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.

地幔和表面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。氧气在地球内的岩浆产生中起作用，以及硫，碳和氢等挥发性元素从固体地球转移到熔融岩石中。当这样的岩浆向地面升起时，火山气会释放，驱动或调节地球大气的组成。因此，越来越多地接受地球地幔氧含量的变化与地球作为行星的宜居性紧密相关。玄武岩岩浆是通过融化地幔而产生的。因此，玄武着火山样品的组成传递了有关基础地幔组成的信息。但是，许多过程将岩浆的组成从深度的发电点到表面喷发。熔体本身的产生，其在浅层地壳中的结晶以及表面附近的火山气体损失都改变了岩浆的成分。因此，必须考虑这些过程以了解地幔的化学特性。先前研究地幔氧含量变化的尝试已经简化了对这些过程的假设。但是，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。先前研究的一个令人震惊的特征是，它们得出了关于地幔中氧气分布的结论。上地幔的氧气含量的当前不确定性对应于大气中存在的许多氧原子。我们认为，这种差异的一部分是由以前研究人员提出的一组假设引起的。因此，我们项目的关键组成部分是使用新的理论和观察性约束来了解岩浆系统中的过程如何修改玄武岩的化学。我们已经仔细选择了我们的目标地质环境：冰岛拥有大量的玄武岩，这些玄武岩在传统的化学成分方面进行了很好的研究，因此为我们提供了基础模型所需的广泛背景信息。一旦我们通过添加新的观察结果来改进模型，我们就可以更好地将地图集中在地球地幔氧化状态的变化上。详细，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由地幔氧化态强烈控制。通过将钒同位素的新约束与被认为对地幔氧敏感的其他地球化学测量相结合，我们可以在冰岛构建氧化态模型。几种独立的化学限制的结合使我们能够确定在这个经典位置下的氧气变化多少。此外，它为社区提供了一种精确的工具，可以提取地幔氧化中的全球变化。