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

地幔羽流的经典概念描述了一种热浮力上升流，它通过整个地幔上升，形成一个薄（约100公里）的煎饼。岩石圈下的热物质形成了热点火山活动。虽然这一理论在其简单性和解释各种观测结果的能力上是优雅的，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，羽流地震层析成像结果显示了一个羽状体起源于下地幔的令人信服的证据，然而，它们也揭示了上地幔中的一个低速体，它看起来太厚，不对称，与经典的热煎饼不符。在南太平洋，分布在广阔的南太平洋超级井中的一组热点往往都是短暂的，年龄进展不一致，并且与大型火成岩省没有联系。因此，这些热点的经典羽流理论几乎被抛弃了，取而代之的是相对较小、寿命较短的“羽流”假说。从一个巨大的，超级羽流的顶部升起？它在中地幔中停滞。对于许多海洋岛屿，包括南太平洋和夏威夷的岛屿，地球化学证据表明，地幔中的基性物质来源？不仅仅是温度过高？有助于火山活动。在研究人员中？最近的数值模拟表明，热浮力强但成分（部分榴辉岩）致密的地幔上涌表现出不规则和随时间变化的形式，可能解释上述许多观测结果。的确,热化学?地幔对流是一个研究热点，但对热化学羽流与地幔相变和岩石圈板块运动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的定量研究却很少。该项目有三个主要目标。(1)探索地幔过渡带和上地幔热化学羽流的物理性质，并将不同形式的上升流作为羽流半径（如超级羽流、夏威夷型羽流）、过热温度和榴辉岩含量等特性的函数进行表征。(2)建立上述性质与岩浆活动的分布、体积和基性含量、膨胀几何形状和地幔地震结构等可普遍应用于全球热点的观测值之间的关系。(3)通过对比模型预测与地球化学和地球物理约束，特别是夏威夷的羽流体波层析成像，验证夏威夷和南太平洋热点地区的热化学羽流假说。这项研究将有助于开发一类新的羽流概念，这些概念既可以由质量不断提高的地球物理和地球化学数据集驱动，也可以通过这些数据集进行测试。热化学对流表现出如此丰富多样的形态和动力机制，因此这项高分辨率的建模研究在发现与热点和其他上地幔过程相关的尚未被认识的行为方面具有良好的潜力。

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.