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Geologic History of the Northern Portion of the South Pole-Aitken

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Geologic History of the Northern Portion of the South Pole-Aitken Journal of Geophysical Research: Planets RESEARCH ARTICLE Geologic History of the Northern Portion of the South 10.1029/2018JE005590 Pole-Aitken Basin on the Moon Special Section: M. A. Ivanov1,2 , H. Hiesinger2 , C. H. van der Bogert2, C. Orgel3 , J. H. Pasckert2 , Planetary Mapping: Methods, and J. W. Head4 Tools for Scientific Analysis and Exploration 1V.I. Vernadsky Institute of Geochemistry and Analyitical Chemistry Russian Academy of Sciences, Moscow, Russia, 2Institut für Planetologie, Westfälische Wilhelms-Universität, Münster, Germany, 3Division of Planetary Sciences and Remote Sensing, Freie Key Points: Universität, Berlin, Germany, 4Earth, Environmental and Planetary Sciences, Brown University, Providence, RI, USA • A geological map of the northern portion of South Pole-Aitken basin was made Abstract We conducted a detailed photogeological analysis of the northern portion of the South • Confirmed and potential volcanic – – materials make up ~8% of the basin Pole-Aitken (SPA) basin (10 60°S, 125 175°W) and compiled a geological map (1:500,000 scale) of this floor region. Our new absolute model age for the Apollo basin, 3.98 + 0.04/À0.06 Ga, provides a lower age limit for • The pre-SPA lunar crust was likely the formation of the SPA basin. Some of the plains units in the study area were formed by distal ejecta from stratified with respect to iron remote craters and basins. The characteristic concentrations of FeO and TiO2 of other plains are indicative of their volcanic origin. The oldest volcanic materials occur near the center of the SPA basin and have an Early Imbrian age of ~3.80 + 0.02/À0.02 Ga. Late Imbrian volcanic activity occurred in and around the Apollo basin. Correspondence to: In total, the volcanic plains cover ~8% of the map area and cannot account for the extensive SPA iron M. A. Ivanov, fl – [email protected] signature. The sources of the signature are the oldest materials on the SPA oor (FeO ~11 14.5 wt%). In contrast, the ejecta composing the SPA rim are significantly poorer in FeO (<7.5 wt%). The signature could be related to the differentiation of the SPA impact melt. However, the spatial segregation of the ancient iron-rich Citation: Ivanov, M. A., Hiesinger, H., van der and iron-poor materials suggests that the SPA iron signature predated the basin. Thus, the signature Bogert, C. H., Orgel, C., Pasckert, J. H., & might be explained by a pre-SPA lunar crust that was stratified with respect to the iron concentrations, so that Head, J. W. (2018). Geologic history of the SPA impact excavated the upper, iron-poorer portion of the crust to form the SPA rim and exposed the the northern portion of the South fl Pole-Aitken basin on the Moon. Journal deeper, iron-richer portion on the oor of the basin. of Geophysical Research: Planets, 123, 2585–2612. https://doi.org/10.1029/ Plain Language Summary We conducted a geological analysis of the northern portion of the South 2018JE005590 Pole-Aitken basin, which is the largest recognized and likely the oldest impact structure on the Moon. Results of our mapping efforts permitted the unraveling of the major sequence of impact and volcanic events Received 28 FEB 2018 that have shaped the basin throughout its evolution and resulted in the discovery of the oldest materials Accepted 13 SEP 2018 Accepted article online 17 SEP 2018 related to the basin formation. Analysis of the distribution and concentrations of iron and titanium in the Corrected 31 OCT 2018 materials of different age within the South Pole-Aitken basin allows the characterization of the structure of Published online 12 OCT 2018 the ancient lunar crust and mantle. These results introduce important constraints on the current models of This article was corrected on 31 OCT the early evolution of the Moon. 2018. See the end of the full text for details. 1. Introduction We studied a region between 10°–60°S and 125°–175°W that represents the northern portion of the South Pole-Aitken basin (SPA), centered on the Apollo basin (Figure 1a). Several important characteristics of the SPA region and, in particular, its northern extent allow us to address a number of key questions regarding lunar evolution, including the constraints that SPA can place on the formation of the lunar magma ocean, the structure of the lunar crust, the formation and evolution of large impact basins, and the history of post-SPA volcanism and cratering. First, the SPA is the largest impact structure on the Moon with dimensions of ~2,400 by 2,000 km (Garrick-Bethell & Zuber, 2009; Hiesinger & Head, 2004; Shevchenko et al., 2007; Spudis et al., 1994; Stuart-Alexander, 1978) and likely is the oldest recognized lunar impact structure (Hiesinger et al., 2012; ©2018. The Authors. This is an open access article under the Wilhelms, 1987). SPA has a discernible morphologic appearance (Stuart-Alexander, 1978; Wilhelms et al., terms of the Creative Commons 