NSF Award
2120193
Magnesium-bearing carbonate minerals, such as calcite and dolomite, are ubiquitous in marine settings and represent an extremely valuable, deep-time record of Earth's past. Subsurface reservoirs comprised of these minerals can also host vast resources that are key to the economic activity and vitality of the country. To effectively and efficiently develop these economic resources, and to use these minerals as viable paleoenvironmental records, it is vital to understand their geologic history. To do this geoscientists often use geochemical proxies to constrain past environmental conditions. This fundamental research uses laboratory experiments to constrain how oxygen isotope compositions, an important geochemical proxy in dolomite and calcite, are impacted by an often overlooked mineralogical parameter called cation ordering. The results of this study will allow geoscientists to both improve their understanding of the natural environments in which dolomite forms on Earth's surface and potentially increase the fidelity of dolomite and other carbonate minerals as a geochemical archive of Earth history. <br/><br/>The use of dolomite as a paleoenvironmental and diagenetic proxy is inhibited by uncertainty in published oxygen isotope water-mineral fractionation values. It is hypothesized here that much of this uncertainty can be attributed to water-mineral fractionation differences between different types of Ca-Mg-carbonate minerals (e.g., very high-Mg calcite, poorly-ordered dolomite and well-ordered dolomite) that have been collectively called ‘dolomite’ in the literature. This hypothesis is rooted in empirical data from the rock record and the laboratory that indicate that these various Ca-Mg-carbonate minerals form in sequence during dolomitization by different crystal growth mechanisms, which leads to vastly different Mg-Ca compositions, Mg-Ca cation ordering, crystalline microstructures, and potentially oxygen isotopic compositions. To test this hypothesis, well-controlled, high-temperature dolomitization experiments will be used where fluid and mineral δ18O will be measured through the sequential mineral transitions from very high-Mg calcite to poorly-ordered dolomite to well-ordered dolomite. How isotopic fractionation varies between these Ca-Mg-carbonate minerals, and the degree to which δ18O values are inherited from each precursor phase, will be assessed. The clumped isotopic composition of each dolomite phase will be measured and used to investigate whether cation ordering affects the clumped isotope acid digestion fractionation factor, another value for which published estimates differ greatly. This research aims to understand the relationship between cation ordering and δ18O in dolomite through well-constrained laboratory experiments. The isotopic framework developed in this project will enable more precise application of dolomite δ18O and 47 measurements in a wide variety of studies. These results will have the potential to enhance the use of stable and clumped isotope proxies in sedimentological studies of natural dolomites, and will allow for better interpretations of the diagenetic conditions of dolomitization. The results may also permit the use of shallow marine dolomites as more robust paleoclimate archives.<br/><br/>This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.
