Joy K H, Snape J F, Nemchin A A, Tartèse R, Martin D M, Whitehouse M J, Vishnyakov V, Pernet-Fisher J F, Kring D A
Department of Earth and Environmental Sciences, School of Natural Sciences, The University of Manchester, Manchester, UK.
Department of Geosciences, Swedish Museum of Natural History, Stockholm, Sweden.
R Soc Open Sci. 2020 Jun 3;7(6):200236. doi: 10.1098/rsos.200236. eCollection 2020 Jun.
Apollo 16 soil-like regolith breccia 65745,7 contains two zircon-bearing clasts. One of these clasts is a thermally annealed silica-rich rock, which mineralogically has affinities with the High Alkali Suite (Clast 1), and yields zircon dates ranging from 4.08 to 3.38 Ga. The other clast is a KREEP-rich impact melt breccia (Clast 2) and yields zircon dates ranging from 3.97 to 3.91 Ga. The crystalline cores of both grains, which yield dates of 3.9 Ga, have undergone shock pressure modification at less than 20 GPa. We interpret that the U-Pb chronometer in these zircon grains has been partially reset by the Imbrium basin-forming event when the clasts were incorporated into the Cayley Plains ejecta blanket deposit. The zircon grains in Clast 1 have been partially decomposed, resulting in a breakdown polymineralic texture, with elevated U, Pb and Th abundances compared with those in the crystalline zircon. These decomposed areas exhibit younger dates around 3.4 Ga, suggesting a secondary high-pressure, high-temperature event, probably caused by an impact in the local Apollo 16 highlands area.
阿波罗16号类似土壤的角砾岩65745,7含有两个含锆石碎屑。其中一个碎屑是经过热退火的富硅岩石，其矿物学特征与高碱岩套（碎屑1）相似，锆石年龄范围为4.08至3.38 Ga。另一个碎屑是富含钾稀土元素的撞击熔结角砾岩（碎屑2），锆石年龄范围为3.97至3.91 Ga。这两个颗粒的晶体核心年龄为3.9 Ga，在低于20 GPa的压力下经历了冲击压力改造。我们认为，当这些碎屑被纳入凯莱平原喷射毯沉积物时，这些锆石颗粒中的铀铅计时仪已被雨海盆地形成事件部分重置。碎屑1中的锆石颗粒已部分分解，形成了一种分解的多矿物结构，与结晶锆石相比，铀、铅和钍的丰度升高。这些分解区域显示出约3.4 Ga的较年轻年龄，表明存在二次高压、高温事件，可能是由当地阿波罗16号高地地区的撞击引起的。
