There are likely many undiscovered impact structures on Earth, but several challenges prevent their detection, including possible concealment beneath large ice sheets. In recent years, geophysical, geochemical, and microphysical evidence has mounted for a ca. 58 Ma impact structure under the Hiawatha Glacier, northwest Greenland. Here, we report evidence for a second, much older hypervelocity impact event in this region, recorded in an impact melt rock sample collected from a glaciofluvial deposit in Inglefield Land. Secondary ion mass spectrometry U-Pb analyses of shock metamorphosed zircon grains yielded a previously unrecorded, Proterozoic best estimate impact age of 1039 ± 16 Ma (mean square of weighted deviates = 2.9). Based on Archean–Proterozoic target rock U-Pb ages obtained from unshocked zircon grains and the location of the melt rock sample along the ice margin, we suggest this sample was derived from a hypervelocity impact structure farther inland, concealed by the Greenland Ice Sheet. This study demonstrates the ability to uncover new impact events in some of the most inaccessible areas on Earth and the possibility of sampling multiple impact structures from one location when examining ex situ material. Our results have implications for current and future Martian and lunar returned samples that demonstrably bear complex impact histories.There is diagnostic evidence for ~200 hypervelocity impact craters on Earth (Schmieder and Kring, 2020; Kenkmann, 2021). However, it is likely that many more undetected structures exist (Hergarten and Kenkmann, 2015), particularly beneath the Greenland and Antarctic ice sheets, which obscure ~10% of Earth's land surface. Identification of diagnostic indicators is required to confirm an impact origin for candidate structures (French and Koeberl, 2010), including either physical (e.g., planar deformation features [PDFs] in quartz) or geochemical evidence (e.g., elevated Ir concentrations). Constraining precise ages of impacts allows for better understanding of the role impact cratering has played in the paleoclimate evolution of Earth (e.g., Schulte et al., 2010). Ideally, impactite lithologies are sampled in situ. When such access is not feasible, samples can be collected distally when topographic evidence links detrital samples to a structure with clear crater morphology (Osinski et al., 2022). This approach was recently used to demonstrate an impact origin for the Hiawatha structure located beneath the Greenland Ice Sheet (Kjær et al., 2018; Garde et al., 2022), which is likely a 57.99 ± 0.54 Ma impact structure based on shocked zircon and monazite U-Pb dating of two detrital impact melt rock samples (Kenny et al., 2022; Hyde et al., 2024). Many other impact structures have been confirmed using similar procedures (e.g., Dypvik et al., 1996; Alwmark et al., 2015).Here, we investigated the sparse impact record of Greenland recorded by five detrital impact melt rock samples recently exhumed from the Greenland Ice Sheet, by combining electron backscatter diffraction (EBSD) and state-of-the-art U-Pb analysis by secondary ion mass spectrometry (SIMS) of variably shocked zircon.Five detrital, pebble-sized impact melt rock samples (HW19-02, HW19-04, HW19-17, HW19-31, and HW19-32) were selected from 40 samples collected proximal to the Hiawatha structure in Inglefield Land, northwest Greenland (Fig. 1A). Two of these samples were collected from a glaciofluvial channel, which is the main drainage channel of the structure, 4 km past the terminus of the protruding Hiawatha Glacier. Three other samples were collected from two locations along the ice margin that conceals the western rim of the structure. PDFs in quartz grains were indexed using a U-stage mounted on a petrographic microscope following Stöffler and Langenhorst (1994). Zircon grains were mechanically separated from each sample, mounted in epoxy, and polished. In total, 119 grains were imaged by backscattered electron (BSE) and cathodoluminescence (CL) imaging, and further microstructural characterization of 18 grains was conducted by EBSD, on an FEI Quanta FEG 650 scanning electron microscope at the Swedish Museum of Natural History. Grains displaying a variety of microtextures were then chosen for U-Pb isotopic composition and age analysis (n = 185) using a CAMECA IMS1280 ion microprobe at the NordSIMS Laboratory, Stockholm, Sweden. Metamict and fractured domains of grains were avoided for U-Pb analysis. To acquire impact ages, granular areas were targeted where Pb loss is more likely complete (e.g., Schmieder et al., 2015b). Grains were repolished to acquire additional data. Further details of laboratory techniques used are given in the Supplemental Material1.All five samples are clast-rich impact melt rocks (Stöffler et al., 2018); samples HW19-02, HW19-04, and HW19-17 comprise a hypocrystalline melt matrix (Fig. 2), whereas HW19-31 and HW19-32 comprise a perlitic and spherulitic glassy matrix, respectively (Fig. S1). All samples contain quartz grains with PDFs. Four of the samples (HW19-02, HW19-04, HW19-31, and HW19-32) and their shock features were described in Hyde et al. (2023), whereas sample HW19-17 is presented here for the first time. Sample HW19-17 is a pebble-sized, orange-gray impact melt rock containing an aphanitic matrix composed of plagioclase microlites, siliceous mesostasis, and secondary smectites (Fig. 2; Fig. S1). The clast load is dominated by quartz clasts, which are commonly recrystallized or partially digested (Fig. S1). PDFs in quartz are heavily decorated with large fluid inclusions (Fig. 2C). Indexing of quartz PDF orientations revealed that {1013} is the most common orientation (28%), followed by {1012} and {1014} (22% and 17%, respectively; Fig. S2D). All samples contain zircon grains; those within polycrystalline clasts commonly appear pristine, whereas those within the melt matrix are often deformed (Fig. 2D).Separated zircon grains from all samples range from undeformed to displaying one or more shock deformation microstructure (Fig. 3; Figs. S3–S4). Unshocked grains showed a variety of textures in CL images (i.e., oscillatory zoning). EBSD imaging of deformed grains revealed planar deformation bands, planar fractures, crystal-plastic lattice strain, and porosity (Fig. 3A; Fig. S4). Additionally, some grains displayed shock recrystallization and shock microtwins (Fig. 3B), and rarely dissociation of zircon to ZrO2 (Figs. S3–S4: Timms et al., 2017). The high-pressure zircon polymorph reidite was not detected in any grain. At least one partially recrystallized grain displayed systematic crystallographic relationships (90° misorientation), indicating former reidite in granular neoblastic (FRIGN) zircon (Fig. S4G; Cavosie et al., 2016, 2018; Timms et al., 2017).The U-Pb data for four of the five samples (excluding HW19-17) individually yielded discordant arrays, trending from the Paleoproterozoic to the Late Paleocene (Fig. 4A; Fig. S5; Supplemental Material). Combined, these data (n = 77) produced a lower concordia intercept age of 50.5 ± 8.6 Ma (Figs. S5I–S5J). Concordant ages from unshocked grains from these same four samples yielded a concordia age at 1928 ± 13 Ma (Fig. 4A; Fig. S5O). In contrast, HW19-17 revealed vastly different U-Pb results: All data from that sample (n = 108) recorded a discordant array trending from the Neoarchean–Paleoproterozoic to the Mesoproterozoic–Neoproterozoic boundary (Fig. 4A; Fig. S5A). A clear correlation between grain microtexture and apparent age was observed (Fig. S5B). Analyses from shock-recrystallized zircon grains gave concordant dates, collectively yielding a best estimate concordia age of 1039 ± 16 Ma (mean square of weighted deviates [MSWD] = 2.9; Figs. 3B and 4). This age was calculated from eight analyses from four neoblastic grains (Figs. S5G–S5H). Critically, none of these eight analyses gave 206Pb/238U ages younger than 976 ± 66 Ma (Fig. 4; Fig. S5A). Furthermore, unshocked zircon grains from