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

在过去的二十年里，由于对地壳上层几公里岩石热历史限制的需求，中低温热年代学在构造和地貌学方面的应用激增。尽管有这种增长，但每一种热年代学应用都受到基本动力学校准和样品内部变化的不确定性的限制，这些不确定性或多或少地对由此产生的地质解释提出了质疑。这些不确定性来自许多来源，但也许最重要的是来自经验实验室阶梯加热扩散和退火实验的解释和推断，这些实验通常在与相关自然条件不同的许多数量级的速率和温度下进行。一些证据，包括从头算动力学模型，自然样品与理想构型的偏差，以及在许多冷却时代，光谱，剖面和轨迹长度数据集中对“有趣的复杂性”的简单观察，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。需要在一个控制良好的自然实验室中对多个温度计进行仔细和深思熟虑的基准测试和相互校准研究，以阐明这些问题，并最终建立更有信心的动力学模型和由此得出的地质解释。在Calk和Naeser于1973年研究的经典自然实验室Little Devils Postpile附近，一个~100 m的玄武岩塞侵入~80 Ma的~8 Ma花岗岩，提出对13种不同的稀有气体和裂变径迹温度计的详细行为进行重新采样和分析。虽然可能由于热液循环和其他影响而变得复杂，但这个自然实验的相对简单的配置将允许构建与侵入有关的热历史的现实模型，从而预测年龄剖面和其他热时学性质作为与接触点距离的函数。通过稀有气体和裂变径迹法在乡村岩石中测定年代的许多不同矿物的丰度，将允许基于许多参数（包括体积年龄、剖面、光谱和径迹长度等）对观察到的热年代学模式进行比较和预测。校准良好的定年系统的相互组合将作为确定解释问题和推断改进其他定年系统校准所需的潜在改进的基准。智力优势：这项集中的合作研究将提供一个相对罕见和需要的机会来测试、改进，并在某些情况下建立动力学校准，这些校准每年被许多地球和行星科学家用于数百种应用。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，允许直接建模，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（如样本内变化）。更广泛的影响:除了有助于在地质界广泛使用的热年代学校准，并有可能建立一个可用于测试其他系统的热年代学“类型地点”外，这项工作还将为几名本科生和研究生提供培训和支持，在UT建立新的裂变径迹定年能力（锆石，钛矿和绿石），并将为约塞米蒂国家公园的游客提供推广机会。通过我们与公园地质学家Gregory Stock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.