Global occurrence trend of high-Ca pyroxene on lunar highlands and its implications

Abstract

We present details of the global distribution of high-Ca pyroxene (HCP)-rich sites in the lunar highlands based on the global data set of hyperspectral reflectance obtained by the SELENE Spectral Profiler. Most HCP-rich sites in the lunar highlands are found at fresh impact craters. In each crater, most of the detection points are distributed on the ejecta, rim, and floor of the impact craters rather than the central peaks, while the central peaks are dominated by purest anorthosite (PAN). This indicates that HCP-rich materials originate from relatively shallower regions of the lunar crust than PAN. In addition, while all ray craters with sizes larger than ∼40 km possess HCP-rich materials, small fresh craters with sizes less than ∼6-10 km do not, indicating that the uppermost mixing layers in the lunar crust are not dominated by HCP. Based on these results, we propose that in the upper lunar crust, a HCP-rich zone overlying the PAN layer exists below the uppermost mixing layer. This HCP-rich zone may originate from interstitial melt during the formation of the flotation anorthositic cumulate, while an impact ejecta origin, impact melt origin, and/or magmatic intrusion into the upper lunar crust may also account for the occurrence of HCP-rich sites in the highlands.

Far‐UV Observations of Lunar Rayed Craters with LRO‐LAMP

“…Another potential explanation for the high reflectances found at the crater cavities and halos is an increase in the high‐calcium pyroxene (HCP) content at those regions. Yamamoto et al () noted that fresh impact craters are coincident with HCP‐rich sites in the highlands, and Tycho crater in particular was found to be especially HCP‐rich. If present in the region, diopside is an HCP that would increase FUV albedo across the full LAMP band pass (Wagner et al, ).…”

Featureless spectra on the Moon as evidence of residual lunar primordial crust

“…On the other hand, there are no FL points inside the Procellarum KREEP Terrain (PKT) and the South Pole‐Aitken (SPA) basin [ Jolliff et al , ]. Furthermore, even the highlands in the nearside (e.g., around the Tycho crater, where the HCP‐rich spectra are found [ Hawke et al , ; Pieters , ; Lucey and Hawke , ; Yamamoto et al , ]) do not exhibit a FL point.…”

“…Furthermore, this may be consistent with an existence of PAN‐bearing farside lunar meteorites, such as the Dhofar 489 group [ Nagaoka et al , ]. Although Parmentier and Liang [], Piskorz and Stevenson [], and Yamamoto et al [] proposed the possibility that there is an interstitial mafic melt layer at the thin, top layer of the crust, such a small amount of mafic does not affect this scenario.…”

“…Each letter corresponds to a location in the list in Table 1. The background is the total crustal thickness map [Ishihara et al, 2009]. where the HCP-rich spectra are found [Hawke et al, 1986;Pieters, 1986;Lucey and Hawke, 1988;Yamamoto et al, 2015]) do not exhibit a FL point.…”

“…Recent remote sensing observations of the Moon have revealed the existence of several kilometer wide sites with exposed end-members of various lunar minerals, namely, olivine-rich sites [Yamamoto et al, 2010[Yamamoto et al, , 2012bIsaacson et al, 2011], purest anorthosite (PAN) sites [Ohtake et al, 2009;Yamamoto et al, 2012a;Donaldson Hanna et al, 2014], orthopyroxene-rich (LCP: low-Ca pyroxene) sites [Nakamura et al, 2009[Nakamura et al, , 2012, clinopyroxene-rich (HCP: high-Ca pyroxene) sites [Ogawa et al, 2011;Yamamoto et al, 2015], and spinel-rich sites [e.g., Sunshine et al, 2010;Pieters et al, 2011;Dhingra et al, 2011;Yamamoto et al, 2013]. The identification of these sites is based on diagnostic absorption bands of 1 μm and 2 μm in continuous reflectance data (hyperspectral data) obtained by remote sensing observations.…”