HW19-17 recorded four concordia ages of 1.8, 1.9, 2.53, and 2.7 Ga, approximately (Fig. 4; Fig. S5).An unambiguous impact origin for all five samples is demonstrated based on PDFs in quartz and shock recrystallization of zircon (Fig. 2; Fig. S4; Hyde et al., 2023). The hypervelocity impact event that formed the enigmatic HW19-17 sample occurred at 1039 ± 16 Ma, from SIMS U-Pb analysis of recrystallized domains of shocked zircon grains (Figs. 3B and 4). This procedure yields precise ages for ancient impact events (e.g., Kenny et al., 2017; Erickson et al., 2020). This impact event, slightly older than the Stenian-Tonian boundary at 1000 Ma, is unknown in Earth's impact record and represents one of the oldest recorded impact events (e.g., Schmieder and Kring, 2020). A ca. 1 Ga impact event contrasts clearly with the other four melt rock samples from Inglefield Land (Fig. 1), which together yield a lower intercept age of 50.5 ± 8.6 Ma, within uncertainty of a 57.99 ± 0.54 Ma zircon U-Pb best estimate impact age based on two melt rock samples collected at the same location (Fig. 4; Fig. S5; Kenny et al., 2022). That ca. 58 Ma impact age is attributed to the Hiawatha impact structure (Fig. 4), so our discovery of a ca. 1 Ga impact event adds substantial complexity to the impact history of northwest Greenland.Additional indirect evidence for two distinct impact events is provided by U-Pb analysis of unshocked zircon grains, interpreted to represent the crystallization ages of target protoliths (Fig. 3A). Within HW19-17, we observed the same 1.9 Ga target rock age that was dominant in the other four samples that recorded the previously identified late Paleocene impact event only (Fig. 4; Fig. S5; Kenny et al., 2022). This age corresponds with known lithologies in the deglaciated foreland of the Hiawatha structure, i.e., ca. 1.9 Ga Etah group paragneiss (Nutman et al., 2008), indicating local provenance of the samples (Kenny et al., 2022). However, U-Pb data from HW19-17 yielded additional target rock ages that are sparse or absent in all our other samples (e.g., two Neoarchean U-Pb ages, ca. 2.53 and ca. 2.7 Ga; Fig. 4C), representing rock ages that are not found at the surface in Inglefield Land (Nutman et al., 2008).Any physical parameters of this new impact event and crater, if still preserved, are as-of-yet undetermined. Impact melt (60–70 GPa; Stöffler et al., 2018) is observed in sub-kilometer-scale structures on Earth (e.g., Kamil crater, Egypt: Ø = 45 m; Fazio et al., 2014). However, Kenkmann (2021) demonstrated a paucity of craters older than 100,000 yr old that are <3 km in diameter. Given the Proterozoic age of this new impact event, the structure likely had an original diameter of several kilometers.The location of this ca. 1 Ga impact event is currently unknown, but it is probable that impact melt rock sample HW19-17 was transported from an impact structure hidden inland beneath the Greenland Ice Sheet (Fig. 1B). Assuming that the sample has been eroded relatively recently, i.e., since the Neogene onset of glaciation in Greenland (Bierman et al., 2016), modern ice-flow directions (Rignot and Mouginot, 2012) suggest that the structure is likely situated beneath the northern ice-drainage basin of the Greenland Ice Sheet (Fig. 1B). Interpolated subglacial geology in northern Greenland (Dawes, 2009) indicates that Neoarchean–Paleoproterozoic bedrock dominates the southern portion of this drainage basin. Further, zircon U-Pb ages in subglacial detrital sediment from the nearby Camp Century ice core are similar to those found in HW19-17 (Figs. 1B and 4; Christ et al., 2023). In sum, these considerations suggest that HW19-17 originated from an impact farther inland than the Hiawatha structure and not in the immediate vicinity of Inglefield Land (Fig. 1).In agreement with these tentative location constraints, we note the earlier discovery of a circular ice-surface expression overlying a gravitational and topographic low, 183 km southeast