Cratonic mantle eclogites as palaeo-dynamic archives

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Mineralogical, elemental and isotopic data show that most eclogite xenoliths, which sample the mantle lithosphere of ancient continental cores, have low-pressure protoliths that were emplaced in palaeo-ocean floors after decompression melting beneath spreading ridges. Like modern mid-ocean ridge basalts, they are therefore probes of the chemical, thermal and redox state of the uppermost convecting mantle (UCOM). Subsequent recycling and metamorphism led to interaction between various oceanic slab components and loss of a partial melt that may have contributed to continental crustal (CC) growth. As rare samples of Archaean and Proterozoic oceanic crust, mantle eclogites may therefore harbour invaluable information on various aspects of palaeo-geodynamics that still awaits full exploitation. The groundwork having been laid for recognising primary and secondary compositional signatures related to their complex multi-stage evolution, this study aims to (1) Investigate the variegated origins of large mantle eclogite suites from different cratons and tectonic settings, using an extensive list of major and trace elements, complemented by radiogenic and stable isotopes and Fe3+/Fe2+ analyses; (2) Use the least differentiated and modified samples to provide new constraints on the chemical, oxidation and thermal state of the ancient UCOM, which strongly affect geodynamics and chemical cycles; (3) Test the link between melt loss from mantle eclogites and regional CC formation from a compositional and chronological perspective. Investigation of eclogite xenoliths from the Colorado Plateau, which were part of the well-studied modern ocean floor subducted as part of the Farallon plate and represent unique modern analogues of ancient eclogites, will allow ascertaining that ancient mantle eclogites can reliably probe the UCOM through time.