Estimating the fluid composition after dolomitization using mass balance equation: comparison of examples from Spain, Canada and France

Abstract

Dolomitization (i.e. replacement of CaCO3 by CaMg(CO3)2) is a major mineralogical replacement process that affects limestones in numerous carbonate platforms, basins and fold-and-thrust belts worldwide. This phenomenon makes an important part of the carbon cycle, and large-scale dolomite geobodies that develop in nature are prime targets for greenhouse gas storage, or are related to ore deposit bearing rare metals, oil and gas reservoirs and geothermy. Yet, the conditions favoring dolomitization remain debated, specifically the major and trace element composition of the reactive aqueous fluid. In this contribution, we quantify the mass transfer between the original calcite and the newly formed dolomite in various natural cases of dolomitization, by coupling EPMA and LA-ICP-MS measurements, following a mass balance approach. This approach also allows to estimate the theoretical composition of an aqueous fluid whose element content would be provided by the reaction (i.e., in equilibrium with dolomite), as well as the partition coefficient for most elements involved in the reaction. This approach was tested using three existing datasets obtained from natural dolomite and original limestone in both Jurassic outcrops of the Layens anticline in the Pyrenees (France), and two from the Middle Devonian Pine Point Formation from the Presqu'ile barrier (Canada). These are complemented with new data acquired from Cretaceous limestones of the Benassal Formation in the Maestrat Basin (Spain). In these areas, dolomitization occurred at different T conditions (~50 to ~300 °C), from different fluid sources (seawater, basinal brines), and in different geodynamic settings. Yet, for all the studied examples, the dolomitization reactions result in similar solid volume variations (-14 to -10 vol%), the fluid in equilibrium with the dolomite have comparable trace element concentrations, and the partition coefficients calculated for all trace elements are consistent with each other. In addition to providing information with regards to the composition of the solid and fluid phases involved in the reaction, these results also suggest similar mechanisms of dolomitization in distinctly different geological contexts.