of the Hiawatha structure (Fig. 1B; MacGregor et al., 2019). This feature was proposed to be a possible second subglacial impact crater (Ø ≥ 36 km) based on remote sensing only and is presumed to be older than the Hiawatha structure, due to its lower depth-to-diameter ratio (MacGregor et al., 2019). However, further investigation is required to test an impact hypothesis for its origin.The absence of well-dated impact craters or deposits dating to 1039 ± 16 Ma (e.g., Schmieder and Kring, 2020) precludes a connection to a specific event and rules out all known Mesoproterozoic impact structures, i.e., the Keurusselkä impact structure (1151 ± 10 Ma; Schmieder et al., 2016) and the Stac Fada impact deposit (1177 ± 5 Ma; Parnell et al., 2011). It is possible that the sample originated from a poorly dated impact structure in Canada (e.g., the Presqu'île impact structure: <2729 Ma; Higgins and Tait, 1990), which at ca. 1 Ga was assembled with Greenland in Rodinia (Pesonen et al., 2012). This is unlikely, however, as it would require HW19-17 to have remained at the surface in northwestern Greenland for an unusually long time, given that there is no plausible ongoing mechanism to transport the sample to Inglefield Land.The most likely scenario is that four of the samples analyzed here are associated with the nearby Hiawatha structure and record the same Late Paleocene impact event (Figs. 1 and 4A; Kenny et al., 2022). The geomorphology of the Hiawatha structure (rim-to-floor depth of 320 ± 70 m; Kjær et al., 2018) is more consistent with a Late Paleocene impact than a ca. 1 Ga event, despite variable high-latitude erosion (Kenkmann, 2021). However, radiometric dating of impact materials collected in situ within the Hiawatha structure is still required to unequivocally confirm this scenario.In contrast, detrital sample HW19-17 records a previously unknown ca. 1 Ga impact event (Fig. 4). This new discovery, alongside the other melt rock samples collected in Inglefield Land, demonstrates the rare occurrence of sampling multiple impact structures at a single location, which occurs infrequently on Earth (e.g., Schmieder et al., 2015a). These results demonstrate that it is imperative to combine isotopic characterization of zircon grains (e.g., U-Pb geochronology) with shock microstructures in detrital material to link them to a specific impact event, especially when working in regions of Earth with sparse impact records.The procedures demonstrated in this study can be a useful analogue for future martian and lunar returned samples, as planetary regolith or surficial breccias demonstrably comprise shocked materials subjected to, or originating from, multiple impact events (e.g., Grange et al., 2013). Searches for detrital samples resembling impactites from known impact structures in moraine or glaciofluvial drainage channels can be used in the search for new impact structures likely hidden under large continental ice sheets (e.g., Hergarten and Kenkmann, 2015). We have shed new light on the impact record of an otherwise inaccessible region and suggest more effort is warranted to search for detrital evidence of new impact events globally, which would help us to understand the succession of impact events that occurred throughout Earth's history.We thank Kerstin Lindén, Heejin Jeon, Sanna Alwmark, and Anders Plan for assistance with data collection. We also thank Kurt H. Kjær and Anders A. Bjørk, and pay tribute to the memory of Jérémie Mouginot, for their assistance with fieldwork. We thank Marc Norman (editor), Gordon Osinski, Nick Timms, and Timmons Erickson for their constructive comments, which significantly improved this manuscript. The NordSIMS laboratory is funded by the Swedish Research Council (grant 2021-00276). This is NordSIMS contribution 759. This work was supported by Swedish Research Council grant 2020-04862 (G.G. Kenny), Geocenter Denmark grant DALIA (Kurt H. Kjær), and the Independent Research Fund Denmark grant 0135-00163B (N.K. Larsen).

