APOLLO 17 SAMPLE 72415-A FRAGMENT OF THE LUNAR MANTLE?

Introduction: An unresolved problem of lunar science is the nature and composition of the lunar mantle. Many models have been published on the composition of the lunar mantle, resulting in a range of possible Mg#s of the bulk rocks and forsterite (Fo) compositions of the constituent olivine. Here we discuss a sample which is a possible fragment of the lunar mantle brought to the surface of the Moon by the Serenitatis impact and collected by Apollo 17 astronauts from the Taurus-Littrow valley. Lunar mantle: No samples of the lunar mantle have yet been unambiguously identified in the Apollo sample collection or the lunar meteorite collection, although [1] described a dunite clast in a brecciated meteorite which may be part of the lunar mantle. However, the lunar mantle has been extensively studied through indirect means. Electromagnetic and seismic data sets place constraints on the interior profile that are consistent with the lunar mantle being composed primarily of olivine and orthopyroxene with lesser amounts of clinopyroxene and garnet. Dunite clasts: Apollo samples 72415–72418 were chipped from a large clast in Boulder 3 at Station 2 in the Taurus-Littrow valley. Astronaut Schmitt recognized the clast as being composed of light pastel green material in a paler matrix and correctly suggested that it was "olivine and something". The samples were described as cataclastic dunites composed of 93% olivine (Fo86-89), 4% plagioclase (An85-97) and 3% pyroxene (En84Wo3Fs10 and En50Wo42Fs4). The dunites also display intergrowths of spinel and different minerals that may have formed as a result of either metamorphism or crystallisation. The dunites were determined to be 4.55 Ga old, and thus considered to be a product of primary lunar differentiation. It is these dunite clasts and their minute blebs of spinel “symplectites” which are the object of this study. Petrological Study: In thin-section, the dunite is a breccia composed mainly of olivine clasts in a very fine-grained matrix (Fig. 1). Olivine clasts show a range of sizes, with the largest measuring 0.6-1.7 mm, smaller ones down to 0.1 mm and fine matrix clasts measuring ~50-100 μm. All olivines have approximately the same Fo (Mg/(Mg+Fe) value of 88, which is at the lower end of the range of lunar mantle compositions suggested by modelling, and surprisingly similar to the Fo content of the terrestrial mantle. Most of the olivine clasts are angular, but a few have more rounded edges. They often show fractures which terminate at their edges and do not continue into the matrix. These irregular fractures are probably due to impact shock. Some olivine grains show undulose extinction and mosaicism (Figure 1), also probably a result of shock metamorphism.