¹Jason C.Lai,²Briony Horgan,¹James F.Bell III,¹Danika F.Wellington
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2019.01.019]
¹School of Earth and Space Exploration, Arizona State University, Tempe, AZ, USA
²Department of Earth, Atmospheric, and Planetary Sciences, Purdue University, West Lafayette, IN, USA
Copyright Elsevier

Much of Mars’ surface is mantled by bright dust, which masks the spectral features used to interpret the mineralogy of the underlying bedrock. Despite the wealth of near-infrared (NIR) and thermal infrared (TIR) data returned from orbiting spacecraft in recent decades, the detailed bedrock composition of approximately half of the Martian surface remains relatively unknown due to dust cover. To address this issue, and to help gain a better understanding of the bedrock mineralogy in dusty regions, Dust Cover Index results from the Mars Global Surveyor Thermal Emission Spectrometer (TES) and analysis of images from the Mars Reconnaissance Orbiter Mars Color Imager (MARCI) were used to identify 63 small localized areas within the classical bright dusty regions of Arabia Terra, Elysium Planitia, and Tharsis Montes as potential “windows” through the dust; that is, areas where the dust cover is thin enough to permit infrared remote sensing of the underlying materials. The mineralogy of each candidate window was inferred using spectra from the Mars Express Observatoire pour la Mineralogie, l’Eau, les Glaces et l’Activité (OMEGA) NIR spectrometer and, where possible, TES. Twelve areas of interest returned spectra that are consistent with mineral species expected to be present at the regional scale, such as high- and low-calcium pyroxene, olivine, and iron-bearing glass. Distribution maps were created using previously defined index parameters for each species present within an area. High-quality TES spectra, if present within an area of interest, were deconvolved to estimate modal mineralogy and to support NIR interpretations. OMEGA data from Arabia Terra and Elysium Planitia are largely similar and indicate the presence of high-calcium pyroxene with significant contributions of glass and olivine, while TES data suggest an intermediate between the established compositions of the southern highlands and Syrtis Major. Limited data from Tharsis are consistent with low-calcium pyroxene mixed with lesser amounts of glass and high-calcium pyroxene. TES data from southern Tharsis correlate well with the previously inferred compositions of the Aonium and Mare Sirenum highlands immediately to the south. Of particular note is the detection of iron-bearing glass as a significant component of all three analyzed regions, especially in Tharsis. Overall, the underlying compositions of the classically dust-covered regions of Mars appear consistent with the compositions of adjacent and other low-albedo (not dust covered) regions of the planet identified in previous studies, with the noted contribution from iron-bearing glass.

¹Sarah E.Roberts,¹Clive R.Neal
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2019.01.023]
¹Department of Civil & Env. Eng. & Earth Sciences, University of Notre Dame, Notre Dame, IN 46556, USA
Copyright Elsevier

Very high potassium or VHK basalts have been described from the Apollo 14 landing site at Fra Mauro on the lunar near side. Many of the known samples are clasts in polymict breccias with a few as <1 cm rocklets in the regolith samples. VHK basalts are distinct from the high-Al basalts that are more common at the Apollo 14 site in that they are enriched in the alkali/alkaline earth elements (e.g., K, Rb, Ba, etc.). The source of this enrichment was initially proposed to be through assimilation of granite by a crystallizing high-Al basaltic magma. However, with the discovery of more VHK basalt clasts from three Apollo 14 polymict breccias, the number of different AFC episodes and granite assimilants had to increase in order to explain the whole rock compositional diversity within the VHK basalt suite.
New work completed on fourteen VHK basalts include Crystal Size Distributions (CSDs) and in-situ chemical analyses and element maps collected by electron probe microanalysis (EPMA). CSDs completed on three VHK basalt samples indicate that these basalts are not impact melts and represent endogenous melts of the lunar interior. Potassium Kα element maps reveal the spatial relationship between the K-rich material and the basalt clasts with attached breccia matrix if present. In-situ EPMA analyses have identified two distinct types of K-rich material: K-feldspar and K-rich glass. With these new data, an additional mechanism for an exogenic petrogenesis of VHK basalts is proposed that involves the impact process. VHK basalts can be divided into two groups based on density and viscosity. VHK-1 basalts formed when a hot impact ejecta covered the granite/felsite-rich matrix material containing high-Al basalt clasts. Heat from the impact ejecta partially melted and possibly evaporated nearby K-bearing materials, which then infiltrated and contaminated the high-Al clasts. VHK-2 basalts are consistent with the hypothesis of granite assimilation by a high-Al magma, as seen in the higher abundances of network-forming elements.