中文翻译：

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大约的证据。格陵兰岛西北部发现1 Ga超高速撞击事件


地球上可能存在许多未被发现的撞击结构，但有一些挑战阻止它们被发现，包括可能隐藏在大冰盖下面。近年来，地球物理、地球化学和微物理证据已经增加了约 100 万。格陵兰岛西北部海华沙冰川下的 58 Ma 撞击构造。在这里，我们报告了该地区第二次更古老的超高速撞击事件的证据，记录在从英格尔菲尔德地冰川沉积物收集的撞击熔岩样本中。对冲击变质锆石颗粒进行二次离子质谱 U-Pb 分析得出了先前未记录的元古代最佳估计撞击年龄为 1039 ± 16 Ma（加权偏差均方 = 2.9）。根据从未受冲击的锆石颗粒获得的太古宙-元古代目标岩石 U-Pb 年龄以及沿冰缘融化岩石样本的位置，我们认为该样本源自内陆更远的超高速撞击结构，被格陵兰冰盖掩盖。这项研究展示了在地球上一些最难以到达的地区发现新撞击事件的能力，以及在检查异位材料时从一个位置对多个撞击结构进行采样的可能性。我们的结果对当前和未来返回的火星和月球样本具有影响，这些样本显然具有复杂的撞击历史。有诊断证据表明地球上有约 200 个超高速撞击坑（Schmieder 和 Kring，2020 年；Kenkmann，2021 年）。然而，很可能存在更多未被发现的结构（Hergarten 和 Kenkmann，2015），特别是在格陵兰岛和南极冰盖下方，这些冰盖遮盖了约 10% 的地球陆地表面。 需要识别诊断指标来确认候选结构的撞击起源（French 和 Koeberl，2010），包括物理证据（例如，石英中的平面变形特征 [PDF]）或地球化学证据（例如，Ir 浓度升高）。限制撞击的精确年龄可以更好地了解撞击坑在地球古气候演化中所发挥的作用（例如，Schulte 等人，2010）。理想情况下，冲击岩岩性是在原位取样的。当这种进入不可行时，当地形证据将碎屑样本与具有清晰陨石坑形态的结构联系起来时，可以在远端收集样本（Osinski 等人，2022）。这种方法最近被用来证明位于格陵兰冰盖下方的 Hiawatha 结构的撞击起源（Kjær 等人，2018；Garde 等人，2022），这可能是基于冲击锆石的 57.99 ± 0.54 Ma 撞击结构以及两个碎屑撞击熔岩样本的独居石 U-Pb 定年（Kenny 等人，2022 年；Hyde 等人，2024 年）。许多其他撞击结构已使用类似的程序得到确认（例如，Dypvik 等人，1996 年；Alwmark 等人，2015 年）。在这里，我们研究了最近从格陵兰岛挖掘出的五个碎屑撞击熔岩样本记录的格陵兰岛稀疏撞击记录。格陵兰冰盖，通过结合电子背散射衍射 (EBSD) 和通过二次离子质谱 (SIMS) 对可变冲击锆石进行最先进的 U-Pb 分析。五个碎屑、卵石大小的冲击熔岩样品 (HW19- 02、HW19-04、HW19-17、HW19-31 和 HW19-32）是从格陵兰西北部英格尔菲尔德地 Hiawatha 构造附近采集的 40 个样本中选出的（图 1A）。 其中两个样本是从冰川河道采集的，该河道是该结构的主要排水道，距突出的海华沙冰川终点 4 公里。另外三个样本是从隐藏着该结构西缘的冰缘沿线的两个位置采集的。遵循 Stöffler 和 Langenhorst (1994) 的方法，使用安装在岩相显微镜上的 U 型台对石英颗粒中的 PDF 进行索引。将锆石颗粒从每个样品中机械分离出来，安装在环氧树脂中并抛光。通过背散射电子 (BSE) 和阴极发光 (CL) 成像总共对 119 个颗粒进行了成像，并通过 EBSD 在瑞典自然历史博物馆的 FEI Quanta FEG 650 扫描电子显微镜上对 18 个颗粒进行了进一步的微观结构表征。