U-Pb Age Record of Zircons from Ancient Lunar Regolith Breccias and the Early Lunar Bombardment

Introduction: The Moon's impact basins and their deposits retain information about the timing, flux and composition of late accretion in the inner solar system. However, isotopic ages of particular basins have been difficult to determine reliably because reheating by later impacts have affected chronometers to variable extent and gardening of impact ejecta has led to additional complexity. Recently, in situ U-Pb geochronology of zirconium minerals and calcium phosphates coupled with petrographic, geological and geochemical studies have provided more evidence for the most likely ages of at least some lunar basins [1-8]. These data can be compared with advanced models of ejecta distribution and the contributions of impact melt from specific basins to the lunar landing sites [9] and with absolute model ages obtained by improved crater counting data (e.g. [10, 11]). It is important to expand the still limited database of reliable lunar impactite ages to better define basin ages. Because of the robustness of zircons, in situ U-Pb ages of these minerals in lunar impactites have the potential to provide clues for the early lunar bombardment history as they are more resistant to resetting than for instance K-Ar ages. The present study is focusing on Apollo 16 and 17 ancient regolith breccias, which contain a mixture of mineral and lithic clasts from different highland and other lithologies. The goal of the study was to test if they contain zircons that can be dated and if ages match those found in other lithologies at these landing sites. Ancient regolith breccias: Four thin sections were screened with SEM for zirconium phases (67035,13; 67115,29; 74115,18; 76565,8). The Apollo 16 samples were collected at the North Ray Crater rim. 67115,29 contains impact melt and some glass coated fragments. 67035,13 contains highly feldspatic matrix, pristine highland material, melt breccias and granulitic clasts. Most of the bigger clasts are anorthosites. 74115,18 is a compacted soil breccia, collected in the ‘light mantle’ area and contains fragments of orange glass spheres and shards, granulites and high-Ti mare basalts. 76565,8 is a dimict breccia rake sample from the North massif with a similar clast inventory. Zircon grains occur as individual mineral clasts in the matrix of the breccias, within larger lithic clasts or as coronas on
ilmenites.

U-Pb age record in zircons: The new data show little to no common Pb in zircons and ancient 207Pb206Pb ages ranging from 3.92 to 4.37 Ga with uncertainties ranging from 9 to 29 Ma (2s). Breccia 67035,13 yields ages of 4.25 to 4.37 Ga, whereas 67115,29 shows an age of 4.21 Ga. Breccia 74115,18 shows age clusters at 3.92-3.93, 4.11-4.17 (most analyses) and 4.34 Ga, 76565,8 at 4.16-4.33 Ga. The oldest age of 4.33 Ga in 76565,8 belongs to a zircon grain attached to clinopyroxene in a evolved lithology. The oldest measured zircon (4.34 Ga) in 74115,18 is an individual grain in a clinopyroxene rich clast, the youngest ages at ~3.9 Ga belong to grains attached to ilmenite or noritic compositions. The 4.11-4.17 Ga cluster occurs in anorthositic clasts with granulitic texture and may reflect 2 or more discrete events or discordance from reheating of older zircon by the 3.92 Ga event. The youngest age of breccia 67035,13 (4.25 Ga) was found for an individual zircon grain associated with olivine and anorthite clasts in the matrix. 67115,29 yields an age of 4.21 Ga, obtained from two individual zircons, one being attached to a feldspar-spinel clast and the other associated with noritic composition. Fe-Ni metal inclusions were found in many of the host clasts or surrounding matrix of the analyzed zircons. Thus, a majority of the zircons in all thin sections can be associated with impact events.

 

Discussion: The results agree with some of the ages and age clusters in lunar breccias obtained previously by in situ U-Pb analyses of zircon, baddeleyite and calcium phosphates [1-8]. In some of these studies, petrographic or microstructural evidence is pointing at the in situ crystallization or recrystallization of these minerals from impact melt or localized shock heating [3, 6, 12]. The presence of metal in dated lithic clasts and similar ages occurring at different landing sites relate these ages to basinforming impact events. Based on chemical and geological arguments, previously published ages at 3.92 and 4.20-4.22 Ga were linked to the formation of the Imbrium and Serenitatis basins, respectively [2, 5,7]. Ages ranging from 4.13 to 4.18 Ga may reflect two or more basin impacts as they are associated with granulite grade metamorphism [8]. Ages near 4.33-4.34 Ga may reflect the formation of one of the earliest preserved basins such as South Pole Aitken, consistent with crater counting ages [10, 11]. With few exceptions, the new ages from the Apollo 16 and 17 breccias confirm the age clusters from previous studies. The data also hint that there may be differences in the abundance of basin ages at the Apollo 16 and 17 landing sites and that ejecta from a few basins dominate the impactor age record at these landing sites [9].

References: [1] Norman & Nemchin (2014) Earth and Planetary Science Letters(388):387-398. [2] Snape et al. (2016) Earth and Planetary Science Letters(451):149-158. [3] Crow et al. (2017) Geochimica et Cosmochimica Acta(202):264-284. [4] White et al. (2020) Nature Astronomy(4):974–978. [5] Cernok et al. (2021) Communications Earth & Environment(2):120. [6] Vanderliek et al. (2021) Earth and Planetary Science Letters(576):117216. [7] Nemchin et al. (2021) Geochemistry 81(1):125683. [8] Becker et al. (2023) GeoBerlin 2023. [9] Liu et al. (2020) Icarus(339):113609. [10] Evans et al. (2018) Journal of Geophysical Research: Planets(123):1596-1617. [11] Orgel et al. (2018) Journal of Geophysical Research: Planets(123):748–762. [12] Kusiak et al. (2022) Contributions to Mineralogy and Petrology(177):112.