SPECTRAL ANALYSIS OF EXPLOSIVE AND EFFUSIVE VOLCANIC EDIFICES IN THE MARIUS HILLS VOLCANIC COMPLEX WITH MOON MINERALOGY MAPPER. M. J. Henderson1 B. H. N. Horgan1 1Purdue University, 610 Purdue Mall, West Lafayette, IN 47907 (marie@purdue.edu).

Introduction: Within Oceanus Procellarum on the

near-side of the Moon, the Marius Hills Volcanic Com-plex (FIGURE 1a; MHVC; 13.3ºN, 47.5ºW) is a 35,000 km2 plateau raised 100-200 m from the surrounding plains with a wide assortment and unusual concentration of volcanic features [1,2]. These volcanic features in-clude domes interpreted as shield volcanoes, lava flows, sinuous rilles, and cones.

The cones in the MHVC were identified initially us-ing visible imagery with morphologies similar to terres-trial cinder cones [1,3]. Cinder cones would require ev-idence that the volcanic edifice was constructed of bal-listic pyroclasts from an explosive, volatile-rich erup-tion. An explosive, volatile-rich eruption produces rap-idly quenched glassy pyroclasts as represented by cin-ders on Earth, and it is expected that the same process would occur on the lunar surface. The Lunar Reconnais-sance Orbiter Wide and Narrow Angle Cameras were utilized to complete a morphological survey of the cin-der cones of the MHVC [4]. Despite the previous anal-yses, the volcanic constructs' compositions are still poorly constrained, partially due to the resolution and techniques required to study the small volcanic cones.

This study uses spectral data collected from the Moon Mineralogy Mapper (M3) visible/near-infrared imaging spectrometer to investigate spectral diversity in MHVC. Previously, we used M3 maps to investigate the spectral properties of proposed cone features visible in the imagery and evaluate the presence of volcanic edi-fices, where visual imagery is ambiguous [5]. Here we assess the implications of the spectral properties of var-ious volcanic features in MHVC to provide greater in-sight into the region's magmatic history.

Background: The Marius Hills Volcanic Complex was a proposed Apollo Landing site, and the first spec-tral reflectivity analyses used Earth-based telescopes [6,7]. These low-resolution observations prevented the ability to distinguish specific volcanic features but indi-cated bulk soil composition similar to the remainder of Oceanus Procellarum [6]. Since the Apollo-era investi-gations, photogeologic and compositional analyses have previously characterized the geologic units within the MHVC. The visual imagery of the region from the Lu-nar Reconnaissance Orbiter Camera (LROC) has been used to distinguish the morphologic properties of the volcanic features within MHVC [4,8].

Compositional analyses included UV-VIS spectral data from Clementine [9,10], and VNIR spectra from M3, using the global 280µm/pixel data [11]. The resolu-tion and methods of previous observations have

prevented the separation and interpretation of the spec-tral signatures from the cones and domes [9,10,11]. However, spectral data were not utilized to investigate specific morphologic features in the MHVC.

Previous investigations of the MHVC developed a hypothesized volcanic evolution timeline [2,9,10,11, 12]. The range and superposition of volcanic features, as well as the spectral diversity of the volcanic units in MHVC, indicate an episodic volcanic history of the re-gion, which may be reflected in the mafic mineralogy.

FIGURE 1: (a) Topography from Lunar Orbiter Laser Altim-eter (LOLA) overlaid on the LROC WAC mosaic. (b) Mosa-icked composite RGB spectral maps from high-resolution M3 data where R= glass band depth, G=1µm band center, B= 2µm band center. The horizontal striping across the M3 frames is due to data variability.

Methods: M3 was an imaging spectrometer with 85 spectral channels on the Chandrayaan-1 lunar orbiter [13,14]. M3 operated in the visible to near-infrared (VNIR; 0.42µm-3.0µm), which is sensitive to absorp-tion bands exhibited by iron-bearing minerals. M3 col-lected data with varying resolution across the mission (75-280 m/pixel) due to variable orbital configurations [13,14]. Here we created an M3 map of the MHVC using 15 high-resolution (140m/pixel) spectral images with bounds 300-312.5ºE and 8-17ºN using mosaicking and continuum removal methods described in [15] and [16].

a

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Once the continuum of the spectrum was removed, sim-ple arithmetical spectral parameters, including the glass band depth, OPX band depth, and CPX band depth, were applied to distinguish between spectral signatures leading to the identification of minerals. Band parame-ters for the shape and position of the 1 and 2 µm iron absorption bands were also calculated based on the tech-niques described in [15]. Spectra were then extracted as averages of ROI’s created using spectral parameters for the localities of cinder cones, volcanic domes, and the mare. Parameter maps were then co-registered to an LROC image to compare to volcanic morphologies [4].