然后使用瑞典斯德哥尔摩 NordSIMS 实验室的 CAMECA IMS1280 离子微探针选择显示各种微观纹理的颗粒进行 U-Pb 同位素组成和年龄分析 (n = 185)。 U-Pb 分析中避免了晶粒的超晶区和断裂区域。为了获得撞击年龄，目标是铅损失更可能完全的颗粒区域（例如，Schmieder 等人，2015b）。对颗粒进行重新抛光以获得额外的数据。补充材料1中给出了所用实验室技术的更多细节。所有五个样品都是富含碎屑的冲击熔岩（Stöffler等人，2018）；样品 HW19-02、HW19-04 和 HW19-17 包含低晶熔体基体（图 2），而 HW19-31 和 HW19-32 分别包含珠光体和球晶玻璃态基体（图 S1）。所有样品均含有带有 PDF 的石英颗粒。 Hyde 等人描述了其中四个样品（HW19-02、HW19-04、HW19-31 和 HW19-32）及其冲击特征。 (2023)，而样品 HW19-17 是首次在此提出。样品 HW19-17 是一种卵石大小的橙灰色冲击熔岩，含有由斜长石微晶石、硅质介稳态和次生蒙脱石组成的隐晶质基质（图 2；图 S1）。碎屑负荷主要是石英碎屑，它们通常被重结晶或部分消化（图S1）。石英中的 PDF 被大量的流体包裹体装饰（图 2C）。石英 PDF 方向的索引显示，{1013} 是最常见的方向 (28%)，其次是 {1012} 和 {1014}（分别为 22% 和 17%；图 S2D）。所有样品均含有锆石颗粒；多晶碎屑中的锆石通常看起来是原始的，而熔体基质中的锆石常常变形（图 2D）。从所有样品中分离出的锆石晶粒的范围从未变形到显示一种或多种冲击变形微观结构（图 3；图 S3-S4） ）。未受冲击的颗粒在 CL 图像中显示出各种纹理（即振荡分区）。变形晶粒的 EBSD 成像显示了平面变形带、平面断裂、晶体塑性晶格应变和孔隙率（图 3A；图 S4）。此外，一些晶粒表现出冲击再结晶和冲击微孪晶（图3B），并且锆石很少解离为ZrO2（图S3-S4：Timms等人，2017）。在任何颗粒中均未检测到高压锆石多晶型雷德石。至少一种部分重结晶晶粒显示出系统的晶体学关系（90° 错误取向），表明粒状新生（FRIGN）锆石中存在以前的重结晶（图 S4G；Cavosie 等人，2016 年、2018 年；Timms 等人，2017 年）。五个样品中的四个（不包括 HW19-17）的 U-Pb 数据分别产生了不一致的阵列，趋势从古元古代到古新世晚期（图 4A；图 S5；补充材料）。综合起来，这些数据 (n = 77) 产生了较低的协和截距年龄 50.5 ± 8.6 Ma（图 S5I-S5J）。来自这四个样品的未冲击谷物的一致年龄得出了 1928±13Ma 的一致年龄（图 4A；图 S5O）。相比之下，HW19-17揭示了截然不同的U-Pb结果：来自该样本的所有数据（n = 108）记录了从新太古代-古元古代到中元古代-新元古代边界的不一致阵列（图4A；图S5A）。观察到颗粒微观纹理和表观年龄之间存在明显的相关性（图S5B）。对冲击再结晶锆石颗粒的分析给出了一致的日期，共同得出了 1039 ± 16 Ma 的最佳估计协和年龄（加权偏差均方 [MSWD] = 2.9；图 3B 和 4）。这个年龄是根据四个新生颗粒的八次分析计算得出的（图S5G-S5H）。重要的是，这八项分析均未得出 206Pb/238U 年龄小于 976± 66 Ma 的年龄（图 4；图 S5A）。此外，来自 HW19-17 的未冲击锆石颗粒记录了四个协和年龄，分别为 1.8、1.9、2.53 和 2.7 Ga（图 4；图 S5）。