Evaluating the Effects of Fluid Chemistry on Dolomite Stoichiometry and Reaction Rate

Abstract

Numerous environmental factors have been shown to affect the process and outcomes of dolomitization. Restricted marine environments, for example, are typically associated with more abundant and more stoichiometric dolomite than deeper subtidal environments. Higher fluid Mg/Ca ratios resulting from the precipitation of gypsum are often invoked to explain this observation, even when evaporite minerals are absent. The new experimental dataset presented here suggests that in addition to the Mg/Ca ratio, the concentrations of the major cations Na, K, Mg, and Ca in solution can also impact dolomite stoichiometry and reaction rate. These findings offer an alternative explanation for the observed relationship between evaporative conditions and dolomite abundance and stoichiometry without the need to invoke precipitation of calcium-bearing evaporites. Nearly 200 individual high-temperature synthesis experiments were completed whereby aragonite ooids or crushed Iceland spar calcite crystals were dolomitized in Mg-Ca-Cl solutions with a constant Mg/Ca ratio of 1.0. Experimental solutions were prepared to mimic fluids found in a wide range of diagenetic environments (e.g., mixing, marine, and evaporative). Teflon-lined Parr bombs were charged with 15 mL of solution and 100 mg of solid reactants. Bombs were heated to 215°C for 2-800 hours. The products were filtered, rinsed with deionized water, and dried in a vacuum desiccator. Powder x-ray diffraction was used to determine relative mineral percentages, dolomite stoichiometry, and the degree of cation ordering of the solid products. Fluid [NaCl] and [KCl] correlate positively with dolomite stoichiometry but negatively with reaction rate. For example, experiments with [NaCl] and [KCl] concentrations of 0.0 M and 4.0 M are characterized by dolomite stoichiometries of 43 and 48 mol% MgCO₃, respectively. In contrast with the NaCl and KCl experiments, higher [Mg] and [Ca] correlate positively with both reaction rate and dolomite stoichiometry over the compositional range of natural diagenetic fluids. Fluid [Ca] and [Mg] concentrations of 0.0 M and 1.0 M, for example, correspond with dolomite stoichiometries of 41 and 45 mol% MgCO₃, respectively. The observations presented here add to our understanding of the fundamental controls on dolomite stoichiometry and reaction rate, and demonstrate that dolomite stoichiometry may be a useful proxy in our effort to elucidate the origin of ancient dolomites.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018