CONTROLS OF HIGH-MG CALCITE ON THE DOLOMITIZATION OF CARBONATE DEPOSITS: AN EXPERIMENTAL STUDY

One of the fundamental requirements of dolomitization is a source of magnesium for the replacement of calcium. Whereas most dolomitization studies focus on fluid sources of magnesium, little is understood about the effects of host rock mineralogy on dolomitization rates. This study examines the potential role of magnesium from high-Mg calcite (HMC) coralline red algae (ave. = 20 mol% MgCO₃) in carbonate sediments on dolomitization reaction rates using high-temperature dolomitization experiments. Laboratory dolomites were produced with modern HMC coralline red algae (Goniolithon) and monocrystalline aragonite in a 1 M Mg-Ca-Cl solution (Mg/Ca = 1.0) at 200 ͒C via a replacement reaction. Solid products were analyzed using standard powdered X-ray diffraction (XRD) to determine mineralogy, percent replacement, and dolomite stoichiometry (mol% MgCO₃).

The overall rate of dolomitization exhibits a strong dependency with the proportion of HMC within the sample. 100% Goniolithon reactants were replaced at rates up to five times faster than monocrystalline aragonite. Mixtures of Goniolithon and monocrystalline aragonite show a corresponding increase in replacement rates with an increase in the proportion of Goniolithon. The stoichiometry of the initial very high magnesium calcite (VHMC) products ranges from 38.7 to 47.4 mol% MgCO₃, and covary strongly with the proportion of high-Mg calcite within the sample. Protodolomites produced from 100% Goniolithon have an average initial stoichiometry of 47.2 mol% MgCO₃, whereas those produced from 100% monocrystalline aragonite have an average initial stoichiometry of 38.9 mol% MgCO₃.

The results of this study (i) provide a potential link between the occurrence of high-Mg calcite rich formation with dolomitization, (ii) provide insights into diagenetic feedbacks within carbonate systems that have not been identified in previous studies, and (iii) illustrate the significance of biologically precipitated carbonates for controlling dolomitization rates.