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，有一个金属地核，一个富含镁硅酸盐的地幔和一个薄的、高度分化的地壳。地壳被细分为刚性板块，在塑性变形的地幔中的大型对流单元的驱动下，刚性板块彼此相对移动。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着几乎所有与板块构造有关的地质过程。在大洋中脊，新的地壳不断地从最上层的地幔物质的融化中形成，并在俯冲带重新回到地幔中。因此，得出的结论是，地幔必须包含曾经在地表的古代地壳碎片，这些地壳碎片在俯冲带被挤压到深部地幔中。通过对源自地幔熔体的火山岩的化学和同位素组成的精确分析，有机会确定地幔中含有循环地壳物质的区域。这种类型的最重要的来源是远离板块边界的火山海洋岛屿；它们喷发出一种叫做海洋岛玄武岩的岩浆类型。它们分散在全球各地，与在大洋中脊开采的更均匀的储层相比，它们表现出较大的化学和同位素异质性。许多研究都集中在描述这些地幔域的性质和起源上，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，我们使用一种新颖的示踪剂，使我们能够确定海洋岛屿玄武岩下地幔中物质的确切来源。在这个项目中，我们已经确定元素铊及其同位素（质量203和205）是识别海洋岛玄武岩中再循环地壳的理想地球化学示踪剂。海洋地壳的沉积物和风化玄武岩具有明显不同于正常地幔岩石的铊同位素组成特征。此外，与地幔相比，这些地壳物质具有非常高的铊浓度，这使我们能够检测到即使是轻微的地幔污染。我们提出的控制铊及其同位素深层循环过程的详细研究将包括：(A)曾经俯冲并挖出地表的地壳岩石（“榴辉岩”）的铊浓度和同位素组成。这些将用于揭示铊在俯冲带深处的输运机制。(B)将确定南库克群岛和圣赫勒拿岛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位置，为了评价俯冲洋壳与洋岛玄武岩的联系，将这些资料与榴辉岩资料进行比较。

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.