Natural age dispersion arising from the analysis of broken crystals. Part I: Theoretical basis and implications for the apatite (U-Th)/He thermochronometer
Natural age dispersion arising from the analysis of broken crystals. Part I: Theoretical basis and implications for the apatite (U-Th)/He thermochronometer
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DOI：
10.1016/j.gca.2013.05.041
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发表时间：
2013-12-01
影响因子：
5
通讯作者：
Fitzgerald, Paul
中科院分区：
文献类型：
--
作者：
Brown, Roderick W.;Beucher, Romain;Fitzgerald, Paul
在过去的十年中，在磷灰石（U-Th）/He热计时法的理论和实践方面都取得了重大进展，现在分析单颗粒等分试样是标准做法，通常被视为最佳做法。这些单独的棱柱形晶体经常破碎，并且是在矿物分离期间沿着磷灰石中的弱基底解理沿着破碎的较大晶体的碎片。这通过在分离的磷灰石颗粒上仅存在1个或不存在清晰的晶体终端的常见情况以及当使用扫描电子显微镜观察颗粒时新断裂的端部的证据清楚地表明。这一点很重要，因为如果整个颗粒中的4 He分布不均匀，例如由于热扩散造成的部分损失，那么碎片将产生彼此不同的年龄以及与整个颗粒年龄不同的年龄。在这里，我们使用一个有限的圆柱体几何形状的数值模型近似4 He的长入和六角棱柱磷灰石晶体内的热扩散。这是用来量化的数量和模式的固有的，自然的年龄分散所产生的分析破碎的晶体。进行了一系列系统的数值实验，探索和量化的模式和行为的分散使用一组5个简单的热历史，代表了一系列合理的地质情况。此外，还进行了一些更复杂的数值试验，以研究在几个真实的数据集中看到的颗粒分散的模式和行为。结果表明，一组单一的碎片年龄（定义为范围除以平均值）的自然分散所产生的单独的碎裂从c。7%，甚至快速（C。10摄氏度/Ma），单调冷却到超过50%的长期，复杂的历史，造成显着的扩散损失的4 He。由碎裂引起的分散程度与颗粒圆柱半径成比例，并且与仅从绝对颗粒尺寸的差异（球当量半径为40-150 lm）预期的分散程度相似。与单个颗粒分析的典型分析不确定性相比，这种分散源是重要的（c）。6%）和标准偏差的多个颗粒分析从一个单一的样品（c。10-20%）。当单个颗粒的U和Th浓度（eU）存在显著差异时，辐射损伤累积对4 He扩散率的影响（使用Flowers等人（2009）的RDAAM模型进行评估）是经历了长期热历史的样品分散的主要原因，并且可能导致eU浓度实际范围内的分散超过100%（即e. 5-100 ppm）。对于复杂的热历史，由晶粒尺寸（半径40-125 lm）、eU浓度（5-150 ppm）和碎裂的合理变化的组合效应引起的预期自然分散通常超过100%。除了增加一个重要的组成部分，自然分散的分析，破碎的效果也起到了解耦和腐败的预期之间的相关性颗粒年龄和绝对粒度，并在较小程度上颗粒年龄和有效铀浓度（eU）。将碎裂明确地视为色散的一个来源，并分析不同的自然色散源如何相互作用，为理解否则看起来混乱的色散模式提供了一个定量框架。这些数值实验的一个重要结果是，它们表明，由碎裂引起的年龄色散模式模仿了整个晶粒内的4 He分布模式，因此提供了关于样品热历史的重要信息源。我们建议，如果研究的主要重点是从（U-Th）/He分析中提取热历史信息，则采样和分析策略应旨在最大化谷物年龄的自然分散，而不是最小化它，并应旨在从每个样品中分析约20-30个谷物。这里得出的主要意见和结论是直接适用于其他热计时器，如磷灰石，金红石和钛的U-Pb系统，其中扩散域近似的物理粒度。(C)2013爱思唯尔有限公司保留所有权利。
Over the last decade major progress has been made in developing both the theoretical and practical aspects of apatite (U-Th)/He thermochronometry and it is now standard practice, and generally seen as best practice, to analyse single grain aliquots. These individual prismatic crystals are often broken and are fragments of larger crystals that have broken during mineral separation along the weak basal cleavage in apatite. This is clearly indicated by the common occurrence of only 1 or no clear crystal terminations present on separated apatite grains, and evidence of freshly broken ends when grains are viewed using a scanning electron microscope. This matters because if the 4 He distribution within the whole grain is not homogeneous, because of partial loss due to thermal diffusion for example, then the fragments will all yield ages different from each other and from the whole grain age. Here we use a numerical model with a finite cylinder geometry to approximate 4 He ingrowth and thermal diffusion within hexagonal prismatic apatite crystals. This is used to quantify the amount and patterns of inherent, natural age dispersion that arises from analysing broken crystals. A series of systematic numerical experiments were conducted to explore and quantify the pattern and behaviour of this source of dispersion using a set of 5 simple thermal histories that represent a range of plausible geological scenarios. In addition some more complex numerical experiments were run to investigate the pattern and behaviour of grain dispersion seen in several real data sets. The results indicate that natural dispersion of a set of single fragment ages (defined as the range divided by the mean) arising from fragmentation alone varies from c. 7% even for rapid (c. 10 degrees C/Ma), monotonic cooling to over 50% for protracted, complex histories that cause significant diffusional loss of 4 He. The magnitude of dispersion arising from fragmentation scales with the grain cylindrical radius, and is of a similar magnitude to dispersion expected from differences in absolute grain size alone (spherical equivalent radii of 40-150 lm). This source of dispersion is significant compared with typical analytical uncertainties on individual grain analyses (c. 6%) and standard deviations on multiple grain analyses from a single sample (c. 10-20%). Where there is a significant difference in the U and Th concentration of individual grains (eU), the effect of radiation damage accumulation on 4 He diffusivity (assessed using the RDAAM model of Flowers et al. (2009)) is the primary cause of dispersion for samples that have experienced a protracted thermal history, and can cause dispersion in excess of 100% for realistic ranges of eU concentration (i. e. 5-100 ppm). Expected natural dispersion arising from the combined effects of reasonable variations in grain size (radii 40-125 lm), eU concentration (5-150 ppm) and fragmentation would typically exceed 100% for complex thermal histories. In addition to adding a significant component of natural dispersion to analyses, the effect of fragmentation also acts to decouple and corrupt expected correlations between grain ages and absolute grain size and to a lesser extent between grain age and effective uranium concentration (eU). Considering fragmentation explicitly as a source of dispersion and analysing how the different sources of natural dispersion all interact with each other provides a quantitative framework for understanding patterns of dispersion that otherwise appear chaotic.An important outcome of these numerical experiments is that they demonstrate that the pattern of age dispersion arising from fragmentation mimics the pattern of 4 He distribution within the whole grains, thus providing an important source of information about the thermal history of the sample. We suggest that if the primary focus of a study is to extract the thermal history information from (U-Th)/He analyses then sampling and analytical strategies should aimtomaximise the natural dispersion of grain ages, not minimise it, and should aim to analyse circa 20-30 grains from each sample. The key observations and conclusions drawn here are directly applicable to other thermochronometers, such as the apatite, rutile and titanite U-Pb systems, where the diffusion domain is approximated by the physical grain size. (C) 2013 Elsevier Ltd. All rights reserved.