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

这项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--地球经过多长时间的吸积形成，地球在最初的几亿年经历了多少化学分化，以及地球大陆地壳在其历史上是如何增长和变化的？这种方法是利用矿物锆石来探测前寒武纪地幔的锶同位素组成。锆石在古老的岩石中很常见，U-Pb年代学可以精确测定其年龄。前寒武纪地幔的锶同位素组成是高度不确定的，因为很难找到在后来的变质和风化过程中保持其原始同位素组成的物质。锆石的锶同位素测量为这个问题提供了可能的解决方案。确定太古宙和元古界地幔的锶同位素组成，可能会提供关于地球的增生史、早期陆壳是否为镁铁质的线索，以及海底风化和俯冲带过程是否可以使地幔中的锶同位素与Hf和Nd同位素脱钩的线索。该项目要做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，并对锆石的Rb-Sr，Sm-ND和O同位素组成进行测量。还必须进行平行工作，以确定年龄未知的锆石的年龄，并验证特定岩石样品中的锆石晶体具有单一的年龄。这些分析将通过热电离质谱、激光烧蚀(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).