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

超越“经典”地幔羽流概念：热化学羽流的上升动力学、地震结构和部分熔融

• 批准号: 1141938
• 负责人: Garrett Apuzen-Ito
• 金额: $ 17.57万
• 依托单位: University of Hawaii
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2012
• 资助国家: 美国
• 起止时间: 2012-08-01 至 2015-07-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1141938&HistoricalAwards=false
• 关键词: Beyond; Classical; Mantle; Plume; Concept

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.

地幔柱的经典概念描述了一种热浮力上升流，它上升穿过整个地幔，为一层薄薄的（约 100 公里）“薄饼”提供能量。热物质在岩石圈下积水并产生热点火山活动。虽然这一理论因其简单性和解释各种观察结果的能力而优雅，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，羽流地震层析成像结果显示了令人信服的证据，证明羽状体起源于下地幔，然而，它们也揭示了上地幔中的低速体，该体看起来太厚且不对称与经典的热煎饼保持一致。在南太平洋，分布在广阔的南太平洋超级涌流中的一组热点往往都是短暂的，显示出不一致的年龄进程，并且与大型火成岩省无关。因此，对于这些热点，经典的羽流理论几乎被抛弃，取而代之的是相对较小、寿命较短的“羽流”假设。从一个巨大的“超级羽”的屋顶上升起它停滞在中地幔中。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，地幔源中的镁铁质物质（不仅仅是温度过高）有助于火山活动。在研究人员中？最近的数值模拟显示，热浮力但成分（部分榴辉岩）致密的地幔上升流表现出不规则且随时间变化的形式，有可能解释上述许多观察结果。事实上，“热化学”？地幔对流是一个热门研究课题，但在定量探索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所做的工作却很少。该项目有 3 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，并表征不同形式的上升流与羽流半径（例如超羽流、夏威夷型羽流）、超温和榴辉岩含量等性质的函数关系。 (2) 建立上述性质与可普遍适用于全球热点的可观测数据之间的关系，例如岩浆活动的分布、体积和基性成分、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，特别是夏威夷的羽流体波层析成像，检验夏威夷和南太平洋热点地区的热化学羽流假说。这项研究将有助于开发一类新的羽流概念，该概念既受质量不断提高的地球物理和地球化学数据集的启发，又可以根据质量不断提高的地球物理和地球化学数据集进行测试。热化学对流表现出如此丰富的形状和动态状态多样性，因此这项高分辨率模拟研究对于发现与相关热点和其他上地幔过程有关的尚未识别的行为具有巨大的潜力。