Earth Planets Space, Vol. 60 (No. 4), pp. 433-444, 2008
Tomoko Arai1, Hiroshi Takeda2, Akira Yamaguchi1,3, and Makiko Ohtake4
1Antarctic Meteorite Research Center, National Institute of Polar Research, 1-9-10 Kaga, Itabashi, Tokyo 173-8515, Japan
2Department of Earth and Planetary Science, The University of Tokyo, Hongo, Tokyo 113-0033, Japan
3Department of Polar Science, The Graduate University for Advanced Studies, 1-9-10 Kaga, Itabashi, Tokyo 173-8515, Japan
4Planetary Science Department, Institute of Space and Astronautical Science (ISAS), Japan Aerospace Exploration Agency (JAXA), 3-1-1 Yoshinodai, Sagamihara 229-8510, Japan
(Received May 7, 2007; Revised December 18, 2007; Accepted December 21, 2007; Online published April 9, 2008)
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 εNd 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.
Key words: Moon, crustal evolution, asymmetry, lunar meteorites, KAGUYA.