1979) and exhibits a prominent topography (Zuber et al., 1994). These characteristics imply that the SPA basin Attribution-NonCommercial-NoDerivs formed within a solid lunar crust. Thus, constraining and/or establishing the age of the SPA will provide an License, which permits use and distri- bution in any medium, provided the important constraint on the time scale of the evolution of the magma ocean on the Moon. original work is properly cited, the use is – non-commercial and no modifications Model simulations of a large impact event at the angle 30° 60° suggest that an SPA-scale event should or adaptations are made. penetrate through entire lunar crust and excavate mantle materials (Melosh et al., 2017). Yet the region IVANOV ET AL. 2585 Journal of Geophysical Research: Planets 10.1029/2018JE005590 Figure 1. Study area that shows location of the Apollo basin (white dashed line) near the rim of the South Pole-Aitken (SPA) basin and large impact craters for reference. The background is a mosaic of wide-angle camera (WAC) images with reso- lution 100 m/pixel. Map in the orthographic projection centered at 35oS, 150oW. inside the SPA does not show strong spectral signatures of olivine (Lucey, 2004), which has been thought to be the major component of the lunar mantle. Recent Kaguya data have observed some local but limited olivine occurrences (Yamamoto et al., 2010). The scarcity of the olivine exposures inside the SPA basin could be explained several ways. It may be that the lunar mantle consists predominantly of low-calcium pyroxene (Melosh et al., 2017). Alternatively, the basin may have been formed during an oblique (30°) impact that involved impactor decapitation (Schultz, 1997; Schultz & Crawford, 2011) such that the impactor was unable to penetrate as deeply into the mantle. An oblique SPA impact and its correspondingly shallower excavation depth seem to be consistent with the presence of a Th anomaly inside the SPA basin (Lawrence et al., 2000). Although the Th concentrations on the SPA floor are lower than within the Oceanus Procellarum KREEP Terrane or the Compton-Belkovich region (Jolliff et al., 2011; Lawrence et al., 2007), they are systematically higher than the typical Th abundance in the feldspathic highlands terrane (Jolliff et al., 2000). This may suggest the presence of a KREEP component in the SPA region. The KREEP-enriched material is thought to be the magma ocean residuum that was sandwiched between the lunar crust and mantle (Jolliff et al., 2000), which potentially could be exposed by the SPA event. Another possibility is the formation and differentiation of a large pool of impact melt inside the SPA basin (Hurwitz & Kring, 2014; Vaughan & Head, 2014). The fractional differentiation of a large volume of melt, if it was undisturbed and fully developed, should cause the separation and sinking of denser olivine cumulates and the concentration of a norite-like component at the top of differentiating body. The SPA region also exhibits a distinct iron signature that has been described using Galileo, Clementine, and Lunar Prospector data (Belton et al., 1992; Pieters et al., 2001; Lawrence et al., 2002). The iron abun- dance within the SPA basin, ~10–12 wt% (Lawrence et al., 2003), is noticeably higher than in its IVANOV ET AL. 2586 Journal of Geophysical Research: Planets 10.1029/2018JE005590 immediate surroundings and is similar to the FeO concentrations in the Schickard-Schiller and Mare Australe regions where cryptomare deposits were detected (Antonenko & Head, 1994; Antonenko et al., 1995; Mustard et al., 1992; Lawrence et al., 2015; Whitten & Head, 2015). The enhanced concentration of FeO on the floor of the SPA basin was interpreted as evidence for the presence of extensive cryptomaria in this area (Gibson & Jolliff, 2011). However, although the entire SPA interior is a region of very thin crust—thinner even than beneath the major mare basalts on the lunar nearside in Oceanus Procellarum (Wieczorek et al., 2013)—the floor of the SPA shows only a minor extent of volcanic plains. The presence of the small and scattered patches of the low albedo, presumably volcanic plains (Head, 1976; Head & Wilson, 2017; Pasckert et al., 2018; Stuart- Alexander, 1978; Wilhelms et al., 1979; Yingst & Head, 1999) on the floor of the SPA basin (Figure 1) connects our study area with another fundamental problem of lunar geology: the history of volcanism on the Moon. Characterization of associations of iron-rich materials with specific rock stratigraphic units is, thus, important for the assessment of the timing and extent of volcanic activity in the SPA region. Here we also encounter questions about the structure and extent of the lunar crust. The idea that crustal thickness is a primary control on the spatial distribution of lunar volcanic materials (Head & Wilson, 1992) is at odds with the limited number and relatively small occurrences of lava plains within the SPA interior, because this area is among the deepest topographic depressions on the Moon and represents an area of thin- ner crust (Wieczorek et al., 2013).
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