Abstract Both mechanical compaction and mineral reactions reduce the porosity and permeability of sedimentary rocks. These processes occur during burial and diagenesis, as well as during contact and regional metamorphism. Although the mechanisms governing porosity reduction have been studied extensively in diagenetic sedimentary systems, relatively limited work has been carried out on metamorphic rocks. Here, we present a petrographic method to isolate and quantify the effects of mineral cementation and mechanical compaction in sandstone altered during contact metamorphism. We sampled rocks from the Inmar formation in Makhtesh Ramon (Israel) - where indurated sandstone is present near contacts with dykes – and analyzed them using high resolution scanning electron microscopy and cathodoluminescence. Using image segmentation to differentiate between quartz cement and detrital grains, we observed both grain rearrangement and brittle fracturing near intrusive contacts. At the highest level of porosity reduction, mechanical compaction reduced porosity by 18–31%, while cementation caused a reduction of 11%. We also found that the maximal level of mechanical compaction, as well as its spatial extent, scale with the size of the intrusion. By contrast, the maximal level of cementation was unaffected by the size of the dyke. Our findings demonstrate that to model porosity and permeability evolution in contact aureoles, both mechanical and chemical processes must be taken into account.

中文翻译：

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砂岩接触变质过程中机械压实和胶结作用的量化

摘要 机械压实和矿物反应都会降低沉积岩的孔隙度和渗透率。这些过程发生在埋藏和成岩作用期间，以及接触和区域变质作用期间。尽管已经在成岩沉积系统中广泛研究了控制孔隙度降低的机制，但对变质岩的研究相对有限。在这里，我们提出了一种岩相学方法来隔离和量化接触变质作用期间改变的砂岩中矿物胶结和机械压实的影响。我们从 Makhtesh Ramon（以色列）的 Inmar 地层中采集了岩石样本 - 那里与岩脉接触的附近存在硬质砂岩 - 并使用高分辨率扫描电子显微镜和阴极发光对其进行分析。使用图像分割来区分石英胶结物和碎屑颗粒，我们观察到侵入接触附近的颗粒重排和脆性破裂。在孔隙度降低的最高水平下，机械压实使孔隙度降低了 18-31%，而胶结使孔隙度降低了 11%。我们还发现，机械压实的最大程度及其空间范围随侵入的大小而变化。相比之下，最大胶结水平不受堤坝大小的影响。我们的研究结果表明，要模拟接触光环中的孔隙度和渗透率演变，必须同时考虑机械和化学过程。机械压实减少了 18-31% 的孔隙度，而胶结减少了 11%。我们还发现，机械压实的最大程度及其空间范围随侵入的大小而变化。相比之下，最大胶结水平不受堤坝大小的影响。我们的研究结果表明，要模拟接触光环中的孔隙度和渗透率演变，必须同时考虑机械和化学过程。机械压实使孔隙度降低了 18-31%，而胶结使孔隙度降低了 11%。我们还发现，机械压实的最大程度及其空间范围随侵入的大小而变化。相比之下，最大胶结水平不受堤坝大小的影响。我们的研究结果表明，要模拟接触光环中的孔隙度和渗透率演变，必须同时考虑机械和化学过程。