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The Distribution of Oxygen in Earth's Mantle

地幔中氧气的分布

基本信息

批准号：
NE/N009568/1
负责人：
Julie Prytulak
金额：
$ 8.44万
依托单位：
Imperial College London
依托单位国家：
英国
项目类别：
Research Grant
财政年份：
2016
资助国家：
英国
起止时间：
2016 至无数据
项目状态：
已结题

来源：
https://gtr.ukri.org/projects?ref=NE%2FN009568%2F1
关键词：
Distribution Oxygen Earth Mantle

项目摘要

The exchange of oxygen between the mantle and surface environment is a key component of Earth's geochemical cycle. Oxygen plays a role in generation of magma within the earth and the transfer of volatile elements like sulfur, carbon and hydrogen from the solid earth into molten rock. When such magma rises towards the surface, volcanic gases are released, driving or regulating the composition of Earth's atmosphere. It is therefore increasingly accepted that variation in the oxygen content of Earth's mantle is closely linked to Earth's habitability as a planet. Basaltic magma is generated by melting the mantle. Therefore, the composition of samples of basaltic volcanoes carries information about the composition of the underlying mantle. However, many processes modify the magma's composition from the point of generation at depth to eruption at the surface. The generation of the melt itself, its crystallisation in the shallow crust and the loss of volcanic gases near the surface all change the composition of magma. It is therefore necessary to account for these processes in order to understand the chemical characteristics of the mantle. Previous attempts to study variation in the oxygen content of the mantle have made simplifying assumptions about these processes. However, progress in theoretical understanding of elemental behaviour indicates that the correcting assumptions need to be revisited. One startling feature of the previous studies is that they come to very different conclusions about the distribution of oxygen in the mantle. The current uncertainty in the oxygen content of the upper mantle corresponds to a number of oxygen atoms that is about 100 times that present in the atmosphere!We think that part of this discrepancy is caused by the sets of assumptions that previous investigators have made. A crucial component of our project is therefore to use new theoretical and observational constraints to understand how the processes in magmatic systems modify the chemistry of basalt. We have carefully chosen our target geologic setting: Iceland has plentiful basalts that are well studied in terms of traditional chemical compositions and therefore provide us with the extensive background information we need to underpin our models. Once we have improved models by adding our new observations, we can better focus our map of the variation in the oxidation state of the Earth's mantle.In detail, our research will involve a great deal of painstaking geochemical work. We aim to use the isotopic composition of the element vanadium, because theoretical work and preliminary experimental studies indicate that the behaviour of vanadium and its isotopes is strongly controlled by mantle oxidation state. By combining new constraints from vanadium isotopes with other geochemical measurements that are thought to be sensitive to mantle oxygen, we can construct a model of oxidation state across the Iceland. The combination of several independent chemical constraints allows us to determine just how much variation in oxygen there is beneath this classic locality. Furthermore, it equips the community with a precise tool to extract global variations in mantle oxidation.

地幔和地表环境之间的氧气交换是地球地球化学循环的关键组成部分。氧在地球内部岩浆的生成以及硫、碳和氢等挥发性元素从固体地球到熔融岩石的转移中发挥着作用。当这些岩浆上升到地表时，火山气体被释放出来，驱动或调节地球大气的组成。因此，越来越多的人认为，地幔含氧量的变化与地球作为行星的宜居性密切相关。玄武岩岩浆是由地幔熔融而产生的。因此，玄武岩火山样品的成分携带了有关下伏地幔成分的信息。然而，许多过程改变了岩浆的成分，从深处的生成点到表面的喷发。熔体本身的产生，其在浅地壳中的结晶，以及地表附近火山气体的损失，都改变了岩浆的组成。因此，有必要解释这些过程，以便了解地幔的化学特征。以前研究地幔中氧含量变化的尝试对这些过程做出了简化的假设。然而，在对基本行为的理论理解方面的进展表明，需要重新审视更正的假设。以前的研究有一个惊人的特点，那就是他们对地幔中氧的分布得出了截然不同的结论。目前上地幔氧含量的不确定性与大气中氧原子的数量约为100倍相对应！我们认为，这种差异的部分原因是之前的研究人员所做的一系列假设。因此，我们项目的一个重要组成部分是使用新的理论和观测约束来了解岩浆系统中的过程如何改变玄武岩的化学成分。我们仔细选择了我们的目标地质环境：冰岛拥有丰富的玄武岩，这些玄武岩在传统化学成分方面得到了很好的研究，因此为我们提供了支持我们的模型所需的广泛背景信息。一旦我们通过添加新的观测数据来改进模型，我们就可以更好地聚焦地球地幔氧化状态的变化地图。具体地说，我们的研究将涉及大量艰苦的地球化学工作。我们的目的是利用元素钒的同位素组成，因为理论工作和初步实验研究表明，钒及其同位素的行为强烈地受到地幔氧化态的控制。通过将钒同位素的新约束与其他被认为对地幔氧敏感的地球化学测量相结合，我们可以构建冰岛全境的氧化状态模型。几个独立的化学约束的组合使我们能够确定这个经典位置下面的氧气有多大的变化。此外，它还为该社区配备了一种精确的工具，以提取地幔氧化的全球变化。