The lunar Mg-suite magmatic rocks are commonly thought to represent mafic intrusions into the anorthositic flotation crust of the lunar magma ocean (LMO). Their geochronology is, therefore, important for constraining evolution models of the LMO. Petrogenetic models of the Mg-suite hold that their parent magmas were derived from primary LMO sources (Mg-cumulates, An-rich plagioclase, and melts enriched in KREEP—potassium, rare earth elements, and phosphorus). Previous radiogenic isotopic age interpretations of Mg-suite and putatively older, related ferroan anorthosites (FANs) overlap over a 200-million-year interval. The Apollo 78238 norite is an exemplary Mg-suite rock, with a relict coarse igneous texture modified by shock metamorphism. In-situ secondary ion mass spectrometry U-Pb analyses of zircon and baddeleyite in 78238 yield discordant arrays, attributed to recent impact metamorphism, with upper intercepts that constrain its crystallization age. The four oldest baddeleyite analyses give a weighted mean ²⁰⁷Pb/²⁰⁶Pb age of 4332 ± 18 Ma (2σ, MSWD = 0.06, P = 0.98), which is interpreted as the crystallization age of the norite. The overlap of the baddeleyite age with previously reported Sm-Nd and Pb-Pb mineral isochron ages for 78238 (Edmunson et al., 2009) supports a moderately fast cooling of the norite. Moreover, it is distinguishably younger than the most precisely dated sample of FAN (Apollo 60025), measured at 4360 ± 3 Ma by Sm-Nd and Pb-Pb mineral isochrons (Borg et al., 2011). Together with the baddeleyite ²⁰⁷Pb/²⁰⁶Pb age of Apollo Mg-suite troctolite 76535 at 4328 ± 8 Ma (White et al., 2020), the chronological record of the 78238 norite indicates a significant Mg-suite magmatic event at 4.33 Ga and a lower age limit on LMO differentiation.

月球镁组岩浆岩通常被认为代表了镁铁质侵入月球岩浆洋（LMO）的斜长岩浮选地壳。因此，它们的年代学对于限制改性活生物体的演化模型很重要。Mg-suite 的岩石成因模型认为它们的母岩浆来自 LMO 的主要来源（Mg 累积、富含 An 的斜长石和富含 KREEP 钾、稀土元素和磷的熔体）。先前对镁组和推定较老的相关铁质斜长岩 (FAN) 的放射性同位素年龄解释在 2 亿年的时间间隔内重叠。Apollo 78238 norite 是一种典型的镁组岩石，具有经冲击变质作用改变的残余的粗糙火成岩结构。原位78238 中锆石和斜锆石的二次离子质谱 U-Pb 分析产生不一致的阵列，归因于最近的撞击变质作用，其上截距限制了其结晶年龄。四个最古老的斜锆石分析给出了加权平均²⁰⁷ Pb/ ²⁰⁶ Pb 年龄为 4332 ± 18 Ma (2 σ, MSWD = 0.06, P = 0.98)，这被解释为紫长石的结晶年龄。斜锆石年龄与先前报道的 78238 的 Sm-Nd 和 Pb-Pb 矿物等时线年龄的重叠（Edmunson 等，2009）支持了紫铁矿的适度快速冷却。此外，它比最精确日期的 FAN 样品（阿波罗 60025）明显更年轻，通过 Sm-Nd 和 Pb-Pb 矿物等时线在 4360 ± 3 Ma 处测量（Borg 等人，2011）。连同Apollo Mg-suite troctolite 76535 在 4328 ± 8 Ma (White et al., 2020) 的斜长石²⁰⁷ Pb/ ²⁰⁶ Pb 年龄（White et al., 2020），78238 norite 的年代记录表明在 4.33 Ga 发生了重要的 Mg-suite 岩浆事件改性活生物体分化的较低年龄限制。