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方面仍然存在着一个主要的模糊性-这段时间没有现存的岩石记录。早期壳幔分异事件的证据可能存在于太古代岩石中，其形式为142 Nd异常，该异常是由地球形成时存在的短寿命146 Sm（半衰期= 1.03亿年）衰变产生的。142 Nd的预期过量水平约为+30 ppm，因此，需要5 ppm或更高的1-sigma不确定性来解决异常。为了完成这项技术上具有挑战性的任务，我们将使用最近安装在达特茅斯的新一代热电离质谱仪（Triton）。对Triton标准的初步测量表明，它能够测量超过+10 ppm的142 Nd异常。我们将制定一个标准操作规程（SOP），以减轻/消除可能的离子光学伪影，以获得精确度和准确度为5 ppm（1-sigma）或更高的Nd同位素结果。在SOP的发展之后，我们将检查来自格陵兰岛Isua的10块早期太古代岩石。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深刻见解。钕同位素测量的前所未有的外部再现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.