Date of Award
Doctor of Philosophy
Geological and Environmental Sciences
Dr. Stephen E. Kaczmarek
Dr. Jay M. Gregg
Dr. William B. Harrison III
Dr. Carla M. Koretsky
The origin of massive dolomites is a contentious issue in geology. Because modern dolomite is rare, interpretation of ancient dolomites requires the use of proxy resources, most of which provide ambiguous information. High-temperature dolomite synthesis experiments have shown that dolomite stoichiometry, the degree of cation ordering, and reaction rate are controlled by Mg:Ca, temperature, molarity, and salinity of the formative solution. Thus, it is posited here that dolomite stoichiometry may be a useful proxy for fluid chemistry, as are stable isotope and trace element composition of ancient dolomites. High-frequency sample intervals (cm-scale) from the Cretaceous Upper Glen Rose Formation (Comanche Platform) and the Eocene Uteland Butte Member (Uinta Basin) were analyzed using X-ray diffraction. Systematic trends in dolomite stoichiometry and cation ordering were observed in both case studies. These trends correspond with facies successions that suggest dolomite abundance, stoichiometry, and cation ordering systematically covary with changes in water depth. We posit that changes in water depth may cause localized changes in fluid chemistry resulting in the observed trends in dolomite stoichiometry. Further, it is proposed that dolomitization occurred very early in these locations, with changes in stoichiometry reflecting evolving physicochemical conditions (e.g., Mg:Ca, temperature, salinity, molarity). To test whether dolomite stoichiometry reflects fluid chemistry we compiled a global database (N = 1,690) of dolomites mineralogy from wide range of ages, geographic locations, depositional environments, and platform types. A suite of statistical analyses performed on the global dataset indicate: (1) dolomite stoichiometry and cation ordering broadly increase with geologic age; (2) significant variations in dolomite stoichiometry and cation ordering throughout the Phanerozoic do not correlate with secular variations in global seawater chemistry; (3) dolomites associated with restricted depositional settingsare more stoichiometric than dolomites associated with open marine settings; and (4) dolomites from shallow ramps and epeiric carbonate platforms are generally more stoichiometric than dolomites from open shelves and isolated carbonate platforms. The primary signal observed in the data is that local environmental conditions associated with platform type and depositional setting are the strongest control on dolomite mineralogy. The observation that more stoichiometric dolomites correlate with shallow and restricted depositional environments is consistent with laboratory experiments that show environmental factors, such as higher Mg:Ca, temperature, and salinity of the dolomitizing fluids yield more stoichiometric dolomite. Second, a weaker secondary signal is also observed such that dolomite stoichiometry and cation ordering both increase with geologic age suggesting that progressive recrystallization driven by mineralogical stabilization, likely in the burial setting, is also occurring. Collectively, these data suggest that most of the dolomites examined formed early in a near-surface setting rather than later in the burial realm, but also imply that spatial and temporal variations in stoichiometry and cation ordering reflect the interplay between local dolomitizing conditions and long-term mineralogical stabilization.
Manche, Cameron J., "Dolomite Stoichiometry as a Proxy for Fluid Chemistry" (2021). Dissertations. 3705.