Olivine-bearing lithologies on the Moon: Constraints on origins and transport mechanisms from M3 spectroscopy, radiative transfer modeling, and GRAIL crustal thickness
Introduction
Olivine is an abundant constituent of the lunar mantle that constrains models of lunar origin and evolution (e.g., Snyder et al., 1992, Elkins-Tanton et al., 2011). Remote detection of surface olivine is a challenge, in part because of its rarity, as evidenced by the almost complete absence of large-scale samples (e.g., dunite clasts) in the lunar sample collection, and also because, in the visible to near-infrared spectra, the signature of olivine can easily be obscured by relatively small amounts of the more common mineral, pyroxene (e.g., Pieters, 1983, Crown and Pieters, 1987). Nonetheless, olivine has been detected on the surface using high-spatial resolution hyperspectral data. Yamamoto et al. (2010) used data from the Kaguya Spectral Profiler (SP) to identify olivine exposures located on the rims of several large impact basins, where the crust was previously thinned as a result of basin excavation. They conclude that the spectra with the indicative olivine signature are consistent with the presence of dunite, with a likely origin from the upper mantle.
Several other studies have estimated olivine composition and investigated the geologic context for the purpose of determining origin and emplacement mechanisms of olivine. Arnold et al. (2016) confirm the presence of olivine at many of the Yamamoto et al. (2010) detection sites with Chandrayaan-1’s Moon Mineralogy Mapper (M3) spectra and estimated olivine abundance using mid-infrared data from the Lunar Reconnaissance Orbiter (LRO) Diviner Lunar Radiometer Experiment. They found that olivine-bearing locations have a wide range of olivine/plagioclase ratios, with most being troctolitic in composition. Powell et al. (2012) also examined Crisium basin using M3 spectra. They detected olivine within mare flows, intrusive landforms, and in areas of thick crust, and conclude that both magmatic and mantle olivine is present. Isaacson et al. (2011) used the Modified Gaussian Model (MGM) to examine the spectral diversity in olivine-rich M3 spectra from Mare Moscoviense, the Copernicus central peak, Aristarchus, and Marius crater. Their MGM-derived absorption-band positions enabled the comparison of the relative Mg (i.e. forsterite) contents of the olivine. Olivine spectra at Moscoviense are Mg-rich but exhibit diversity that suggests varied Fe/Mg contents. The most Mg-rich spectra are similar in composition to olivine found in Mg-suite rocks that are ∼Fo90. Coperincus has spectrally and compositionally homogeneous olivine that is also relatively Mg-rich, which indicates a single magmatic or mantle source, rather than a source that evolved over time. They conclude that the relatively Mg-rich composition suggests a plutonic origin, because olivine in mare basalt is usually more Fe-rich (e.g., Papike et al., 1976). However, the diversity in composition at Moscoviense requires a different process than that responsible for olivine at Copernicus. Olivine spectra from Marius and Aristarchus had contamination from pyroxene and an unknown phase, respectively, preventing the estimation of Mg#. The spectral differences indicate different lithologies than olivine exposures at Copernicus and Moscoviense. Mustard et al. (2011) also identified olivine-rich M3 spectra in impact melt and ejecta deposits on the southeastern wall and rim of Aristarchus and conclude that they originate from a shallow pluton, an olivine-rich region of Imbrium ejecta, or olivine-rich Procellarum basalts that were either excavated or melted by the impact and subsequently crystallized.
Studies have also identified olivine with evidence for types of origin other than mantle and plutonic. Staid et al. (2011) used M3 spectra to confirm the presence of Fe-rich olivine in fresh material in high-Ti maria on the western nearside. The stratigraphic evolution and Fe-rich compositions of these basalts indicate that the magma sources were evolved residual melts, rather than assimilation of primitive (Mg-rich) mantle olivine. Spectrally similar mare basalts are scarce outside the Procellarum-Imbrium region but do occur as small flows or ponds in areas of late-stage regional volcanism, supporting their interpretation that the olivine in high-Ti maria originated from evolved residual melts.
Using the LRO Narrow Angle Camera (NAC) and Kaguya Terrain Camera (TC), Dhingra et al. (2015) investigated the geologic context and albedo differences of olivine occurrences identified at Copernicus Crater with M3 and found evidence for multiple styles of origin. The northern wall has smooth deposits that contain olivine. The spectra are lower in albedo than other olivine exposures, suggesting that abundant or coarse-grained olivine resides in a dark matrix. They propose that these exposures originated from olivine-bearing clasts in the target material that were entrained into a mafic impact melt, which cooled rapidly. Mare basalt was part of the pre-impact lithology, which might have been the source for the mafic material that is necessary in order to explain the opaque, mafic glass. They also found olivine in the central peak and crater floor, which they evoke a different origin in order to explain. Olivine on the central peak is mixed with plagioclase and likely to have been uplifted crustal material from a depth of ∼15 km. Olivine exposed at high-standing mounds on the crater floor represent large broken, unmelted fragments that were embedded in the impact melt and might be from the same source material as the central peak. As these studies demonstrate, it is important to establish the geologic context and the composition in order to interpret the origins and transport mechanisms, as well as any genetic relationships, of olivine exposures within a region.
