A new model of lunar crust: asymmetry in crustal composition and evolution

Abstract

Earlier models of lunar crustal formation as a simple flotation of ferroan anorthosites (FAN) do not account for the diverse crustal composition revealed by feldspathic lunar meteorites and granulites in the Apollo samples. Based on the integrated results of recent studies of lunar meteorites and global chemical and mineralogical maps, we propose a novel asymmetric crust model with a ferroan, noritic, nearside crust and a magnesian, troctolitic farside crust. Asymmetric crystallization of a primordial magma ocean can be one possibility to produce a crust with an asymmetric composition. A post-magma-ocean origin for a portion of the lunar crust is also possible and would account for the positive ε Nd value for FAN and phase equilibria. The formation of giant basins, such as the South Pole-Aitken (SPA) basin may have significant effects on resurfacing of the early lunar crust. Thus, the observed surface composition of the feldspathic highland terrane (FHT) represents the combined results of magma ocean crystallization, post-magma-ocean magmatism and resurfacing by basin formation. The Mg/(Mg+Fe) ratios, rock types, and mineral compositions of the FHT and the South Pole-Aitken basin Terrane (SPAT) obtained from the KAGUYA mission, coupled with further mineralogical and isotopic studies of lunar meteorites, will facilitate an assessment of the feasibility of the proposed crust model and improve understanding of lunar crustal genesis and evolution.

Heterogeneity in lunar anorthosite meteorites: implications for the lunar magma ocean model

“…Lunar anorthosites tend to be more magnesian in composition and bridge the gap between FAN and Mg-suite rocks (see also [34,48]). This is also true of the lunar meteorite plagioclase Eu-anomaly (figure 4), which bridges the gap between FANs and Mg-suite rocks.…”

Seismic Tomography of the Moon

“…Returned samples from the Apollo 12, 14 and 15 sites were found to contain highly elevated concentrations of incompatible trace elements, and these rocks were given the name KREEP, because of their high levels of potassium (K), rare earth elements (REE), and phosphorous (P). Although the absolute concentrations of these elements vary among the KREEP-rich samples, their relative concentrations tend to be roughly consistent (Warren and Wasson 1979;Arai et al 2008Arai et al , 2010.…”

“…Many researchers have suggested that a dichotomy in geological processes may have existed between the lunar nearside and farside (e.g., Warren and Wasson 1979;Arai et al 2008Arai et al , 2010Yamashita et al 2010, and many references therein). The two sides exhibit distinct differences in the surface topography (Araki et al 2009), gravity field (Namiki et al 2009;Yan et al 2012), crustal thickness (Hikida and Wieczorek 2007;Ishihara et al 2009;Wieczorek et al 2013), and composition (e.g., Arai et al 2008Arai et al , 2010.…”

“…8.21): the Procellarum KREEP Terrane (PKT), the Feldspathic Highlands Terrane (FHT), and the South Pole-Aitken Terrane (SPAT), largely on the basis of geochemical, petrological, and geophysical criteria that indicate each terrane experienced a fundamentally different geological evolution (Jolliff et al 2000;Arai et al 2008). The PKT and FHT are likely originated from an early global-scale differentiation.…”

“…The two sides exhibit distinct differences in the surface topography (Araki et al 2009), gravity field (Namiki et al 2009;Yan et al 2012), crustal thickness (Hikida and Wieczorek 2007;Ishihara et al 2009;Wieczorek et al 2013), and composition (e.g., Arai et al 2008Arai et al , 2010. The dichotomy may be a combined result of asymmetric crystallization of a primordial magma ocean, post-magma-ocean magmatism and resurfacing by basin formation (Arai et al 2008). The analysis of the returned lunar samples and meteorites revealed that the Moon's evolution was drastically different from that of the Earth (e.g., Warren and Wasson 1979;Arai et al 2010, and references therein).…”