The effects of magnesium concentration in high-magnesium calcite allochems on dolomitization: Insights from high-temperature dolomite synthesis experiments

Abstract

ABSTRACT Selective dolomitization of HMC allochems in Neogene carbonates is a common phenomenon. It has been proposed that the higher magnesium concentrations [Mg] in these allochems promotes dolomitization. To directly investigate the effects of [Mg] (reported as mol% MgCO3) in biogenic HMC on dolomitization, high-temperature (200 °C) dolomitization experiments were conducted. Dolomite reaction rate, stoichiometry, and microstructures were tracked during dolomitization for a variety of biogenic HMC reactants, including Goniolithon, Lithothamnion, Lithophyllum, Corallina officinalis, Heterocentrotus mamillatus, and Mellita quinquiesperforata. Solids were characterized using standard powdered X-ray diffraction (XRD) and scanning electron microscopy (SEM). In general, biogenic HMC reactants are progressively replaced by protodolomite products between 2 and 46 hours. In the experiment using Corallina officinalis, no products were detected by XRD after 46 hours. Initial protodolomite products are subsequently replaced by ordered dolomite as the reactions proceed. Dolomitization rate and protodolomite stoichiometry do not correlate with [Mg] of the biogenic HMC reactants. Dolomitized HMC skeletons reliably retain the original microstructure of the HMC reactants, consistent with the microcrystalline mimetic textures observed in nature. These findings collectively suggest that under the conditions investigated, dolomitization is less affected by the concentration of magnesium in the HMC reactants relative to other known factors, such as temperature, reactant mineralogy, organic content, skeletal texture, and fluid chemistry. These results imply that the observed correlation between coralline red algae abundance and global dolomitization events in the Neogene is not driven by the elevated magnesium concentration of the HMC reactants.