Compositional variability of Lunar basalts in selected basins using Chandrayaan-1 data

Abstract

The work presented in this thesis is aimed to understand the evolution of two lunar basins- Mare Serenitatis and Mare Nectaris based on the mineralogy and compositional diversity of the basaltic units that exist in the basin. Hyperspectral data from Moon Mineralogy Mapper (M3) onboard Chandrayaan-1 was used for the study. Reflectance spectra of fresh craters from the basins were analyzed to study the nature and location of the spectral absorption features around 1- and 2 µm respectively, arising due to the electronic charge transition of Fe2+ in the crystal lattice of pyroxenes and/or olivine. M3 images were utilized to obtain Integrated Band Depth (IBD) parameter images. Based on the spectral variations observed in the IBD mosaic, thirteen spectral units were mapped in Mare Serenitatis. Minimum Noise Fraction (MNF) transform is very useful technique to select meaningful bands that can represent the spectral variability of a large dataset. In this study, it is demonstrated that use of some selected MNF bands combined with continuum removed image at 1µm in RGB combination can capture the spectral variations more effectively as compared to the IBD parameter. The combination has been used for delineating the spectral units of Mare Nectaris basin and to map the various lithologies present in the basin. A total of five spectral units were mapped in Mare Nectaris. The detailed mineralogy of the delineated units was investigated utilizing the reflectance spectra. For this purpose, reflectance spectra from the fresh craters present across the basin were collected and studied. Using both 1 µm (band I) and 2 µm (band II) absorptions, band parameters i.e., band centers, band depth and Band Area Ratio (BAR) were calculated and utilized to characterize the various lithologies excavated by the craters along with their distribution within the basin. Based on the band parameters analysis, it is shown that Mare Serenitatis basaltic units are primarily dominated by low to intermediate end of high-Ca pyroxenes. Similar analysis for Nectaris basin suggests presence of intermediate to high-Ca pyroxenes in Nectaris basalts with more enrichment in Ca content as compared to the Serenitatis basin. Pyroxenes are the most abundant mineral in the mare basalts and serve as petrologic indicators. Utilizing both band I and band II centers, pyroxene chemistry can be determined based on the equations outlined by Gaffey et al., (2002). The pyroxene composition of the Mare Serenitatis based on the equations, is found to range from ~Wo18En32Fs50 to Wo31En14Fs55 having a mean composition of ~Wo28En20Fs52 indicating a sub-calcic to calcic augite compositional range. The pyroxene composition of Mare Nectaris is clustered in three clouds that cover almost the full range of pyroxenes from low-Ca pyroxene to high Ca augites i.e. from Wo56-5En40-2Fs4-93. Dark-haloed craters (DHCs) provide information about the crypto-mare units that are hidden deposits and represent ancient volcanic flows. Mare Nectaris contains some dark-haloed craters on the western edge of the basin. Three DHCs Beaumont L, DH-1 and DH-2 craters present in Mare Nectaris were also studied for their composition and morphology which led to the identification of olivine-rich deposits excavated by Beaumont L. Further mapping of olivine-rich craters in the Nectaris basin in this work confirmed that the olivine-rich material in Nectaris basin represents compositionally distinct lava flows which have not mentioned in earlier studies. From the overall analysis, it is concluded that the two basins studied differ from each other in terms of mineralogy, volcanic history and overall mineralogical evolution. This study concludes that no compositional trend has been observed in a spatial context with time across the Serenitatis basin, which is suggestive of a probably stable homogeneous basaltic source region. In case of Mare Nectaris, the basalts are much diverse and large variations exists in mafic mineralogy in terms of Ca content of pyroxenes and olivine content associated with the different basaltic units. Nectaris basalts which are enriched in Ca content are consistent with a melt generated from orthopyroxene and plagioclase rich source magma that can generate a more calcic melt and crystallizes more calcium rich pyroxene as compared to the source. Mare Serenitatis, on the contrary represents a very low-calcium mantle orthopyroxene source that enabled crystallization of a somewhat lower Ca pyroxene than the source. The study, thus, has great implications in understanding the mineralogical evolution of two lunar basins using the information derived from the hyperspectral data.