Investigating Mantle Recycling and the Origin of the HIMU Component with Stable Thallium Isotopes

我们的星球有一个同心结构，有一个金属核心，一个富含镁硅酸盐的地幔和一个薄的，高度分化的地壳。地壳被细分为坚硬的板块，这些板块在塑性变形的地幔中的大型对流单元的驱动下相对移动。这种对流是地幔释放热量的主要机制，它直接或间接地驱动了几乎所有与板块构造有关的地质过程。新的地壳在大洋中脊不断地从熔化的最上层地幔物质中形成，并在俯冲带被循环回到地幔。因此，得出的结论是，地球地幔中一定含有曾经在地表，但在俯冲带被推入地幔深处的古代地壳碎片。通过精确分析源自地幔熔体的火山岩的化学成分和同位素组成，有机会确定地幔中含有循环地壳物质的区域。这种类型最重要的来源是远离板块边界的火山岛；它们喷发出一种称为洋岛玄武岩的岩浆类型。它们分布在全球各地，与在大洋中脊开采的更均质的储集层相比，它们表现出很大的化学和同位素不均质性。许多研究集中于描述这些地幔区域的性质和起源，但迄今为止使用的大多数地球化学工具只能提供模棱两可的信息。在这个项目中，我们使用了一种新的示踪剂，它使我们能够确定洋岛玄武岩下地幔中物质的确切来源。在这个项目中，我们已经确定了元素Tl及其同位素(质量203和205)是识别洋岛玄武岩中循环地壳的理想地球化学示踪剂。大洋地壳的沉积物和风化玄武岩具有与正常地幔岩石明显不同的特征铊同位素组成。此外，与地幔相比，这些地壳物质的铊浓度非常高，这使我们能够检测到回收壳对地幔的轻微污染。我们提议对控制铊及其同位素深部再循环的过程进行详细研究，包括：(A)曾经俯冲并折返地表的地壳岩石(‘榴辉岩’)的铊浓度和同位素组成。这些将被用来揭开铊在俯冲带深处的传输机制。(B)将确定南库克群岛和圣赫勒拿玄武岩样品的铊同位素和浓度数据。这是两个重要的洋岛玄武岩产区，并将这些数据与榴辉岩数据进行对比，以评价俯冲洋壳与洋岛玄武岩的联系。

Our planet has a concentric structure with a metallic core, a magnesio-silicate rich mantle and a thin, highly differentiated crust. The crust is subdivided into rigid plates that move relative to each other driven by large convection cells in the plastically deforming mantle. This convection is the dominant mechanism by which heat is released from the mantle, and it drives - directly or indirectly - virtually all geologic processes related to plate tectonics. New crust is constantly formed at mid-ocean ridges from melting uppermost mantle materials, and it is recycled back into the mantle at subduction zones. It has, therefore, been concluded that the Earth's mantle has to contain fragments of ancient crust that was once at the surface, but was driven into the deep mantle at subduction zones. There is a chance to identify domains in the mantle that contain recycled crustal materials through the precise analysis of the chemical and isotopic composition of volcanic rocks that are derived from melts sourced in the mantle. The most important source of this type are volcanic ocean islands that are located far away from plate boundaries; they erupt a magma type called ocean island basalts. They are scattered all around the globe and are known to exhibit large chemical and isotopic heterogeneity compared with the more homogenous reservoir that is tapped at mid ocean ridges. Numerous studies have focused on characterizing the nature and origin of these mantle domains, but most of the geochemical tools employed to date are only capable of providing ambiguous information. In this project we use a novel tracer that allows us to determine the exact origin of the material in the mantle beneath ocean island basalts.In this project we, we have identified the element thallium and its isotopes (masses 203 and 205) as the ideal geochemical tracer to identify recycled crust in ocean island basalts. Sediments and weathered basalts of the oceanic crust have characteristic thallium isotopic compositions that are distinctly different from the composition of normal mantle rocks. In addition, these crustal materials have very high thallium concentrations compared to the mantle, which enables us to detect even minor contamination of the mantle with recycled crust. Our proposed detailed study of the processes controlling deep recycling of thallium and its isotopes will include: (A) Thallium concentrations and isotopic compositions of crustal rocks that were once subducted and exhumed back to the surface ('eclogites'). These will be used to unravel the transport mechanism for thallium deep inside subduction zones. (B) Thallium isotope and concentration data for basalt samples from the Austral-Cook islands and St. Helena will be determined. These are two important ocean-island basalt localities, and the data will be compared to the eclogite data in order to evaluate the connection of subducted oceanic crust and ocean island basalts.