Collaborative Research: Experimental Investigation of Actinide Partitioning in Zircon and its Applications to Geochronology

合作研究：锆石中锕系元素分配的实验研究及其在地质年代学中的应用

• 批准号: 1654637
• 负责人: Noah McLean
• 金额: $ 14.36万
• 依托单位: University of Kansas Center for Research Inc
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2017
• 资助国家: 美国
• 起止时间: 2017-07-01 至 2022-06-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1654637&HistoricalAwards=false
• 关键词: Collaborative; Research; Experimental; Investigation; Actinide

One of the most accurate and useful ways of determining the age of rocks that formed more than about 500,000 years ago is uranium-lead (U-Pb) geochronology. Earth scientists use U-Pb geochronology to put together the geologic history of entire regions and of specific events, like the mass extinction of all non-avian dinosaurs about 66 million years ago or the catastrophic eruptions of supervolcanoes like the one currently centered at Yellowstone. The mineral zircon is often utilized because it is abundant, durable, and readily incorporates uranium into its crystal structure. But it excludes thorium, whose isotope 230Th is part of the naturally occurring isotopic decay chain from 238U to 206Pb. Calculating a date from the relative abundances of 206Pb and 238U therefore requires a correction for the missing 230Th. Existing experimental and observational constraints on the way U and Th behave when zircon crystallizes from a melt are imprecise, so we propose to quantify this behavior in a series of controlled laboratory experiments. The results of this study will reduce the largest source of uncertainty in dating young zircons and improve the accuracy of U-Pb dates, improving our ability to tell time during geologic processes. The attainment of more accurate timing of the geologic timescale is important to geologists of all disciplines, from paleontology to planetary cosmochemistry to geobiology. The data from this study will be broadly disseminated to the geoscience community. The data will be packaged and presented in an interactive visualization, hosted on the web, to illustrate why a correction must be made to geochronology data, how to calculate the magnitude of the correction, and how the correction will vary with compositional parameters. The results of this grant will also support the admission and mentoring of students in underrepresented groups and support the professional and laboratory development of two early career investigators. Synthetic zircon will be grown at high temperature from melts doped with U and Th that mimic natural magmas at a range of temperatures, pressures, and compositions. The liquids will be quenched so that the melt turns into glass. Zircons will be separated from their coexisting glass and using high precision and high-spatial-resolution techniques, the abundance and distribution of U and Th in each phase will be measured. The experiments proposed will result in precise determination of the zircon/melt uranium and thorium partition coefficients under a wide variety of naturally occurring conditions. This data will be fit to a multidimensional surface using maximum likelihood regression techniques, so that the ratio of partition coefficients can be calculated for any set of known parameters. Measurement uncertainties from the isotope dilution analyses, as well as any observed experimental scatter will be incorporated into these uncertainties, and we expect that they will conservatively be on the order of 1-5%. This is compared to the existing experimental data, which has yielded uncertainties on the order of 50-100%.

确定大约50万年前形成的岩石年龄的最准确和最有用的方法之一是铀-铅(U-Pb)年代学。地球科学家使用U-Pb地质年代学来组合整个地区和特定事件的地质史，比如大约6600万年前所有非鸟类恐龙的大规模灭绝，或者像目前以黄石公园为中心的超级火山的灾难性喷发。矿物锆石经常被利用，因为它丰富、耐用，而且很容易将铀融入其晶体结构中。但它不包括钍，它的同位素230是从238U到206Pb的自然发生的同位素衰变链的一部分。因此，根据206Pb和238U的相对丰度计算日期需要对丢失的第230个元素进行修正。现有的关于U和Th在熔体中结晶时U和Th行为的实验和观测限制是不精确的，所以我们建议在一系列受控的实验室实验中对这种行为进行量化。这项研究的结果将减少年轻锆石测年中最大的不确定因素，并提高U-Pb测年的准确性，提高我们在地质过程中判断时间的能力。对于从古生物学到行星宇宙化学再到地球生物学的所有学科的地质学家来说，获得更准确的地质时间刻度是很重要的。这项研究的数据将广泛传播给地球科学界。这些数据将被打包并在网上的交互式可视化中展示，以说明为什么必须对地质年代学数据进行校正，如何计算校正的幅度，以及校正将如何随着组成参数的变化而变化。这笔赠款的结果还将支持录取和指导任职人数不足的群体的学生，并支持两名早期职业调查员的专业和实验室发展。合成锆石将在高温下从掺有U和Th的熔体中生长出来，这些熔体在一定的温度、压力和成分范围内模拟天然岩浆。液体会被冷却，这样熔体就会变成玻璃。锆石将从其共存的玻璃中分离出来，并使用高精度和高空间分辨率的技术，将测量U和Th在每个相中的丰度和分布。拟议的实验将导致在各种自然条件下精确测定锆石/熔体铀和钍的分配系数。这些数据将使用最大似然回归技术拟合到多维表面，从而可以针对任何一组已知参数计算分配系数的比率。同位素稀释分析的测量不确定度以及任何观测到的实验散射都将被纳入这些不确定度中，我们预计它们将保守地在1-5%的数量级上。这与现有的实验数据相比，产生了50%-100%的不确定度。