Abstract

U–Pb geochronology of shocked monazite can be used to date hypervelocity impact events. Impact-induced recrystallisation and formation of mechanical twins in monazite have been shown to result in radiogenic Pb loss and thus constrain impact ages. However, little is known about the effect of porosity on the U–Pb system in shocked monazite. Here we investigate monazite in two impact melt rocks from the Hiawatha impact structure, Greenland by means of nano- and micrometre-scale techniques. Microstructural characterisation by scanning electron and transmission electron microscopy imaging and electron backscatter diffraction reveals shock recrystallisation, microtwins and the development of widespread micrometre- to nanometre-scale porosity. For the first time in shocked monazite, nanophases identified as cubic Pb, Pb₃O₄, and cerussite (PbCO₃) were observed. We also find evidence for interaction with impact melt and fluids, with the formation of micrometre-scale melt-bearing channels, and the precipitation of the Pb-rich nanophases by dissolution–precipitation reactions involving pre-existing Pb-rich high-density clusters. To shed light on the response of monazite to shock metamorphism, high-spatial-resolution U–Pb dating by secondary ion mass spectrometry was completed. Recrystallised grains show the most advanced Pb loss, and together with porous grains yield concordia intercept ages within uncertainty of the previously established zircon U–Pb impact age attributed to the Hiawatha impact structure. Although porous grains alone yielded a less precise age, they are demonstrably useful in constraining impact ages. Observed relatively old apparent ages can be explained by significant retention of radiogenic lead in the form of widespread Pb nanophases. Lastly, we demonstrate that porous monazite is a valuable microtexture to search for when attempting to date poorly constrained impact structures, especially when shocked zircon or recrystallised monazite grains are not present.

中文翻译：

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海华沙撞击结构冲击独居石的微观结构和同位素分析：孔隙度的发展及其在撞击坑测年中的用途

摘要

冲击独居石的 U-Pb 地质年代学可用于确定超高速撞击事件的年代。撞击引起的重结晶和独居石中机械孪晶的形成已被证明会导致放射性铅损失，从而限制撞击年龄。然而，对于冲击独居石中孔隙率对 U-Pb 系统的影响知之甚少。在这里，我们通过纳米和微米级技术研究了格陵兰岛海华沙撞击构造的两种撞击熔岩中的独居石。通过扫描电子和透射电子显微镜成像以及电子背散射衍射进行的微观结构表征揭示了冲击再结晶、微孪晶以及广泛的微米至纳米级孔隙的发展。首次在冲击独居石中观察到纳米相，如立方 Pb、Pb ₃ O ₄和白铅矿 (PbCO _{3 )。}我们还发现了冲击熔体和流体相互作用的证据，以及微米级熔体承载通道的形成，以及通过涉及预先存在的富铅高密度团簇的溶解-沉淀反应来沉淀富铅纳米相。为了揭示独居石对冲击变质作用的响应，通过二次离子质谱法完成了高空间分辨率 U-Pb 定年。再结晶晶粒显示出最先进的 Pb 损失，并且与多孔晶粒一起产生协和截距年龄，该年龄处于先前确定的归因于 Hiawatha 撞击结构的锆石 U-Pb 撞击年龄的不确定范围内。尽管单独使用多孔颗粒得出的年龄不太精确，但它们在限制冲击年龄方面显然很有用。观察到的相对较老的表观年龄可以通过以广泛的 Pb 纳米相形式显着保留放射性铅来解释。最后，我们证明，当试图确定约束不良的冲击结构时，特别是当不存在冲击锆石或重结晶独居石晶粒时，多孔独居石是一种有价值的微观结构。