Convection clashes: Plume splitting beneath eastern Australia

地幔对流是一个基本的动态过程，深刻地影响着我们星球的表面，影响着板块构造、长期海平面变化和气候等多种过程。对流需要在下沉到地幔深处的物质和上升到地表的物质之间保持平衡。虽然我们知道下降流物质主要是由俯冲板片，最终下沉数千公里到地幔，所需的深部上升流物质的位置，持续时间和动力学更加模糊。这种上升流最著名的地表迹象是板内火山活动（火山活动位于远离板块边界的地方），通常与从下地幔上升的热物质的羽状物（或“热点”）有关。当这些羽流的宽头到达地表产生大的火成岩省（LIP）时，就发生了特别大量的板内火山活动。这些LIP通过释放大量的气体（如硫酸）影响世界大气，改变气候，对海洋、大气和生物产生破坏性影响（即，大规模预防）。在LIP之后的热点火山的踪迹也被证明是发现构造板块过去运动的有力工具。由于这些原因，了解板内火山活动的起源和演化是地球科学的重要组成部分。热点板内火山活动的经典例子是太平洋板块上的夏威夷：一系列火山岛和水下山脉（“海山”）向西北延伸，距离活跃喷发的夏威夷岛越远，它们就越古老。然而，地球上的板内火山活动是非常多样的。许多地区并不符合热柱从深地幔上升的经典模型，而是似乎是由上地幔的过程引起的，或者具有深、浅混合的特征。对于本项目，澳大利亚东部海域是研究板内火山活动形成过程的理想区域。这一地区被三个平行的板内火山链穿过，这些火山在3500万到600万年前同时爆发。这些火山高达5公里，直径100公里，几乎完全淹没在海洋之下。链条的长寿命和特殊的年龄增长是经典的深层上涌源的有力指标，但三条链条的配置挑战了我们对地球基本驱动力的理解。无论是三个间隔很近的羽流（相距约500公里），还是一个上涌层，都不符合我们对底层物理的理解：它们要么是不稳定的，要么是在地球地幔对流模型中没有观察到的。相反，这些观测结果表明，当它接近地表时，可能是由于地幔中的障碍物或地幔对流中的漩涡造成的深层上升流分裂。这一提议建立在与澳大利亚的合作基础上，澳大利亚已经资助了一次为期28天的航行（价值约180万英镑），以收集岩石样本并进行地球物理研究。这次航行的目标将是两个海洋链，以及Tasmantids以北的Louisiade高原（一个10万平方公里的隆起海底区域，可能是LIP）。我们将使用多方面的方法来研究这些火山，结合年表（以确定它们的年龄），化学（以确定什么类型的地幔熔化）和地球动力学建模（以检查形成火山的地幔过程）。地球动力学模型还将应用于加那利群岛和科摩罗群岛（分别位于非洲西部和东部），以研究地球其他地方板内火山背后的机制。这个项目将使我们深入了解神秘的板内火山活动的形成，以及从地幔深处流出的物质在上升时如何与障碍物相互作用。

Convection of the Earth's mantle is a fundamental dynamic process that profoundly influences the surface of our planet, affecting processes as diverse as plate tectonics, long-term sea level change, and climate. Convection requires a balance between material sinking deep into the mantle and rising towards the surface. Whilst we know that downwelling material is dominated by subducting slabs that eventually sink thousands of km into the mantle, the locations, durations, and dynamics of the required deep upwelling material are much more ambiguous. The best-known surface indication of such upwelling is intraplate volcanism (volcanism located far from plate boundaries), classically associated with plumes (or 'hotspots') of hot material rising from the lower mantle. Particularly voluminous examples of intraplate volcanism occur when the broad heads of these plumes reach the surface to produce large igneous provinces (LIPs). These LIPs affect the world's atmosphere via the release of massive amounts of gases like sulphuric acid, changing the climate with damaging effects on the ocean, atmosphere, and biology (i.e., mass extinctions). The trails of hotspot volcanoes that come after the LIP have also proved a powerful tool in discovering the past motions of tectonic plates. For these reasons, understanding the origins and evolution of intraplate volcanism is an important part of Earth science.The classic example of hotspot intraplate volcanism is Hawaii on the Pacific plate: a series of volcanic islands and submerged undersea mountains ('seamounts') that stretches away to the northwest, becoming progressively older the further they are from the actively erupting island of Hawaii. However, intraplate volcanism on Earth is very diverse. Many localities do not fit the classic model of a hot plume rising from the deep mantle, but instead appear to have been caused by processes in the upper mantle or have a mix of deep and shallow characteristics.For this project, the seas off Eastern Australia are an ideal region for studying the processes involved in the formation of intraplate volcanism. This region is crossed by not one, but three sub-parallel chains of intraplate volcanoes, which erupted simultaneously between 35 and 6 million years ago. These volcanoes are up to 5 km high and 100 km across, and are almost entirely submerged beneath the ocean. The long life and exceptional age progression of the chains are strong indicators of a classic deep upwelling source, but the configuration of the three chains challenges our understanding of this fundamental driving force of our planet. Neither three closely spaced plumes (~500 km apart) nor an upwelling sheet fit well with our understanding of the underlying physics: they are either unstable or are not observed in models of Earth's mantle convection. Instead, these observations suggest a deep upwelling splitting as it nears the surface, perhaps due to obstacles in the mantle, or eddies in the mantle convection.This proposal builds on a collaboration with Australia, who has already funded a 28 day voyage (worth ~£1.8 million) to collect rock samples and carry out geophysical studies. The voyage will target the two marine chains, as well as the Louisiade Plateau (a 100,000 square km area of raised seafloor that could be a LIP) north of the Tasmantids. We will study these volcanoes using a multi-faceted approach combining chronology (to determine their ages), chemistry (to determine what type of mantle melted), and geodynamic modelling (to examine the processes in the mantle that formed the volcanoes). The geodynamic models will also be applied to the Canary and Comoros Islands (west and east of Africa, respectively) to examine the mechanisms behind intraplate volcanoes elsewhere on the planet. This project will give us significant insight into the formation of enigmatic intraplate volcanism and how material flowing from deep in the Earth's mantle interacts with obstacles as it rises.