Probing Mantle Heterogeneity: A Petrological Reconciliation for Geochemistry and Seismology

探测地幔异质性：地球化学和地震学的岩石学协调

• 批准号: NE/J021539/1
• 负责人: John Maclennan
• 金额: $ 40.29万
• 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: 2013
• 资助国家: 英国
• 起止时间: 2013 至 无数据
• 项目状态: 已结题
• 来源: https://gtr.ukri.org/projects?ref=NE%2FJ021539%2F1
• 关键词: Probing; Mantle; Heterogeneity; Petrological; Reconciliation

In cutaway cartoon models of the Earth, the mantle is depicted as a uniform layer stretching from about 20 miles beneath our feet to a depth of 1800 miles (or 2890 km). The mantle is the largest part of the Earth system and despite its deep obscurity, has a role in shaping the long-term evolution of the planet and its environment. The magma that feeds volcanic eruptions and supplies the cocktail of chemical elements required for the maintenance of a habitable planet is generated by melting in the mantle. The loss of heat from the Earth over billions of years drives motions in the mantle which control the movement of tectonic plates. These stirrings also lead to geologically rapid motions of the Earth's surface, which change the shape of the oceans, alter ocean circulation and shift climatic patterns.Even the deepest drill-cores have failed to penetrate pristine mantle rocks. Given this inaccessibility, scientists have had to build their models of Earth's deep interior based on indirect means. Studies of the chemistry of meteorites provide clues about the bulk composition of the planet. Rare slivers of mantle are exposed in mountain belts when plates collide and certain exotic types of volcanic eruptions bring small lumps of mantle to the surface. However, there remains a fear that these extraordinary samples do not provide a balanced view of the deep mantle. Earth scientists have therefore combined the compositional evidence from meteorites and mantle samples with two further types of observations in order to develop their models. The first is seismic data acquired by tracking the progress of seismic waves through the Earth after large earthquakes. Mineral transformations at certain depths produce large changes in the properties of mantle material so that seismic waves are reflected at these depths. The pattern of reflections is sensitive to the composition of the mantle and are important tests of compositional models. The second further type of observation comes from the study of the composition of basalts, the melts of the mantle that are erupted at volcanoes across the globe. In order to estimate mantle compositions it is necessary to understand how the melting process modifies the composition of the basalt melt away from that of the solid mantle source. After consideration of these observations, a consensus was reached that the composition of the mantle was effectively uniform, and the favoured mantle composition is referred to as pyrolite. However, isotope geochemists, studying the composition of oceanic basalts, found evidence for strong variations in mantle chemistry. At first, these isotopic variations were not seen as a challenge to the pyrolite model, because they were not thought to correspond to variations in mantle mineralogy. However, recent controversial research has concluded that the isotopic variations observed in basalts correlate with mineralogical variations. In particular, these researchers have suggested that the mantle under ocean island volcanoes, such as Hawaii, is very different in composition to pyrolite. This conclusion, if correct, changes the way in which Earth Scientists think about the mantle.Our aim is to test this controversial research, and to develop a new model of mantle heterogeneity, by taking a novel interdisciplinary approach. We have access to a unique archive of samples from a wide range of ocean island groups and will analyse the composition of crystals in these samples to constrain the composition of the mantle. We will then test these compositional estimates by calculating the expected response of each composition to the passage of seismic waves. This approach makes use of recently developed models of mineral properties. Finally, we will compare these expected seismic responses to those actually observed under the island groups, using an updated dataset of seismic observations and newly refined seismic techniques.

