CSEDI: Dynamics and Melting of a Heterogeneous Mantle: Importance to the Geochemical Variability of Hotspot and Mid-Ocean Ridge Magmas

提出的研究将解决固体地球动力学中的两个问题：(1)如何调和洋中脊玄武岩（MORBs）和热点洋岛玄武岩（OIBs）之间的地球化学差异，这些玄武岩被广泛地用化学层状地幔解释，与全地幔（即单层）对流的地球物理证据。（二）确定热点地区观察到的熔岩地球化学地理差异的起源。该假说认为，组成不均匀的地幔的熔融和韧性流动的变化导致了oib和morb的许多关键的地球化学特征。这一假设将通过模拟上地幔三维粘性流动、热传输和熔融的数值模型进行验证。模型的建立将基于热点是由浮力上升流或地幔柱与岩石圈底部相互作用形成的假设。熔化计算将结合不同成分材料熔化压力-温度条件下的实验室实验数据。计算结果将量化特定物理条件与喷发岩浆中痕量元素和Sr、Nd、Pb和He同位素地球化学预测的地理变动性之间的关系，在三种构造情景中存在足够的数据集进行比较：板块内热点（夏威夷）、以洋中脊为中心的热点（冰岛）和远离洋中脊但靠近洋中脊的热点（加拉帕戈斯群岛）。问题(1)将通过确定模型是否要求地幔柱（可能来自下地幔）在成分上与上地幔不同来解决。问题（II）将通过确定解释在上述类型热点观测到的空间地球化学变异性质所需的物理条件来解决。热点岛的起源和地幔整体对流与双层对流的问题具有广泛的科学意义，高中和大学教科书对这些问题的处理就证明了这一点。调查人员已经并将继续为当地的高中生和大学生举办关于这些主题的讲座和课外研讨会。大部分申请的预算将支持博士生，为一个全面的，跨学科的论文提供所需的资金。

The proposed study will address two problems in solid Earth dynamics: (I) How to reconcile the geochemical distinctions between mid-ocean ridge basalts (MORBs) and hotspot ocean island basalts (OIBs) which are widely interpreted in terms of a chemically layered mantle, with geophysical evidence for whole mantle (i.e., single layer) convection. (II) Determining the origin of the geographic variations in lava geochemistry observed at hotspots. The hypothesis is that variations in melting and ductile flow of a compositionally heterogeneous mantle cause many of the key geochemical characteristics of OIBs and MORBs. This hypothesis will be tested using numerical models that simulate the three-dimensional viscous flow, heat transport, and melting in the upper mantle. Models will be constructed based on the assumption that hotspots are formed by buoyant upwellings, or mantle plumes, interacting with the base of the lithosphere. The melting calculations will incorporate laboratory experimental data on the pressure-temperature conditions for melting of materials with different compositions. Computational results will quantify the relationships between specific physical conditions and predicted geographic variability of trace-element and Sr, Nd, Pb, and He isotope geochemistry in erupted magmas at three tectonic scenarios where adequate data sets exist for comparisons: an intraplate hotspot (Hawaii), a hotspot centered on a mid-ocean ridge (Iceland), and a hotspot off of, but near a mid-ocean ridge (Galapagos). Problem (I) will be addressed by determining whether models do or do not require mantle plumes--which could come from the lower mantle--to be compositionally distinct from the upper mantle. Problem (II) will be addressed by determining the physical conditions needed to explain the nature of spatial geochemical variability observed at the above types of hotspots. The origin of hotspot islands and the question of whole- versus two-layer mantle convection are of broad scientific interest, as evidenced by their treatment in high school and college textbooks. The investigators have given, and will continue giving, lectures and extracurricular seminars on these topics to local high school and undergraduate students. Most of the requested budget will support a Ph. D. student, providing funds needed for a comprehensive, cross-disciplinary dissertation.