The Importance of the Size and Distribution of Mantle Heterogeneities to the Interpretation of Isotope Ratios and Trace Element Abundances in Basalts

像夏威夷这样的大洋岛屿是由火山爆发形成的。为火山提供养分的岩浆是由地球内部的部分融化产生的。在大洋岛屿和大洋中脊的海底取样的大洋玄武岩，为了解难以接近的地球的组成、岩浆在地幔中产生和运输的过程和时间尺度提供了重要的线索。地球化学的一个基本观察是，地幔中海洋玄武岩的源区是不均匀的，由不同大小和分布的化学和岩性的不同区域组成。由于这些非均质地幔源的部分熔融作用，地表喷发出种类繁多的玄武岩岩浆。地幔非均质性的大小和分布是未知的，但可能在控制玄武岩中观察到的化学变化方面发挥了重要作用。将玄武岩的非均质性映射到地幔源的化学和岩性非均质性是地球化学长期存在的问题之一。这种映射是非线性的，其数学表达式是未知的。本课题旨在研究地幔非均质大小和分布对海洋玄武岩同位素比值和微量元素丰度的影响。该项目的结果将使地球化学家能够建立玄武岩及其地幔源之间的非线性映射。该项目将为本科生提供研究机会，并通过培养研究生和博士后学者，培养具有竞争力的STEM劳动力，从而推进STEM教育。本研究的主要目的是：(1)寻找玄武岩及其地幔源非线性映射的函数形式；(2)建立理解和解释时空相关海洋玄武岩同位素比值和微量元素丰度变化的框架；(3)通过有时间或空间地球化学数据的案例研究对新模型进行检验。为了建立非线性映射，人们需要知道非均质性的性质（化学与岩性、大小和分布）、熔融方式（批量与分式）、熔体迁移过程（通道化与颗粒级多孔流动），以及残余矿物与渗透熔体之间化学不平衡的程度。将在地球化学文献中广泛使用的熔融模型中加入时间和空间变量。这些随时间变化的熔融模型将使地球化学家能够系统地研究地幔源非均质性的大小和分布在控制萃取熔体中同位素比率和不相容微量元素丰度变化中的作用。二维模型将使用一维随时间变化的熔化模型来构建。这些物理上更真实的模型将用于研究大洋中脊玄武岩和大洋岛屿玄武岩中Sr-Nd-Hf-Pb同位素的变化和选择不相容的微量元素比率。这项工作的一个重要成果是为岩石学家和地球化学家提供了一套新的解释工具，用于研究空间和时间相关的地幔和地幔衍生样品。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Oceanic islands such as Hawaii are formed by volcanic eruptions. Magmas that feed the volcanos are produced by partial melting in the interior of the Earth. Oceanic basalts, which are sampled at oceanic islands and along mid-ocean ridges on the seafloor, provide important clues to understanding the composition of the inaccessible Earth and the processes and time scales of magma generation and transport in the mantle. A fundamental observation in geochemistry is that the source regions of oceanic basalts in the mantle are heterogeneous and consist of chemically and lithologically distinct domains of variable size and distribution. Partial melting of such heterogeneous mantle sources gives rise to a diverse range of basaltic magmas erupted on the surface. The size and distribution of mantle heterogeneities are unknown, but may have played an important role in controlling chemical variations observed in basalts. One of the long-standing problems of geochemistry is the mapping of heterogeneities observed in basalts into the chemical and lithological heterogeneities in the mantle source. Such a mapping is nonlinear and its mathematical expression is unknown. The thesis of this project is to investigate how the size and distribution of mantle heterogeneities affect the isotope ratios and trace element abundances in oceanic basalts. Results from this project will enable geochemists to establish the nonlinear mapping between basalts and their mantle sources. The project would advance STEM education by providing research opportunities for undergraduate students and by developing of a competitive STEM workforce through training of graduate students and postdoc scholar. The primary objectives of this proposed study are (1) to find the functional forms for the nonlinear mapping between basalts and their mantle sources; (2) to develop a framework for understanding and interpreting variations in isotope ratios and trace element abundances in spatially and temporally associated oceanic basalts; and (3) to test the new models through selected case studies where temporal or spatial geochemical data are available. To establish the nonlinear mapping, one needs to know the nature of heterogeneity (chemical vs. lithological, size and distribution), the style of melting (batch vs. fractional), the process of melt migration (channelized vs. grain-scale porous flow), and the extent of chemical disequilibrium between residual minerals and the percolating melt. Time and spatial variables will be added to the widely used melting models in the geochemical literature. These time-dependent melting models will enable geochemists to systematically study the roles of the size and distribution of heterogeneities in the mantle source in controlling the variations of isotope ratios and incompatible trace element abundances in extracted melts. Two-dimensional models will be constructed using the one-dimensional time-dependent melting models. These physically more realistic models will be used to examine variations of Sr-Nd-Hf-Pb isotope and selected incompatible trace element ratios in mid-ocean ridge basalts and ocean island basalts. An important outcome of the proposed work is a set of new interpretation tools to petrologists and geochemists in their studies of spatially and temporally correlated mantle and mantle-derived samples.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.