The Distribution of Oxygen in Earth's mantle

地幔中氧气的分布

• 批准号: NE/N009886/1
• 负责人: John Maclennan
• 金额: $ 36.46万
• 依托单位: University of Cambridge
• 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%2FN009886%2F1
• 关键词: Distribution; Oxygen; Earth; mantle

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交换是地球地球化学循环的关键组成部分。氧气在地球内部岩浆的生成和挥发性元素（如硫、碳和氢）从固体地球转移到熔融岩石中起着重要作用。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，地幔中氧含量的变化与地球作为一颗行星的可居住性密切相关。玄武岩岩浆是由地幔熔融产生的。因此，玄武岩火山样本的成分包含了地幔成分的信息。然而，从深处的生成点到地表的喷发，许多过程改变了岩浆的成分。熔体本身的产生，它在浅层地壳中的结晶以及地表附近火山气体的流失都改变了岩浆的组成。因此，为了了解地幔的化学特征，有必要解释这些过程。以前研究地幔氧含量变化的尝试对这些过程做出了简化的假设。然而，对元素行为的理论理解的进展表明，需要重新审视纠正假设。之前的研究有一个惊人的特点，那就是它们对地幔中氧气的分布得出了非常不同的结论。目前上地幔氧含量的不确定性对应于氧原子的数量，大约是大气中氧原子数量的100倍！我们认为，这种差异的部分原因是由以前的研究人员所做的一系列假设造成的。因此，我们项目的一个关键组成部分是利用新的理论和观测约束来理解岩浆系统中的过程如何改变玄武岩的化学性质。我们仔细选择了我们的目标地质环境：冰岛有丰富的玄武岩，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模型所需的广泛背景信息。一旦我们通过添加新的观测结果来改进模型，我们就可以更好地集中我们的地幔氧化态变化图。具体来说，我们的研究将涉及大量艰苦的地球化学工作。我们的目标是利用元素钒的同位素组成，因为理论工作和初步的实验研究表明，钒及其同位素的行为在很大程度上受地幔氧化态的控制。通过将钒同位素的新限制与其他被认为对地幔氧敏感的地球化学测量相结合，我们可以构建一个横跨冰岛的氧化状态模型。几个独立的化学约束条件的结合，使我们能够确定在这个经典位置下，氧气的变化有多大。此外，它为群落提供了一个精确的工具来提取地幔氧化的全球变化。

项目成果

A multi-proxy investigation of mantle oxygen fugacity along the Reykjanes Ridge

• DOI: 10.1016/j.epsl.2019.115973
• 发表时间: 2020-02
• 期刊: Earth and Planetary Science Letters
• 影响因子: 5.3
• 作者: D. Novella;J. Maclennan;O. Shorttle;J. Prytulak;B. Murton