Distribution of Anorthositic and Noritic Material in the Apollo Basin

Mathematics – Logic

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[5410] Planetary Sciences: Solid Surface Planets / Composition, [5420] Planetary Sciences: Solid Surface Planets / Impact Phenomena, Cratering, [5464] Planetary Sciences: Solid Surface Planets / Remote Sensing, [6250] Planetary Sciences: Solar System Objects / Moon

Scientific paper

The Moon Mineralogy Mapper (M3), an instrument on Chandrayaan-1, is an imaging spectrometer with high spatial and spectral resolution covering the wavelength range from 430-3000 nm. Data gathered over northeastern South Pole-Aitken Basin (SPA) enable a detailed compositional study of the materials associated with the Apollo Basin. Both the Galileo and Clementine missions revealed the Apollo Basin to be spectrally and geologically unique. With Galileo SSI data, the interior of Apollo was found to be similar in albedo, visible spectral slope, and mafic content to areas west of Apollo in SPA, suggesting that Apollo exposed a thick and extensive unit of mafic crustal material [1]. Using Clementine UVVIS data, anorthositic materials were hypothesized to exist in the rings of Apollo and are amongst the few probable exposures of such materials in SPA [2-4]. These materials were thought to form from a differentiated SPA-melt sheet or represent exposed crustal material surrounded by the more mafic SPA melt. If we assume that the original crust within SPA’s transient cavity was removed, then the presence of anorthositic material at Apollo can be used to constrain the extent of the SPA transient cavity [5]. M3 data suggest that the anorthositic material associated with Apollo is pervasive in both the inner ring and the smooth floor (as soils) across the interior of the basin and has been covered by mare deposits in both the center and south of the basin. The anorthositic material in Apollo is spectrally similar to material located outside of SPA in the feldspathic highlands. The anorthositic material observed in Apollo’s rings lacks the diagnostic plagioclase absorption features indicative of unshocked crystalline anorthosite, such as those identified in the rings of the Orientale basin [6]. There are also several exposures of mafic materials within Apollo, in addition to the basalts. These other mafic exposures are associated with craters larger than 5 km in diameter, mostly clustered around the center of the basin, and have exposed a low-Ca pyroxene-bearing material, interpreted to be noritic. Previous analyses found that the interior of SPA, as a whole, contained noritic materials [4], which were interpreted as SPA-derived impact-melt breccias. If we assume that Apollo excavated through a veneer of SPA impact-melt and/or SPA ejecta and retained its own anorthositic melt material, the noritic material exposed in Apollo must be derived from the secondary, more mafic portions of the crust [1]. One of the largest craters in the Apollo basin, Dryden, contains noritic materials in its floor and ejecta, suggesting that noritic material is found at depths of at least 5 km in the northern interior of Apollo. That the inner ring of Apollo does not contain a mafic signature suggests that its materials are from a somewhat different and shallower region of the anorthositic crust, perhaps near a transition to more mafic materials. References: [1]Head,et al.,1993,JGR,17149-17181.[2]Morrison and Bussey,1997,LPSC 28,1501.[3]Morrison,1998,LPSC 29,1657.[4] Pieters,et al.,2001,JGR,28001-28022.[5]Petro and Pieters,LPSC 33,1848.[6]Pieters,2009,LPSC 40,2157.

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