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

在数百万年的平均时间里，一座海山或岛屿产生熔岩的速度控制着它的最终形状和大小。沿着长长的海山链，如夏威夷-帝王链，观察到的海山和岛屿体积周期性地变化，表明导致它们形成的熔岩产生率发生了变化。形成这些岛屿的熔岩的来源被认为是地幔热柱的融化：来自地幔深处并不断上升到地表的热的、化学富集物的静止管道。然而，与观测相反，持续上升的管道将产生几乎恒定的熔岩产生速率；该项目旨在解决中断或扰动持续上升的地幔热柱的过程，从而沿着热点岛链产生熔岩的速率。了解控制熔岩产生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处动态之间的基本联系。在地幔内有两个位置上涌热柱可能受到扰动：1)核-地幔边界，在那里异常致密的物质可能被并入到热柱来源中，从而改变上涌速率；2)在中地幔，地幔岩石的相态或矿物结构的突然变化可能改变通过这些转变的热柱的密度，从而改变热柱的上涌速率。上升流速率的变化可能在这些机制之间有所不同，并将导致地表熔岩产生率的时间变化，这取决于相关机制的不同。这项拟议工作的目的是利用实验室实验和3D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上涌的变化将影响地球表面的熔岩产量和成分，从而改变岛链的形状和大小的规模、长度和时间尺度。熔岩喷发的化学成分和流量在不同的热点海山链之间存在显著差异；一些热点具有几乎一致的化学来源和火山产出(例如Kerguelen)，另一些热点在数百万年内周期性地变化(例如夏威夷)，而其他热点似乎随着时间的推移而缓慢减少(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的约束很差。这项拟议的工作将量化两种深地幔机制对热柱上升的影响程度，进而观察到热点的表面表现：1)来自大的低剪切波速度省(LLSVP)的物质夹带，以及2)与地幔过渡带的相互作用。该项目将通过联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可预测的地幔热柱上升流的大小、长度和时间尺度的变化，从而在地球表面产生熔体和喷发的熔岩成分。第一个目标将是记录自然界地表变异性的范围，方法是汇编一个全球数据库，其中包括过剩热点岩浆活动、火山之间的间隔以及沿年龄递增的热点轨迹的地球化学数据(常量元素和微量元素以及放射性同位素)。接下来，研究人员将进行互补的实验室和数值实验，以分别量化上述每一种机制的物理性质。最后，包括这两种机制的数值模拟将评估它们对地幔热柱表面表现的影响。实验室实验将在装满葡萄糖糖浆的玻璃墙储罐中进行，作为地幔的类似物。使用3D、有限差分、单元标记代码，将为每个过程运行两套数值模拟：1)与实验室实验具有相同条件的初始模拟以验证数值方法，以及2)扩展到更类似于地球的条件。在整个拟议的工作中，从自然热点轨迹汇编的观测数据库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.