Impact history of the Apollo 17 landing site revealed by U‐Pb SIMS ages
Impact history of the Apollo 17 landing site revealed by U‐Pb SIMS ages
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U-Pb SIMS 年龄揭示阿波罗 17 号着陆点的撞击历史
DOI：
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发表时间：
2017
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影响因子：
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通讯作者：
J. Bellucci
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作者：
F. Thiessen;A. Nemchin;J. Snape;M. Whitehouse;J. Bellucci
从阿波罗17号着陆点收集的四个质地不同的角砾岩样品（72255，76055，76015，76215）中获得了钙磷酸盐的二次离子质谱（西姆斯）U-Pb年龄，试图确定它们是否代表一个或多个撞击事件。结合岩石学关系的推断，确定的年龄可能表明在3920 ± 3 Ma，3922 ± 5 Ma和3930 ± 5 Ma（误差均为2σ）发生了两次或可能三次不同的撞击事件。通过计算撞击盆地和大陨石坑的连续喷出物半径以及它们预期的喷出物厚度来寻找角砾岩的可能来源，我们得出结论，Nectaris，Crisium，Serenitatis和Imperial是可能的候选人。如果先前的解释是正确的，即在北地块采集的微嵌晶角砾岩代表Serenitatis喷出物，那么平均207 Pb/206 Pb年龄为3930 ± 5 Ma（2σ），确定了Serenitatis盆地的形成。在南地块取样的角砾岩中出现锆石，含有钙磷酸盐，产生的年龄为3922 ± 5 Ma（2σ），可能表明角砾岩起源于Procellarum KREEP Ekee（PKT）内，而Imperial盆地似乎是唯一可能来源于它们的盆地。然而，这一解释意味着，所有的盆地都在短时间（<11 Ma）内形成，突出了全球地层约束，样品解释和年代学数据之间的严重矛盾。或者，两个微嵌晶角砾岩的年龄稍大可能是由于一些磷酸盐颗粒中保存的U-Pb系统不完全重置的结果。根据现有的数据集，不能排除这种可能性。
Secondary ion mass spectrometry (SIMS) U‐Pb ages of Ca‐phosphates from four texturally distinct breccia samples (72255, 76055, 76015, 76215) collected at the Apollo 17 landing site were obtained in an attempt to identify whether they represent a single or several impact event(s). The determined ages, combined with inferences from petrologic relationships, may indicate two or possibly three different impact events at 3920 ± 3 Ma, 3922 ± 5 Ma, and 3930 ± 5 Ma (all errors 2σ). Searching for possible sources of the breccias by calculating the continuous ejecta radii of impact basins and large craters as well as their expected ejecta thicknesses, we conclude that Nectaris, Crisium, Serenitatis, and Imbrium are likely candidates. If the previous interpretation that the micropoikilitic breccias collected at the North Massif represent Serenitatis ejecta is correct, then the average 207Pb/206Pb age of 3930 ± 5 Ma (2σ) dates the formation of the Serenitatis basin. The occurrence of zircon in the breccias sampled at the South Massif, which contain Ca‐phosphates yielding an age of 3922 ± 5 Ma (2σ), may indicate that the breccia originated from within the Procellarum KREEP terrane (PKT) and the Imbrium basin appears to be the only basin that could have sourced them. However, this interpretation implies that all basins suggested to fall stratigraphically between Serenitatis and Imbrium formed within a short (<11 Ma) time interval, highlighting serious contradictions between global stratigraphic constraints, sample interpretation, and chronological data. Alternatively, the slightly older age of the two micropoikilitic breccias may be a result of incomplete resetting of the U‐Pb system preserved in some phosphate grains. Based on the currently available data set this possibility cannot be excluded.