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The Composition of Lunar Central Peaks Relative to Lunar Samples

Introduction: Impact crater central peaks offer a unique opportunity to remotely examine material that was once deep in the crust (1-3). They also offer the added advantage that they consist of material that has undergone less extensive rock-type mixing typical of other surface exposures potentially offering a less confounded account of lunar geologic history. Here we examine a series of peaks using existing Clementine eight band visible and near-infrared reflectance spectra. With this information we can infer the compositional diversity of the lunar crust both laterally and vertically. Placing these data in the context of lunar sample allows us to make petrologic inferences into lunar crustal formation. Method of Analysis: Crater central peaks are analyzed by comparing Clementine reflectance spectra to radiative transfer modeled spectra. The radiative transfer approach of (4-6), mineral optical constant data of (7) and iron optical constant data of (8) are used in this model. Similar implementations of the model used here are reported by (9, 10), and explained in detail by (11). The mineral mapping approach is similar to (12) and (10) but differs by including both Clementine UVVIS and NIR data to enable better mineralogical interpretations. Clementine near-infrared spectra are calibrated to Earth-based telescopic spectra from various locations on the lunar nearside. Documentation for these correction procedures and their results are posted on the USGS website (http://astrogeology.usgs.gov/Projects/ClementineNIR/) courtesy of Denevi and Lucey (personal communication). Central Peak Composition Relative to Lunar samples: Recently, in support of interpretation of remote sensing analyses we compiled modal mineral abundances and corresponding mafic Mg’ (10). This information can be used as a starting point to make direct comparisons between cataloged lunar samples and mineral abundances inverted from spectral studies of lunar surface rocks. These mineral data are displayed on modifed Stöffler diagrams in figure 1 (13). Inverted mineralogy of central peak spectra show a large diversity of average lithologies but most of them are dominantly mafic (Fig. 1). Peaks with mafic model mineralogy also show many similarities to olivinebearing gabbronorite Mg-suite rocks of the lunar sample collection. However, peak pyroxene abundances show more diversity with orthopyroxene-rich gabbroic norite and clinopyroxene-rich noritic gabbro lithologies both being present. Several peaks modeled also have anorthositic mineralogies and show similarities to both lunar FANs and anorthositic Mg-suite rocks. The typical method to discern lunar petrologic suites is to compare mafic mineral Mg’ with one of the numerous “plagiophile loving” elements or ratios (e.g., Ca/Ca+Na, Al, Ti/Sm, Al/Eu, etc.) (14). At present remote sensing cannot measure these plagiophile ratios. What we can determine is mafic Mg’ and plagioclase abundance. Using the compilation of modal mineralogy and Mg’ for ~100 lunar FAN and Mg-suite rocks by (10), we plot plagioclase abundance directly versus Mg’ in figure 2. This plot effectively separates the lunar FAN and Mg’petrologic suites (though it cannot separate alkali anorthosites verses FAN). This offers a tool to make direct comparisons between remotely determined impact crater central peak composition and the