Lunar megaregolith mixing by impacts: Spatial diffusion of basin melt and its implications for sample interpretation
Lunar megaregolith mixing by impacts: Spatial diffusion of basin melt and its implications for sample interpretation
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月球巨风土因撞击而混合：盆地融化的空间扩散及其对样本解释的影响
DOI：
10.1016/j.icarus.2019.113609
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发表时间：
2020
期刊：
影响因子：
3.2
通讯作者：
J. Oberst
中科院分区：
文献类型：
--
作者：
Tiantian Liu;G. Michael;K. Wünnemann;H. Becker;J. Oberst
月球撞击盆地的形成年龄是了解内太阳系晚期吸积历史的关键。此外，正确解释盆地来源的碰撞熔融（“盆地熔融”）的物源和同位素年龄是必不可少的校准月球年代学功能。然而，月球近地表盆地融化的丰度还没有得到很好的了解。一系列长期的后续撞击事件使盆地融化，改变了其丰度和粒度分布。我们开发了一个数值模型来研究这个过程中的Monte Carlo方法在空间分辨模型。在全球范围内，并在阿波罗14-17号和月球20号采样点跟踪喷出物中的熔融部分，并与月球撞击熔融角砾岩的K-Ar年龄分布进行了比较。结果发现，非常大的SPA盆地以及相对较晚形成的Imperial盆地产生的熔体应该在近地表（顶部一米）占主导地位。模拟结果表明，阿波罗14-17号和月球20号站点的熔体成分受到附近中晚期盆地的强烈影响。阿波罗14- 17号和月球20号样品中均含有来自塞伦尼塔蒂斯盆地和SPA盆地的熔体，阿波罗16号、阿波罗17号和月球20号样品中均含有Nectaris熔体，阿波罗14-17号和月球20号样品中均含有来自塞伦尼塔蒂斯盆地和SPA盆地的熔体。在阿波罗14-17号和月球20号取样点，东方熔体没有明显的混合。一般来说，除了一个突出的年龄峰值在3.9 Ga（与Imperial盆地），该模型预测明显的丰度峰值较老的盆地熔体（>3.9 Ga），往往是缺乏从着陆点的撞击岩的K-Ar年龄分布。盆地熔体的扩散特征表明，未来的采样旨在收集早期盆地熔体，晚撞击坑的重新挖掘区大于几十公里的直径内盆地可能提供最高丰度的熔体从早期盆地。
The formation ages of lunar impact basins are critical to understanding the late accretion history of the inner solar system. Furthermore, the correct interpretation of the provenance and isotopic dates of basin-derived impact melt (‘basin melt’) is essential for the calibration of lunar chronology function. However, abundances of basin melt in the lunar near-surface are not well understood. Basin melt has been gardened by a long sequence of subsequent impact events, altering its abundance and size distribution. We developed a numerical model to investigate this process by means of the Monte Carlo method in a spatially resolved model. The fraction of melt in ejecta was tracked globally and at the Apollo 14–17 and Luna 20 sampling sites and was compared with K-Ar age distributions of lunar impact melt breccias. It was found that melt produced by the very large SPA basin as well as the relatively late-forming Imbrium basin should be dominant in the near-surface (top one meter). The simulation shows that the melt component at the Apollo 14–17 and Luna 20 sites is strongly affected by nearby mid- to late-forming basins. Imbrium melt should be abundant in Apollo 14–17 samples; Crisium melt is the most significant component of basin-sourced melt in Luna 20 samples; all the Apollo 14–17 and Luna 20 samples could include melt from Serenitatis and the SPA basin; Nectaris melt should occur in Apollo 16, Apollo 17 and Luna 20 samples; and Orientale melt has no significant mixing in the Apollo 14–17 and Luna 20 sampling sites. In general, besides a prominent age peak at 3.9 Ga (related to the Imbrium basin), the model predicts pronounced abundance peaks of older basin melt (>3.9 Ga) which tend to be absent from distributions of K-Ar ages of impactites from landing sites. The diffusion characteristics of basin melt suggest that for future sampling aimed at collecting early basin melt, the re-excavation zones of late impact craters larger than tens of kilometer in diameter inside basins may provide the highest abundances of melt from early basins.