Helium, Sr, Nd, and Pb Isotope Geochemistry of Intraplate Mafic Igneous Rocks in Eastern Pacific: Implications for the Origin of Linear Volcanic Chains

摘要：Castillo（0550237）板块内熔岩高3He/4He特征的起源是当今地球科学中最具争议的话题之一。总的来说，该特征的起源可以归结为两个相互竞争的模型：外围模型，该模型认为高3He/4He熔岩起源于地幔的浅层部分，其中包含了回收的外星物质和/或经历了(U + Th)/ 3He耗尽过程；而羽流模型，该模型声称高3He/4He熔岩起源于更原始的，相对未脱气的下地幔部分。本研究试图通过检验哪一种模型能做出有效的预测，来确定两种模型中哪一种能更好地解释板内熔岩高3He/4He特征的起源。外围模型预测：(1)所有板内熔岩具有相似的组成特征；(2)部分板内熔岩具有较高的3He/4He比率，而其他板内熔岩具有较低的3He/4He比率；(3)He与放射性成因同位素特征之间没有相关性。羽流模型预测：(1)局部板内火山熔岩的氦源特征与形成主要线性火山链的熔岩不同；(2)这些局部板内火山熔岩的3He/4He比值不高。本研究利用东太平洋化石扩张中心和裂脊的基性火成岩He-Sr-Nd-Pb同位素组成及其相应的主微量元素化学特征对模型进行检验，以验证数据是否支持模型。更广泛的影响：该项目支持加利福尼亚州的研究人员，并构成一名学生博士论文的一部分，该学生将负责大部分化学和同位素分析以及分析结果的解释。所有的分析都将支持使用斯克里普斯海洋学研究所现有的分析基础设施。该研究还支持与加州海洋科学卓越教育中心（COSEE-CA）合作，与公众，特别是科学教育者和学生分享研究的关键方面。它包括与在线科学教育资源Earthguide （http://earthguide.ucsd.edu/）的创建者合作，制作一个交互式在线动画，重点关注构成海洋地壳的岩浆和熔岩的形成。还涵盖了板块构造的驱动力，地球隐藏内部的性质，行星流变学，以及我们目前理解的地球物理和岩石学证据。

ABSTRACT Castillo (0550237)Intellectual merit: The origin of the high 3He/4He signature of some intraplate lavas is one of the most controversial topics in Earth Science today. In general, the origin of the signature boils down to two competing models: the perisphere model, which states that high 3He/4He lavas originate from the shallow part of the mantle that contains recycled extraterrestrial materials and/or experienced (U + Th)/ 3He depletion processes, and the plume model, which claims that high 3He/4He lavas originate from a more primitive, relatively undegassed portion of the lower mantle. This research tries to determine which of the two models better explains the origin of high 3He/4He signature of intraplate lavas by testing which model makes valid predictions. The perisphere model predicts: (1) all intraplate lavas exhibit similar compositional characteristics, (2) some intraplate lavas have high whereas others have low 3He/4He ratios and (3) there is no correlation between He and radiogenic isotopic signatures. The plume model predicts: (1) lavas from localized intraplate volcanoes have a helium source signature different from those of lavas that form major linear volcanic chains and (2) these localized intraplate volcanoes do not have high 3He/4He ratios. This research uses He-Sr-Nd-Pb isotopic compositions of mafic igneous rocks from fossil spreading centers and fissure ridges in the eastern Pacific, as well as their corresponding major and trace element chemistry to test the models to see which is supported by the data. Broader Impacts: This project supports investigators in the state of California and form part of a Ph.D. dissertation of a student who will be responsible for much of the chemical and isotopic analyses and interpretation of the analytical results. All the analyses will support use of existing analytical infrastructure at the Scripps Institution of Oceanography. The research also supports work with the California Center for Ocean Sciences Education Excellence (COSEE-CA) to share key aspects of the research with the public, particularly science educators and students. It involves working with the creators of Earthguide (http://earthguide.ucsd.edu/), an online science educational resource to produce an interactive online animation feature focusing on the formation of magmas and lavas that comprise the oceanic crust. Also covered are the driving forces of plate tectonics, the nature of the Earths hidden interior, planetary rheology, and the geophysical and petrological evidence on which our current understanding is based.