Ages and stratigraphy of lunar mare basalts in Mare Frigoris and other nearside maria based on crater size‐frequency distribution measurements

Abstract

We report on ages derived from impact crater counts for exposed mare basalt units in the northern part of the lunar nearside hemisphere (Mare Frigoris), the eastern and northeastern part of the nearside hemisphere (Lacus Temporis, Joliot, Hubble, Goddard, Mare Marginis, and Mare Smythii), the central part of the nearside hemisphere (Palus Putredinis, Mare Vaporum, and Sinus Medii), and the southwestern part of the nearside hemisphere (Grimaldi, Crüger, Rocca A, Lacus Aestatis, and Schickard). In Mare Frigoris, we dated 37 basalt units, showing ages from 2.61 to 3.77 Gyr, with most units being formed in the late Imbrian period between 3.4 and 3.8 Gyr ago. In Mare Vaporum we dated six spectrally homogeneous units that show model ages of 3.10 to 3.61 Gyr. Our model ages of basalts in Mare Marginis range from 3.38 to 3.88 Gyr and are mostly older than basalts in Mare Smythii (3.14–3.48 Gyr). The model ages of four units in Sinus Medii indicate that the basalts in this region formed 3.63 to 3.79 Gyr ago. We find an excellent agreement of our crater size‐frequency model ages of the Palus Putredinis area, which contains the Apollo 15 landing site, with the radiometric ages of Apollo 15 samples. According to our crater counts, basalts in Palus Putredinis are 3.34 Gyr old and this compares favorably with the radiometric ages of 3.30–3.35 Gyr of the olivine‐normative and quartz‐normative basalts of the Apollo 15 landing site. Lacus Aestatis is a small irregular‐shaped mare patch in the southwestern nearside and shows an Imbrian age of 3.50 Gyr; basalts in Lacus Temporis in the northeastern nearside formed between 3.62 and 3.74 Gyr ago and are, therefore, older than the basalts in Lacus Aestatis. We found that basalts in craters of the southwestern nearside (Schickard, Grimaldi, Crüger, and Rocca A) are also mostly younger than basalts in craters of the northeastern nearside (Hubble, Joliot, and Goddard). While basalt ages vary between 3.16 and 3.75 Gyr in the southwest, basalts in the northeast are 3.60–3.79 Gyr old. These results confirm and extend the general distribution of ages of mare basalt volcanism and further underline the predominance of older mare basalt ages in the eastern and southern nearside and in patches of mare peripheral to the larger maria, in contrast to the younger basalt ages on the western nearside (Oceanus Procellarum).