[M-IS21] Planetary Volcanology

[MIS21-02] The VIS-NIR spectroscopic Study of Silicic Volcanic Complex on the Moon

*Ryotaro Araki¹, Kazuto Saiki² (1.Osaka University, Graduate School of Science, 2.Ritsumeikan University, Research Organization of Science and Technology)

Keywords:Lunar volcano, Compton-Belkovich Thorium Anomaly, Visible-near infrared spectroscopy, silicic volcano, remote sensing

Most of volcanic terrains on the moon are basaltic, but silicic ones are also found with their unique spectroscopic features. Most of them are found at the rim of lunar maria, and they are associated with the crystallization differentiation processes of mare basaltic magma (Glotch et al., 2019). The Compton-Belkovich Volcanic Complex (CBVC) is the only known silicic volcano present in the highlands far from lunar maria. It includes a volcanic caldera and lava domes and covers an area spanning approximately 30 km. Previous studies have shown that it exists in the same region as the Compton-Belkovich Thorium Anomaly, which is known to exhibit local concentrations of thorium (e.g., Gillis et al., 2002; Jolliff et al., 2011). CBVC is an interesting and important region for understanding the lunar interior dynamics and its evolution.The study of CBVC is based only on remote sensing observations from orbit. Bhattacharya et al. (2013) reported the composition of CBVC based on VIS-NIR spectroscopic data obtained by the Moon Mineralogy Mapper (M3) on the Chandrayaan-1. One of the significant features of CBVC is the prominent absorption band at a wavelength of 2.9μm, which is more pronounced towards the central part of the caldera. This suggests that H2O or OH groups were supplied as volatiles. Additionally, they indicated the presence of Fe-Mg spinel judging from the absorption feature at 2μm in boulder clusters near the pit crater.Chauhan et al. (2015) examined the morphology of CBVC using Lunar Reconnaissance Orbiter Narrow Angle Camera images with a minimum resolution of 50 cm. According to their analysis, CBVC has a depression structure as a volcanic caldera surrounded by three domes: East dome, West dome, and North dome. The central caldera and the domes have a few pit craters, boulder clusters as volcanic rocks, flows, and small silicic lava domes.In order to understand the formation process of CBVC, we utilized data from M3, Spectral Profiler (SP), and Multiband Imager (MI) obtained by the SELENE to investigate the topographical features, mineral distribution, and water distribution of CBVC. We made the map of water band strength at 3μm on the region of CBVC (Fig. 1). Comparing with the morphological interpretations suggested by Chauhan et al. (2015), it was reconfirmed that the feature becomes stronger towards the central caldera. In addition, we found that the signal is weakened in small lava domes which are thought to have formed in the later stages of activity. This may indicate a change in the activity type on CBVC. After the caldera formation, we thought that explosive eruptions occurred at the vent and volcanic ash containing a large amount of volatile were deposited. The small lava domes might have been formed by silicic magma without volatiles in latest stages of CBVC formation. On the other hand, it was confirmed that the distribution of the 2μm absorption band depth has no correlation with the topographic features except for the central pit craters. The data from SP and MI with higher spatial resolution than M3 showed that the band depth in the 2 μm correlated with that in the 1μm. This result suggests that pyroxene, not spinel, is distributed in the pit craters.
Figure 1. (a) A 750 nm reflectance image of CBVC obtained from M3, and (b) a binary image indicating the range of absorption strength of 0.17 or greater at 2.8-2.9μm, suggesting the presence of H2O or OH groups. We found the weaker absorption at the locations of the four small domes that are defined by Chauhan et al. (2015).