Sr Isotopic Evolution of the Mantle: Constraints on Accretion, Recycling, and Crustal Growth

DePaoloe-0208671拟议的研究涉及使用古代火成岩的同位素地球化学来评估与地球早期演化有关的问题-地球通过吸积形成需要多长时间，地球在最初的数亿年中经历了多少化学差异，以及地球的大陆地壳在其历史上是如何生长和变化的？ 利用古老岩石中普遍存在的、可精确定年的锆石矿物，探讨前寒武纪地幔的锶同位素组成。 前寒武纪地幔的锶同位素组成是高度不确定的，因为它是很难找到的材料，通过后期的变质作用和风化保持其原始同位素组成。 锆石的锶同位素测量为这一问题提供了一个可能的解决方案。确定太古代和元古宙地幔的Sr同位素组成可能会提供有关地球增生历史的线索，早期大陆地壳是否是镁铁质的，以及海底风化和俯冲带过程是否可以从地幔中的Hf和Nd同位素中分离Sr同位素。该项目的工作包括从适当的地点采集岩石样品，分离锆石和其他矿物，并测量锆石的Rb-Sr、Sm-Nd和O同位素组成。 同时，还必须对年龄未知的锆石进行测年，并验证特定岩石样品中的锆石晶体具有单一年龄。 分析将通过热电离质谱法、激光烧蚀（用于O）和高分辨率离子微探针（SHRIMP）进行。

DePaoloEAR-0208671The proposed research involves using the isotope geochemistry of ancient igneous rocks to evaluate issues related to the early evolution of the earth - how long did it take for the Earth to form by accretion, how much chemical differentiation did the Earth experience in the first few hundred million years, and how did the Earth's continental crust grow and change through its history? The approach is to use the mineral zircon, which is common in old rocks and can be precisely dated by U-Pb geochronology, to probe the strontium isotopic composition of the Earth's mantle in Precambrian time. The Sr isotopic composition of the Precambrian mantle is highly uncertain because it is difficult to find materials that preserve their original isotopic composition through later episodes of metamorphism and weathering. Strontium isotopic measurements of zircon offer a possible resolution to this problem. Defining the Sr isotopic composition of the Archean and Proterozoic mantle would potentially provide clues about the accretion history for the Earth, whether early continental crust was mafic, and whether ocean floor weathering and subduction zone processes can decouple Sr isotopes from Hf and Nd isotopes in the mantle. The work to be done for this project includes collecting rock samples from appropriate localities, separating zircon and other minerals, and making measurements of the Rb-Sr, Sm-Nd, and O isotopic composition of the zircons. Parallel work must also be done to date the zircons where the age is not known, and to verify that zircon crystals in a particular rock sample have a single age. The analyses will be done by thermal ionization mass spectrometry, laser ablation (for O), and high-resolution ion microprobe (SHRIMP).