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

白云石微观结构和白云石化反应机制：对选定白云石体进行综合 TEM、岩石学、地球化学和现场研究

• 批准号: 9316969
• 负责人: Lawrence Hardie
• 金额: $ 15.64万
• 依托单位: Johns Hopkins University
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 1994
• 资助国家: 美国
• 起止时间: 1994-02-15 至 1996-10-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=9316969&HistoricalAwards=false
• 关键词: Dolomite; Microstructures; Reaction; Mechanisms; Dolomitization

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.

小行星9316969 在过去10年左右的时间里，TEM研究揭示了天然钙钛矿内部结构的真实性质，揭示了在一到几千埃的尺度上发生的异质微结构的多样性。 从沉积白云岩的电子显微镜可以清楚地看出，“白云石问题”的根本原因之一在于，在低地层温度下，白云石晶体结构中固有的阳离子有序和相分离存在能量障碍。 这种无法达到热力学平衡的情况导致了许多由组成和/或结构不同的域构建的折衷大厦，这些域的尺寸只有几百埃。 因此，钙白云石显示出一系列结构修饰，并且用电子显微镜表征为异质微观结构，例如化学或结构调制。 如果我们要在将矿物白云石的原子尺度特征与岩石白云石的地层体的微观和宏观特征结合起来方面取得进展，那么整合实地、岩相地球化学、SEM和TEM观测的案例研究是必不可少的。 我们建议进行这样一个综合研究，比较三个白云岩机构彼此不同的时代，规模和风格的白云岩化。 我们选择的三个白云岩体是北方意大利的三叠纪晚期石灰岩、巴哈马和佛罗里达的全新世潮坪碳酸盐岩以及阿巴拉契亚山脉中部的寒武-奥陶纪台地碳酸盐岩。 我们之所以选择这些特殊的含碳酸盐岩，是因为（1）我们对所有这些碳酸盐岩都有丰富的经验，（2）它们呈现出一系列的碳酸盐化特征，但它们都有一个最重要的共同特征，即它们都在露头和显微镜尺度上保留了碳酸盐岩-CaCO 3相界，（3）它们记录了从低温潮上环境（全新世和三叠纪灰岩）到浅埋-深埋环境（寒武-奥陶纪灰岩）到200 ℃热液环境（三叠纪灰岩）的各种各样的灰岩化环境。 通过对白云岩体进行比较研究，这些白云岩体具有与其不同的白云石化环境相关的显着不同的微观和宏观性质，我们希望在原子尺度上揭示白云岩的反应机制和微观结构以及通过TEM和AEM分析确定的晶内地球化学的诊断差异。 特别是，我们将尝试：（1）将白云石显微结构和精细尺度化学变化与硅化环境联系起来;（2）将白云石显微结构与反应机制联系起来，如晶体生长到被动空隙中，纤维保持和纤维破坏置换，以及退火重结晶;（3）将联合收割机1和2结合起来，将反应机制与硅化环境联系起来。 白云岩的成因一直是地球化学中尚未解决的主要问题之一。 解决这个“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。