基于石英中的 PDF，证明了所有五个样品的明确撞击起源。和锆石的冲击再结晶（图 2；图 S4；Hyde 等人，2023）。根据对冲击锆石晶粒的再结晶域的 SIMS U-Pb 分析，形成神秘 HW19-17 样品的超高速撞击事件发生在 1039 ± 16 Ma（图 3B 和 4）。此过程可得出古代撞击事件的精确年龄（例如，Kenny 等人，2017 年；Erickson 等人，2020 年）。 这次撞击事件比 1000Ma 的 Stenian-Tonian 边界稍早，在地球的撞击记录中是未知的，并且代表了有记录的最古老的撞击事件之一（例如，Schmieder 和 Kring，2020）。大约。 1 Ga撞击事件与来自英格尔菲尔德地的其他四个熔岩样本形成鲜明对比（图1），这四个熔岩样本共同产生了50.5 ± 8.6 Ma的较低截距年龄，在57.99 ± 0.54 Ma锆石U-Pb最佳估计撞击的不确定性范围内年龄基于在同一地点收集的两个熔岩样本（图 4；图 S5；Kenny 等人，2022）。那个约。 58 Ma撞击年龄归因于Hiawatha撞击结构（图4），因此我们发现了大约58 Ma撞击年龄。 1 Ga撞击事件极大地增加了格陵兰西北部撞击历史的复杂性。对未受冲击的锆石颗粒的U-Pb分析提供了两个不同撞击事件的额外间接证据，解释为代表目标原岩的结晶年龄（图3A）。在 HW19-17 中，我们观察到了相同的 1.9 Ga 目标岩石年龄，这在其他四个样本中占主导地位，这些样本仅记录了先前确定的古新世晚期撞击事件（图 4；图 S5；Kenny 等，2022）。这个年龄与 Hiawatha 构造消融前陆的已知岩性相对应，即大约 1000 年。 1.9 Ga Etah群副片麻岩（Nutman等人，2008），表明样本的本地来源（Kenny等人，2022）。然而，HW19-17 的 U-Pb 数据产生了额外的目标岩石年龄，这些目标岩石年龄在我们所有其他样本中很少或不存在（例如，两个新太古代 U-Pb 年龄，约 2.53 和约 2.7 Ga；图 4C），代表英格尔菲尔德地表未发现的岩石年龄（Nutman 等人，2008 年）。这一新撞击事件和陨石坑的任何物理参数（如果仍保留下来）尚未确定。 在地球上亚公里级结构中观察到了撞击融化（60–70 GPa；Stöffler 等，2018）（例如，埃及卡米尔陨石坑：Ø = 45 m；Fazio 等，2014）。然而，Kenkmann（2021）证明，年龄超过 10 万年且直径 <3 公里的陨石坑很少。考虑到这次新撞击事件的元古代年龄，该结构的原始直径可能有几公里。 1 Ga撞击事件目前尚不清楚，但撞击熔岩样本HW19-17很可能是从格陵兰冰盖下方隐藏的内陆撞击结构输送来的（图1B）。假设该样本最近被侵蚀，即自格陵兰岛新近纪冰川作用开始以来（Bierman 等人，2016 年），现代冰流方向（Rignot 和 Mouginot，2012 年）表明该结构可能位于格陵兰冰盖北部的冰排盆地（图1B）。格陵兰岛北部的插值冰下地质学（Dawes，2009）表明，新太古代-古元古代基岩主导了该流域盆地的南部。此外，附近 Camp Century 冰芯的冰下碎屑沉积物中的锆石 U-Pb 年龄与 HW19-17 中发现的相似（图 1B 和 4；Christ 等，2023）。总之，这些考虑表明 HW19-17 起源于比 Hiawatha 构造更远的内陆撞击，而不是紧邻英格尔菲尔德地（图 1）。与这些暂定位置限制一致，我们注意到早期发现的圆形冰面表达覆盖了海华沙构造东南 183 公里处的重力和地形低点（图 1B；MacGregor 等人，2019）。 