Spectral evidence for alkaline rocks and compositional diversity among feldspathic light-toned terrains on Mars

Abstract

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 dominated by plagioclase (e.g., anorthosite) and/or by spectrally neutral materials (e.g., quartz, K-spar, some volcanic glasses, some Fe2+−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 (~20–25°S, ~40–47°E). 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, feldspathic materials likely consist of a plagioclase-dominated composition with silica content below ~60 wt%. However, in at least one location in Terra Sabaea, lithologies with significant alkali feldspar (trachyandesitic to trachytic) and with silica contents up to ~60–65% are inferred from THEMIS observations. Terra Sabaea feldspathic materials could contain silica contents that straddle the boundary between intermediate-to-felsic lithologies; however, highly silicic or quartz-bearing compositions (e.g., rhyolites, granite, dacites) are not associated with the locations studied. Last, these results indicate that NIR-based plagioclase detections are associated with differing lithologies.