CALCULATION OF SIMULTANEOUS ISOTOPIC AND TRACE-ELEMENT VARIATIONS DURING WATER-ROCK INTERACTION WITH APPLICATIONS TO CARBONATE DIAGENESIS
CALCULATION OF SIMULTANEOUS ISOTOPIC AND TRACE-ELEMENT VARIATIONS DURING WATER-ROCK INTERACTION WITH APPLICATIONS TO CARBONATE DIAGENESIS
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DOI：
10.1016/0016-7037(90)90128-8
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发表时间：
1990-11-01
影响因子：
5
通讯作者：
HANSON, GN
中科院分区：
文献类型：
--
作者：
BANNER, JL;HANSON, GN
方程的开发用于计算的微量元素浓度和放射性和稳定同位素组成的同时发生在水-岩相互作用的变化。该方程一般用于模拟多孔介质中的化学和同位素变化，并在此应用于碳酸盐岩和沉积物的成岩作用。控制成岩碳酸盐岩最终地球化学组成的变量包括原始沉积物和流体的组成、水：岩比、分馏因子、分配系数、开放与封闭系统行为以及孔隙度。由于成岩流体中氧和碳浓度的极大差异，碳酸盐矿物与流体δ 18 O值平衡时的水岩比（<10）比与流体δ 13 C值平衡时的水岩比（10 3）低3个数量级。87 Sr-86 Sr比值受到不同程度的影响。为了在淡水成岩作用期间重置海相灰岩的87 Sr 86 Sr值，需要与平衡δ 13 C值计算的水岩比相似的水岩比，而富含Sr-Ca的卤水可以在低水岩比下重置灰岩的87 Sr 86 Sr值，与平衡δ 18 O值计算的水岩比相似。水：成岩作用中岩石比值大于10 3时，碳酸盐岩沉积物的稀土元素（REE）配分模式和Nd同位素组成才会受到影响。不同同位素体系对水-岩相互作用的响应存在较大的相对差异，这些差异转化为同位素和微量元素协变图的特征趋势，可用于区分（1）不同的水-岩相互作用模型和（2）不同的过程，如水-岩相互作用、流体混合和矿物端元混合。一个例子是使用O和Sr同位素的同时变化来模拟石灰岩的淡水成岩作用。淡水-石灰岩相互作用途径与Sr-Ca交换分配系数（K Sr-Ca D）值在0.01至0.15的数量级范围内无关，因为平衡两种同位素系统所需的水：岩石比存在很大差异。同一体系中碳、氧同位素的变化可用于区分水岩相互作用、流体混合和端元方解石混合。这些建模方法适用于密西西比河的伯灵顿Keokuk形成的区域广泛的花岗岩。与其早期白云岩（白云岩I）相比，Burlington-Keokuk组第二代交代白云岩（II）具有较高的87Sr86Sr比值，较低的δ 18O值和Sr浓度，相似的δ 13C值、Nd同位素组成和稀土元素模式。多阶段模型计算可以通过白云石I的重结晶来解释白云石II数据，其中白云石II的δ 18 O值记录了样品的水-岩相互作用历史的相对较小和较晚的部分，而白云石中的Sr保留了相同历史的较早和较大的部分。重结晶过程的影响，层外卤水在一个相对开放的系统相对于Sr，而C和稀土元素不受影响。
Equations are developed for the calculation of the simultaneous variations in trace element concentrations and radiogenic and stable isotopic compositions that occur during water-rock interaction. The equations are of general use for modeling chemical and isotopic variations in porous media and are applied here to the diagenesis of carbonate rocks and sediments. The variables which control the ultimate geochemical composition of diagenetic carbonates include the composition of the original sediment and fluid, water: rock ratio, fractionation factors, distribution coefficients, open vs. closed system behavior, and porosity. Owing to the extreme differences in the concentrations of oxygen and carbon in diagenetic fluids, carbonate minerals equilibrate with fluid δ 18 O values at three orders of magnitude lower water: rock ratios (< 10) than the water: rock ratios at which they equilibrate with fluid δ 13 C values (10 3). 87 Sr 86 Sr ratios are affected at variable rates. In order to reset the 87 Sr 86 Sr value of a marine limestone during freshwater diagenesis, water: rock ratios similar to those calculated for equilibration of δ 13 C values are required, while Sr-Ca-rich brines can reset the 87 Sr 86 Sr value of a limestone at low water: rock ratios, similar to those calculated for equilibration of δ 18 O values. Water: rock ratios exceeding 10 3 are required to affect the rare earth element (REE) patterns and Nd isotopic compositions of carbonate sediments during diagenesis. These large, relative differences in the response of different isotopic systems to water-rock interaction translate into characteristic trends on isotope and trace element covariation diagrams that can be used to distinguish between (1) different models for water-rock interaction and (2) different processes such as water-rock interaction, mixing of fluids, and mixing of mineral endmembers. An example is the use of simultaneous variations of O and Sr isotopes in modeling the freshwater diagenesis of limestones. Fresh-water-limestone interaction pathways are independent of Sr-Ca exchange distribution coefficient (K Sr-Ca D) values over an order of magnitude range of 0.01 to 0.15, because of the large differences in the water: rock ratios necessary to equilibrate the two isotopic systems. Carbon vs. oxygen isotopic variations in the same system can be used to distinguish between water-rock interaction, mixing of fluids, and mixing of end-member calcites. These modeling approaches are applied to the regionally extensive dolomites of the Mississippian Burlington-Keokuk Formation. Compared to its early dolomite precursor (dolomite I), second-generation replacement dolomite (II) in the Burlington-Keokuk Formation has higher 87 Sr 86 Sr ratios, lower δ 18 O values and Sr concentrations, and similar δ 13 C values, Nd isotopic compositions, and REE patterns. A multistage model calculation can account for the dolomite II data via recrystallization of dolomite I, whereby the δ 18 O values of dolomite II record a relatively minor and late portion of the water-rock interaction history of the samples while Sr in the dolomites preserves an earlier and larger segment of the same history. The recrystallization process was effected by extraformational brines in a relatively open system with respect to Sr, while C and the REE were unaffected.