Testing the Validity of Muscovite as a Continuous History Thermochronometer

温度变化对岩石的化学演化很重要。形成地貌的构造变形和侵蚀导致了岩石中记录的温度变化。矿床的形成和油气聚集的形成在一定程度上是温度驱动的。在过去的50年里，地质学家开发出了利用矿物中发现的放射性衰变产物来解码温度历史的方法。虽然腐烂产物以众所周知的规律积累，但它们也可能在高温下从矿物中消失。在矿物颗粒的生命周期中，剩余的净衰变产物和测量的矿物年龄代表了这些与温度相关的稳定增长和损失相互竞争的过程的结果。研究不同矿物对温度的不同反应可能是很有力量的。研究人员需要更广泛的抗蚀变矿物，以在尽可能广泛的温度范围内使用。这个团队将确定普通的白色云母是否可以用于温度历史研究。这种矿物很容易用钾-氩法测定年龄，耐风化，并显示出能够在300˚C到400˚C温度范围内工作的希望，这一温度范围是其他矿物所不能很好覆盖的。除了演示文稿和出版物外，这个团队还将为学生和公众制作关于矿物测年及其如何支持关于地球的基础研究的外联视频。该项目的目标是开发一种新的方法来确定275˚C到425˚C范围内地壳岩石的温度历史。几份已发表的报告表明，对矿物白云母的40Ar/39Ar分析可以解决这一范围。为了测试白云母不仅可以作为测年目标，还可以作为测量岩石冷却期间温度的手段，研究人员将对一套具有良好特征的白云母样品进行扩散和测年实验，这些样品具有已知的温度历史，可以与他们的实验预测进行比较。他们还将选择样品进行重新加热实验，旨在检查白云母样品对更短时间尺度、更高温度的热脉冲的反应。使白云母成为一个特别吸引人的研究对象的是这样一种说法，即白云母颗粒不是作为一个简单的扩散系统表现出来的，而是表现出多扩散域行为，因此每个白云母颗粒实际上是对Ar具有部分重叠保留率的子样品的集合。如果是这样的话，单个样本可以记录超过100˚C或更多的热历史的连续片段。此外，有人建议，可以通过40Ar/39Ar实验室阶梯加热分析获得关于白云母扩散性质的样品特定信息，这将支持恢复比应用从有限数量的已发表的扩散研究中获得的通用动力学数据更准确的热历史。这项研究将测试白云母分析可以揭示一系列热历史信息的提议，以及这些分析还可以产生准确的扩散动力学数据的建议。所涉及的工作将成为博士论文和本科生项目的一部分。为了广泛地传达他们的工作，该团队将开发针对本科生水平的教学材料，内容涉及地质年代学主题，如什么样的不同类型的人可以做这种工作，他们为什么做这项工作，以及如何做。这些材料将与与动画专业人士合作准备的一系列视频整合在一起，并在发布前在课堂上进行测试。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

Temperature change is important to a rock's chemical evolution. The tectonic deformation and erosion that shape landscapes cause temperature changes that are recorded in rocks. Ore deposit formation and the creation of hydrocarbon accumulations are in part temperature-driven. Over the past 50 years, geologists have developed ways use radioactive decay products found in minerals to decode temperature histories. While decay products accumulate at a well-known regular rate, they can also be lost from minerals at elevated temperatures. Over the life span of a mineral grain, the net remaining decay product and thus the measured mineral age represents the outcome of these competing processes of steady gain and temperature-dependent loss. Studying different minerals with different responses to temperature can be powerful. Researchers need a broader range of alteration-resistant minerals to use for as wide a range of temperatures as possible. This team will determine whether the common white mica can be used for temperature-history studies. This mineral is easily dated by the potassium-argon method, is resistant to weathering, and shows promise in being able to work over the 300 ˚C to 400˚C temperature range, an interval not well covered by other minerals. In addition to presentations and publications, this team will develop outreach videos for students and the general public about mineral dating and how it supports basic research about the Earth.The goal of this project is to develop a new means of determining the temperature histories of crustal rocks across the range 275˚C to 425˚C. Several published reports suggest that 40Ar/39Ar analyses of the mineral muscovite can address this range. To test how useful muscovite might be as not just a dating target but also as a means to measure temperatures during rock cooling, the researchers will carry out diffusion and dating experiments on a suite of well characterized muscovite samples that have known temperature histories that can be compared to predictions from their experiments. They will also choose samples for reheating experiments designed to examine the response of muscovite samples to shorter timescale, higher-temperature thermal pulses. What makes muscovite a particularly appealing target for study are suggestions that rather than behaving as a simple diffusing system, muscovite grains show multi-diffusion-domain behavior, such that each grain of muscovite is in effect a collection of subsamples having partially overlapping retentivities for argon. If so, an individual sample could record a continuous segment of a thermal history that extends over some 100˚C or more. Moreover, it has been proposed that sample-specific information about muscovite diffusion properties can be obtained as a byproduct of 40Ar/39Ar laboratory step heating analysis, which would support the recovery of more accurate thermal histories than would be possible by applying generic kinetic data obtained from a limited number of published diffusion studies. The research will test both the proposal that muscovite analyses can reveal a range of thermal-history information and that these analyses can also yield accurate diffusion-kinetics data. The work involved will form part of a Ph.D. dissertation as well as an undergraduate project. To convey their work broadly, the team will develop instructional materials targeted at the undergraduate level on such geochronology topics as what sort of diverse people can do this kind of work, why they do it, and how. These materials will be integrated with a series of videos prepared in collaboration with animation professionals, and tested in classes before release.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.