Rock Composition, Dolomite Stoichiometry, and Rock/Water Reactions in Dolomitic Carbonate Rocks
Rock Composition, Dolomite Stoichiometry, and Rock/Water Reactions in Dolomitic Carbonate Rocks
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白云质碳酸盐岩中的岩石成分、白云石化学计量和岩石/水反应
DOI：
10.1086/628901
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发表时间：
1984
期刊：
影响因子：
--
通讯作者：
D. Peacor
中科院分区：
文献类型：
--
作者：
C. Sperber;B. Wilkinson;D. Peacor
通过X射线衍射分析，评价了来自北美各种中生代地区和沉积环境的白云质碳酸盐岩单元的白云石含量和白云石成分。白云石的组成范围为48 - 57摩尔%，具有明确的模式，约为51%（近化学计量白云石）和55%（明显钙质白云石）。白云岩的百分比值也表现出明显的双峰分布，尽管定义不太明确，但模式为97%（白云岩）和20%（白云质石灰岩）白云岩。类似的分布特征大多数其他已发表的数据上古生代的英质碳酸盐。这些数据表明，两个独立的过程可能会导致两个不同的群体的沉积白云岩：（1）在成岩封闭系统中的镁方解石溶解方解石和白云石沉淀反应，以及（2）白云岩起源于成岩开放系统中的溶解过程中的碳酸钙前体的外来镁离子的存在。此外，这些数据还证明了白云岩含量与白云岩成分之间的成因关系。特别是在古生代层序中，大多数白云岩由化学计量的白云岩组成，而大多数灰钙质石灰岩则含有钙质白云岩的菱形。这种关系表明：（1）在形成时，许多古代海洋泥晶岩含有大量的含镁方解石，因此为白云石的形成提供了内部镁供应，（2）尽管钙白云石是亚稳定的，但在封闭的成岩系统中它是非反应性的，（3）石灰岩中钙质白云岩的存在可作为评价岩石性质的一个有用标准。碳酸盐岩序列成岩稳定过程中的含水率。
Dolomitic carbonate rock units from a variety of Phanerozoic localities and depositional environments throughout North America were evaluated with respect to their dolomite content and dolomite composition by X-ray diffraction analysis. Dolomite compositions range from 48 to 57 mole% $$CaCO_{3}$$ with well-defined modes at 51% (nearly stoichiometric dolomite) and 55% (distinctly calcian dolomite) $$CaCO_{3}$$. Values of percentage of dolomite also exhibit a pronounced, albeit less well-defined, bimodal distribution with modes at 97% (dolostones) and 20% (dolomitic limestones) dolomite. Similar distributions characterize most other published data on Phanerozoic dolomitic carbonates. These data suggest that two separate processes may lead to two distinct populations of sedimentary dolomite: (1) dolomitic limestones originate in diagenetically closed systems during magnesian calcite dissolution-calcite and dolomite precipitation reactions, and (2) dolostones originate in diagenetically open systems during the dissolution of calcium carbonate precursors in the presence of allochthonous magnesium ions. In addition, these data document a genetic relationship between amount of dolomite and dolomite composition. Particularly in Paleozoic sequences, most dolostones consist of stoichiometric dolomite whereas most dolomitic limestones contain rhombs of calcian dolomite. This relationship suggests: (1) at the time of their formation, many ancient marine micrites contained appreciable amounts of magnesian calcite, and therefore an internal supply of magnesium for dolomite formation, (2) although calcian dolomite is metastable, in closed diagenetic systems it is non-reactive, and (3) the presence of calcian dolomite in limestones may serve as a useful criterion in the evaluation of rock/water ratios during the diagenetic stabilization of carbonate sequences.