Zircon microcrystals are found in many planetary crustal rocks and nanoscale research on grains from well-characterized impact environments on Earth provide a baseline for reconstructing extreme shock and thermal histories elsewhere. However, using zircon to date large impact events can be challenging given that shock-related isotopic re-setting of U–Pb ratios, when measured at micrometre scale, is often incomplete and difficult to interpret as the underlying Pb migration mechanisms are unclear. To better understand shocked zircon U–Pb systematics, we performed atom probe tomography and electron microscopy with the previous SIMS analyses of two shock metamorphosed Mesoarchean zircon grains from deep (≥ 15 km) beneath the centre of the 2.020 Ga giant Vredefort impact structure. We find evidence of two types of impact-related nanoscale Pb mobility. In one grain, clustering has produced ~ 10 nm diameter bodies of radiogenic Pb, unsupported by U and co-located with Al, with an average 207 Pb/ 206 Pb ratio of ~ 0.50 ( n = 4); the value extant in the grain at the time of impact. Conversely, nearby nanodomains exhibit randomly distributed radiogenic Pb, U and Al and yield 206 Pb/ 238 U dates consistent with 100% loss of pre-impact radiogenic Pb atoms during shock metamorphic processes. Notably, domains with multiple Pb clusters occur within micrometres of domains that experienced 100% Pb loss, precluding a uniform radial pattern of thermally-driven Pb diffusion at the grain scale. These cases of broadly coeval clustering and outward Pb mobility during geologically instantaneous shock metamorphism point to unusually rapid, multi-path diffusion processes within sub-micrometre volumes which, when averaged, yield normally discordant U–Pb dates. The isolation of spatially variable styles of Pb retention and loss at nanoscale amidst classical grain-scale shock microstructures shows promise for recognizing and resolving bombardment histories in planetary crusts using zircon.

中文翻译：

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太古代锆石中撞击触发的纳米级 Pb 聚集和 Pb 损失域

在许多行星地壳岩石中都发现了锆石微晶，对来自地球上特征鲜明的撞击环境的晶粒的纳米级研究为重建其他地方的极端冲击和热历史提供了基线。然而，使用锆石来确定大型撞击事件可能具有挑战性，因为在微米尺度上测量时，U-Pb 比率的冲击相关同位素重新设置通常不完整且难以解释，因为潜在的 Pb 迁移机制尚不清楚。为了更好地了解冲击锆石 U-Pb 系统学，我们对来自 2.020 Ga 巨型 Vredefort 撞击结构中心下方（≥ 15 公里）的两个冲击变质中古宙锆石颗粒进行了原子探针断层扫描和电子显微镜分析，之前的 SIMS 分析。我们发现了两种与撞击相关的纳米级铅迁移率的证据。在一颗颗粒中，聚集产生了直径约 10 nm 的放射性 Pb 体，不受 U 支持并与 Al 共存，平均 207 Pb/206 Pb 比率约为 0.50（n = 4）；冲击时谷物中现存的价值。相反，附近的纳米域表现出随机分布的放射成因 Pb、U 和 Al，并产生 206 Pb/238 U 日期，这与冲击变质过程中冲击前放射成因 Pb 原子的 100% 损失一致。值得注意的是，具有多个 Pb 簇的域出现在经历 100% Pb 损失的域的微米范围内，排除了在晶粒尺度上热驱动 Pb 扩散的均匀径向模式。在地质瞬时冲击变质作用期间，这些广泛同时期的聚集和向外的 Pb 迁移的情况表明，亚微米体积内异常快速的多路径扩散过程，平均时，产生通常不一致的 U-Pb 日期。在经典的晶粒尺度冲击微结构中，在纳米尺度上分离出空间可变的 Pb 保留和损失样式，这显示出使用锆石识别和解决行星地壳中的轰击历史的前景。