Experimental Replacement of Zircon by Melt‐Mediated Coupled Dissolution‐Precipitation Causes Dispersion in U–Pb Ages

IF 3.5 2区 地球科学 Q1 GEOLOGY Journal of Metamorphic Geology Pub Date : 2024-08-28 DOI:10.1111/jmg.12795
Jeremy L. Asimus, Nathan R. Daczko, Jean‐Antoine Gazi, Isra S. Ezad, Ivan Belousov, Thomas Rodemann, Jacqueline A. Halpin, Sandra Piazolo
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Abstract

Zircon geochronology provides critical information on the rates and durations of geological processes and enables researchers to explore deep time. However, some zircon datasets show a continuum of concordant ages (‘smear’) without well‐defined age populations. These age smears are typically interpreted to represent variable loss of radiogenic Pb or protracted geological events lasting tens of millions of years. Coupled dissolution‐precipitation replacement of zircon has been suggested as one process that may produce these complex age datasets. Here, we react fragments of the well characterised Mud Tank zircon standard with natural intermediate and mafic melts (0.9 GPa, 1100–1180°C) to test if short‐term exposure to a melt can modify the geochronological patterns of zircon. Our observations show that within a short duration (18 h to 3.5 days), most Mud Tank zircon fragments display microstructural and/or chemical evidence for modification by dissolution at fragment boundaries along with partial replacement by coupled dissolution‐precipitation processes. The replaced zircon domains have U–Pb ages that smear over one hundred million years, between 764–647 Ma, illustrating variable mobility and redistribution of the U and Pb isotopes. Our experiments demonstrate that zircon modified by coupled dissolution‐precipitation replacement may not faithfully record the age or duration of geological events and that investigation of zircon microstructure in high‐resolution backscattered electron, cathodoluminescence imaging and/or Raman mapping is needed to better understand complex zircon geochronological datasets.
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熔融介导的溶解-沉淀耦合实验置换锆石导致 U-Pb 年代的分散
锆石地质年代学提供了有关地质过程的速率和持续时间的重要信息,使研究人员能够探索深部时间。然而,一些锆石数据集显示出连续的一致年龄("涂抹"),而没有明确的年龄群。这些年龄斑点通常被解释为放射性铅的可变损失或持续数千万年的漫长地质事件。锆石的溶解-沉淀耦合置换被认为是产生这些复杂年龄数据集的一个过程。在这里,我们将特性良好的泥潭锆石标准碎片与天然的中间熔体和岩浆熔体(0.9 GPa, 1100-1180°C)进行反应,以测试短期暴露于熔体是否会改变锆石的地质年代模式。我们的观察结果表明,在短时间内(18 小时至 3.5 天),大多数泥罐锆石碎片显示出微观结构和/或化学证据,表明碎片边界处的溶解以及溶解-沉淀耦合过程的部分置换改变了锆石。被置换的锆石区域的 U-Pb 年龄在 1 亿年(764-647 Ma)之间,说明了 U 和 Pb 同位素的可变流动性和再分布。我们的实验表明,通过溶解-沉淀耦合置换改变的锆石可能无法忠实地记录地质事件的年龄或持续时间,因此需要通过高分辨率反向散射电子、阴极发光成像和/或拉曼绘图来研究锆石的微观结构,以便更好地理解复杂的锆石地质年代数据集。
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来源期刊
CiteScore
6.60
自引率
11.80%
发文量
57
审稿时长
6-12 weeks
期刊介绍: The journal, which is published nine times a year, encompasses the entire range of metamorphic studies, from the scale of the individual crystal to that of lithospheric plates, including regional studies of metamorphic terranes, modelling of metamorphic processes, microstructural and deformation studies in relation to metamorphism, geochronology and geochemistry in metamorphic systems, the experimental study of metamorphic reactions, properties of metamorphic minerals and rocks and the economic aspects of metamorphic terranes.
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