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

拟议的研究将涉及固体地球动力学中的两个问题：(I)如何调和大洋中脊玄武岩(MORBs)和热点洋岛玄武岩(OIB)之间的地球化学差异，这两种玄武岩被广泛地解释为化学分层的地幔，并有整个地幔(即单层)对流的地球物理证据。(2)确定在热点地区观察到的熔岩地球化学地理变化的来源。假设是成分不均匀的地幔的熔融和韧性流动的变化导致了OIBs和MORBs的许多关键的地球化学特征。这一假设将用数值模型来验证，这些模型模拟了上地幔中的三维粘性流动、热传输和融化。模型将建立在这样的假设基础上，即热点是由浮力上升流或地幔热柱与岩石圈底部相互作用形成的。熔化计算将包括关于不同成分材料熔化的压力-温度条件的实验室实验数据。计算结果将量化在三种构造情景下喷发岩浆中微量元素和锶、钕、铅、氦同位素地球化学的预测地理变异性之间的关系，在这三种构造情景中，有足够的数据集用于比较：板内热点(夏威夷)，以大洋中脊为中心的热点(冰岛)，以及远离大洋中脊但靠近大洋中脊的热点(加拉帕戈斯)。问题(I)将通过确定模型是否要求地幔热柱--可能来自下地幔--在成分上与上地幔不同来解决。将通过确定解释在上述类型热点观察到的空间地球化学变异性的性质所需的物理条件来解决问题(2)。热点岛屿的起源和整个地幔对流与双层地幔对流的问题具有广泛的科学兴趣，高中和大学教科书中的处理证明了这一点。调查人员已经并将继续就这些主题向当地高中生和本科生进行讲座和课外研讨会。所要求的大部分预算将支持一名博士生，为一篇全面的、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.