Stepwise Reaction Pathway and Temperature-Dependence of Dolomitization of Aragonite Ooids

High-temperature laboratory synthesis experiments indicate that ordered, stoichiometric dolomite forms via a series of metastable precursors, but the exact reaction pathway remains a poorly understood aspect of dolomitization. In this study we use over one-hundred high-temperature experiments to evaluate the effects of temperature on (a) the rate of dolomitization, (b) evolution of cation order and (c) product composition over the range of 160–250°C. Reactions were conducted in Teflon-lined stainless steel reaction vessels containing 0.1 g of aragonite ooids (300–354 μm size fraction) from the Ambergris Shoal, Turks and Caicos Islands, B.W.I. and 15 mL of a 0.875 M Mg-Ca solution (Mg/Ca = 1.0). Dolomitization reactions were periodically terminated and solid products were analyzed using powder X-ray diffraction to determine the relative abundances of the mineral phases present, their composition and degree of cation order. Aragonite ooid data exhibit significantly less uncertainty than previously published data for ground Iceland spar calcite reactants and thus provide a more detailed picture of the dolomitization reaction. Data indicate that regardless of temperature, the first phase formed is always disordered, very high magnesium calcite (VHMC) with 40–45 mol% MgCO3. At lower temperatures the VHMC forms less stoichiometric products initially. VHMC is subsequently replaced by poorly ordered dolomite later in the reaction after more than 90% of the initial aragonite ooid reactants have been consumed. During this phase of the reaction, cation ordering increases simultaneously with increases in stoichiometry of reaction products. Longer reactions indicate that cation order and dolomite stoichiometry continue to increase with time, but at slower rates than earlier in the reaction. Data show that the rate at which VHMC replaces aragonite ooids, and the overall rate of dolomitization, exhibit strong, non-linear temperature dependence where temperatures below 200°C exhibit significantly slower rates than temperatures above 200°C. The induction period of the reaction also exhibits strong non-linear temperature dependence. Higher temperatures dramatically affect the rate of cation ordering compared to lower temperature reactions. Results from high-temperature experiments suggest that the reaction pathway of dolomitization in nature may be more complicated than a simple dissolution-reprecipitation reaction between a CaCO3 precursor and fully-ordered dolomite