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Zircon (U-Th)/He dates from radiation damaged crystals: A new damage-He diffusivity model for the zircon (U-Th)/He thermochronometer.

机译：锆石（U-Th）/ He起源于辐射损坏的晶体：一种新的锆石（U-Th）/ He测温仪的损伤-He扩散率模型。

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摘要

Zircon (U-Th)/He (zircon He) dating has become a widely used thermochronologic method in the geosciences. Practitioners have traditionally interpreted (U-Th)/He dates from zircons across a broad spectrum of chemical compositions with a single set of 4He diffusion kinetics derived from only a handful of crystals (Reiners et al., 2004). However, it has become increasingly clear that a "one-size-fits-all" approach to these kinetics is inadequate, leading to erroneous conclusions and incongruent data. This dissertation develops a more grain-specific approach by showing the fundamental role that intracrystalline radiation damage plays in determining the He diffusivity in a given zircon. I present three appendices that seek to quantify the radiation damage effect on He diffusion in zircon, explain how this effect manifests in zircon He dates, and show how to exploit such manifestations to better constrain sample thermal histories. Of particular importance, this dissertation represents the first comprehensive study to concentrate on the entire damage spectrum found in natural zircon and also the first to show that two different mechanisms affect He diffusion in zircon in different ways across this spectrum.;In the first appendix, I and my fellow co-authors describe results from a series of step-heating experiments that show how the alpha dose of a given zircon, which we interpret to be correlated with accumulated radiation damage, influences its He diffusivity. From 1.2 x 1016 &agr;/g to 1.4 x 1018 &agr;/g, He diffusivity at a given temperature decreases by three orders of magnitude, but as alpha dose increases from ~2 x 1018 &agr;/g to 8.2 x 1018 &agr;/g, He diffusivity then increases by about nine orders of magnitude. We parameterize both the initial decrease and eventual increase in diffusivity with alpha dose with a function that describes these changes in terms of increasing abundance and size of intracrystalline radiation damage zones and resulting effects on the tortuosity of He migration pathways and dual-domain behavior. This is combined with another equation that describes damage annealing in zircon. The end result is a new model that constrains the coevolution of damage, He diffusivity, and He date in zircon as a function of its actinide content and thermal history.;The second and third appendices use this new model to decipher zircon He datasets comprising many single grain dates that are correlated with effective uranium (eU, a proxy for the relative degree of radiation damage among grains from the same sample). The model is critical for proper interpretation of results from igneous settings that show date-eU correlations and were once considered spurious (appendix B). When applied to partially reset sedimentary rocks, other sources of date variability, such as damage and He inheritance, have to be considered as well (appendix C).
机译：锆石（U-Th）/氦（锆石He）测年法已成为地球科学中广泛使用的热年代学方法。从业人员传统上解释（U-Th）/ He起源于锆石，涵盖了广泛的化学成分，并且具有仅来自少数几个晶体的一组4He扩散动力学（Reiners等，2004）。但是，越来越清楚的是，对这些动力学采用“一刀切”的方法是不够的，从而导致了错误的结论和不一致的数据。本文通过显示晶体内辐射损伤在确定给定锆石中He扩散系数中的基本作用，开发出一种更具晶粒特异性的方法。我提供了三个附录，这些附录试图量化辐射损伤对锆石中He扩散的影响，解释该效应如何在锆石He枣中表现出来，并展示如何利用这些表现形式更好地限制样品的热历史。特别重要的是，本论文代表了第一项针对天然锆石中整个损伤谱的综合研究，并且首次表明了两种不同的机制以不同的方式影响He在锆石中的扩散。我和我的其他合著者描述了一系列逐步加热实验的结果，这些实验显示了给定锆石的α剂量如何影响我们的He扩散率，我们将其解释为与累积的辐射损伤相关。从1.2 x 1016 a / g到1.4 x 1018 a / g，给定温度下的He扩散率降低了三个数量级，但是随着α剂量从〜2 x 1018 a / g增至8.2 x 1018 a / g。 / g，然后He扩散率增加大约9个数量级。我们用阿尔法剂量参数化了扩散率的初始降低和最终增加，该函数描述了这些变化，涉及晶内辐射损伤区的丰度和大小的增加以及对He迁移途径的曲折性和双畴行为的影响。这与描述锆石中的损伤退火的另一个方程式结合在一起。最终结果是一个新模型，该模型根据其act系元素含量和热历史来限制锆石中的损伤，He扩散率和He数据的共同演化;第二和第三个附录使用该新模型来解密包含许多元素的锆石He数据集与有效铀（eU，同一样品中谷物之间的相对辐射损伤程度的代表）相关的单个谷物日期。该模型对于正确解释来自火成岩环境的结果至关重要，该结果显示了日期与eU的相关性，曾经被认为是伪造的（附录B）。当用于部分重置沉积岩时，还必须考虑其他日期变化的来源，例如破坏和He的继承（附录C）。

著录项

  • 作者

    Guenthner, William Rexford.;

  • 作者单位

    The University of Arizona.;

  • 授予单位 The University of Arizona.;
  • 学科 Geology.;Geochemistry.
  • 学位 Ph.D.
  • 年度 2013
  • 页码 260 p.
  • 总页数 260
  • 原文格式 PDF
  • 正文语种 eng
  • 中图分类
  • 关键词

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