Effect of smectite dehydration on pore water geochemistry in the shallow subduction zone: An experimental approach
Effect of smectite dehydration on pore water geochemistry in the shallow subduction zone: An experimental approach
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DOI：
10.1029/2012gc004212
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发表时间：
2012-10-25
影响因子：
3.5
通讯作者：
Kopf, A. J.
中科院分区：
文献类型：
--
作者：
Huepers, A.;Kopf, A. J.
人们普遍认为，成岩蒙皂石到伊利石的转变会导致收敛边缘处普遍存在的孔隙水淡水化。然而，实验固结测试表明，随着有效应力的增加，蒙脱石也会部分脱水。为了阐明这一过程，我们对富含粉质蒙皂石的粘土岩进行了三项热液固结试验，应力 (P) 高达 70 MPa，温度 (T) 分别为 20 ℃、60 ℃ 和 100 ℃。分析了试验过程中排出的流体的主量元素和微量元素，以评估随着 PT 条件增加的脱水和流体-岩石相互作用。结果表明，当有效应力超过 1.3 MPa 时，液体开始清新。在实验过程中，蒙皂石层间含水量从 27 wt-% 降至约 20 wt-%，相当于层间塌陷从 18.5 埃降至约 15.4 埃。释放的夹层H(2)0占固结沉积物释放的总流体体积的17%。主要和挥发性元素地球化学受温度和蒙皂石层间塌陷的影响。层间塌陷的特征是吸收钾和钙，同时从蒙脱石中释放硼。将实验结果应用于巴巴多斯增生边缘表明，应力依赖性蒙皂石脱水是一种重要的流体来源机制，并且是大量流体清新的原因。模拟的蒙脱石脱水表明，蒙脱石层间含水量从表面的 27 wt-% 下降到 2 公里深度处的大约 22 wt-%。这导致氯度值低至 460 mM。
The diagenetic smectite to illite transition is widely accepted to cause ubiquitous pore water freshening at convergent margins. However, experimental consolidation tests show that smectite also dehydrates partially with increasing effective stress. To shed light on this process three hydrothermal consolidation tests on a silty smectite-rich claystone were conducted with stresses (P) up to 70 MPa and at constant temperatures (T) of 20 degrees C, 60 degrees C and 100 degrees C. Fluids expelled during the tests were analyzed for major and trace elements to evaluate dehydration and fluid-rock interaction with increasing PT conditions. The results document that fluid freshening starts when the effective stress exceeds 1.3 MPa. The smectite interlayer water content decreases from 27 wt-% to similar to 20 wt-% during the experiments, which is equivalent to an interlayer collapse from 18.5 to similar to 15.4 angstrom. The released interlayer H(2)0 accounts for up to 17% of the total fluid volume released from the consolidating sediment. Major and volatile element geochemistry is influenced by temperature and smectite interlayer collapse. The interlayer collapse is characterized by K and Ca uptake while is B released from smectite. Application of experimental results to the Barbados accretionary margin shows that stress dependent smectite dehydration is an important fluid source mechanism and accounts for substantial fluid freshening. Modeled smectite dehydration suggests a decrease of smectite interlayer water content from 27 wt-% at the surface down to similar to 22 wt-% at 2 km depth. This leads to chlorinity values as low as 460 mM.