Beyond the 'Classical' Mantle Plume Concept: Upwelling Dynamics, Seismic Structure, and Partial Melting of Thermochemical Plumes

地幔柱的经典概念描述了一个热浮力上升流，通过整个地幔上升，饲料薄（~100公里）？煎饼？热物质在岩石圈下形成积水，并产生热点火山活动。虽然这一理论在其简单性和解释各种观测的能力方面是优雅的，但最近的发现表明，这种理想化可能不再适用于所有热点。例如，在典型的夏威夷热点地区，PLUME地震层析成像结果显示了一个源自下地幔的羽状体的令人信服的证据，然而，它们也揭示了上地幔中的低速体，它看起来太厚和不对称，与经典的热饼不一致。在南太平洋，一群热点居住在广阔的南太平洋超级井中，每个热点往往是短暂的，显示出不一致的年龄进展，并且没有连接到一个大的火成岩省。因此，经典的羽流理论几乎被抛弃了这些热点，让位于假设，相对较小，短暂的？铅？从一个巨人的屋顶上升起超级羽毛？停滞在中地幔中对于许多大洋岛屿，包括南太平洋和夏威夷，地球化学证据表明，镁铁质物质的地幔来源？不仅仅是温度过高有助于火山活动。在研究人员？最近的数值模拟表明，热浮力但成分（部分榴辉岩）致密的地幔隆起显示出不规则和随时间变化的形式，有可能解释上述许多观察结果。是吗？热化学？地幔对流是一个十分活跃的研究课题，但在定量研究热化学羽流与地幔相变和移动岩石圈板块相互作用的动力学过程、熔融行为、地球物理表现和地球化学后果方面的工作还很少。(1)探索地幔过渡带和上地幔中热化学羽流的物理学，并描述不同形式的隆起作为羽流半径（例如超级羽流，夏威夷型羽流），超额温度和榴辉岩含量等属性的函数。(2)建立上述属性和可观测值之间的关系，这些可观测值通常适用于全球热点，如岩浆活动的分布、体积和镁铁质含量、膨胀几何形状和地幔地震结构。(3)通过将模型预测与地球化学和地球物理约束进行比较，特别是夏威夷的PLUME体波层析成像，检验夏威夷和南太平洋热点的热化学羽流假说。这项研究将有助于发展一种新的羽流概念，这种概念既受到质量不断提高的地球物理和地球化学数据集的启发，又可以根据这些数据集进行测试。热化学对流显示出如此丰富的多样性的形状和动力机制，因此，这种高分辨率的模拟研究具有很好的潜力，发现尚未认识到的行为，是相关的热点和其他上地幔过程。

The classical concept of mantle plumes describes a thermally buoyant upwelling that rises through the entire mantle to feed a thin (~100 km) ?pancake? of hot material ponding beneath the lithosphere and spawn hotspot volcanism. While this theory is elegant in its simplicity and its ability to explain a variety of observations, recent discoveries suggest that this idealization may no longer be tenable for all hotspots. At the archetypal Hawaiian hotspot, for example, the PLUME seismic tomography results shows compelling evidence for a plume-like body originating in lower-mantle, however, they also reveal a low-velocity body in the upper mantle that appears far too thick and asymmetric to be consistent with a classical thermal pancake. In the South Pacific, a cluster of hotspots populating the broad South Pacific Superswell each tend to be short-lived, show inconsistent age progressions, and are not connected to a large igneous province. Consequently, the classical plume theory has all but been discarded for these hotspots, giving way to the hypothesis that relatively small, short-lived ?plumelets? rising from the roof of a giant ?superplume? that is stagnating in the mid mantle. For many ocean islands, including those in the South Pacific and Hawaii, geochemical evidence reveals that mafic materials in the mantle source?not only excess temperature?contribute to volcanism. In the researchers? recent numerical simulations, mantle upwellings that are thermally buoyant but compositionally (partially eclogite) dense show irregular and time-dependent forms with potential for explaining many of above observations. Indeed, ?thermochemical? mantle convection is topic of vigorous research but very little work has been done to quantitatively explore the dynamical processes, melting behavior, geophysical manifestations, and geochemical consequences of thermochemical plumes interacting with mantle phase changes and a moving lithospheric plate.The project has 3 main objectives. (1) Explore the physics of thermochemical plumes in the mantle transition zone and upper mantle and characterize the different forms of upwellings as a function of properties such as plume radius (e.g. superplume, Hawaiian-type plume), excess temperature, and eclogite content. (2) Establish relationships between the above properties and observables that can apply generally to hotspots world-wide such as the distribution, volume, and mafic content of magmatism, swell geometry, and mantle seismic structure. (3) Test the thermochemical plume hypothesis for hotspots in Hawaii, and the South Pacific by comparing model predictions with geochemical and geophysical constraints, especially the PLUME body wave tomography for Hawaii. This study will help develop a new class of plume concepts that is both motivated by, and can be tested against geophysical and geochemical data sets of ever increasing quality. Thermochemical convection displays a such rich diversity of shapes and dynamic regimes, and therefore this high-resolution modeling study has excellent potential for discovering yet unrecognized behaviors that are relevant hotspots and other upper mantle processes.