CSEDI Collaborative Research: Optical investigations of a mantle plume laboratory model

地幔内羽流的概念在将深部地幔对流与火山活动、地形抬升和地球化学特征的地表表现联系起来方面发挥了核心作用。然而，这一概念仍然不完整。有两个问题仍有待解决，以完成我们对地幔热柱及其在地球动力学和地球化学演化中的作用的理解。第一个，也是激烈争论的主题，是内部加热的对流地幔中羽流的存在和生存，与板块运动相关的大范围流动占主导地位。第二个问题涉及洋岛玄武岩的同位素特征所揭示的地球化学变化程度，我们认为这是地幔羽流的表面表现。为了解决这两个问题，PI Lithgow-Berelloni，Cotel，Whitehead和Hart正在进行均匀和分层流体中单一热柱和成分羽流的实验室实验。本实验的主要目标是：首先，准确测量和表征羽流演化过程中的浮力通量随初始热流密度和流体组成的变化规律；其次，分析大尺度环流引起的剪切对羽流破碎和管道内搅拌的影响。重点在于使用新的非侵入性光学技术(粒子图像测速、激光诱导荧光、光偏转和用于数字粒子图像测温的热色液晶)。这些方法除了可以同时测量组成和温度外，还可以直接测量三维流动结构。在接下来的三年里，PIs打算提供严格的标度关系，以阐明--导致稳定的羽流和稳定的导管或上升时羽流破裂的条件--与地震成像有关的羽流结构的时空变化--携带物质在热和热化学羽流中存活和相互作用的尺度--与大洋岛屿基岩的同位素地球化学变异性的联系。资助的项目涉及四个具有从地球物理流体动力学到地球化学的广泛科学和技术专长的PI之间的密切合作。因此，这些实验的结果可能会引起地球科学界不同阶层的兴趣。所有四个PI都参与了实验设计和结果的解释。

The concept of plumes within the mantle of the earth has played a central role in linking deep mantle convection to surface expressions of volcanism, topographic uplift, and geochemical signatures. However, the concept continues to remain incomplete. Two issues remain to be resolved to complete our understanding of mantle plumes and their role in the dynamical and geochemical evolution of the planet. The first, and the subject of vigorous controversy, is the very existence and survival of plumes in an internally heated, convecting mantle dominated by the large-scale flow associated with plate motions. The second relates to the degree of geochemical variation revealed by the isotopic signatures of ocean island basalts, which we identify as the surface expression of mantle plumes.In order to address both issues, PIs Lithgow-Bertelloni, Cotel, Whitehead and Hart are conducting laboratory experiments on single thermal and compositional plumes in homogeneous and stratified fluids. The primary goals of the present experiments are first, to accurately measure and characterize the buoyancy flux throughout an evolving plume as a function of the initial heat flux, and fluid composition, and second, to analyze the effects of shearing induced by the large-scale circulation on plume breakup and on stirring within the conduit. The emphasis lies in the use of new non-intrusive optical techniques (Particle Image Velocimetry, Laser-Induced Fluorescence, light deflection, and thermochromic liquid crystals for Digital Particle Image Thermometry). These can directly measure the three-dimensional flow structure in addition to giving the simultaneous measurements of composition and temperature. Over the next three years the PIs intend to provide rigorous scaling relationships to illuminate - The conditions that lead to steady plumes and steady-state conduits or plume breakup upon ascent- Spatial and temporal variations in plume structure as it relates to seismic imaging- The scales at which entrained material survives and interacts in thermal and thermochemical plumes- The link with the isotopic geochemical variability of ocean island basalt.The funded project involves close collaboration between four PIs with widely varying scientific and technical expertise ranging from geophysical fluid dynamics to geochemistry. Hence, the results of these experiments are likely to be of interest to diverse segments of the Earth Sciences community. All four PIs are involved in the experimental design and interpretation of the results.