A Critical Look at Dolomitization by Evaporative Fluids

Dolomitization models commonly invoke a replacement reaction between evaporative fluids and a limestone precursor. These models assume that evaporative fluids are powerful dolomitizing agents, particularly when Ca-sulfate precipitation drives up the Mg/Ca ratio of the fluids. The principle line of evidence offered in many published studies is the co-occurrence of evaporite minerals and dolomite. However, recent studies from a wide variety of ancient shallow water, peri-tidal marine dolomites have turned up little petrological data in support of dolomitization by evaporative fluids. Petrographic relationships in many of these examples show that dolomite formed prior to the evaporite minerals. Mineralogical data also exhibit values and patterns that are difficult to explain using an evaporative fluid model. Stable isotope data are particularly unconvincing because the δ18O values of many of these dolomites are not significantly different than those expected for dolomites formed in marine fluids. In this paper, new and published petrographical, geochemical, and mineralogical data are presented from a global survey of peritidal dolomites. Collectively, these data suggest that these dolomites likely formed in normal marine fluids. The findings further imply that: (i) evaporative fluids are not as important to dolomitization as commonly assumed, (ii) dolomites are commonly recrystallized and therefore contain an unreliable record of the initial dolomitizing fluids, and/or (iii) the evaporative fluid dolomite model has been applied more broadly than is warrented by the data.