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

原始和再生的地球化学特征是如何在地幔深处共存的？

基本信息

批准号：
NE/P002331/1
负责人：
Margaret Hartley
金额：
$ 50.1万
依托单位：
University of Manchester
依托单位国家：
英国
项目类别：
Research Grant
财政年份：
2017
资助国家：
英国
起止时间：
2017 至无数据
项目状态：
已结题

来源：
https://gtr.ukri.org/projects?ref=NE%2FP002331%2F1
关键词：
How did primordial recycled geochemical

项目摘要

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.

挥发性元素在固体地球、海洋和大气之间的传输和循环塑造了我们星球的演变。对流和构造过程维持着挥发分与长期地幔储层之间的化学通量。然而，控制不同的地幔水库的波动性预算的机制仍然不确定。主要的问题包括：地球是如何获得它最初的挥发性库存的？地幔对大气亏损的封闭年龄是多少？地壳再循环是如何改变地幔挥发性储层的？地幔中原始挥发分和再循环挥发分的空间分布是什么？该项目的主要目标是以高精度地球化学数据和详细的统计模型为基础，对地幔中原始来源和再循环来源之间的空间和时间关系提供新的认识。我们将实现这一目标，通过询问结合惰性气体同位素组成和卤素含量在一个特殊的套件冰下喷发的玄武岩从冰岛地图的高分辨率横断面地幔柱。这些样品保留了来自古老来源和浅对流地幔的熔体的地球化学特征，这些熔体在地质时期被熔体提取和再循环广泛地改变。冰岛新火山区喷发的玄武岩提供了无与伦比的高分辨率、空间连续的地幔样品，使冰岛成为研究地幔增生、分异、放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、Ar）提供了原始地幔域的关键地球化学示踪剂，这些地幔域自地球形成以来基本上保持不变。与此相反，流体流动卤素（F，Cl，Br，I）被重新引入到地幔的俯冲海水，沉积物和改变大洋地壳在破坏性的板块边界，因此是关键的示踪剂回收材料在地幔中。目前，对惰性气体、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着一个尚未解决的分歧。亲岩同位素和卤素提供了大量的证据，证明地幔的混合、拉伸和再循环创造了冰岛地下高度异质的岩性和空间结构。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。以前的工作的一个关键限制是，亲石元素和同位素，主要挥发分和惰性气体已在不同的样品集进行了分析，这样我们就不能联合收割机的灵敏度不同的元素，以获得强大的地幔源的约束。通过将这些数据纳入一个统一的框架，该项目首次提供了同时询问多个地球化学代用指标的机会，以探测原始地幔源和再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，提供新的独立约束条件，对地幔中的卤素和惰性气体去耦，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氪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和再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，并将为正在进行的对其他行星体的挥发性预算和挥发性循环进行建模的努力提供参考。