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

要确定50多万年前形成的岩石的年龄，最准确、最有用的方法之一是铀铅（U-Pb）地质年代学。地球科学家利用U-Pb年代学将整个地区和特定事件的地质历史结合在一起，比如大约6600万年前所有非鸟类恐龙的大灭绝，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，因为它储量丰富，经久耐用，并且很容易将铀结合到它的晶体结构中。但它不包括钍，其同位素230Th是从238U到206Pb的自然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缺失的230Th进行校正。当锆石从熔体中结晶时，U和Th的行为方式的现有实验和观测约束是不精确的，因此我们建议在一系列受控的实验室实验中量化这种行为。这项研究的结果将减少年轻锆石测年的最大不确定性来源，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。对所有学科的地质学家来说，从古生物学到行星宇宙化学再到地球生物学，获得更精确的地质时间标度是很重要的。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以交互式可视化的形式呈现在网络上，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，以及校正将如何随成分参数的变化而变化。这笔拨款的结果还将支持代表性不足群体的学生入学和指导，并支持两名早期职业研究者的专业和实验室发展。合成锆石将在高温下从掺入U和Th的熔体中生长，这些熔体在一定的温度、压力和成分下模拟天然岩浆。液体会被冷却，这样熔体就会变成玻璃。将锆石从共存的玻璃中分离出来，采用高精度、高空间分辨率技术，测量各相中U、Th的丰度和分布。所提出的实验将导致在各种自然条件下精确测定锆石/熔融铀和钍的分配系数。该数据将使用最大似然回归技术拟合到多维曲面上，以便可以为任何已知参数集计算分配系数的比率。来自同位素稀释分析的测量不确定度，以及任何观察到的实验散射将被纳入这些不确定度，我们预计它们将保守地在1-5%的数量级。这与现有的实验数据进行了比较，实验数据产生了50-100%的不确定性。

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%.