Ages of mare basalts on the lunar nearside

Abstract

Abstract. The chronology of lunar volcanism is based on radiometric ages determined from Apollo and Luna landing site samples, regional stratigraphic relationships, and crater degradation and size-frequency distribution data for units largely defined prior to the end of the Apollo program. Here we report on new crater size-frequency distribution data for 139 spectrally and morphologically defined basalt units which are exposed in six nearside impact basins (Australe, Tranquillitatis, Humboldtianum, Humorum, Serenitatis, and Imbrium). Crater size-frequency distribution measurements are a statistically robust and accurate method to derive absolute model ages of unsampled regions of the Moon. Compared to crater degradation ages, crater size-frequency ages, performed on spectrally defined units, offer significant improvements in accuracy. Our investigation showed that (1) in the investigated basins, lunar volcanism was active for at least 1.5-2.0 b.y., starting at about 3.9-4.0 b.y. and ceasing at-2.0 b.y., (2) most basalts erupted during the late Imbrian Period at about 3.6-3.8 b.y., (3) significantly fewer basalts were emplaced during the Eratosthenian Period, (4) basalts of Copernican age were not found in any of the investigated basins, (5) lunar basin-filling volcanism probably started within -100 m.y. after the formation of the individual basins. We also assessed the relationship between impact basin age and the history of mare basalt emplacement in these basins. We found that (1) in all pre-Nectarian basins (Australe and Tranquillitatis) as well as in the Humboldtianum basin, which is of Nectarian age, the distribution of surface ages is clearly dominated by only a single peak in the number of erupted units at 3.6-3.8 b.y., (2) in the younger basins (Humorum, Serenitatis, and Imbrium) a second peak at 3.3-3.5 b.y. is observed, (3) basalt eruptions younger than 2.6 b.y. occur only intermittently, and (4) in the youngest basins, Serenitatis and Imbrium, we see an extended period of active basin-filling volcanism (1.5-1.6 b.y.) which is 500 m.y. longer than in the Australe and Humorum and even -1.0 b.y. longer than in Tranquillitatis and Humboldtianum.

The topography and gravity of Mare Serenitatis: implications for subsidence of the mare surface

“…A recent analysis by Arkani-Hamed et al (1999) suggests that basins flooded to local rather than a global equipotential surface. Although the mare basalts filled the Serenitatis basins to an equipotential level, the total accumulation of the mare sequence is estimated to have occurred over a period of 800 myr Head, 1979, 1980) to 1.4 byr (Hiesinger et al, 2001). As the basalts flooded the basin over this period, the newly formed mare floors subsided.…”

“…Subsidence continued during and after the emplacement of the intermediate and youngest units, forming the mare ridges. Units in Mare Serenitatis estimated to be younger than 3.0 byr have been identified (Boyce, 1976) and some may be as young as 2.44 byr (Hiesinger et al:, 2001). The total thickness of the basalt sequence in Serenitatis and other mascon maria is estimated to be 2 4 km or as much as 3-6 km (Sjogren et al, 1974;Head, 1979, 1980;Arkani-Hamed, 1998;Williams and Zuber, 1998).…”

“…4) concluded that the basalts in this area are younger than the central mare unit. Hiesinger et al (2001) mapped units in west-central Serenitatis that are intermediate to young in age, ranging from 3.6 to 2.9 byr. Gravity data indicate a -60 mgal relative low in the positive anomaly (Figs.…”

Volcanic history of the Imbrium basin: A close-up view from the lunar rover Yutu

“…The presence of the KREEPy materials, indicated by high concentrations of radionuclides U, Th, and K (9), around the rims of the Imbrium basin suggests that they are likely the basin-forming ejecta deposits. At least three main lava flows, dated from 3.5 Ga to 2.0-2.3 Ga (7,12), have been recognized in Mare Imbrium with distinct FeO and TiO 2 concentrations (13,14), which brought up interior information of this KREEP-rich terrain. The old and low-Ti basalt unit has been sampled by the Apollo 15 mission that landed at the eastern rim of the Imbrium basin.…”

“…They were formed about 3.9 Ga (7,8), probably by the late heavy bombardment, and then filled with dark lava flows derived from partial melting of the lunar mantle, within a period mainly during 3.8-3.1 Ga (7). The Imbrium basin is the largest and was formed on Procellarum KREEP [potassium (K), rare earth elements (REE), and phosphorus (P)] Terrane (9), a unique terrain highly enriched in U, Th, and K radionuclides and other incompatible trace elements referred to as KREEP (10) and considered as the last residual melt of the Lunar Magma Ocean (11).…”