Transport-controlled hydrothermal replacement of calcite by Mg-carbonates
Transport-controlled hydrothermal replacement of calcite by Mg-carbonates
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DOI：
10.1130/g36934.1
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发表时间：
2015-09-01
期刊：
影响因子：
5.8
通讯作者：
Putlitz, Benita
中科院分区：
文献类型：
--
作者：
Jonas, Laura;Mueller, Thomas;Putlitz, Benita
白云石化是最重要的成岩作用之一，但白云石化的反应速率和时间尺度一直存在争议。我们进行了实验，其中在200摄氏度下，单一方解石晶体与富镁流体的反应导致形成由菱镁矿和中间时间的富钙白云石组成的分区反应边缘。从详细的文件的微观结构和化学成分的反应边缘的演变，我们推断出一个动力学模型方解石镁碳酸盐置换。化学梯度的结构形成元素Mg和Ca在反应边缘和边缘厚度的演变强烈表明，通过多孔反应边缘的扩散传输控制的整体反应速率。产物相的组成是动力学控制的，并且记录界面流体的局部组成，而不需要储层流体的过饱和。白云石化过程的反应输运模型假定白云石化速率取决于白云岩沉淀速率，这与我们的实验证据相矛盾。在自然系统中模拟碳酸盐置换需要详细了解控制孔隙流体中元素物理化学输运性质的微观结构的演变。
Dolomitization is one of the most important diagenetic processes, but the reaction rate and time scale of dolomitization remain a topic of controversy. We conducted experiments in which the reaction of single calcite crystals with a Mg-rich fluid at 200 degrees C leads to the formation of a zoned reaction rim consisting of magnesite and, for intermediate times, Ca-rich dolomite. From detailed documentation of the evolution of the microtexture and chemical composition of the reaction rim, we infer a kinetic model for calcite replacement by Mg-carbonates. The chemical gradient for the structure-forming elements Mg and Ca in the reaction rim and the evolution of the rim thickness strongly indicate that the overall reaction rate is controlled by diffusive transport through the porous reaction rim. The composition of the product phases is kinetically controlled and records the local composition of the interfacial fluid without requiring oversaturation of the reservoir fluid. Reactive transport models on dolomitization processes assume that the rate of dolomitization depends on the rate of dolomite precipitation, which is contradictory to our experimental evidence. Modeling carbonate replacement in natural systems requires detailed knowledge on the evolution of the microstructure controlling the physicochemical transport properties of elements in the pore fluid.