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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/N009568/2
负责人：
Julie Prytulak
金额：
$ 5.86万
依托单位：
Durham University
依托单位国家：
英国
项目类别：
Research Grant
财政年份：
2018
资助国家：
英国
起止时间：
2018 至无数据
项目状态：
已结题

来源：
https://gtr.ukri.org/projects?ref=NE%2FN009568%2F2
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.

地幔与地表环境之间的氧交换是地球地球化学循环的重要组成部分。氧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的过程中起着重要作用。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。玄武岩浆是由地幔熔融产生的。因此，玄武质火山样本的成分携带着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、在地壳浅层的结晶以及地表附近火山气体的流失都改变了岩浆的成分。因此，为了理解地幔的化学特征，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了简化的假设。然而，元素行为的理论理解的进展表明，需要重新审视纠正的假设。先前研究的一个令人吃惊的特点是，他们对地幔中氧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的氧原子数量大约是大气中的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假设造成的。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性质。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改进了模型，我们就可以更好地关注地幔氧化态变化的地图。具体来说，我们的研究将涉及大量艰苦的地球化学工作。我们的目标是使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，我们可以构建整个冰岛的氧化态模型。几个独立的化学约束的组合使我们能够确定在这个经典位置之下氧的变化有多大。此外，它还为该社区提供了一个精确的工具来提取地幔氧化的全球变化。