Collaborative Research: Experimentally Evaluating the Relationship between Cation Ordering and Oxygen and Clumped Isotope Fractionation in Dolomite

含镁碳酸盐矿物，如方解石和白云石，在海洋环境中无处不在，代表了地球过去极有价值的深时间记录。由这些矿物组成的地下储层也可以容纳大量资源，这些资源对国家的经济活动和活力至关重要。为了有效地开发这些经济资源，并将这些矿物作为可行的古环境记录，了解它们的地质历史是至关重要的。为了做到这一点，地球科学家经常使用地球化学代理来限制过去的环境条件。这项基础研究使用实验室实验来限制氧同位素组成（白云岩和方解石的重要地球化学指标）如何受到一个经常被忽视的矿物学参数——阳离子排序的影响。这项研究的结果将使地球科学家提高他们对地球表面白云岩形成的自然环境的理解，并有可能增加白云岩和其他碳酸盐矿物作为地球历史地球化学档案的保真度。已公布的氧同位素水矿物分馏值的不确定性抑制了白云岩作为古环境和成岩代用物的作用。这里假设，这种不确定性很大程度上可以归因于不同类型的ca - mg -碳酸盐矿物（例如，高镁方解石，无序白云石和有序白云石）之间的水矿物分选差异，这些矿物在文献中被统称为“白云石”。这一假设是基于岩石记录和实验室的经验数据，这些数据表明，在白云化过程中，这些不同的ca - mg -碳酸盐岩矿物通过不同的晶体生长机制按顺序形成，这导致了Mg-Ca组成、Mg-Ca阳离子顺序、晶体微观结构和潜在的氧同位素组成的巨大差异。为了验证这一假设，将采用控制良好的高温白云化实验，通过从高镁方解石到无序白云岩再到有序白云岩的顺序矿物转变，测量流体和矿物的δ18O。将评估这些钙镁碳酸盐矿物之间的同位素分馏如何变化，以及δ18O值从每个前驱相继承的程度。每个白云岩相的团块同位素组成将被测量，并用于研究阳离子排序是否影响团块同位素酸消解分馏因子，这是另一个与已发表的估计差异很大的值。本研究旨在通过约束良好的实验室实验了解白云岩中阳离子有序与δ18O的关系。该项目开发的同位素框架将使白云岩δ18O和47的测量在各种研究中得到更精确的应用。这些结果将有可能加强在天然白云岩的沉积学研究中使用稳定和块状同位素代用物，并将允许更好地解释白云石化的成岩条件。这些结果也允许使用浅海白云岩作为更可靠的古气候档案。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

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. 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.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.