Young Volcanism in the Northern Oceanus Procellarum and Southern Apollo Basin, Chang'e-5 and Chang'e-6 Landing Sites

YOUNG VOLCANISM IN THE NORTHERN OCEANUS PROCELLARUM AND SOUTHERN APOLLO BASIN, CHANG’E-5 AND CHANG’E-6 LANDING SITES. Yuqi Qian1, James Head2, Long Xiao3, Harald Hiesinger4, Carolyn H. van der Bogert4, Guochun Zhao1, 1Department of Earth Sciences, The University of Hong Kong ([email protected]), 2Department of Earth, Environmental and Planetary Sciences, Brown University, 3School of Earth Sciences, China University of Geosciences, 4Institute for Planetology, University of Münster. Introduction: Volcanic activities are the dominant endogenic geological process on terrestrial planets in the Solar System [1], whose magma formed by partial melting of the solid interior. Originating from the accretion of planetesimals that release kinetic energy and heated by the decay of short half-life radioactive elements, terrestrial planets are hot at origin but cool down through time. The volcanic deposits link the interior and exterior of a planet, with critical implications for its internal thermal state. The timescale of volcanism, i.e., when did volcanism on a planet start and end, contains the most crucial information on its thermal evolution that needs to be constrained but not clear even for the Earth [2]. In addition to the Earth, the Moon is the only other planet with confirmed samples from known locations. The most ancient lunar volcanism started as early as 4.3 Ga by the end of the magma ocean solidification as recorded by meteorites [3]. What’s even more mysterious is the ceasing time of lunar volcanism. With a smaller size, the Moon loses heat more efficiently than other terrestrial planets. Most lunar volcanism occurred during the Late Imbrian Epoch, and its flux receded dramatically after 3.0 Ga [4]. According to crater counting measurements, large-scale lunar volcanic eruptions faded at ~1.0 Ga [4], perhaps with even younger localized eruptions as irregular mare patches (<100 Ma) [5], ring-moat dome structures (several 100 Ma) [6], or pyroclastic beads (~120 Ma) [7]. So far, only Chang’e-3 (CE-3), Chang’e-5 (CE5), and Chang’e-6 (CE-6) missions of the Chinese Lunar Exploration Program investigated young mare regions on the Moon in situ, including which CE-5 and CE-6 returned 1731 g and 1935 g samples from northern Oceanus Procellarum and southern Apollo basin, respectively, shedding light on a comprehensive understanding of young lunar volcanism. Young Volcanism at the northern Oceanus Procellarum: The CE-5 landing site (43.06ºN, 51.92ºW) is located to the Em4 unit of northern Oceanus Procellarum (Fig. 1A). This area is covered by widespread intermediate-Ti basalts (6 wt.%) with an area of 37,000 km2, a mean thickness of ~51 m, and a volume between ~1,450 and 2,350 km3 [8]. Isotopic dating of these basalts yields an age of 2.0 Ga in the Eratosthenian Period [9-10], and the volcanic beads in the returned soils even extending the regional volcanism to ~120 Ma [7]. No eruption sources (e.g., fissures, cones, domes) were identified within Em4, except for a complex sinuous rille system crossing its middle. This complex sinuous rille system was regarded as one single sinuous rille previously [11]. However, detailed morphology analyses suggest it is composed of four sinuous rilles, i.e., Rima Sharp, Mairan, Harpalus, and Louville [12]. Rima Sharp and Mairan formed at ~2.0 Ga and 1.4 Ga from North and South Vents, respectively. At initial stage of their eruptions, sheet flows were formed. Lavas from the North Vent overlaid on the pre-existing Imbrian-aged low-Ti basalts and covered the entire Em4 region. Lavas from South Vent are limited to the Southeast of Em4. At the terminal stage of their eruptions, channelization of lavas enhanced thermal erosions during the persistent eruption and produced sinuous rille channels. Mare basalts collected by CE-5, highly likely represent lavas from the North Vent. Young Volcanism at the southern Apollo basin: The CE-6 landing site (41.63ºN, 153.98ºW) is located to the southern mare unit of Apollo basin (Fig. 1B) with a total area of 9,329 km2, between the boundary of Southern Mare-West (W) and East (E). CE-6 sampled the intermediate-Ti mare basalts (6 wt.