Can Scanned Cathodoluminescence (SEM-CL) of Quartz Silt be Applied to Provenance Studies of Mudstones? A Feasibility Study

有记录的地球历史大部分包含在泥岩和页岩中，泥岩和页岩是大陆大陆风化和侵蚀形成的细粒沉积物。然而，泥岩是人们最不了解的沉积岩类型，从泥岩中提取信息是困难的。沉积物研究中的一项基本任务是确定来源，即岩石的成分是如何起源并到达其沉积地点的。虽然我们有非常复杂的方法来研究砂岩，但泥岩的细小颗粒长期以来一直无法详细地破译它们的历史。不幸的是，泥岩的主要成分粘土矿物化学不稳定，在埋藏和成岩作用中发生了重大变化。因此，很难在事后确定原始粘土矿物是什么以及它们来自哪里。碎屑石英颗粒的丰度次之，在化学和机械性能上都很强。原则上，它们将是检验保存完好的物源记录的理想成分，但在过去，它们在物源研究中的前景有限。扫描阴极发光(SEM-CL)的最新发展使我们能够观察到以前未被识别的石英颗粒的结构特征。它在沉积物主要源岩(深成岩、火山岩、变质岩)中的应用有力地表明，通过SEMCL下可见的结构特征，可以区分不同条件下形成的石英颗粒。这为沉积颗粒的深入研究创造了新的机会。由于它允许对细小颗粒进行检查，如石英粉砂颗粒，因此它具有研究泥岩来源的潜力。通过对晚泥盆世泥岩中石英粉砂的扫描电子显微镜-化学发光分析表明，与变质石英、深成岩和火山石英有关的结构仍然可识别，很有可能成为泥岩研究的重要工具。我们希望在扫描电子显微镜-化学发光特征的基础上制定区分变质、深成和火山石英颗粒的标准，然后检查这些特征在土壤形成和沉积过程中保留的可靠性。一旦这项技术的应用有了坚实的基础，扫描电子显微镜-化学发光技术将成为研究各种含石英沉积岩的一种非常重要的工具。由于沉积记录以泥岩为主，了解其石英成分的来源将对合理解释地质记录有很大帮助。我们的变质岩、深成岩和火山岩石英的扫描电子显微镜-化学发光特征目录也应该促进火成岩和变质岩石学家对这些特征背后的过程和原因的研究，并导致对火成岩和变质岩起源的新见解。由于观察到的扫描电子显微镜-化学发光特征反映了在火成岩和变质岩形成过程中发生的基本过程，因此这项研究的结果也应该适用于其他世界的岩石研究。具体地说，它们可能提供解释石英颗粒的新方法，这些颗粒很有可能在不久的将来从火星上被取回。

0073995SchieberMost of recorded earth history is contained in mudstones and shales, fine-grained sediments derived from weathering and erosion of continental landmasses. Yet, mudstones are the least understood sedimentary rock type, and extracting information from them is difficult. A basic task in the study of sediments is the determination of provenance, meaning how the constituents of a rock originated and got to their site of deposition. Whereas we have quite sophisticated ways of doing this for sandstones, the fine grain size of mudstones has long defied efforts to decipher their history in detail.Clay minerals, the dominant constituent of mudstones, are unfortunately chemically labile and altered substantially during burial and diagenesis. As a consequence, it is very difficult to determine after the fact what the original clay minerals were and where they came from. Detrital quartz grains are second in abundance, and are chemically and mechanically very resistant. In principle they would be the ideal component to examine for a well-preserved provenance record, but in the past they only showed limited promise for provenance studies.The recent development of scanning cathodoluminescence (SEM-CL) allows us to observe previously unrecognized textural features in quartz grains. Its application to principal source rocks (plutonic, volcanic, metamorphic) of sediments strongly suggests that quartz grains that originate under different conditions can be differentiated through textural features visible under SEMCL. This has created new opportunities for the in depth examination of sedimentary particles. Because it allows the examination of small particles, such as quartz silt grains, it has potential for the provenance study of mudstones. Examination of quartz silt in Late Devonian mudstones by SEM-CL shows that textures related to metamorphic, plutonic, and volcanic quartz are still recognizable, and that it is very likely that SEM-CL could become an important tool in the study of mudstones.This study is the first step in a systematic study of source rock related SEM-CL features. We want to developing criteria by which to differentiate metamorphic, plutonic, and volcanic quartz grains on the basis of SEM-CL features, and then examine how reliably these features are retained trough soil forming and sedimentary processes. There is very good potential that once there is a solid foundation for the application of this technique, SEM-CL will become a very important tool in the study of all kinds of quartz bearing sedimentary rocks. Because the sedimentary record is so strongly dominated by mudstones, understanding where its quartz component originates will contribute greatly to a sensible interpretation of the geologic record. Our catalog of SEM-CL features in metamorphic, plutonic, and volcanic quartz should also stimulate research by igneous and metamorphic petrologists into processes and causes behind these features, and lead to new insights into the origin of igneous and metamorphic rocks. Because the observed SEM-CL features reflect fundamental processes that occur during the formation of igneous and metamorphic rocks, the results from this study should also be applicable to the study of rocks from other worlds. Specifically, they may provide new ways of interpreting quartz grains that will in all likelihood be retrieved from Mars in the near future.