Cratonic mantle eclogites as palaeo-dynamic archives

矿物学、元素和同位素数据表明，在古大陆岩心的地幔岩石圈中，大多数榴辉岩包体具有扩张脊下减压融化后嵌入古海底的低压原岩。因此，与现代洋中脊玄武岩一样，它们是最上层对流地幔（UCOM）化学、热和氧化还原状态的探测器。随后的再循环和变质作用导致了各种海洋板块组分之间的相互作用，以及可能有助于大陆地壳（CC）生长的部分熔体的丧失。作为太古宙和元古代海洋地壳的稀有样本，地幔榴辉岩可能蕴藏着古地球动力学各个方面的宝贵信息，这些信息仍有待充分利用。为识别其复杂的多阶段演化相关的初级和次级成分特征奠定了基础，本研究的目的是：(1)利用大量的主微量元素清单，结合放射性成因和稳定同位素以及Fe3+/Fe2+分析，研究来自不同克拉通和构造背景的大型地幔榴辉岩套的多样化起源；(2)利用最小分化和修饰样品对古UCOM的化学、氧化和热状态提供了新的约束，这些约束强烈影响地球动力学和化学循环；(3)从成分和年代学角度检验地幔榴辉岩熔体损失与区域CC形成之间的联系。科罗拉多高原的榴辉岩捕虏体是作为法拉隆板块的一部分被充分研究的现代海底的一部分，代表了古代榴辉岩的独特的现代类似物，对它的研究将使我们能够确定古代地幔榴辉岩能够可靠地穿越时间探测UCOM。

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.