Improving the 40Ar/39Ar geochronometer

本项目旨在显著提高40Ar/39Ar和K-Ar定年方法的准确性。这两种方法都是基于钾的自然放射性衰变，钾是岩石和其他地质材料中的一种常见元素。这些方法是地球、行星和生物历史上最重要的测年方法之一，因为它们可以应用于不同的材料，而且它们的有效适用时间范围是任何此类方法中最大的。然而，尽管这些方法的精度极好且不断提高，即实验可重复性，但由于对衰变常数的了解程度以及校准所需标准物的同位素组成，这些方法的准确性受到限制。因此，虽然40Ar/39Ar和K-Ar定年可以解决小的年龄差异并精确确定地质过程的速率，但将其结果与其他方法（例如放射性碳或U-Pb）的年龄数据进行比较最多只能在1%的水平上有效。本项目所要实现的改进将使地质学、地球物理学、古生物学（包括古人类学）、考古学和行星科学等领域受益。该项目将结合使用K同位素和多收集器电感耦合等离子体质谱（MC-ICP-MS）和热电离体质谱（TIMS）进行浓度测量的选定标准的40K和40Ar浓度测量，以及使用专用移液管系统和惰性气体体质谱进行40Ar浓度测量。40K衰变常数将通过在精心设计的合成基质中使用高度富集40K的钾的生长实验和通过液体闪烁计数的分解活性测量来确定。这些测量将允许对40Ar/39Ar地质年代学中使用的主要矿物标准的同位素组成和40K衰变常数进行新的确定。基于长入的衰变常数测量需要大约十年的时间来积累足够的40Ar以进行足够精确的测量，因此精确制备玻璃作为起始材料将是本提案的重点。许多活动将在具有相关专门知识的同事监督下的专门实验室进行。大部分数据采集和样品制备将由一名博士后研究员进行，他将由PI的Renne和Morgan共同指导。博士后将在质谱、实验岩石学和核化学方面获得广泛的经验，并将在伯克利的40Ar/39Ar实验室和美国地质调查局（丹佛）获得重要的经验。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。

This project aims to significantly improve the accuracy of the 40Ar/39Ar and K-Ar dating methods. Both of these methods are based on the natural radioactive decay of potassium, a common element in rocks and other geological materials. These methods are among the most important methods for dating events in Earth, planetary and biological history because they can be applied to diverse materials and their effective time range of applicability is the greatest of any such method. However, despite excellent and continually improving precision, i.e., experimental reproducibility, these methods are limited in accuracy by how well the decay constants are known, as well as the isotope composition of standards needed for calibration. Thus, while 40Ar/39Ar and K-Ar dating can resolve small age differences and precisely determine rates of geological processes, comparing their results with age data from other methods (e.g., radiocarbon or U-Pb) is only valid at the 1% level at best. The improvements to be realized in this project will benefit the fields of geology, geophysics, paleontology (including paleoanthropology), archeology, and planetary science.The project will combine measurements of 40K and 40Ar concentration in selected standards using K isotopic and concentration measurements by Multicollector Inductively Coupled Plasma Mass Spectrometry (MC-ICP-MS) and Thermal Ionization Mass Spectrometry (TIMS), and 40Ar concentration measurements using a purpose-built pipette system and noble gas mass spectrometry. The 40K decay constant will be determined both by ingrowth experiments using highly 40K-enriched potassium in strategically-designed synthetic matrix, and by disintegration activity measurements via liquid scintillation counting. These measurements will allow for new determinations of the isotope composition of primary mineral standards used in 40Ar/39Ar geochronology, and the 40K decay constant. The ingrowth-based decay constant measurement requires an estimated ten years to accumulate sufficient 40Ar for adequately precise measurement, hence exacting preparation of glasses as starting material will be the focus of the present proposal. Many of the activities will be conducted in specialized laboratories overseen by colleagues with relevant expertise. Much of the data acquisition and sample preparation will be conducted by a postdoctoral researcher who will be jointly supervised by PI’s Renne and Morgan. The postdoc will gain broad experience in mass spectrometry, experimental petrology, and nuclear chemistry, and will also gain significant experience in the 40Ar/39Ar laboratories in Berkeley and the US Geological Survey (Denver).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.