Extreme ductile deformation of fine-grained salt by coupled solution-precipitation creep and microcracking: Microstructural evidence from perennial Zechstein sequence (Neuhof salt mine, Germany)
Extreme ductile deformation of fine-grained salt by coupled solution-precipitation creep and microcracking: Microstructural evidence from perennial Zechstein sequence (Neuhof salt mine, Germany)
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DOI：
10.1016/j.jsg.2012.01.024
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发表时间：
2012-04
影响因子：
3.1
通讯作者：
P. Závada;G. Desbois;A. Schwedt;Ondrej Lexa;J. Urai
中科院分区：
文献类型：
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作者：
P. Závada;G. Desbois;A. Schwedt;Ondrej Lexa;J. Urai
微结构研究表明，德国Neuhof矿Zechstein-Wera盐系中暴露的强烈褶皱细粒盐的韧性流动是由盐岩颗粒的溶液-沉淀(SP)蠕变和微破裂耦合活动所调节的。岩盐集合体的颗粒核含有残留的沉积微构造，具有直的和人字形的流体包裹体轨迹(FITS)，并被反映不同盐析出事件的两个同心地幔所包围。大量的粒内或穿晶微裂纹起源于FITS尖端，并优先沿沉积岩芯和周围再沉淀岩盐的地幔界面扩展。这些微裂纹被解释为格里菲斯张性裂纹。由于晶界滑动(GBS)产生的应力集中，在晶界三重结或晶界台阶处开始产生微裂纹。固体或流体包裹体经常改变微裂纹的扩展过程或裂纹终止于这些包裹体。由于含有微裂纹的内地幔被腐蚀，并被没有微裂纹的外地幔所包围，因此微裂纹被解释为反映集料的瞬时破坏。微破裂在Wera盐的卤化过程中对SP-GBS蠕变的延续和增强起着基本的作用，因为穿晶裂纹(1)在横切配合时提供了额外的流体进入晶界网络，(2)通过分裂颗粒来减小颗粒尺寸。此外，进入晶界的额外流体也是由非保守的晶界迁移提供的，这些迁移前进到与颗粒的承载核心相匹配的位置。所描述的岩盐-卤水系统中晶界扩散流动的微观结构和力学和化学反馈的调整可与通过晶界滑动(GBS)耦合变形机制变形的其他岩石-流体或岩石-熔体集合体相似。
Microstructural study revealed that the ductile flow of intensely folded fine-grained salt exposed in an underground mine (Zechstein-Werra salt sequence, Neuhof mine, Germany) was accommodated by coupled activity of solution-precipitation (SP) creep and microcracking of the halite grains. The grain cores of the halite aggregates contain remnants of sedimentary microstructures with straight and chevron shaped fluid inclusion trails (FITs) and are surrounded by two concentric mantles reflecting different events of salt precipitation. Numerous intra-granular or transgranular microcracks originate at the tips of FITs and propagate preferentially along the interface between sedimentary cores and the surrounding mantle of reprecipitated halite. These microcracks are interpreted as tensional Griffith cracks. Microcracks starting at grain boundary triple junctions or grain boundary ledges form due to stress concentrations generated by grain boundary sliding (GBS). Solid or fluid inclusions frequently alter the course of the propagating microcracks or the cracks terminate at these inclusions. Because the inner mantle containing the microcracks is corroded and is surrounded by microcrack-free outer mantle, microcracking is interpreted to reflect transient failure of the aggregate. Microcracking is argued to play a fundamental role in the continuation and enhancement of the SP–GBS creep during halokinesis of the Werra salt, because the transgranular cracks (1) provide the ingress of additional fluid in the grain boundary network when cross-cutting the FITs and (2) decrease grain size by splitting the grains. More over, the ingress of additional fluids into grain boundaries is also provided by non-conservative grain boundary migration that advanced into FITs bearing cores of grains. Described readjustments of the microstructure and mechanical and chemical feedbacks for the grain boundary diffusion flow in halite-brine system are proposed to be comparable to other rock-fluid or rock-melt aggregates deforming by the grain boundary sliding (GBS) coupled deformation mechanisms.