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

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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.