Results: An RGB composite image is shown in (Figure 1b), where red represents the glass band depth, green is the 1µm band center, and blue is the 2µm band center. Therefore, green typically indicates orthopyrox-ene, yellow can indicate increased glass, and blue typi-cally indicates clinopyroxene. The horizontal variability across the M3 frames in Figure 1b maps is due to reso-lution and sensitivity changes. Both the surface of the plateau and surrounding mare present as pink/blue in Figure 1b, indicating they may have erupted from the same source. These areas are spectrally distinct from the materials composing the domes and cones, which ap-pear green and yellow, respectively, in Figure 1b.

Spectral analysis of the MHVC confirms that dis-tinct spectral signatures are associated with different ge-ologic units, including volcanic cones and domes (Fig-ure 2). M3 spectra for MHVC units show distinctive iron-bearing absorption bands near 1- and 2-µm. Vol-canic cones of all shape classes in the MHVC display a symmetric band at 1.05-1.1 μm in addition to a band at 2.0 µm, consistent with significant glass in the cones, as inferred from the parameter map in Figure 1b. The glass-rich spectral interpretation suggests that the vol-canic cones were likely formed in an explosive eruption resulting in cinders. In contrast, volcanic dome struc-tures exhibit M3 spectra with strong bands centered near 0.95µm and 2.05 µm, consistent with orthopyroxene.

Discussion: Volcanic History: The range of volcan-ism in MHVC is further amplified by the region's spec-tral diversity. Combining the morphology and the spec-tral data, we hypothesize the magma evolution of the re-gion was long-lived and episodic. The formation of the plateau, possibly as an enormous shield volcano, may have been the first volcanic episode [12]. Next, the su-perimposed OPX-rich domes and glass-rich cones were built with the eruption of primitive and relatively vola-tile-rich magma [9,10,11]. Lastly, more evolved, CPX-bearing magmas erupted to form the youngest basalt flows in MHVC that embayed the volcanic domes and cones on the plateau, then followed rilles and channels to the mare units of Oceanus Procellarum [11]. In this scenario, MHVC could uniquely preserve ancient and possibly pre-mare volcanic materials.

Future Exploration: Marius Hills is proposed as a priority target for returning to the Moon [18]. The long-lived volcanism visible through multiple volcanic units in MHVC would be ideal for collecting samples to be returned to Earth for dating and analyses within close proximity. Spectral mapping would be imperative for planning a mission to MHVC either by a proposed long-lived rover capable of traversing slopes [18] or human explorers. References: [1] McCauley, J.F. (1967), IMAP. [2] Whitford-Stark, J.L., and J.W. Head (1977), LPS, 8, 2705–2724. [3] Wood, C.A. (1979), LPSC. [4] Lawrence, S.J. et al. (2013), JGR:Planets, 118(4), 615–634 [5] McBride, M.J. et al. (2018) LPSC, 49, #2798.[6] McCord, T.B. et al. (1972), The Moon, 5(1), 52–89 [7] Pieters, C., and T.B. McCord (1976) LPS, 7, 2677–2690. [8] Gustafson, J.O., et al. (2012), JGR 117, E00H25. [9] Weitz, C., and J.W. Head (1999), JGR:Oceans, 104(E8), 18933–18956. [10] Heather, D.J., et al. (2003), JGR: Planets, 108(E3), 5017 [11] Besse, S., et al. (2011), JGR, 116(E6), E00G13 [12] Spudis, P.D. et al. (2013), JGR: Plan-ets, 118(5), 1063–1081. [13] Pieters, C.M. et al. (2009), Curr Sci. [14] Green, R.O. et al. (2011), JGR:Planets, 116(E10), E00G19. [15] Horgan, B.H.N et al. (2014), ICARUS, 234, 132–154. [16] Bennett, K.A., et al. (2016), ICARUS, 273, 296–314. [17] Jawin, E.R., et al. (2019), ESS, 6(1), 2–40. [18] Stopar, J.D., et al. (2016). LEAG. Abstract # 5074.

Figure 2:M3 extracted spectra of cones (blue) and domes (green) compared to laboratory spectra (gray)

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