Zircon Microstructures in Large, Deeply Eroded Impact Structures and Terrestrial Seismites

IF 3.5 2区 地球科学 Q1 GEOCHEMISTRY & GEOPHYSICS Journal of Petrology Pub Date : 2023-10-25 DOI:10.1093/petrology/egad079
Adam Andreas Garde, Leif Johansson, Nynke Keulen, Anja Schreiber, Richard Wirth
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Abstract

Abstract Extraterrestrial cratering was a critical component in the evolution of the early Earth, but discovery of very ancient and deeply exhumed impact structures remains elusive, as identification tools are in short supply. The recognition of such structures is hindered by post-cratering geological processes, whereby impact-induced features common in younger, upper-crustal craters may be lost. In this study we make a detailed analysis of planar microstructures in zircons from four large, confirmed impact structures (Manicouagan, Rochechouart, Sudbury, Vredefort) preserved at different crustal levels, from two previously described non-impact seismites in SW Norway and northern Italy, and from the 3.0 Ga Maniitsoq structure in West Greenland. A total of ~3400 zircon grains were studied using exterior and interior scanning and transmission electron microscopy. We show for the first time that shocked zircons contain two successive, principally different types of planar microstructures, only one of which is diagnostic of impact. Closely spaced, contiguous microplanes (CP) are formed first, presumably by the shock wave. In Manicouagan and Rochechouart zircons the exterior CPs have ultrathin interior counterparts of straight dislocation arrays, as identified in Manicouagan zircon using transmission electron microscopy. They have the same close spacing and orientations as the exterior CPs and are abundantly decorated with tiny pores down to less than 50 nm across. These interior CPs are identical to shock-induced decorated, partly annealed amorphous planar microstructures in quartz (planar deformation features, PDFs) and are interpreted as such. The second type is open planar fractures (PF). They are widely and irregularly spaced and texturally younger than the CPs. They re-use and displace the CP orientations, which they cut in stepwise fashion. We interpret these PFs as formed by impact-induced seismic shaking in the wake of the shock wave. We confirm two previous reports of isolated planar fractures in zircons from non-impact seismites, showing that PFs per se are not impact-diagnostic. There are no CPs in any of these zircons. Zircons from different parts of the Maniitsoq structure contain CPs in various states of preservation besides PFs, corroborating that this very large and very deeply exhumed structure resulted from an extraterrestrial impact.
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大型深蚀冲击构造与陆相震积岩中的锆石微结构
地外陨石坑是早期地球演化的重要组成部分,但由于缺乏识别工具,发现非常古老且深挖的撞击结构仍然难以捉摸。这种结构的识别受到陨石坑后地质过程的阻碍,由此在较年轻的上地壳陨石坑中常见的撞击诱发的特征可能会丢失。在这项研究中,我们详细分析了保存在不同地壳水平的4个大型、已确认的撞击构造(Manicouagan、Rochechouart、Sudbury、Vredefort)、挪威西南部和意大利北部的两个先前描述的非撞击震生岩以及西格陵兰岛3.0 Ga的Maniitsoq构造的锆石的平面微观结构。采用内外扫描电镜和透射电镜对3400颗锆石颗粒进行了研究。我们首次发现,受冲击锆石包含两种连续的、主要不同类型的平面微结构,其中只有一种是对冲击的诊断。密集的、连续的微平面(CP)首先形成,可能是由冲击波形成的。在Manicouagan和Rochechouart锆石中,外部CPs具有超薄的直接位错阵列内部对应物,这在Manicouagan锆石中通过透射电子显微镜发现。它们具有与外部CPs相同的紧密间距和方向,并且具有丰富的小孔,直径小于50纳米。这些内部CPs与石英中由冲击引起的装饰、部分退火的非晶态平面微结构(平面变形特征,pdf)相同,并被解释为如此。第二种类型为开放性平面骨折(PF)。它们广泛而不规则地间隔,在结构上比cp年轻。它们重新使用和替换CP方向,它们以逐步的方式切割。我们解释这些PFs是由冲击波后的冲击引起的地震震动形成的。我们证实了先前关于非冲击震积岩中锆石中孤立平面裂缝的两篇报道,表明PFs本身并不能诊断冲击。这些锆石里都没有CPs。manitsoq构造不同部位的锆石除了含有PFs外,还含有各种保存状态的CPs,这证实了这个非常大且非常深的挖掘结构是由外星撞击造成的。
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来源期刊
Journal of Petrology
Journal of Petrology 地学-地球化学与地球物理
CiteScore
6.90
自引率
12.80%
发文量
117
审稿时长
12 months
期刊介绍: The Journal of Petrology provides an international forum for the publication of high quality research in the broad field of igneous and metamorphic petrology and petrogenesis. Papers published cover a vast range of topics in areas such as major element, trace element and isotope geochemistry and geochronology applied to petrogenesis; experimental petrology; processes of magma generation, differentiation and emplacement; quantitative studies of rock-forming minerals and their paragenesis; regional studies of igneous and meta morphic rocks which contribute to the solution of fundamental petrological problems; theoretical modelling of petrogenetic processes.
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