Volcanism on farside of the Moon: New evidence from Antoniadi in South Pole Aitken basin

Abstract

The lunar surface is characterized by asymmetric distribution of its volcanic deposits. The nearside contains about 90% of the mare basalts, while there are a few on the farside with some are in the South Pole Aitken (SPA) basin, which is the largest and oldest impact basin on the Moon. Although the mare volcanism on the nearside of the Moon has been very well characterized, our understanding of the farside is relatively poor. Our geological mapping using Lunar Reconnaissance Orbiter (LRO) and Chandrayaan-1 Moon Mineralogy Mapper (M3) data of 143km Antoniadi impact basin provides new insights into volcanic processes in the southern SPA. Antoniadi has excavated the SPA floor as deep as 9km. Distribution of secondary craters around Antoniadi shows that the basin was formed by oblique impact with the direction of 50° counter-clockwise from 178°E longitude. Our geological mapping has found, for the first time, several volcanic cones (0.2–2.5km diameter) in association with the mare basalt flows. As per morphological classification, these are C, E, N and P type volcanic cones with abundant volcanic fragments in their summit areas. The base and summit diameters of these cones are similar to those of Marius Hills on the nearside of the Moon. The cumulative size frequency distributions of the summit fragments provide a range of power index values between −3 and −9, suggesting the involvement of dynamic fragmentation of the volcanic materials during the eruptive processes. The grabens in the basin floor show evidence for fissure eruption of the mare basalts. The graben near the central peak exposes about 140m thick basalt sequence containing a minimum of five individual lava layers. The crater size–frequency distributions of the mare deposits suggest an age of 1.6Ga for the flows in the central part of the basin, while 2.6Ga for those on the outer floor near the basin wall, indicating the episodic volcanism in the basin. The young volcanism of 1.6Ga is reported for the first time from SPA. The M3 data reveal that the basin materials contain abundant low-Ca pyroxene, while the mare basalts with high-Ca pyroxene. We calculated the liquidus density of mare basalts based on FeO and TiO2 content, which suggest a range of density from 2.7 to 2.8g/cm3, which would be lower than a gross lunar crust. This would imply the emplacement of basaltic lavas through dikes by simple magma buoyancy or over-pressurization of magma chambers, if they present at the crust-mantle boundary when the magma density is greater than the host crust. The 1.6Ga old volcanism in Antoniadi suggests that the mantle beneath the SPA has been conducive for young magmatism, possibly due to presence of KREEP like material in the mantle.