Helium diffusion systematics inferred from continuous ramped heating analysis of Transantarctic Mountains apatites showing age overdispersion
Helium diffusion systematics inferred from continuous ramped heating analysis of Transantarctic Mountains apatites showing age overdispersion
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通过对横贯南极山脉磷灰石的连续斜坡加热分析推断出的氦扩散系统学显示年龄过度分散
DOI：
10.1016/j.gca.2021.07.015
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发表时间：
2021
影响因子：
5
通讯作者：
McDannell, Kalin T.
中科院分区：
文献类型：
--
作者：
Guo, Hongcheng;Zeitler, Peter K.;Idleman, Bruce D.;Fayon, Annia K.;Fitzgerald, Paul G.;McDannell, Kalin T.
磷灰石（U-Th）/He热年代学的应用一直受到阻碍扩散系统学的不完全理解，导致单颗粒年龄分散体往往显示的样品，特别是那些从老的，缓慢冷却的设置。我们应用连续斜坡加热（CRH）的方法，磷灰石套房从大教堂岩石在跨南极山脉（TAM），具有较高的年龄分散，以解释复杂的过程，4 He在磷灰石扩散。检查132磷灰石颗粒从总共6个样品，我们证实了早期的磷灰石（U-Th）/He的结果表明，测得的AHe年龄至少有三倍的样品内的分散与年龄和有效铀浓度（eU）或晶粒尺寸之间没有明显的关系。CRH结果对这些磷灰石产生了两组。那些年龄较小，其特点是单峰递增4 He气体释放曲线，表现出简单的体积扩散行为。相比之下，具有较老年龄的谷物通常显示出在高温下以尖锐尖峰和/或延长的气体释放形式的复杂气体释放（即，≥800 ℃）。简单的行为磷灰石仍然表现出相当大的年龄分散，超过了什么晶粒尺寸，辐射损伤，和分析的不确定性可以解释，但这种分散似乎与4 He扩散动力学的变化。筛选AHe年龄从简单的行为年轻磷灰石颗粒与动力学信息，从这些颗粒表明，采样区域经历了缓慢冷却之前，快速冷却（岩石折返）a.35马。这一解释与其他研究一致，表明此时的剥露率增加，可能与始新世-渐新世气候转变时冰川作用的开始有关。通过简单地去除外部气体释放成分来校正较旧的磷灰石年龄的尝试产生了一些对于样品的地质环境来说太年轻的年龄，这表明导致复杂的实验室释放行为的因素可以影响预期的放射性成分以及那些明显无关的成分。根据我们的观察，我们推断许多磷灰石颗粒含有各种各样的缺陷，这些缺陷对动力学变化的贡献明显超过了与辐射损伤相关的变化，并得出结论：（1）CRH分析可以作为常规的AHe定年筛选工具，并提供了揭示一级动力学变化的机会;（2）依赖模型的年龄校正是可能的，但需要一些方法来估计在扩散闭合前后掺入颗粒中的4 He组分的广泛比例，（3）解释高度分散的AHe年龄需要评估单个颗粒扩散动力学，超出辐射损伤模型的预测。
Application of apatite (U-Th)/He thermochronology has been hindered by incomplete understanding of diffusion systematics that leads to the single-grain age dispersion often displayed by samples, particularly those from older, slowly cooled settings. We applied the continuous ramped heating (CRH) method to an apatite suite from Cathedral Rocks in the Transantarctic Mountains (TAM) that have high age dispersion in order to explain processes that complicate4He diffusion in apatite. Examining 132 apatite grains from a total of six samples, we confirmed earlier apatite (U-Th)/He results showing that measured AHe ages have at least three-fold intra-sample dispersion with no obvious relationships between ages and effective uranium concentration (eU) or grain size. CRH results on these apatites yielded two groups. Those with younger ages, characterized by unimodal incremental4He gas-release curves, displayed simple volume diffusion behavior. In contrast, grains with older ages generally show complex gas release in the form of sharp spikes and/or extended gas-release at high temperatures (i.e., ≥800 °C). Simply-behaved apatites still show considerable age dispersion that exceeds what grain size, radiation damage, and analytical uncertainty can explain, but this dispersion appears to be related to variations in4He diffusion kinetics. The screened AHe ages from simply-behaved younger apatite grains together with kinetic information from these grains suggest that the sampled region experienced slow cooling prior to rapid cooling (rock exhumation) beginningca.35 Ma. This interpretation is consistent with other studies indicative of an increase in exhumation rates at this time, possibly related to the initiation of glaciation at the Eocene-Oligocene climate transition. An attempt to correct older apatite ages by simply removing extraneous gas-release components yielded some ages that are too young for the samples’ geologic setting, suggesting that the factors that lead to complex laboratory release behavior can impact both the expected radiogenic component as well as those that are apparently extraneous. From our observations, we infer that many apatite grains contain imperfections of varying kinds that contribute significantly to kinetic variability beyond that associated with radiation damage and conclude that: (1) CRH analysis can serve as a routine screening tool for AHe dating and offers opportunities to reveal first-order kinetic variations; (2) model-dependent age correction may be possible but would require some means of estimating the broad proportions of4He components incorporated into grains before and after closure to diffusion, and (3) interpretation of highly dispersed AHe ages requires assessment of individual-grain diffusion kinetics beyond that predicted by radiation-damage models.
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