Snape Joshua F, Nemchin Alexander A, Whitehouse Martin J, Li Qiu-Li, Liu Yu, Timms Nicholas E, Erickson Timmons, Benedix Gretchen K
Department of Earth and Environmental Sciences, University of Manchester, Manchester M13 9PL, UK.
Department of Applied Geology, Curtin University, Perth, Western Australia 6845, Australia.
R Soc Open Sci. 2024 May 15;11(5):231963. doi: 10.1098/rsos.231963. eCollection 2024 May.
The Apollo 14 and 16 missions returned several samples commonly interpreted as crystalline impact melt, with ages of approximately 3800-3850 Ma. Previous work has suggested that these rocks formed in one or more pre-Imbrium basin forming impact(s). By contrast, recent ages determined for a range of lunar breccias provide compelling evidence that the Imbrium basin was formed at approximately 3920 Ma. Using an approach previously demonstrated in lunar basalts, Pb-Pb isochron ages are determined for two of these proposed impact melt samples (14078: 3848 ± 4 Ma; and 68415: 3834 ± 11 Ma). In the case of 14078, the least radiogenic Pb isotopic compositions measured here are interpreted as representing the initial Pb isotopic composition of the sample. This value indicates derivation from a source (or sources) with high U/Pb ratios (approx. 2400), similar to those predicted for the Apollo 14 high-Al and very high-K basalts. It was not possible to determine an equivalent initial Pb isotopic composition for 68415, but Pb isotope evolution models indicate that the sample would have been derived from lithologies with lower U/Pb source ratios (approx. 1000). In both cases, the samples are interpreted as having been formed by impacts local to the Apollo 14 and Apollo 16 landing sites.
阿波罗14号和16号任务带回了几个通常被解释为结晶撞击熔体的样本，其年龄约为3800 - 3850百万年。先前的研究表明，这些岩石是在一个或多个雨海盆地形成前的撞击事件中形成的。相比之下，最近对一系列月球角砾岩测定的年龄提供了令人信服的证据，表明雨海盆地形成于约3920百万年。采用先前在月球玄武岩中证明的方法，对其中两个提议的撞击熔体样本测定了铅 - 铅等时线年龄（14078：3848 ± 4百万年；68415：3834 ± 11百万年）。对于14078样本，此处测量的放射性最低的铅同位素组成被解释为代表该样本的初始铅同位素组成。该值表明其源自具有高U/Pb比值（约2400）的一个或多个源，类似于对阿波罗14号高铝和超高钾玄武岩预测的比值。无法确定68415样本的等效初始铅同位素组成，但铅同位素演化模型表明该样本源自U/Pb源比值较低（约1000）的岩性。在这两种情况下，样本都被解释为是由阿波罗14号和16号着陆点附近的撞击形成的。
