Variations in Hotspot Volcanism as a Key to Understanding Deep Mantle Dynamics

平均数百万年，海底山或岛屿的熔岩产生速度决定了其最终的形状和大小。沿着长长的海山链，如夏威夷-皇帝链，观察到的海山和岛屿体积的变化是间歇性的，这表明导致它们形成的熔岩产生速度的变化。形成这些岛屿的熔岩的来源被认为是地幔柱的融化：由热的、化学成分丰富的物质组成的上涌的、静止的管道，它们起源于地幔深处，并不断上升到地表。然而，与观测结果相反，持续上涌的管道将产生几乎恒定的熔岩产生速率；该项目旨在研究中断或扰乱持续上涌的地幔柱的过程，从而研究热点岛链上熔岩产生的速度。了解控制熔岩生成速率随时间变化的过程，将有助于深入了解地表火山与地球深部动力学之间的基本联系。在地幔中有两个位置上升流羽流可能受到扰动：1)核幔边界，异常致密的物质可能被纳入羽流源并改变上升流速率；2)在中地幔，地幔岩石的相位或矿物结构的突变可以改变密度，从而改变通过这些过渡的羽流的上升流速率。上涌速率的变化可能在这些机制之间有所不同，并将导致表面熔岩产生速率的时变变化，这种变化取决于所负责的机制。这项工作的目的是利用实验室实验和三维数值模拟相结合的方法来量化由深部地幔过程引起的地幔柱上涌变化的幅度、长度和时间尺度，这些变化将影响地球表面熔岩的产生和成分，改变岛链的形状和大小。热点海山链之间喷发熔岩的化学成分和通量存在显著差异；一些热点地区的化学来源和火山喷发几乎是一致的（如凯尔盖伦岛），另一些热点地区在数百万年里偶尔发生变化（如夏威夷），还有一些热点地区似乎随着时间的推移而缓慢减少（如路易斯维尔岛）。控制这种热点间变异性以及热点内变异性的过程受到的约束很差。所提出的工作将量化两种深部地幔机制对地幔柱上涌的影响程度，进而观测到热点的地表表现：1)来自大低剪切波速度省（LLSVP）的物质携带，以及2)与地幔过渡带的相互作用。该项目将通过结合实验室和数值实验来限制这两种机制的影响，以量化深部地幔动力学如何导致地幔柱上涌的可预测幅度、长度和时间尺度的变化，从而导致地球表面的熔体产生和喷发的熔岩成分。第一个目标将是通过建立一个全球数据库来记录地表自然变化的范围，该数据库包括过量热点岩浆活动、火山之间的间隔以及沿着年龄增长的热点轨迹的地球化学数据（主要和微量元素以及放射性成因同位素）。接下来，研究人员将进行互补的实验室和数值实验，分别量化上述每种机制的物理特性。最后，数值模拟包括这两种机制将评估它们对地幔柱表面表现的影响。实验室实验将在装满葡萄糖糖浆的玻璃壁罐中进行，葡萄糖糖浆是地幔的类似物。使用三维、有限差分、细胞内标记代码，将对每个过程运行两组数值模拟：1)在与实验室实验相同的条件下进行初始模拟，以验证数值方法；2)扩展到更类似地球的条件。在整个拟议的工作中，从自然热点轨道上收集的观测数据将限制实验室和数值结果。

Averaged over millions of years, the rate of lava production at a seamount or island controls its final shape and size. Along long seamount chains, such as the Hawaiian-Emperor chain, observed seamount and island volumes change episodically indicating changes in the lava production rate responsible for their formation. The source of lavas forming these islands is believed to be the melting of mantle plumes: upwelling, stationary conduits of hot, chemically enriched material that originate from deep within the Earth's mantle and rise continuously to the surface. However, in contrast to observations, a continuously upwelling conduit would produce a nearly constant lava production rate; this project aims to address the processes that interrupt or perturb a continuously upwelling mantle plume, and, thus, the rate of lava production along hotspot island chains. Understanding the processes that control changes in lava production rate through time will provide insights into the fundamental connections between volcanoes at the surface and the dynamics of the deep Earth. There are two locations within the mantle where upwelling plumes are likely to be perturbed: 1) the core-mantle boundary, where anomalously dense material may be incorporated into the plume source and change the upwelling rate, and 2) in the mid-mantle, where abrupt changes in the phase, or mineral structure, of mantle rocks can alter the density and, thus, upwelling rate of plumes passing through these transitions. Changes in the upwelling rates will likely differ between these mechanisms and will result in time-varying changes in lava production rate at the surface that differ depending upon the mechanism responsible. The purpose of the proposed work is to use a combination of laboratory experiments and 3D numerical simulations to quantify the magnitude, length, and time scales over which variations in mantle plume upwelling caused by deep mantle processes will affect lava production and compositions at the Earth's surface, changing the shape and size of island chains. The chemistry and flux of erupted lavas differ strikingly between hotspot seamount chains; some hotspots have nearly uniform chemical sources and volcanic output (e.g., Kerguelen), others vary episodically over millions of years (e.g., Hawaii), and yet others appear to decrease slowly with time (e.g., Louisville). The processes that control this inter-hotspot variability, as well as intra-hotspot variations, are poorly constrained. The proposed work will quantify the degree to which two deep-mantle mechanisms affect plume upwelling and, in turn, observed surface manifestations of hotspots: 1) entrainment of material from Large Low Shear-wave Velocity Provinces (LLSVP), and 2) interaction with the mantle transition zone. This project will constrain the impact of these two mechanisms through combined laboratory and numerical experiments to quantify how deep mantle dynamics lead to predictable magnitudes, length-, and time-scales of variations in mantle plume upwelling and, consequently, melt production and erupted lava compositions at the Earth's surface. The first objective will be to document the range of surface variability in nature by assembling a global database of excess hotspot magmatism, spacing between volcanoes, and geochemical data (major and trace elements, and radiogenic isotopes) along age-progressive hotspot tracks. Next, the investigators will conduct complementary laboratory and numerical experiments to quantify the physics of each of the above mechanisms individually. Finally, numerical simulations including both mechanisms will assess their impact on surface manifestations of mantle plumes. Laboratory experiments will be conducted in glass-walled tanks filled with glucose syrup as an analogue for the mantle. Using a 3D, finite-difference, marker-in-cell code, two sets of numerical simulations will be run for each process: 1) initial simulations with identical conditions to the laboratory experiments to verify the numerical approach, and 2) extension to more Earth-like conditions. Throughout the proposed work, the compiled database of observations from natural hotspot tracks will constrain the laboratory and numerical results.