The dolomite problem: Control of precipitation kinetics by temperature and saturation state

Abstract

The mineral dolomite and the uncertainties surrounding its origin have attracted the attention of earth scientists for over a century. The core of the dolomite "problem" is the apparent paradox posed by the paucity of dolomite in modern marine depositional environments versus its relative abundance in the sedimentary rock record. Solving this problem requires knowledge of the conditions under which the mineral forms and the rate of precipitation under those conditions. As a working hypothesis, it is suggested that the precipitation rate of dolomite may be quantified and modeled in a manner similar to other carbonate minerals through application of a rate law that represents the rate as a simple function of saturation index, r = k(Ω - 1)n. This hypothesis is tested in a series of experiments by measuring the steady state rate of dolomite precipitation in a dolomite-seeded flow reactor through analysis of reacted fluid chemistry. By varying temperature from approx 100° to 200°C and saturation index (Ω) from near saturation to ∼100, sufficient data were collected to solve for the reaction order and Arrhenius rate constant (k = A exp {-(∈A/RT)}) of this rate law. The dolomite produced in these experiments was variable in composition but typically a calcium-rich protodolomite, forming syntaxial overgrowths on the seed material. At the highest supersaturations obtained, formation of distinct nucleation centers was observed. These experiments do confirm a strong temperature dependence for the precipitation reaction (activation energy ∈A = 31.9 kcal mol-1) and moderate dependency on saturation index (n = 2.26, log A = 1.05). The experimental findings of this paper suggest that the abundance of dolomite in the sedimentary rock record reflects, at least in part, environmental changes in temperature and seawater chemistry over geologic time.