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

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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.