%) of Southern Mare-W, overlaying on low-Ti basalts which might be exposed to the east of the sampling site. The thickness of Southern Mare-W decreases from west to east (from ~150 to 65 m) until meeting wrinkle ridges [13]. The western boundary of Southern Mare-W and Apollo basin rim close to Chaffee S develops abundant volcanic features, including floor-fractured craters (intrusive), sinuous rilles (extrusive), and even more may be buried by the ejecta of Chaffee S. Therefore, this location might be the lava source for the intermediate-Ti basalts. After eruption, they flowed east with decreasing thickness [13]. In addition, other mare and cryptomare eruptions were also identified in and surrounding the Apollo basin, crater size-frequency distribution measurements indicate the volcanism could last from 4.1 to 1.8 Ga in the area [13], similar to the nearside. All of them could be sampled by CE-6 either from local basalts or delivered by distal impacts. Isotopic dating of basaltic fragments in the CE-6 lunar soils have determined at least 2 episodes of eruptions in the region, including the dominant local 2.8 Ga low-Ti mare [14-16] and the exotic 4.2 Ga high-Al cryptomare eruptions [15]. Control of Pre-Existing Topography on Lava Flowing Processes: For both the CE-5 (Em4) and CE-6 (Southern Mare-W) sampled lava flows, it was found that the pre-existing topography significantly impacted the lava flowing processes [8, 13], which might be a common process on the Moon [17]. The formation of both Rima Sharp and Mairan was controlled by the predating highlands, underlying mare basalts, and continuously evolving wrinkle ridges. The channel of Rima Mairan parallels with the stretch of wrinkle ridges in some locations of Em4, indicating pre-existing topography of wrinkle ridge guided the lavas that forming Rima Mairan. In addition, the boundary between Southern Mare-W and Southern Mare-E correlates well with a wrinkle ridge (WR3). This wrinkle ridge might exist before the emplacement of the ~2.8 Ga intermediate-Ti eruption. These flows likely have no sufficient velocity to overtop the ridge, and thus would pond behind protoWR3 and then solidify to form Southern Mare-W. For both wrinkle ridges at the CE-5 and CE-6 unit, they continued to grow even after mare emplacement with the cooling of the Moon. Control of Crustal Thickness on Magma Eruption Processes: For both the CE-5 and CE-6 sampling regions, their volcanic activities were significantly influenced by crustal thickness, especially the CE-6 site in Apollo basin [13]. Dikes in intermediate-thickness crust (e.g., Oppenheimer crater) tend to stall beneath the crater floor, spreading laterally to form sill and floor-fractured crater; dikes below the crust thinned by Apollo basin event, reach directly to the surface and erupt to form widespread lava flows such as Southern Mare basalts; and dikes in thick crust stall before being able to reach the surface and form basaltic dike intrusions. For the Em4 unit region in northern Oceanus Procellarum, it is located between two rings of the Imbrian basin with thinned crustal thickness, therefore, the magma in this region preferring to erupt to the surface. Altogether, both the volcanic activities at the CE-5 and CE-6 sites imply that crustal thickness might be a key control factor for the asymmetrical distribution of mare basalts for the nearside and farside [18]. Mantle Source Evolution: We found for both the northern Oceanus Procellarum (CE-5) and southern Apollo basin (CE-6), there are at least two episodes of eruptions. The earlier one erupted at the Imbrian Period with low-Ti compositions [13, 19]), and the latter eruption occurred at the Eratosthenian Period [9-10, 14-16] and formed mare basalts with relatively high Ti abundance, overlaying on low-Ti basalts. This phenomenon is also observed across the Procellarum KREEP Terrance [20], indicating the transition of lunar mantle source [21] and the relatively high-Ti mare source might evolve from the source that produced low-Ti basalts [22]. Figure 1. (A) CE-5 landing site in northern Oceanus Procellarum. The blue lines represent sinuous rilles. 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