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

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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.