仅基于遥感，这一特征被认为是可能的第二个冰下撞击坑（Ø ≥ 36 km），并且由于深度直径比较低，推测比 Hiawatha 结构更古老（MacGregor 等人， 2019）。然而，需要进一步调查来检验其起源的撞击假说。由于缺乏可追溯到 1039 ± 16 Ma 的撞击坑或沉积物（例如，Schmieder 和 Kring，2020），排除了与特定事件的联系并排除了这一可能性所有已知的中元古代撞击构造，即 Keurusselkä 撞击构造（1151 ± 10 Ma；Schmieder 等，2016）和 Stac Fada 撞击矿床（1177 ± 5 Ma；Parnell 等，2011）。该样本可能源自加拿大一个年代不详的撞击构造（例如，Presqu'île 撞击构造： <2729 Ma；Higgins 和 Tait，1990），其年代大约为 1990 年。 1 Ga 与格陵兰岛在罗迪尼亚组装（Pesonen 等，2012）。然而，这不太可能，因为考虑到没有合理的持续机制将样本运送到英格尔菲尔德地，HW19-17 需要在格陵兰岛西北部的地表停留异常长的时间。最可能的情况是：这里分析的四个样本与附近的 Hiawatha 结构有关，并记录了相同的古新世晚期撞击事件（图 1 和 4A；Kenny 等人，2022）。 Hiawatha 构造的地貌（边缘到地面的深度为 320 ± 70 m；Kjær 等人，2018）比大约古新世晚期的影响更符合古新世晚期的影响。 1 Ga 事件，尽管高纬度侵蚀存在变化（Kenkmann，2021）。然而，仍然需要对海华沙结构内现场收集的撞击材料进行放射性测年，以明确证实这一情况。相比之下，碎屑样本 HW19-17 记录了先前未知的约。 1 Ga撞击事件（图4）。这一新发现与在英格尔菲尔德地收集的其他熔岩样本一起，证明了在一个地点对多个撞击结构进行采样的罕见情况，这在地球上很少发生（例如，Schmieder 等人，2015a）。这些结果表明，必须将锆石颗粒的同位素表征（例如，U-Pb 地质年代学）与碎屑材料中的冲击微观结构结合起来，将它们与特定的撞击事件联系起来，特别是在地球上撞击记录稀疏的地区工作时。本研究中演示的程序可以作为未来火星和月球返回样本的有用模拟，因为行星风化层或地表角砾岩显然包含遭受或源自多次撞击事件的冲击材料（例如，Grange 等人，2013 年）。从冰碛或冰川河流排水通道中已知的撞击结构中搜索类似于撞击岩的碎屑样本，可用于寻找可能隐藏在大型大陆冰盖下的新撞击结构（例如，Hergarten 和 Kenkmann，2015）。我们对一个无法进入的地区的撞击记录有了新的认识，并建议需要付出更多努力来寻找全球新撞击事件的碎屑证据，这将有助于我们了解地球历史上发生的一系列撞击事件。我们感谢Kerstin Lindén、Heejin Jeon、Sanna Alwmark 和 Anders Plan 寻求数据收集方面的帮助。我们还感谢 Kurt H. Kjær 和 Anders A. Bjørk，并向 Jérémie Mouginot 表示敬意，感谢他们在实地工作中提供的协助。 我们感谢马克·诺曼（编辑）、戈登·奥辛斯基、尼克·蒂姆斯和蒂蒙斯·埃里克森的建设性意见，这些意见极大地改进了本手稿。 NordSIMS 实验室由瑞典研究委员会资助（拨款 2021-00276）。这是 NordSIMS 贡献 759。这项工作得到了瑞典研究委员会拨款 2020-04862 (G.G. Kenny)、Geocenter 丹麦拨款 DALIA (Kurt H. Kjær) 和丹麦独立研究基金拨款 0135-00163B (N.K. Larsen) 的支持。