U and Pb distribution in eclogites: implications for in-situ U-Pb dating of high-pressure metamorphism through rock-forming minerals

了解和量化共存变质矿物之间的主微量元素分布具有重要意义，因为它们被用作许多地质过程的示踪剂。最近激光消融ICPMS领域的仪器进步和技术发展提高了灵敏度，使我们能够更详细地研究俯冲带的过程和反应。因此，出现了新的研究领域，如低铀矿物的地质年代学。变质低铀矿物的U-Pb测年仍处于起步阶段，但与传统的溶解技术相比，快速的数据采集、数据处理和高空间分辨率具有巨大的优势。由于主要矿物相的递进生长在很大程度上取决于含水矿物的分解，对这些不同区域的分析和定年，以及将它们与特定的变质反应联系起来的可能性，可能会给我们提供一个更完整的俯冲各阶段的时间框架。因此，对俯冲带变质作用过程中控制铀和铅（再）分布及其与主要造岩矿物结合的过程的研究将是理解这些元素动员的重要一步，这将是对岩石年代学的重要贡献，这是一种将岩石学和地质年代学数据联系起来的跨学科方法。本研究的主要目的是获得主要榴辉岩形成矿物中U和Pb分布的新信息，从而获得其对低U矿物相（即石榴石、斜辉石、角闪洞）U-Pb年代学的约束。在本项目中，我们将研究来自三个地质区域的各种榴辉岩，它们记录了i)低温变质作用（500-550ºC，阿曼As Sifah榴辉岩），ii)中温矿物组合（约800ºC，西班牙Cabo Ortegal杂岩）和iii)高温共生作用（>850ºC，捷克Moldanubian地区）。这将使我们能够i)确定各种P-T历史的天然岩石中的实际U和Pb分布，ii)评估原岩体组成对矿物U和Pb含量的影响，iii)研究除石榴石外其他主要相定年的可能性，以及iv)预测哪些是主要矿物U-Pb定年的最佳岩性。最后，我们将确定三个地区石榴石的年代，这将有助于揭示每个地区高压岩石中不同事件的地质历史。

Understanding and quantifying the major and trace-element distribution between co-existing metamorphic minerals are of great interest as they are used as tracers of many geological processes. Recent instrumental advances and technical developments in the field of laser-ablation ICPMS have improved the sensitivity, allowing us to investigate subduction-zone processes and reactions in much greater detail. As a consequence, new research fields have emerged as the geochronology of low-U minerals for example. The U-Pb dating of metamorphic low-U minerals is still in its infancy, but the rapid data acquisition, data processing and high spatial resolution represent a huge benefit as compared to conventional dissolution techniques. As the prograde growth of major mineral phases largely depends on the breakdown of hydrous minerals, the analysis and dating of these distinct domains and the possibility to link them with a specific metamorphic reaction will likely give us a much more complete time frame of the various stages of subduction. Thus, the study of the processes controlling the (re)distribution of U and Pb and their incorporation in the major rock-forming minerals during subduction-zone metamorphism will represent a significant step forward in the understanding the mobilization of those elements, which will be an important contribution to petrochronology, the interdisciplinary approach linking petrological and geochronological data. The major aim of this study is to gain new information about U and Pb distribution in major eclogite-forming minerals and subsequently, its constraints for the U-Pb geochronology of low-U mineral phases (i.e., garnet, clinopyroxene, amphibole). In the course of this project, we will investigate a variety of eclogites from three geological areas that record i) low-temperature metamorphism (500–550 ºC, As Sifah eclogites, Oman), ii) medium-temperature mineral assemblages (ca. 800 ºC, Cabo Ortegal complex, Spain) and iii) high-temperature paragenesis (>850 ºC, Moldanubian Zone, Czech Republic). This will allow us to i) determine the practical U and Pb distribution in natural rocks of various P–T histories, ii) evaluate the influence of the protolith bulk composition in the U and Pb content of minerals, iii) study the possibility of dating other major-phases apart from garnet, and iv) predict which are the best lithologies for U-Pb dating of major-minerals. Finally, we will date garnet from the three localities, which will help to unravel the geological history of different events in the high-pressure rocks of each area.