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

在数百万年的平均时间里，海山或岛屿的熔岩生成速度决定了其最终的形状和大小。沿着沿着长海山链，如夏威夷-皇帝海山链，观察到海山和岛屿的体积发生周期性变化，表明形成海山和岛屿的熔岩产生率发生变化。形成这些岛屿的熔岩的来源被认为是地幔柱的熔化：上涌，热的，化学富集的物质的固定管道，起源于地球地幔深处，并不断上升到表面。然而，与观测相反，一个持续上涌的管道将产生一个几乎恒定的熔岩生产率;本项目旨在解决中断或扰动一个持续上涌的地幔柱的过程，因此，熔岩生产率沿着热点岛链。了解控制熔岩生产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，上涌羽流可能会受到扰动：1）核幔边界，在那里，异常致密的物质可能会进入羽流源并改变上涌速率; 2）中地幔，在那里，地幔岩石的相态或矿物结构的突然变化会改变密度，从而改变通过这些过渡的羽流的上涌速率。上升流速率的变化在这些机制之间可能会有所不同，并将导致地表熔岩产生速率随时间变化，这些变化取决于负责的机制。拟议的工作的目的是使用实验室实验和三维数值模拟相结合，以量化的幅度，长度和时间尺度的变化，在地幔柱上涌引起的深地幔过程将影响熔岩生产和成分在地球表面，改变形状和大小的岛链。爆发熔岩的化学性质和流量在热点海山链之间有着惊人的差异;有些热点具有几乎相同的化学来源和火山产出（例如，Kerguelen），其他的则在数百万年内发生变化（例如，夏威夷），还有一些似乎随时间缓慢减少（例如，路易斯维尔）。控制这种热点间变化以及热点内变化的过程受到很差的约束。 拟议的工作将量化两个深地幔机制影响羽流上涌的程度，反过来，观察到的热点的表面表现：1）夹带的物质从大低剪切波速度省（LLSVP），2）与地幔过渡区的相互作用。该项目将通过实验室和数值实验相结合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生产和喷发的熔岩成分。第一个目标将是通过收集一个全球数据库，记录自然界中地表变化的范围，该数据库包括热点岩浆活动过剩、火山间距以及沿沿着年龄递增热点轨迹的地球化学数据（主要和微量元素以及放射性同位素）。接下来，研究人员将进行补充的实验室和数值实验，以量化上述每个机制的物理特性。最后，包括这两种机制的数值模拟将评估其对地幔柱表面表现的影响。实验室实验将在装满葡萄糖浆的玻璃壁罐中进行，作为地幔的模拟物。 使用3D有限差分标记单元代码，将对每个过程运行两组数值模拟：1）与实验室实验相同条件的初始模拟，以验证数值方法，以及2）扩展到更类似地球的条件。在整个拟议的工作中，汇编的数据库的观察，从自然热点轨道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.