在地球的切片卡通型号中，地幔被描述为均匀的层，从我们的脚下约20英里到1800英里（或2890 km）的深度。地幔是地球系统中最大的部分，尽管它具有深厚的晦涩性，但在塑造地球及其环境的长期演变方面发挥了作用。通过地幔中的融化产生了饲喂火山喷发和供应的岩浆维护宜居行星所需的化学元素的鸡尾酒。数十亿年来，地球上的热量损失使地幔中的动作控制了构造板的运动。这些搅拌还导致地球表面的地质快速运动，从而改变了海洋的形状，改变了海洋循环并移动气候模式。即使是最深的钻孔也无法穿透原始的地幔岩石。鉴于这种无法获得的性，科学家必须基于间接手段建立地球深处的模型。对陨石化学的研究提供了有关地球的整体组成的线索。当板碰撞时，在山皮带中暴露了罕见的地幔片，某些外来的火山喷发将小块地幔带到了地面。但是，人们仍然担心这些非凡的样本并不能提供深层地幔的平衡视图。因此，地球科学家将来自陨石和地幔样品的组成证据与另外两种类型的观察结果结合在一起，以开发其模型。第一个是通过在大地震后跟踪地震波的进度获得的地震数据。在某些深度处的矿物转化会在地幔材料的性质上发生巨大变化，从而在这些深度上反映了地震波。反射模式对地幔的组成敏感，并且是组成模型的重要测试。第二种类型的观察来自研究玄武岩的组成，即在全球火山上爆发的地幔的融化。为了估计地幔组成，有必要了解熔融过程如何修饰玄武岩的组成融化从固体地幔源的组成上。在考虑了这些观察结果之后，达成了共识，即地幔的组成有效均匀，而受青睐的地幔组成称为黄利石。但是，研究海洋玄武岩组成的同位素地球化学主义者发现了地幔化学差异很大的证据。首先，这些同位素变化并不被视为对黄铁矿模型的挑战，因为它们不被认为与地幔矿物学的变化相对应。然而，最近有争议的研究得出的结论是，在玄武岩中观察到的同位素变化与矿物学变化相关。特别是，这些研究人员提出，海洋岛火山下的地幔（例如夏威夷）与黄铁矿的组成截然不同。如果正确的话，这个结论会改变地球科学家对地幔的思考方式。我们的目的是测试这项有争议的研究，并通过采用一种新颖的跨学科方法来开发一种新的地幔异质性模型。我们可以访问来自众多海洋岛群的独特样品存档，并将分析这些样品中晶体的组成，以限制地幔的组成。然后，我们将通过计算每个组成对地震波通过的预期响应来测试这些组成估计。这种方法利用了最近开发的矿物质模型。最后，我们将使用更新的地震观测数据集和新精炼的地震技术进行比较这些预期的地震反应与在岛屿群体下实际观察到的那些反应。

项目成果

The Composition of Melts from a Heterogeneous Mantle and the Origin of Ferropicrite: Application of a Thermodynamic Model

非均质地幔熔体的组成和铁苦铁矿的起源：热力学模型的应用

• DOI: 10.17863/cam.6875
• 发表时间: 2016
• 作者: Jennings E

Melting of Peridotites through to Granites: A Simple Thermodynamic Model in the System KNCFMASHTOCr

• DOI: 10.1093/petrology/egy048
• 发表时间: 2018-05-01
• 期刊: JOURNAL OF PETROLOGY
• 影响因子: 3.9
• 作者: Holland, Tim J. B.;Green, Eleanor C. R.;Powell, Roger
• 通讯作者: Powell, Roger

Global influence of mantle temperature and plate thickness on intraplate volcanism.

• DOI: 10.1038/s41467-021-22323-9
• 发表时间: 2021-04-06
• 期刊: Nature communications
• 影响因子: 16.6
• 作者: Ball PW;White NJ;Maclennan J;Stephenson SN
• 通讯作者: Stephenson SN

Olivine-hosted melt inclusions as an archive of redox heterogeneity in magmatic systems

• DOI: 10.1016/j.epsl.2017.09.029
• 发表时间: 2014-12
• 期刊: Earth and Planetary Science Letters
• 影响因子: 5.3
• 作者: M. Hartley;O. Shorttle;J. Maclennan;Y. Moussallam;M. Edmonds