^1,2K. L. Siebach, ²M. B. Baker, ²J. P. Grotzinger, ¹S. M. McLennan, ³R. Gellert, ⁴L. M. Thompson, ¹J. A. Hurowitz
Journal of Geophysical Research Planets (in Press) Link to Article [DOI: 10.1002/2016JE005195]
¹Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA, USA
²Present address: Department of Geosciences, SUNY Stony Brook, Stony Brook, NY, USA
³Department of Physics, University of Guelph, Guelph, Ontario, Canada
⁴Planetary and Space Science Centre, University of New Brunswick, Fredericton, Canada
Published by arrangement with John Wiley & Sons

Sedimentary rocks are composed of detrital grains derived from source rocks, which are altered by chemical weathering, sorted during transport, and cemented during diagenesis. Fluvio-lacustrine sedimentary rocks of the Bradbury group, observed on the floor of Gale crater by the Curiosity rover during its first 860 sols, show trends in bulk chemistry that are consistent with sorting of mineral grains during transport. The Bradbury group rocks are uniquely suited for sedimentary provenance analysis because they appear to have experienced negligible cation-loss (i.e., open-system chemical weathering) at the scale of the Alpha Particle X-ray Spectrometer bulk chemistry analyses based on low Chemical Index of Alteration values and successful modeling of >90% of the (volatile-free) targets as mixtures of primary igneous minerals. Significant compositional variability between targets is instead correlated to grain size and textural characteristics of the rocks; the coarsest-grained targets are enriched in Al2O3, SiO2, and Na2O, whereas the finer-grained targets are enriched in mafic components. This is consistent with geochemical and mineralogical modeling of the segregation of coarse-grained plagioclase from finer-grained mafic minerals (e.g., olivine and pyroxenes), which would be expected from hydrodynamic sorting of the detritus from mechanical breakdown of subalkaline plagioclase-phyric basalts. While the presence of a distinctive K2O-rich stratigraphic interval shows that input from at least one distinctive alkali-feldspar-rich protolith contributed to basin fill, the dominant compositional trends in the Bradbury group are consistent with sorting of detrital minerals during transport from relatively homogeneous plagioclase-phyric basalts.

^1,2Rebecca J. Smith,²Briony H. N. Horgan,³Paul Mann,³Edward A. Cloutis,⁴Philip R. Christensen
Journal of Geophysical Research Planets (in Press) Link to Article [DOI: 10.1002/2016JE005112]
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
³Department of Geography, University of Winnipeg, Winnipeg, Manitoba, Canada
⁴School of Earth and Space Exploration, Arizona State University, Tempe, AZ, USA
Published by arrangement with John Wiley & Sons

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 (SEM) 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.

¹Briony H. N. Horgan,²Rebecca J. Smith,³Edward A. Cloutis,³Paul Mann,⁴Philip R. Christensen
Journal of Geophysical Research Planets (in Press) Link to Article [DOI: 10.1002/2016JE005111]
¹Department of Earth, Atmospheric, and Planetary Sciences, Purdue University, West Lafayette, IN, USA
²School of Earth and Space Exploration, Arizona State University, Tempe, AZ, USA
³Department of Geography, University of Winnipeg, Winnipeg, Manitoba, Canada
⁴School of Earth and Space Exploration, Arizona State University, Tempe, AZ, USA
Published by arrangement with John Wiley & Sons

Acid leached rinds and coatings occur in volcanic environments on Earth and have been identified using orbital spectroscopy on Mars, but their development is poorly understood. We simulated long-term open-system acid weathering in a laboratory by repeatedly rinsing and submerging crystalline and glassy basalts in pH ~ 1 and pH ~ 3 acidic solutions for 213 days, and compared their visible/near-infrared (0.3-2.5 µm) and thermal-infrared (5-50 µm) spectral characteristics to their microscopic physical and chemical properties from scanning electron microscopy (SEM). We find that while alteration at moderately low pH (~3) can produce mineral precipitates from solution, it has very little spectral or physical effect on the underlying parent material. In contrast, alteration at very low pH (~1) results in clear silica spectral signatures for all crystalline samples while glasses exhibit strong blue concave up near-infrared slopes. SEM indicates that these spectral differences correspond to different modes of alteration. In glass, alteration occurs only at the surface and produces a silica-enriched leached rind, while in more crystalline samples, alteration penetrates the interior to cause dissolution and replacement by silica. We confirm that glass is more stable than crystalline basalt under long-term acidic leaching, suggesting that glass could be enriched and common in terrains on Mars that have been exposed to acid weathering. Leached glasses are consistent with both OMEGA and TES spectra of the martian northern lowlands, and may contribute to the high-silica phases detected globally in TES Surface Type 2. Thus, both glass-rich deposits and acidic weathering may have been widespread on Mars.