Implications for the origins of pure anorthosites found in the feldspathic lunar meteorites, Dhofar 489 group

Hiroshi Nagaoka,Nobuyuki Hasebe,Hiroshi Takeda,Shigekazu Yoneda,Makiko Ohtake,Akira Yamaguchi,Yuzuru Karouji

doi:10.1186/1880-5981-66-115

Abstract

Remote observation by the reflectance spectrometers onboard the Japanese lunar explorer Kaguya (SELENE) showed the purest anorthosite (PAN) spots (>98% plagioclase) at some large craters. Mineralogical and petrologic investigations on the feldspathic lunar meteorites, Dhofar 489 and Dhofar 911, revealed the presence of several pure anorthosite clasts. A comparison with Apollo nearside samples of ferroan anorthosite (FAN) indicated that of the FAN samples returned by the Apollo missions, sample 60015 is the largest anorthosite with the highest plagioclase abundance and homogeneous mafic mineral compositions. These pure anorthosites (>98% plagioclase) have large chemical variations in Mg number (Mg# = molar 100 × Mg/(Mg + Fe)) of each coexisting mafic mineral. The variations imply that these pure anorthosites underwent complex formation processes and were not formed by simple flotation of plagioclase. The lunar highland samples with pure anorthosite and the PAN observed by Kaguya suggest that pure anorthosite is widely distributed as lunar crust lithology over the entire Moon.

Highlights

On the basis of mineralogical and geochemical investigations of the returned Apollo lunar samples, we determined that ferroan anorthosite (FAN) is the main component of a lunar feldspathic crust (e.g., Taylor et al 1991)
Mineral chemistry and petrography of Dhofar 489 Dhofar 489 is a feldspathic breccia consisting of mineral fragments and lithic and glassy clasts embedded in a much finer-grained dark crystalline matrix (Figure 1a)
Several pure anorthosite clasts have been found in the Dhofar samples, and 60015 is an anorthosite with high abundance of plagioclase and uniform composition in comparison with other FAN samples

Summary

Introduction

On the basis of mineralogical and geochemical investigations of the returned Apollo lunar samples, we determined that ferroan anorthosite (FAN) is the main component of a lunar feldspathic crust (e.g., Taylor et al 1991). In the global lunar magma ocean (LMO) model, the early lunar feldspathic crust was produced by the crystallization and flotation of plagioclase from the LMO (e.g., Warren 1985). The samples returned by Apollo missions were collected from a relatively small and geochemically restricted anomalous region of the central nearside of the Moon (Warren and Kallemeyn 1991). The presence of rocks with high plagioclase abundances at some large craters, according to the Kaguya data, was reported by Ohtake et al (2009), who proposed the lithology term purest anorthosite (PAN) for rocks with plagioclase content higher than 98%

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