Rock Composition, Dolomite Stoichiometry, and Rock/Water Reactions in Dolomitic Carbonate Rocks

Dolomitic carbonate rock units from a variety of Phanerozoic localities and depositional environments throughout North America were evaluated with respect to their dolomite content and dolomite composition by X-ray diffraction analysis. Dolomite compositions range from 48 to 57 mole% $$CaCO_{3}$$ with well-defined modes at 51% (nearly stoichiometric dolomite) and 55% (distinctly calcian dolomite) $$CaCO_{3}$$. Values of percentage of dolomite also exhibit a pronounced, albeit less well-defined, bimodal distribution with modes at 97% (dolostones) and 20% (dolomitic limestones) dolomite. Similar distributions characterize most other published data on Phanerozoic dolomitic carbonates. These data suggest that two separate processes may lead to two distinct populations of sedimentary dolomite: (1) dolomitic limestones originate in diagenetically closed systems during magnesian calcite dissolution-calcite and dolomite precipitation reactions, and (2) dolostones originate in diagenetically open systems during the dissolution of calcium carbonate precursors in the presence of allochthonous magnesium ions. In addition, these data document a genetic relationship between amount of dolomite and dolomite composition. Particularly in Paleozoic sequences, most dolostones consist of stoichiometric dolomite whereas most dolomitic limestones contain rhombs of calcian dolomite. This relationship suggests: (1) at the time of their formation, many ancient marine micrites contained appreciable amounts of magnesian calcite, and therefore an internal supply of magnesium for dolomite formation, (2) although calcian dolomite is metastable, in closed diagenetic systems it is non-reactive, and (3) the presence of calcian dolomite in limestones may serve as a useful criterion in the evaluation of rock/water ratios during the diagenetic stabilization of carbonate sequences.