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Potential, premises, and pitfalls of interpreting illite argon dates - A case study from the German Variscides
Earth-Science Reviews ( IF 12.1 ) Pub Date : 2022-07-20 , DOI: 10.1016/j.earscirev.2022.104133
Mathias Hueck , Klaus Wemmer , Anna K. Ksienzyk , Rebecca Kuehn , Nadia Vogel

K-Ar and Ar/Ar illite geochronology are powerful and versatile tools that can be used to provide age constraints on diverse geological processes such as diagenesis, hydrothermalism, and low-temperature metamorphism and deformation in the brittle and ductile-brittle regimes. However, the interpretation of illite Ar dates can be difficult, especially since results from grain-size fractionated samples are typically grain-size dependent, with the finest fractions yielding the youngest dates. This is usually the result of a mixed isotopic signal, caused by a physical mixture between different populations, by partial isotopic and/or chemical resetting, or by a recurrent crystallization history. In order to extract meaningful ages from such datasets, two main lines of interpretation exist: assuming that end fractions (i.e., the finest and coarsest materials of a sample) constitute limit ages, or extrapolating “pure” authigenic and inherited ages using quantifiable mineralogical parameters (such as polytype composition). The two strategies are tested using a well-constrained case study from very low-grade metamorphic samples from the Rhenish Massif (Germany). Six fractionated samples from a continuous outcrop produce internally consistent K-Ar ages that can be interpreted in terms of limit ages and are supported by illite “crystallinity” and polytype composition. Nonetheless, extrapolation of dates from clastic metasedimentary rocks into “pure” end-member ages (illite age analyses - IAA) result in geologically meaningless results that could only be recognized as such because of the analysis of intercalated detritus-free metatuffs. The reason for this is that some of the fundamental premises for the extrapolation of illite ages are not satisfied. These premises are: authigenic and inherited illite populations must be homogeneous in their mineralogical compositions; authigenic illite must have been crystallized during a single near-instantaneous event; and both authigenic and inherited populations must have the same K content. The case study demonstrates the importance of interrogating the limitations and the premises of any treatment of geochronological data in order to avoid over-interpretation. Both strategies for interpreting illite dates are complementary and none is intrinsically better than the other. Because of this complexity, it is important to validate and complement illite argon geochronology by acquiring large datasets and performing additional analyses.



中文翻译:

解释伊利石氩日期的潜力、前提和陷阱——来自德国 Variscides 的案例研究

K-Ar 和 Ar/Ar 伊利石地质年代学是功能强大且用途广泛的工具,可用于为各种地质过程(如成岩作用、热液作用和低温变质作用)提供年龄限制和脆性和韧脆状态下的变形。然而,伊利石 Ar 年代的解释可能很困难,特别是因为粒度分级样品的结果通常取决于粒度,最细的部分产生最年轻的年代。这通常是混合同位素信号的结果,由不同种群之间的物理混合、部分同位素和/或化学重置或反复结晶历史引起。为了从这些数据集中提取有意义的年龄,存在两条主要的解释路线:假设末端部分(即样品中最细和最粗的材料)构成极限年龄,或使用可量化的矿物学参数推断“纯”自生和遗传年龄(如多型组合)。使用来自莱茵河地块(德国)的极低品位变质样品的受约束的案例研究对这两种策略进行了测试。来自连续露头的六个分馏样品产生内部一致的 K-Ar 年龄,可以用极限年龄来解释,并得到伊利石“结晶度”和多型成分的支持。尽管如此,从碎屑推断日期将变质沉积岩转化为“纯”端元年龄(伊利石年龄分析 - IAA)导致在地质学上毫无意义的结果,只能通过对无碎屑变质岩的分析来识别。其原因是伊利特年龄外推的一些基本前提不满足。这些前提是: 自生和遗传的伊利石种群的矿物成分必须是同质的;自生伊利石必须在单个近乎瞬时的事件中结晶;并且自生种群和遗传种群必须具有相同的 K 含量。该案例研究表明,为了避免过度解释,询问对地质年代学数据的任何处理的局限性和前提的重要性。解释伊利特日期的两种策略是互补的,没有一种在本质上比另一种更好。由于这种复杂性,重要的是通过获取大型数据集和执行额外的分析来验证和补充伊利石氩地质年代学。

更新日期:2022-07-20
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