Acid weathering of basalt and basaltic glass: 2. Effects of microscopic alteration textures on spectral properties

Abstract

Acid alteration has long been proposed for the Martian surface, and so it is important to understand how the resulting alteration textures affect surface spectra. Two basaltic materials of varying crystallinity were altered in two different H2SO4 solutions (pH 1 and pH 3) for 220 days. The unaltered and altered samples were studied in the visible and near infrared (VNIR) and thermal infrared (TIR), and select samples were chosen for scanning electron microscopy analysis. Materials altered in pH 3 solutions showed little to no physical alteration, and their spectral signatures changed very little. In contrast, all materials altered in pH 1 acid displayed silica‐rich alteration textures, and the morphology differed based on starting material crystallinity. The more crystalline material displayed extensive alteration reaching into the sample interiors and had weaker silica spectral features. The glass sample developed alteration layers tens of microns thick, exhibiting amorphous silica‐rich spectral features that completely obscured the substrate. Thus, the strong absorption coefficient of silica effectively decreases the penetration depth of TIR spectral measurements, causing silica abundances to be grossly overestimated in remote sensing data. Additionally, glass samples with silica layers exhibited distinct concave up blue spectral slopes in the VNIR. Spectra from the northern lowland plains of Mars are modeled with high abundances of amorphous silica and exhibit concave up blue spectral slopes and are thus consistent with acid altered basaltic glass. Therefore, we conclude that large regions of the Martian surface may have formed through the interaction of basaltic glass with strongly acidic fluids.

Acidic weathering of basalt and basaltic glass: 1. Near‐infrared spectra, thermal infrared spectra, and implications for Mars

“…They attributed the spectral features to the accumulation of silica‐rich depositional coatings over time. Our companion paper found silica‐rich layers on the basalt glass sand samples in this study, and so we also assign the combination of ~1100, 465, and 1220 cm −1 features to silica‐rich alteration layers [ Smith et al ., ].…”

“…On the other hand, alteration penetrated the interior of the glassy basalt sand grain samples (ICE170), leaving parts of the surface of the grains relatively unaltered, so that the fraction of silica‐rich material measured in the thermal infrared was much less. The silica‐rich phases in both the basalt glass and the glassy basalt were fractured and porous, so we do not think porosity is causing the spectral difference in this case [ Smith et al ., ].…”

“…Finer crystals have higher surface area to volume ratios than do large crystals and thus dissolve more quickly. In SEM images of BAS205 presented in the companion paper to this study [ Smith et al ., ], plagioclase, olivine, and pyroxene are all easily identified as large crystals of similar sizes. In contrast, BAS101 and BSB101 tend to exhibit only large plagioclase crystals, surrounded by a glassy matrix.…”

“…SEM observations of altered samples in our companion paper confirm that there are significant physical differences between partially glassy and fully glassy samples [ Smith et al ., ]. Of the samples examined using SEM, only the fully glassy sample (BAS101) showed clear evidence for the development of silica‐enriched rinds or coatings on the exterior of the sample.…”

“…This process may also be responsible for producing leached layers in glasses [ Crovisier et al ., ; Geisler et al ., , ]. See our companion paper for a more in‐depth discussion of this topic [ Smith et al ., ]. For continuity with previous Mars‐relevant work [e.g., Minitti et al ., ; Chemtob et al ., ; Horgan and Bell , ], here we will continue to refer to these leaching and/or dissolution fronts on glass as leached rinds.…”