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

热点火山活动的变化是理解深部地幔动力学的关键

• 批准号: 1520856
• 负责人: Eric Mittelstaedt
• 金额: $ 22万
• 依托单位: Regents of the University of Idaho
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2015
• 资助国家: 美国
• 起止时间: 2015-07-01 至 2019-06-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1520856&HistoricalAwards=false
• 关键词: Variations; Hotspot; Volcanism; Key; Understanding

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.

平均数百万年，海山或岛屿的熔岩产生速度控制着其最终的形状和大小。沿着长海山链，例如夏威夷-皇帝链，观察到的海山和岛屿体积会发生间歇性变化，表明导致其形成的熔岩生产率发生变化。形成这些岛屿的熔岩的来源被认为是地幔柱的融化：来自地幔深处并不断上升到地表的热的、化学丰富的物质的上升、固定管道。然而，与观察相反，连续上升的管道会产生几乎恒定的熔岩生产率；该项目旨在解决中断或扰乱持续上涌的地幔柱的过程，从而解决热点岛链沿线熔岩产生的速度。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处动态之间的基本联系。地幔中有两个位置可能会扰动上升流羽流：1）核心-地幔边界，异常致密的物质可能会融入羽流源并改变上升流速率；2）在中地幔中，地幔岩石的相或矿物结构的突然变化可以改变密度，从而改变通过这些转变的羽流的上涌率。这些机制之间的上升流速率的变化可能会有所不同，并且会导致地表熔岩生产率随时间变化，具体变化取决于负责的机制。拟议工作的目的是结合实验室实验和 3D 数值模拟来量化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着差异。一些热点地区的化学来源和火山喷发量几乎一致（例如，凯尔盖朗），另一些热点地区则在数百万年的时间里偶尔发生变化（例如，夏威夷），而另一些地区则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。 拟议的工作将量化两种深部地幔机制影响羽流上升的程度，进而观察到热点的表面表现：1）来自大低剪切波速度省（LLSVP）的物质夹带，以及2）与地幔过渡带。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，以量化深层地幔动力学如何导致地幔柱上涌变化的可预测幅度、长度和时间尺度，从而导致熔体产生和喷发熔岩成分在地球表面。第一个目标是通过收集过量热点岩浆作用、火山间距以及沿年龄渐进热点轨迹的地球化学数据（主量元素和微量元素以及放射性同位素）的全球数据库来记录自然界表面变化的范围。接下来，研究人员将进行补充实验室和数值实验，以分别量化上述每种机制的物理原理。最后，包括这两种机制的数值模拟将评估它们对地幔柱表面表现的影响。实验室实验将在装有葡萄糖浆的玻璃壁罐中进行，作为地幔的类似物。 使用 3D、有限差分、单元内标记代码，将为每个过程运行两组数值模拟：1）使用与实验室实验相同的条件进行初始模拟，以验证数值方法，2）扩展到更多类似地球的条件。在整个拟议工作中，自然热点轨迹观测数据的汇编将限制实验室和数值结果。