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，代表着地球过去极其宝贵的深层记录。由这些矿物组成的地下储藏库还可能蕴藏着对该国经济活动和活力至关重要的大量资源。为了有效和高效地开发这些经济资源，并将这些矿物用作可行的古环境记录，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指标来约束过去的环境条件。这项基础研究使用实验室实验来限制氧同位素组成--白云石和方解石中的重要地球化学指标--如何受到一种经常被忽视的矿物学参数--阳离子有序性--的影响。这项研究的结果将使地球科学家能够提高他们对地球表面白云石形成的自然环境的了解，并有可能提高白云石和其他碳酸盐矿物作为地球历史的地球化学档案的保真度。白云石作为古环境和成岩作用的替代物，由于公布的氧同位素水矿物分馏值的不确定性而受到限制。这里假设，这种不确定性在很大程度上可以归因于不同类型的钙镁碳酸盐矿物(例如，极高镁方解石、有序的白云石和有序的白云石)之间的水-矿物分馏差异，这些矿物在文献中统称为“白云石”。这一假说来源于岩石记录和实验室的经验数据，这些数据表明，这些不同的钙镁碳酸盐矿物在白云石化过程中以不同的晶体生长机制依次形成，导致了截然不同的镁钙组成、镁钙阳离子有序性、晶体微结构和潜在的氧同位素组成。为了验证这一假设，将使用控制良好的高温白云石化实验，通过从非常高的镁方解石到无序白云石再到有序白云石的顺序矿物转变来测量流体和矿物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值从每个前驱物相继承的程度。将测量每个白云岩相的块状同位素组成，并用来研究阳离子有序性是否影响块状同位素酸消化分馏因子，这是另一个发表的估计有很大差异的值。本研究旨在通过受限的实验室实验，了解白云石中阳离子有序性与δ-18O的关系。该项目开发的同位素框架将使白云石δ18O和47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稳定和块状同位素替代物在天然白云岩沉积学研究中的使用，并将允许更好地解释白云岩作用的成岩条件。结果也可能允许使用浅海白云岩作为更强大的古气候档案。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势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.