Magnesian Rock Types in the Lunar Highlands: Remote Sensing Using Data from Lunar Prospector and Clementine

Introduction: Several types of magnesian highlands rocks occur in the lunar samples including the mafic Mg-suite rocks, magnesian feldpathic granulites, and magnesian anorthosites. Some mafic magnesian rocks are easily accommodated into the magma ocean model, as large variations in Mg/Fe are expected as the magma ocean crystallizes, forming source regions for magnesian melts. The origin of others are more enigmatic including the Mg-troctolites and magnesian anorthosites as melts saturated in plagioclase ought to exhibit low Mg/Fe ratios [1]. The distribution of magnesian rocks is unknown, though observations of lunar meteorites and feldspathic granulites suggests that magnesian feldspathic material is distributed Moon-wide except in KREEP dominated regions [2]. Remote sensing of large crater central peaks has demonstrated the existence of infrequent mafic lithologies throughout the lunar highlands, and these have been associated with the mafic Mg-suite, lacking a suitable alternative in the sample collection [3]. However, no direct estimates of the Mg/Fe ratio for these locations has been offered as yet, and it has been suggested that magnesian mafic rocks are related in some way to KREEP and confined to the Procellarum KREEP terrane (PKT) [4]. Mg’ (Mg/(Mg+Fe)) has a striking effect on reflectance spectra of lunar mafic minerals (e.g. Figure 1) allowing estimation of Mg’ from spectral remote sensing. Methodology: We have developed methods for detecting magnesian lithologies using spectral reflectance data. The one presented here employs a radiative transfer model to compute thousands of mixtures of the common lunar minerals, including the effects of space weathering and variations in Mg’, and these mixtures are compared to measured spectra to deduce the relative abundance and chemistry of these minerals. This method is an enhancement of an existing mineral mapping algorithm [5]; here we add an explicit report of the Mg’ of the mineral assemblage detected. We applied the algorithm to a 1 km resolution mosaic of Clementine data. The algorithm only applies to spectra of immature lunar material and we used an optical maturity [6] of 0.3 to limit our analysis to immature locations. We then used the algorithm of [5] to interpolate the data to produce a continuous map at 10 km spatial sampling. The result is an image of Mg’. To validate these data we compute an Mg’ image from Lunar Prospector iron and magnesium data at 5 degree spatial sampling [7], and after appropriate filtering of the Clementine data compare the two. The histogram of the two data sets is shown in Figure 2. The Prospector Mg’ data exhibit a mode of Mg’~70, which is essentially the same as the Luna 20 regolith, the feldspathic fraction of the Apollo 16 regolith, and the feldspathic lunar meteorites [2]. The Clementinederived Mg’ data are systematically higher than the Prospector estimate by five units of Mg’, but the distribution is strikingly similar. Given the challenging nature of this measurement, the relatively close agreement gives us confidence to proceed with analysis based on the 10 km resolution Clementine Mg’ image. Results: Figure 3 shows the distribution of plagioclase-rich material (>85%, using data of [5]) including color-coding magnesian (Mg’ >70) and ferroan (Mg’ < 70) compositions. While there is overlap in occurrences, the ferroan compositions are concentrated on the nearside and the lunar south, and magnesian compositions occur more frequently on the farside and the lunar north. This distribution is consistent with the distribution inferred from study of feldspathic lunar meteorites [2] where this KREEP-poor material (presumably originating far from the PKT) is magnesian, while the ferroan material, sampled in the Apollo missions, occurs principally on the nearside. We also mapped the occurrence of magnesian mafic material. Figure 4 is the distribution of material with less than 60% plagioclase, dominated by orthopyroxene (as this mineral is the most distinctive with respect to Mg’ variations), and exhibiting an Mg’ greater than 70. Concentrations of this material occurs within South Pole-Aitken Basin near Apollo, north and west of Serenitatis, and the western Central Highlands. Conclusions: These observations support the inference from the lunar meteorites that magnesian feldspathic material is widespread, but occurs mainly distant from the Apollo zone. Mafic magnesian material is not confined to the PKT but also appears to occur within the mafic SPA Terrane. It appears to be absent from the magnesian anorthosite regions, supporting a source other than mafic magnesian lithologies for the chemistry of the magnesian anorthosites.