Regional illitization in bentonite beds from the East Slovak Basin based on isotopic characteristics (K-Ar, δ18O and δD) of illite-type nanoparticles
Regional illitization in bentonite beds from the East Slovak Basin based on isotopic characteristics (K-Ar, δ18O and δD) of illite-type nanoparticles
复制标题
基于伊利石型纳米粒子同位素特征（K-Ar、δ18O 和 δD）的东斯洛伐克盆地膨润土层区域伊利石化
作者：
N. Clauer;M. Honty;A. Fallick;V. Šucha
摘要 研究了从东斯洛伐克盆地核膨润土层的混合层伊利石-蒙脱石中提取的纳米伊利石型晶体，以研究其成核和生长的时间、持续时间和物理/化学条件。该研究包括将结果应用于评估区域伊利化程度。靠近含盐地层，纳米粒子经历了更强烈的伊利特化，持续时间相同，但母体盐水的氧和氢同位素特征发生了变化。膨润土层的地层年龄与自生晶体的 K-Ar 年龄的比较表明，伊利石化始于中中新世塞拉瓦阶（下萨尔马提亚）沉降，持续 17 至 11.5 Ma。伊利化的持续时间被划分在 14.5 Ma 和 9.0 Ma 之间，直到最近在 0 Ma 进一步发生，具体取决于宿主床的地理位置。计算的沉降速率从小于 300 m/Ma 到大于 500 m/Ma，热梯度范围从 ~60°C/km 到小于 50°C/km。纳米伊利石的 K-Ar 年龄表明要么是短期伊利石化（沉积后不久开始），要么是长期伊利石化（沉积后明显较晚才开始）。伊利石δ18O随着晶体尺寸的增加变化不大，而δD变化显着。在大多数样品中，通过不同标准确定的结晶温度在晶体生长过程中没有显着变化，但相互作用的流体的氧和氢同位素组成根据样品位置、直接环境、伊利化的时间和持续时间而变化。膨润土层中伊利石的稳定同位素组成具有降低的流体-岩石比率，可以通过识别局部过程而不是大范围的过程来深入了解沉积盆地的流体-温度状况。伊利石化作用是间歇性的，沿着不同的结晶路径，持续时间各异，并且在低孔隙度、低渗透率膨润土床中具有不同程度的作用。根据当地条件，即使在明显同质的床层中，伊利化也可能有所不同。这与具有大规模流体连通性的多孔含水层的情况形成对比
Abstract Nanometre-sized illite-type crystals extracted from mixed-layer illite-smectite of cored bentonite beds from the East Slovak Basin were studied to investigate timing, duration and physical/chemical conditions of their nucleation and growth. The study includes application of the results to an evaluation of the regional extent of illitization. Close to salt-bearing formations, the nanometric particles underwent a more intense illitization, of identical duration, but with changing oxygen and hydrogen isotope characteristics of the parental brines. A comparison of the stratigraphic ages of the bentonite beds with the K-Ar ages of the authigenic crystals shows that illitization started during the Middle-Miocene Serravalian (Lower Sarmatian) subsidence that lasted from 17 to 11.5 Ma. Duration of illitization was bracketed between 14.5 and 9.0 Ma, with further episodes until recently at 0 Ma, depending on the geographic location of the host beds. The calculated subsidence rate varies from less than 300 m/Ma to more than 500 m/Ma and the thermal gradient ranges from ~60°C/km to less than 50°C/km. The K-Ar ages of the nanometric illite point to either short-duration illitization when the onset was soon after sedimentation, or longduration when the onset was significantly later after sedimentation. The illite δ18O varies little with increasing crystal size, whereas the δD changes significantly. In most samples the crystallization temperature determined by different criteria did not change significantly during crystal growth, but the oxygen and hydrogen isotopic compositions of the interacting fluids varied depending on sample location, immediate environment, timing and duration of illitization. The stable isotope composition of illite in bentonite beds with reduced fluid-rock ratios gives insight into the fluid-temperature regime of sedimentary basins through identifying local processes rather than wide-scale ones. Illitization is episodic with variable duration along different crystallization pathways, and to various extents in low-porosity, low-permeability bentonite beds. Depending on local conditions, illitization may vary even within apparently homogeneous beds. This contrasts with the situation for porous aquifers having large-scale fluid connectivity