Probing Mantle Heterogeneity: A Petrological Reconciliation for Geochemistry and Seismology

在地球的剖面图卡通模型中，地幔被描绘成一个均匀的层，从我们脚下大约20英里延伸到1800英里（或2890公里）的深度。地幔是地球系统中最大的部分，尽管它非常晦涩，但它在塑造地球及其环境的长期演变中起着重要作用。岩浆为火山喷发提供燃料，并提供维持宜居星球所需的化学元素混合物，这些岩浆是由地幔融化产生的。数十亿年来地球热量的流失推动了地幔的运动，而地幔又控制着构造板块的运动。这些搅动还会导致地球表面的地质快速运动，从而改变海洋的形状，改变海洋环流并改变气候模式。即使是最深的钻芯也无法穿透原始的地幔岩石。由于难以接近，科学家们不得不基于间接手段建立地球深层内部的模型。对陨石化学的研究为了解地球的主要成分提供了线索。当板块碰撞时，罕见的地幔碎片在山带中暴露出来，某些奇特类型的火山喷发将小块地幔带到地表。然而，人们仍然担心，这些非同寻常的样本并不能提供一个关于深部地幔的平衡观点。因此，地球科学家将来自陨石和地幔样本的成分证据与另外两种类型的观测结合起来，以建立他们的模型。第一种是在大地震后通过跟踪地震波在地球上的传播过程而获得的地震数据。某些深度的矿物变化会使地幔物质的性质发生很大变化，因此地震波在这些深度会被反射。反射模式对地幔成分很敏感，是地幔成分模型的重要检验。第二种进一步的观察来自对玄武岩成分的研究，玄武岩是全球各地火山喷发出的地幔融化物。为了估计地幔成分，有必要了解熔融过程如何改变玄武岩的成分，使其远离固体地幔源的成分。在考虑了这些观测结果后，达成了一个共识，即地幔的组成实际上是均匀的，并且最有利的地幔组成被称为软锰矿。然而，同位素地球化学家在研究海洋玄武岩的组成时，发现了地幔化学发生强烈变化的证据。起初，这些同位素变化并没有被视为对软锰矿模型的挑战，因为它们不被认为与地幔矿物学的变化相对应。然而，最近有争议的研究认为，在玄武岩中观察到的同位素变化与矿物变化有关。特别是，这些研究人员提出，海洋岛屿火山（如夏威夷）下的地幔在成分上与软锰矿非常不同。如果这个结论是正确的，它将改变地球科学家对地幔的看法。我们的目标是测试这一有争议的研究，并通过采用新的跨学科方法开发一个新的地幔非均质性模型。我们可以访问来自各种海洋岛屿群的独特样本档案，并将分析这些样本中的晶体组成，以限制地幔的组成。然后，我们将通过计算每种成分对地震波通过的预期响应来测试这些成分估计。这种方法利用了最近开发的矿物性质模型。最后，我们将使用更新的地震观测数据集和最新改进的地震技术，将这些预期地震反应与在岛群下实际观测到的地震反应进行比较。

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.