Acid weathering of basalt and basaltic glass: I. Near-infrared spectra, thermal-infrared spectra, and implications for Mars

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.

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