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

经典的地幔热柱概念描述了一股热力上升流，它上升穿过整个地幔，形成一个薄的(约100公里)？薄煎饼？形成岩石圈下面的热物质，并产生热点火山作用。虽然这一理论在其简单性和解释各种观察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令人信服的证据，证明了起源于下地幔的羽状天体，然而，它们也揭示了上地幔中的一个低速天体，看起来太厚和不对称，与经典的热煎饼不一致。在南太平洋，分布在广阔的南太平洋超级巨井上的一群热点往往都是短暂的，显示出不一致的年龄进程，并且与一个大的火成岩省没有联系。因此，经典的羽流理论几乎被这些热点所抛弃，取而代之的是相对较小、寿命较短的羽毛？从一个巨大的超大烟柱的屋顶升起？这在地幔中段停滞不前。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，地幔来源中的镁铁质物质--不仅是过高的温度--促成了火山活动。在研究人员身上？最近的数值模拟显示，地幔上涌具有热浮力，但成分(部分榴辉岩)致密，呈现出不规则的、与时间相关的形式，有可能解释上述许多观测结果。的确，？热化学？地幔对流是一个热门的研究课题，但对热化学热柱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工作很少。(1)探索地幔过渡带和上地幔中热化学羽流的物理学，并描述不同形式的上涌，作为羽流半径(如超强羽流、夏威夷型羽流)、过高温度和榴辉岩含量等性质的函数。(2)建立上述性质与岩浆活动的分布、体积和镁铁质含量、隆起几何形状、地幔地震结构等可普遍适用于全球热点地区的观测数据之间的关系。(3)通过比较模式预测与地球化学和地球物理约束，特别是夏威夷的羽状体波层析成像，检验夏威夷和南太平洋热点地区的热化学羽流假说。这项研究将有助于发展一类新的羽流概念，这类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推动，也可以针对这些数据集进行测试。热化学对流表现出如此丰富的形状和动力机制，因此这种高分辨率的模拟研究对于发现相关热点和其他上地幔过程中尚未被认识的行为具有很好的潜力。

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.