calcite, aragonite, and dolomite drives the fluid from a region of calcite saturation to a region of calcite undersaturation, resulting in dolomitization.Seawater is a better dolomitizing fluid than a seawater-fresh water mixing-zone fluid from thermodynamic, kinetic and mass-balance considerations. The rate of dolomitization increases with increasing salinity, which is reflected in the fact that dolomite commonly occurs in hypersaline marine environments.I hypothesize the existence of dolomite domains in magnesian calcite, and propose a "dolomite-domain" model to explain the microscopic process for dolomitization of magnesian calcite. Dolomitization occurs along domain boundaries, where calcite is dissolved while dolomite precipitates as overgrowth on dolomite domains. This model can explain the preservation of fabrics and geochemical signatures during dolomitization of magnesian calcite.The wide variety of Middle Ordovician dolomites in the Illinois basin provides an opportunity to apply the above theoretical considerations. Both the Everton and Joachim Formations contain extensive dolomites that are associated with evaporite minerals and have other supratidal features. Those dolomites were formed in hypersaline environments during deposition and early diagenesis. The dolomites of Platteville and Galena Groups are not associated with evaporites and supratidal features. Nevertheless, petrographic observations, geochemical data, and theoretical consideration suggest that they also were mostly formed during deposition and early diagenesis in normal marine to hypersaline environments which are closely associated with the Wisconsin Arch. Later diagenesis by meteoric fluids and/or basinal brines might have altered oxygen isotopic composition of the dolomites. Hydrothermal fluids which had migrated through the granitic basement might have been responsible for the high $\sp{87}$Sr/$\sp{86}$Sr and low $\delta\sp{18}$O values of some dolomites in northern Illinois as well as for the source of metals in nearby Mississippi Valley Type (MVT) ore deposits. These conclusions are supported by geochemical simulations.U of I OnlyETDs are only available to UIUC Users without author permissio