Visible/near Infrared Spectral Characterization of Brecciated Mare Basalt Flow and Surface Mare Basalt

Introduction: Lunar mare basalts are products of partial melting of the lunar interior, thus are keys to understand the chemical compositions of the source mantle. However, the iron-enrichment of the mare basalt samples returned by Apollo and Luna missions, compared to the terrestrial basalts, have led to an inference that the mare basalt hand specimen do not represent the primary magma composition, due to pre-eruptive fractionation in sub-crustal magma chambers and/or post-eruptive stratification of relatively hick basalt flows and lava ponds. In the latter case, the basalt samples do not have the average compositions of the whole basalt flow unit [1]. Instead, they just represent the upper part of the basalt flow which is depleted in phenocryst by gravitational settling within the flow and the complementary phenocryst-enriched cumulates at the base of the flow. Basaltic clasts found in the Luna 24 soils support this inference: Several mare-type clasts of Luna 24 successfully reconstruct the differentiated layered basalt flow with Luna 24 glass composition [2]. Recently recovered lunar meteorites have boostered this case. A dimict breccia NWA 773 includes clasts of Mg-rich olivine cumulate and Fe-rich basalts [3, 4]. A regolith breccia, MET 01210 also contains Fe-rich basalt clasts and the Mg-enriched counterparts [5]. Thus, the true mare basalt composition is to be elucidated based on the composition of the entire basalt flow units, not on those of the Fe-rich surface basalts which are dominant in the most Apollo/Luna basalt hand specimen. In this study, Visible and Near Infrared spectral properties were studied for a brecciated mare basalt flow unit and a surface basalt from lunar meteorites, in order to understand the spectral disparity between the two, further aiming to deduce the true mare-basalt composition from reflectance spectra of mare breccias distributed around relatively fresh craters penetrating basalt flows, utilizing the multi-spectral data of global lunar surfaces with high (∼ 500 m) spatial resolution acquired by SELENE mission. Samples: A lunar meteorite NWA 773 (NWA) is a dimict breccia including two distinct lithologies of Mg-rich olivine cumulate and Fe-rich low-Ti (LT) mare basalt [3, 4] (Fig.1). Previous studies [3, 4] revealed that the two lithologies are petrogenetically connected and are probably derived from a single, relatively thick, basalt stratiform. Thus, NWA is expected to represent the average composition of a basalt unit. In contrast, LAP 02205 (LAP) is a crystalline mare basalt similar to Fe-enriched Apollo 12 and 15 LT basalts [6, 7] (Fig. 1) and an example for a typical surface basalt. NWA is more magnesian than LAP: bulk-rock Mg/(Mg+Fe) atomic ratio for NWA is 69 for the Mg-rich gabbro and 56 for the breccia with mixtures of Mg-rich cumulate and Fe-rich basalt, while that for LAP 02205 ranges from 30 to 36. Their pyroxene compositions are shown in Fig. 2. Olivine compositions are Fo68-61 for NWA gabbro and Fo62-48 for LAP. Methods: Bidirectional reflectance spectra from 0.3 – 2.6 μm nm were measured using a UV-Visible-Near IR spectrophotometer of Institute of JAXA. A 30o incident angle and a 0o emission angle were chosen for the measurements. Two detectors were used: a photomultiplier in the range of 0.3 – 0.85 μm and a PBS cell in the range of 0.85 – 2.6 μm. Rock chips of LAP 02205, 28 weighing 0.099 g and those of NWA 773, M23 of 0.14 g were crushed and sieved with 75 and 105 μm meshes. The powdered samples of 75 – 105 μm micron in size were used for the spectral measurement. A curve fitting model using the modified Gaussian Model (MGM) [8] was performed to resolve the reflectance spectra containing composite absorption features into individual absorption bands. Mineralogical analyses were done for two thin sections LAP 02205, 35 and NWA 773, M23-1 by JEOL 8200 Electron Microprobe at National Institute of Polar Research. Results and Discussions: The reflectance spectra of the two samples are shown in Fig. 3. They display strong absorption peaks at around 1.0 μm and 2.0 μm. These absorption features are more extensive in LAP than NWA. The absolute value of the obtained reflectance is also higher in LAP than NWA. This is consistent with the higher maturity of the brecciated NWA comparative to the crystalline LAP, though NWA is fairly immature as the maturity standard of typical regolith. The reflectance spectra of NWA is apparently shifted to the shorter wavelength, in good agreement with the more magnesian compositions than LAP. These spectral disparities of the two samples, especially the lateral spectral shift were investigated with respect to the difference in the individual absorption bands resulting in the composite absorption signatures around 1.0 μm and 2.0 μm which are derived from Fe electron transition absorption of pyroxene and olivine. To resolve individual absorption bands, the MGM deconvolution was performed for each of the reflectance spectra. The optimization of parameters was performed by initially starting with given wavelength, depth and width of absorption peak center which correspond to the representative Lunar and Planetary Science XXXVII (2006) 1895.pdf