Dolomite Microstructures and Reaction Mechanisms of Dolomitization: An Integrated TEM, Petrographic, Geochemical and Field Study of Selected Dolomite Bodies

在过去10年左右的时间里，透射电镜研究揭示了天然钙质白云岩内部结构的真实性质，揭示了在一到几千埃尺度上发生的非均质微观结构的多样性。从沉积白云岩的电镜观察可以清楚地看出，“白云岩问题”的根本原因之一是白云岩晶体结构在低温下固有的阳离子有序和相分离存在能量障碍。由于无法达到热力学平衡，导致了许多由组成和/或结构上不同的域组成的妥协大厦，其尺寸只有几百埃。因此，钙质白云岩表现出一系列的结构修饰，并通过电子显微镜通过非均质微观结构（如化学或结构调制）进行表征。如果我们要将矿物白云岩的原子尺度特征与岩石白云岩的地层体的微观和宏观特征结合起来，那么结合现场、岩石地球化学、扫描电镜和透射电镜观察的案例研究是必不可少的。我们拟对3个白云化时代、规模、样式不同的白云岩体进行综合对比研究。我们选择研究的三个白云岩体是意大利北部的三叠纪拉特马尔石灰岩，巴哈马和佛罗里达的全新世潮滩碳酸盐，以及阿巴拉契亚中部的寒武纪-奥陶纪台地碳酸盐。我们之所以选择这些含白云石的碳酸盐岩，是因为(1)我们对它们都有丰富的研究经验；(2)它们呈现出一系列白云化特征，但它们有一个最重要的共同特征，即它们在露头和显微镜尺度上都保留了白云石-碳酸钙相边界；(3)记录了从低温潮上条件（全新世-三叠纪白云岩）到浅埋-深埋条件（寒武系-奥陶系白云岩）到200℃热液条件（三叠纪白云岩）的多种成岩环境。通过对不同白云化环境下微观和宏观性质差异显著的白云岩体进行对比研究，我们希望通过TEM和AEM分析揭示白云岩在原子尺度上的反应机理和微观结构的诊断差异，以及晶内地球化学的诊断差异。特别是，我们将尝试：1)将白云岩的微观结构和精细化学变化与白云化环境联系起来；(2)将白云石微观结构与晶体生长成被动空洞、保留织物和破坏织物的替换以及退火再结晶等反应机制联系起来；(3)结合1和2，将反应机理与白云化环境联系起来。白云岩的成因一直是地球化学研究中未解决的重大问题之一。解决这一“白云岩问题”具有相当大的经济意义，因为许多沉积白云岩体是天然气、石油和热液矿床的宿主。

9316969 Hardie In the last 10 or so years that TEM studies have unveiled the true nature of the internal architecture of natural calcian dolomites, revealing diversity of heterogeneous microstructures occurring on a scale of one to a few thousand Angstroms. It is clear from electron microscopy on sedimentary dolomites that one of the root causes of the "dolomite problem" lies in the existence of energy barriers to cation ordering and phase separation inherent in the dolomite crystal structure at low formation temperatures. This inability to reach thermodynamic equilibrium leads to a host of compromise edifices built of compositionally and/or structurally different domains with dimensions of only a few hundred Angstroms. Thus, calcian dolomites display a range of structural modifications and are characterized with the electron microscope by heterogeneous microstructures, such as chemical or structural modulations. Case studies integrating field, petrographic geochemical, SEM, and TEM observations are essential if we are to make progress in bringing together the atomic-scale features of the mineral dolomite with the microscopic and macroscopic features of stratigraphic bodies of the rock dolomite. We propose to carry out such an integrated study that compares three dolomite bodies different from each other in age, scale, and style of dolomitization. The three dolomite bodies wee have chosen for study are the Triassic Latemar Limestone of northern Italy, the Holocene tidal flat carbonates of the Bahamas and Florida, and the Cambro-Ordovician platform carbonates of the central Appalachians. We have chosen these particular dolomite-bearing carbonates because (1) we have extensive experience with all of them, (2) they present a spectrum of dolomitization features yet they have in common a most important characteristic, namely, they all preserve dolomite-CaCO3 phase boundaries at both the outcrop and the microscope scales, (3)they record a variety of do lomitization environments that range from low-temperature supratidal conditions (Holocene and Triassic dolomites) through shallow to deep burial conditions (Cambro- Ordovician dolomites) to 200C hydrothermal conditions (Triassic dolomites). By making the study a comparative one carried out on dolomite bodies with significantly different microscopic and macroscopic properties related to their different environments of dolomitization, we hope to uncover diagnostic differences in the reaction mechanisms and in the microstructures of the dolomites at the atomic scale and in the intracrystalline geochemistry as determined by TEM and AEM analyses. In particular, we will attempt to: 1) relate dolomite microstructures and fine-scale chemical variations to the environment of dolomitization; (2) relate dolomite microstructures to reaction mechanisms such as crystal growth into passive voids, fabric-retentive and fabric-destructive replacement, and annealling recrystallization; (3) combine 1 and 2 to link reaction mechanism to dolomitization environment. The origin of dolomite remains one of the major unresolved problems in geochemistry. Resolution of this "dolomite problem" has considerable economic significance because many sedimentary dolomite bodies are hosts to gas, oil, and hydrothermal ore deposits.