In this study, we use M3 data to search for olivine at Crisium, Nectaris, and Humorum basins, three areas where Yamamoto et al. (2010) detected olivine. We also search for olivine near Roche and Tsiolkovsky craters, where Andrews-Hanna et al. (2013) identified a possible massive dike, similar to one found at Crisium that is associated with olivine (Yamamoto et al., 2010, Powell et al., 2012). Unlike Spectral Profiler, which Yamamoto et al. (2010) used for detecting olivine, M3 is an imaging spectrometer, so each spectrum and its geologic context is evident in the same data. An olivine index algorithm based on the weighted depth and breadth of the 1-µm olivine absorption helped us identify locations with relatively high abundances of olivine. Topography and slope maps, created using Lunar Orbiter Laser Altimeter (LOLA) data (Smith et al., 2010a), crustal thickness models (Wieczorek et al., 2013) from the Gravity Recovery and Interior Laboratory (GRAIL) (Zuber et al., 2013) and LOLA (Smith et al., 2016) data sets, and images from the LRO Wide Angle Camera (WAC) and Narrow Angle Camera (NAC) (Robinson et al., 2010) allow us to put olivine detections into precise geologic and geophysical contexts through identification of features not spatially resolvable by M3. Furthermore, this is the first study to estimate the olivine abundance of each location using radiative transfer modeling with M3 data. Here, we use radiative transfer modeling to estimate both Mg# (MgO/(MgO + FeO)) and mineral abundances (i.e., olivine, orthopyroxene, clinopyroxene, and plagioclase) for each M3 olivine-bearing spectrum. In addition to identifying the locations of olivine exposures, we seek to establish both the origin of exposed olivine and the means by which it was transported to the surface or near-surface.
Section snippets
Hypotheses for the origin of observed olivine and mechanisms of transport to surface
We identify five different explanations for exposures of lunar olivine, based on where they originated and how they were transported to the surface. We consider that these olivine exposures either (1) originated from the primary mantle or mafic lower crust, (2) are igneous in origin, or (3) crystallized from differentiated impact melt; and that these olivine-bearing materials were transported to the surface either (A) by basin or complex crater impact excavation or (B) magmatically. We focus on
M3 data
The M3 imaging spectrometer has a spatial resolution of 140 m and its imaging capability allowed us to associate olivine-bearing spectra with geologic features (Fig. 4). The M3 spectra have high spectral resolution (83 bands from 540 to 2980 nm) that can be used to distinguish mineral signatures. We used level 2 M3 reflectance data, which is thermally, geometrically, and photometrically corrected (Green et al., 2011, Lundeen et al., 2012, Besse et al., 2013). Some of the M3 data have a low
Results
We produced M3 mosaics of our four study areas, Crisium, Nectaris, and Humorum basins and Roche/Tsiolkovsky craters. The olivine index algorithm was applied to each M3 mosaic from which we confirmed a total of 111 M3 pixels (140 m per pixel) with olivine-bearing spectra throughout the four study areas. By comparing the M3 olivine spectra with the library of radiative transfer modeled spectra with an Mg# of 65 and 90 (Fig. 9), we found that only six olivine-bearing spectra had best spectral
Discussion
The integration of the geophysical settings and estimated mineral abundances allow for the interpretation of the origins and the transport mechanisms of the olivine exposures examined in this study. Possible transport mechanisms include excavation of the mantle, lower crust, or impact melt by basin-forming impact, or magmatic emplacement of olivine-rich basalts, cumulates, or xenoliths. Furthermore, mineral abundances also provide insight into the lunar mantle overturn and the origin of olivine
Conclusions
The application of a radiative transfer model to high-resolution M3 data have allowed for estimation of mineral abundances of olivine-bearing material at Crisium, Humorum, and Nectaris basins and near Roche and Tsiolkovsky craters. Modeled crustal thickness estimates from GRAIL and LOLA (Wieczorek et al., 2013), LOLA topography maps, and WAC and NAC images were used to examine the geophysical and morphologic settings and put our olivine locations into geologic context. Detailed investigations
Acknowledgments
The authors are grateful for helpful comments from Francis Nimmo and Jim Head, as well as advice on radiative transfer modeling from Paul Lucey. This work was supported by the NASA GRAIL mission under contract to the Massachusetts Institute of Technology and Jet Propulsion Laboratory/Caltech, as well as NASA GRAIL Guest Scientist grant NNX12AP48G to P. J. M. This is SOEST contribution #10023, HIGP contribution #2258, and LPI contribution #2053.
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