SPECTRAL EVIDENCE FOR COMPOSITIONAL DIVERSITY IN FELDSPATHIC LIGHT-TONED NOACHIAN TERRAINS ON MARS (Invited Presentation)

Previously reported plagioclase detections in the near-IR (NIR) are associated with crater-exposed subsurface materials as well as light-toned bedrock plains with poor crater retention. Because Fe-bearing plagioclase has a weak ~1.25 µm feature that can be easily obscured by stronger absorptions in most pyroxenes and olivines, candidate lithologies must be (a) dominated by plagioclase (e.g. anorthosite), (b) contain plagioclase crystals that are coarse grained compared to mafic materials and/or (c) dominated by spectrally neutral materials (e.g. quartz, K-spar, some volcanic glasses, some Fe-poor pyroxenes). To further understand the lithologies associated with NIR plagioclase detections, we investigated the spectral nature of these materials with THEMIS and CRISM data, focused on six light-toned plains exposures in Nili Patera caldera and Terra Sabaea/NE Noachis Terra. Thermal infrared data from THEMIS can discriminate between some of the candidate lithologies; however, it has a lower spatial resolution (100 m/pix) than CRISM (18 m/pix) thus increasing the likelihood of subpixel mixing of rock and sand. We forward modeled a range of spectral mixtures of mafic sands with alkaline and subalkaline volcanic lithologies spanning the total alkali-silica diagram to identify the suite of rock compositions that could plausibly fit the THEMIS observations. In Nili Patera, the feldspathic rock cannot be more silicic than andesite; a plagioclase-dominant lithology is most consistent with both the NIR and TIR observations. Because the ~1.25 µm feature is readily apparent over this unit, and because the THEMIS data are not consistent with a silicic composition, the matrix material is likely a fine-grained, low-Fe pyroxene or mafic glass. Anorthositic compositions are also spectrally consistent with the NIR and TIR observations, but the high TI and minimal regolith suggests a more friable material, inconsistent with coarsely crystalline rocks. In Terra Sabaea, THEMIS spectra suggest silica contents below ~60 wt% but in at least one location may contain significant alkali feldspar and is spectrally consistent with a trachytic lithology. These results indicate that high-silica compositions are not associated with the locations studied, but also that NIR-based feldspar detections are associated with differing lithologies.