SGER: Development of High-Precision Neodymium Isotope Mass Spectrometry to Search for Crust-Mantle Differentiation on Early Earth

在确定地球的硅酸盐部分是否在其历史的前5亿年中经历了分化——这段时间没有现存的岩石记录——仍然存在一个主要的歧义。早期壳幔分异事件的证据可能以142异常的形式存在于太古宙岩石中，这些异常来自于地球形成时存在的短命的146Sm（半衰期为1.03亿年）的衰变。142的预期过量水平约为+30 ppm，因此，需要1 σ不确定性为5ppm或更高，以解决异常。为了完成这项技术上具有挑战性的任务，我们将使用达特茅斯大学最近安装的新一代热电离质谱仪（Triton）。对Triton标准的初步测量表明，它能够测量142中超过+ 10ppm的异常。我们将制定一个标准操作程序（SOP）来减轻/消除可能的离子光学伪像，以获得精确和准确的Nd同位素结果，在5ppm（1西格玛）或更好。随着SOP的发展，我们将研究格陵兰岛Isua的10块早期太古宙岩石。拟议研究的更广泛影响包括教学和培训一名积极性很高的毕业生Rasmus Andreasen，以及开发使用热电离质谱法精确和超精确测量Nd同位素的新技术。此外，这项研究有望为早期地球分化的历史提供新的和深刻的见解。nd同位素测量前所未有的外部可重复性将为地球和行星科学的不同领域开辟新的研究途径。

EAR-0336405SharmaA major ambiguity remains in ascertaining whether the silicate portion of the Earth underwent differentiation during the first 500 million years of its history-a time for which there is no extant rock record. The evidence of early crust-mantle differentiation event(s) may be present in Archean rocks in the form of 142Nd anomalies produced from the decay of short-lived 146Sm (half-life = 103 million years), which was present at the time of formation of the Earth. The expected level of excess in 142Nd is about +30 parts per million (ppm) and, therefore, a 1-sigma uncertainty of 5 ppm or better is needed to resolve the anomaly. To accomplish this technically challenging task, we will use a new generation thermal ionization mass spectrometer (Triton) recently installed at Dartmouth.Initial measurements of standards on Triton have shown that it is capable of measuring anomalies in 142Nd exceeding +10 ppm. We will develop a standard operating procedure (SOP) to mitigate/eliminate possible ion optical artifacts in order to obtain Nd isotope results that are precise and accurate at 5 ppm (1-sigma) or better. Following the development of the SOP we will examine ten early Archean rocks from Isua, Greenland.Broader impacts of the proposed study include teaching and training of a highly motivated graduated student, Rasmus Andreasen, and development of new techniques of accurate and super-precise measurements of Nd isotopes using thermal ionization mass spectrometry. Also, the study is expected to provide new and profound insights into the history of early Earth differentiation. The unprecedented external reproducibility of Nd-isotope measurements will open up new avenues of investigations in diverse areas of earth and planetary sciences.