How did primordial and recycled geochemical signatures come to coexist in the Earth's deep mantle?

挥发性元素在固体地球、海洋和大气之间的运输和循环塑造了我们星球的进化。长期地幔储层的挥发物化学通量由对流和构造过程维持。然而，控制不同地幔储层不稳定收支的机制仍然不确定。主要的问题包括：地球是如何获得它最初的不稳定库存的？地幔对大气损失的封闭年龄是什么时候？地壳再循环如何改变地幔挥发性储层？地幔中原始挥发物和循环挥发物的空间分布是怎样的？该项目的主要目标是在高精度地球化学数据和详细的统计模型的基础上，对地幔中原始资源和回收资源之间的时空关系提供新的认识。我们将通过分析冰岛一组特殊的冰下喷发玄武岩的稀有气体同位素组成和卤素含量来实现这一目标，这些玄武岩可以绘制出横跨地幔柱的高分辨率样带。这些样品保存了来自古代来源和浅层对流地幔的熔体的地球化学特征，这些特征已经被地质时期的熔体提取和再循环广泛地修改了。冰岛新火山带喷发的玄武岩提供了无与伦比的高分辨率、空间连续的地幔样本，使冰岛成为研究地幔吸积、分化、脱气和再循环历史的理想天然实验室。惰性惰性气体（He, Ne, Ar, Kr, Xe）提供了自地球形成以来基本保持不变的原始地幔域的关键地球化学示踪剂。相比之下，流动卤素（F、Cl、Br、I）则通过海水、沉积物和蚀变洋壳在破坏性板块边界的俯冲作用重新引入地幔，因此是地幔中循环物质的关键示踪剂。目前对稀有气体、卤素和其他地球化学示踪剂（如亲石同位素（Sr, Nd, Pb））的现有解释之间存在一个未解决的二分法。亲石同位素和卤素提供了丰富的证据，表明地幔中的混合、拉伸和再循环在冰岛地下形成了高度不均匀的岩性和空间结构。然而，稀有气体同位素表明，冰岛地幔的某些部分保留了原始的、未经处理的地球化学特征。先前工作的一个关键限制是，亲石元素和同位素、主要挥发物和稀有气体已经在不同的样品集中进行了分析，因此我们无法将不同元素的灵敏度结合起来，以获得对地幔源的可靠约束。通过将这些数据整合到一个单一的连贯框架中，该项目首次提供了同时询问多个地球化学代理的机会，以探索原始和循环地幔源之间的时空关系。我们将使用最先进的质谱技术，为地幔中卤素和惰性气体的解耦提供新的独立约束，以及地球早期不同地幔储层中卤素损失的程度。我们将首次对地球原始地幔的钾同位素组成进行高精度分析，并对地幔中的碘丰度进行新的精确测定。我们将首次对稀有气体、卤素和亲石同位素数据进行综合空间统计分析，以确定地幔柱中原始储层和循环储层的位置和分布，为地幔的结构和动力学提供关键见解。我们的研究结果将为解释地球上其他海洋岛屿的地幔地球化学和空间结构提供指导，并将为其他行星体的挥发性预算和挥发性循环建模提供支持。

The transport and cycling of volatile elements between the solid Earth, oceans and atmosphere has shaped the evolution of our planet. Chemical fluxes of volatiles to and from long-term mantle reservoirs are maintained by convective and tectonic processes. However, the mechanisms that control the volatile budgets of distinct mantle reservoirs remain uncertain. Major questions include: How did the Earth acquire its initial volatile inventory? What was the closure age of the mantle to atmosphere loss? How has crustal recycling modified mantle volatile reservoirs? What is the spatial distribution of primordial and recycled volatiles in the mantle? The key objective of this project is to provide a new understanding of spatial and temporal relationships between primordial and recycled sources in the Earth's mantle, underpinned by high-precision geochemical data and detailed statistical modelling. We will achieve this by interrogating combined noble gas isotope compositions and halogen contents in an exceptional suite of subglacially erupted basalts from Iceland that map a high-resolution transect across a mantle plume. The samples preserve geochemical signatures of melts from both ancient sources and the shallow convecting mantle, which has been extensively modified by melt extraction and recycling over geologic time. The unparalleled high-resolution, spatially continuous sample of the mantle afforded by erupted basalts from Iceland's neovolcanic zones makes Iceland an ideal natural laboratory for investigating the history of accretion, differentiation, outgassing and recycling in the mantle. The inert noble gases (He, Ne, Ar, Kr, Xe) provide key geochemical tracers of primordial mantle domains that have remained largely unmodified since the Earth's formation. In contrast, fluid-mobile halogens (F, Cl, Br, I) are reintroduced to the mantle by the subduction of seawater, sediment and altered oceanic crust at destructive plate boundaries, and are therefore key tracers of recycled material in the mantle. There is at present an unresolved dichotomy between existing interpretations of noble gases, halogens and other geochemical tracers such as lithophile isotopes (Sr, Nd, Pb). Lithophile isotopes and halogens provide abundant evidence that mixing, stretching and recycling in the mantle has created highly heterogeneous lithological and spatial structure beneath Iceland. However, noble gas isotopes indicate that some parts of the Icelandic mantle retain primordial, unprocessed geochemical signatures. A crucial limitation of previous work is that lithophile elements and isotopes, major volatiles and noble gases have been analysed in different sample sets, such that we cannot combine the sensitivities of different elements to obtain robust constraints on mantle sources. By bringing these data into a single coherent framework, this project provides the first opportunity to interrogate simultaneously multiple geochemical proxies to probe the spatial and temporal relationships between primordial and recycled mantle sources.We will use state-of-the-art mass spectrometry techniques to provide new independent constraints on halogen and noble gas decoupling in the mantle, and the extent of halogen loss from different mantle reservoirs in the early Earth. We will provide the first high-precision analysis of the Kr-isotopic composition of the Earth's primordial mantle, and a new, precise determination of iodine abundance in the mantle. We will perform the first combined spatial statistics analysis of noble gas, halogen and lithophile isotope data to determine the location and distribution of primordial and recycled reservoirs in a mantle plume, providing key insights into the structure and dynamics of the mantle. Our results will provide a guide to interpreting mantle geochemical and spatial structure at other ocean islands on Earth, and will feed into ongoing efforts to model volatile budgets and volatile cycling on other planetary bodies.