40Ar/39Ar K-feldspar Thermochronology: Exploring Argon Transport

学术价值：钾长石40Ar/39Ar热年代学的使用越来越多，需要继续评估方法和了解对Ar输送的控制。更好地了解微观结构如何对应或控制Ar的传输，是钾长石热年代学所有应用的关键。考虑到这种方法在世界范围内的广泛应用，进一步了解Ar的传输机制和边界是非常重要的。用多扩散域(MDD)模型解释了Ar系统和钾长石微结构之间的复杂关系，在该模型中，Ar的存留和释放由晶体中的非均质结构域控制。基于大量的分析数据，这个模型似乎对一些钾长石是可行的，但不是所有的。建议开展一项合作研究，将同位素分析与样品的矿物学特征相结合，以阐明这种不同行为的原因。将对天然和实验生产的长石进行研究，以评估Ar从钾长石中迁移出来的途径。这个项目的一个独特优势是，这两个PI带来了不同的、协同的和部分相互冲突的观点。Heizler积累了一个广泛的数据库，在众多独立地质约束的背景下结合了K长石数据和热年代学模型；Parsons参与了Ar地质年代学的某些方面，广泛开展了K长石矿物学的实验研究，并对MDD模型提出了挑战。双方都认识到，为了更好地了解钾长石热年代学和钾长石热年代学的用途这一共同目标，必须将思想和哲学结合起来。美国西部正在进行的钾长石MDD热年代学揭示了一些重要的地区性见解，如果能够更好地理解数据集的复杂性，这些洞察力可以演变为地区性和面向过程的突破。到目前为止的结果表明：1)大不整合是至少两次主要折返作用的产物，这两次折返作用与约1.1Ga Grenville造山作用和约800 Ma的Rodinia裂谷作用相吻合；2)在一些断裂上识别出不一致的年龄，从而确定了断裂的起始和重新活动的时间；3)断续的热液钾长石生长时期似乎与构造幕有关。对新出现的解释和假设的改进需要在具有充足支持数据的研究领域内进行。皮海兹勒提供了这方面的经验。皮帕森斯的参与带来了钾长石微结构的广泛知识，这将有助于评估钾长石热年代学。他将监督颗粒成像分析，记录显微结构，评估二次钾长石生长，并帮助采用不同矿物域的微观采样技术进行氩分析。广泛影响：由于钾长石热年代学在地热资源、矿床、石油资源评估和火山灾害等相关社会问题上有着广泛的应用，因此对该方法的持续评估至关重要。这项提议将支持对研究生的培训，促进两个PI的交叉培训-特别是为Pi Heizler提供一个学习钾长石表征的基础知识和微观结构解释的机会。美国西部的努力是一个广泛合作的一部分，这个合作涉及许多NSF PI和来自不同机构的数十名研究生和本科生，他们寻求提高他们解释40Ar/39Ar数据的能力。该项目将与这一更大的努力相结合，并将对其产生影响。

Intellectual Merit: The growing use of K-feldspar 40Ar/39Ar thermochronology requires continued evaluation of the methods and understanding of the controls for argon transport. Better understanding of how microtextures correspond to or control argon transport is key to all applications of K-feldspar thermochronology. Considering the growing worldwide use of the method it is important to further understanding of the mechanisms of and boundaries for argon transport.The complex relationship between argon systematics and microtextures of K-feldspar have been explained in terms of a multi-diffusion domain (MDD) model, in which Ar retention and release is controlled by heterogeneous structural domains in crystals. Based on extensive analytical data, this model seems viable for some but not all K-feldspars. A collaborative study is proposed to integrate isotopic analysis with mineralogical characterization of samples to elucidate the causes for this varied behavior. Both natural and experimentally produced feldspars will be studied to evaluate Ar transport pathways out of K-feldspar. A unique strength of this project is that the two PIs bring different, synergistic, and partially conflicting perspectives. Heizler has amassed an extensive database that combines K-feldspar data and thermochronologic models within the context of numerous independent geologic constraints; Parsons has been involved in some aspects of argon geochronology, worked extensively on experimental studies of K-feldspar mineralogy, and has challenged the MDD model. Both recognize that it is essential to combine ideas and philosophies towards the common goal of better understanding K-feldspar argon systematics and the utility of K-feldspar thermochronology. On-going K-feldspar MDD thermochronology from throughout the western USA has revealed some important regional insights that can evolve towards regional and process-oriented breakthroughs if the complexities of the dataset can be better understood. Results so far indicate: 1) a 'Great Unconformity' was the product of at least two major episodes of exhumation that coincide with the ca. 1.1 Ga Grenville orogeny and the ca. 800 Ma rifting of Rodinia; 2) Discordant ages are recognized across some faults that allow determination of the timing of fault initiation and reactivation; 3) Periods of punctuated hydrothermal K-feldspar growth appear to correlate to tectonic episodes. Refinement of emerging interpretations and hypotheses needs to be done within study areas with ample supporting data. PI Heizler provides experience in this area. Involvement of PI Parsons brings extensive knowledge of K-feldspar microtextures that will facilitate evaluation of the K-feldspar thermochronology. He will supervise grain-imaging analyses, document microtextures, evaluate secondary Kfeldspar growth and help with microsampling techniques of different mineralogical domains for argon analysis.Broader Impacts: Because K-feldspar thermochronology has wide application to relevant societal issues such as geothermal resources, ore deposits, petroleum resource assessment and volcanic hazards, continued evaluation of the method is critical. This proposal will support training of a graduate student, foster the cross-training of both PIs - notably providing PI Heizler an opportunity to learn fundamentals of K-feldspar characterization and interpretation of microtextures. The western USA effort is part of a broad collaboration that involves numerous NSF PIs and dozens of graduate and undergraduate students from a variety of institutions who seek to enhance their ability to interpret 40Ar/39Ar data. This project will be integrated with and will impact this larger effort.