Abstract A geochemical study of ancient (Permian to Neogene) dolomites that were formed under widely different salinities revealed certain distinct characteristics: (1) marine (non-evaporitic) dolomites vary in their calcium content from stoichiometric to calcian (57 mole percent, and their sodium content is 150-350 ppm; (2) dolomites in association with gypsum cover the same range of stoichiometry as the marine dolomites, but their sodium content reaches much higher values (as high as approximately 2,700 ppm). Furthermore, a negative correlation is present between excess calcium and sodium; and (3) dolomites in association with halite have an almost ideal stoichiometric composition and a sodium content overlapping the lower range of marine dolomites (150-270 ppm). To explain these characteristics, a model is presented that states that ionic ratios in dolomites are related to the respective ratios in mother solutions. Those solutions are considered to have evolved through a combination of surface evaporation and modification in pore water, mainly through dolomitization. Accordingly, our model indicates that the compositional variations in the "evaporitic" dolomites correlate well with the expected Ca/Mg and Na/Ca ratios in modified evaporitic sea water. The "marine" dolomites, however, formed under varying degrees of modification, the two extremes being non-modified (open-system) solutions and fully modified (closed-system) solutions. In that context, the deviations from stoichiometric composition of the marine dolomites reflect different degrees of modification. Thus, advanced modification is expressed in higher Ca/Mg in solution and more calcian dolomite. The distribution of δ 18 O in the studied dolomites is consistent with anticipated variations, where δ 18 O increases at the first stage of evaporation and tends to decrease beyond a certain high salinity. The conclusions of the present study relate only to the main trends of changes that occur in dolomites forming under a wide range of salinity and ignore the fine details. They indicate, however, that the geochemical characteristics of ancient dolomites reflect the imprints of their depositional and diagenetic environments.

Abstract Authigenic calcian dolomite is a common but rarely abundant (≤20%) component of Neogene deep-water (475-2,767 m) carbonates peripheral to the Florida-Bahamas Platform. Dolomite concentrations as high as 57% of the carbonate fraction occur in the Miocene of west Florida, however, and as much as 86% dolomite has been found in a hardground from the Bahamas. Dolomite occurs principally as pore-filling euhedral rhombs (5–20 μm) that precipitated in situ, as well as by replacement of calcite at disconformities. Stable isotope ratios (oxygen and carbon) suggest dolomite precipitation from deep, cold, seawater-derived fluids, and trace-element (Sr) concentrations suggest strontium-rich aragonitic(?) precursors. Preliminary 87 Sr/ 86 Sr data suggest substantial lag times for dolomite precipitation and contamination by "old" strontium. Because of the high diagenetic potential of periplatform carbonates, Bahamian deep-water dolomites appear to be a natural consequence of shallow subsurface (<60 m) burial diagenesis. In contrast, carbonate ramp slope sediments from west Florida, which have a much lower initial diagenetic potential, are punctuated by discrete concentrations of authigenic dolomite, which may represent paleoceanographically controlled dolomite "events." Overall, our data indicate that deep-water dolomite has had little difficulty in precipitating from normal marine-derived fluids.