Collaborative Research: Little Devils Postpile Revisited: Intercalibration of Thermochronometer Kinetics in a Contact Aureole

接触热年代学在地壳上部几公里岩石热历史约束的要求下，中低温热年代学在过去二十年中在构造和地貌应用中得到了迅速发展。尽管有这种增长，但每个热年代学应用都受到基本动力学校准和样品内变化的不确定性的限制，这些不确定性在不同程度上引发了对由此产生的地质解释的问题。这些不确定性的来源很多，但也许最重要的是对实验室阶梯加热扩散和退火实验的解释和外推，这些实验通常在与相关自然条件相差许多数量级的速度和温度下进行。有几条证据，包括从头算动力学模型、自然样品与理想化组态的偏差，以及在许多冷却年龄、光谱、轮廓和径迹长度数据集中简单地观察到“令人感兴趣的复杂性”，表明我们对常规使用的基本热年代学动力学校准的理解存在重大差距。需要对一个控制良好的自然实验室的多个温度计时器进行仔细和深思熟虑的基准和相互校准研究，以阐明这些问题，并最终建立对动力学模型和从它们得出的地质解释的更多信心。有人提议重新采样和分析小魔鬼桩附近的13个不同的稀有气体和裂变径迹温度计时器的详细行为。小魔鬼桩是Calk和Naeser于1973年研究的一个经典的天然实验室，在那里，一个~100m的玄武岩塞在~8 Ma侵入~80 Ma的花岗岩中。尽管可能受到热液循环和其他影响的复杂影响，但这个自然实验的相对简单的配置将允许构建与侵入相关的热历史的现实模型，从而预测年龄和其他热计时特性作为距离接触距离的函数。在围岩中，许多不同矿物的丰度可以用惰性气体和裂变径迹方法测年，这将使我们能够根据许多参数(包括。散体年龄、分布、光谱和轨道长度等)。校准良好的测年系统的相互组合将作为基准，以确定解释问题，并推断改进其他测年系统校准所需的潜在改进。智力优势：这项专注和合作的研究将提供一个相对难得和必要的机会来测试、完善，在某些情况下建立动力学校准，许多地球和行星科学家每年使用数百种应用。这将通过在实验室无法实现的条件下进行的自然实验来完成，通过相对简单的接触关系进行直截了当的建模，但有合理的机会揭示许多地质应用中可能经常遇到的复杂性(例如样品内的变化)。广泛的影响：除了有助于地质界广泛使用的热年代学校准，并可能建立有助于测试其他系统的热年代学“类型位置”，这项工作将为几名本科生和研究生提供培训和支持，在德克萨斯大学建立新的裂变径迹测年能力(锆石、钛铁矿和绿帘石)，并将为约塞米蒂国家公园的游客提供外展机会通过我们与公园地质学家格雷戈里·斯托克博士的合作。

Thermochronometer Kinetics in a Contact AureoleDriven by the demand for constraints on thermal histories of rocks in the upper few kilometers of the Earth's crust, intermediate- and low-temperature thermochronology has proliferated in tectonic and geomorphic applications in the last two decades. Despite this growth, every thermochronologic application is limited by uncertainties in fundamental kinetic calibrations and intra-sample variation that to one degree or another raise questions about geologic interpretations derived from them. These uncertainties arise from many sources, but perhaps most importantly from the interpretation and extrapolation of empirical laboratory step-heating diffusion and annealing experiments that are usually performed at rates and temperatures many orders of magnitude different from relevant natural conditions. Several lines of evidence, including ab initio kinetic models, deviations of natural samples from idealized configurations, and simply observations of "intriguing complexities" in many cooling-age, -spectra, -profile and track-length data sets, suggest the existence of significant gaps in our understanding of routinely used fundamental thermochronologic kinetic calibrations. A careful and deliberate benchmarking and intercalibration study of multiple thermochronometers from a well-controlled natural laboratory is needed to illuminate these issues and to ultimately establish more confidence in kinetic models and geologic interpretations derived from them. It is proposed to resample and analyze the detailed behavior of 13 different noble-gas and fissiontrack thermochronometers in profiles adjacent to Little Devils Postpile, a classic natural laboratory studied by Calk and Naeser in 1973, where a ~100 m basalt plug intruded ~80-Ma granitoid rock at ~8 Ma. Although possibly complicated by hydrothermal circulation and other effects, the relatively simple configuration of this natural experiment will allow construction of realistic models of the thermal histories associated with the intrusion, hence prediction of profiles of age and other thermochronometric properties as a function of distance from the contacts. The abundance of many different minerals dateable by both noble gas and fission-track methods in the country rock will then allow comparison between and predicted the observed thermochronologic patterns based on many parameters (incl. bulk ages, profiles, spectra, and track lengths, etc.). Inter-combinations of well-calibrated dating systems will serve as benchmarks to identify interpretive problems and to infer potential refinements needed to improve calibrations of other dating systems.Intellectual Merit: This focused and collaborative study will provide a relatively rare and needed opportunity to test, refine, and in some cases establish kinetic calibrations used by many Earth and planetary scientists for hundreds of applications each year. This will be accomplished using a natural experiment performed under conditions not achievable in the laboratory, through a relatively simple contact relationship that allows for straightforward modeling, but with reasonable opportunity for revealing complexities (such as intrasample variations) that are likely to be commonly encountered in many geologic applications.Broader Impacts: Besides contributing to thermochronologic calibrations widely used in the geologic community, and potentially establishing a thermochronologic "type locality" useful for testing other systems, this work will provide training and support for several undergraduate and graduate students, establish new fission-track dating capabilities (in zircon, titanite, and epidote) at UT, and will provide for outreach opportunities to visitors at Yosemite National Park, through our collaboration with Park Geologist Dr. Gregory Stock.