AMORPHOUS MATERIALS FORMED BY RAPID EVAPORATION OF BASALT WEATHERING SOLUTIONS: IMPLICATIONS FOR AMAZONIAN WEATHERING ON MARS

The current Martian surface environment is cold and relatively dry, with regionally, seasonally and diurnally variable atmospheric water vapor pressure. Similar conditions likely persisted through most of the Amazonian. Weathering under these conditions might be driven by small amounts of transient, circum-neutral pH liquid water (e.g. frost melt) interacting with the Martian surface. Aqueous solutions resulting from weathering would rapidly evaporate or freeze under these conditions, potentially forming amorphous materials since little time is allowed to develop well-crystalline precipitates. However, this process is under-constrained and poorly understood. Therefore, we are performing laboratory weathering experiments to better understand the types of weathering products that might form under Amazonian conditions on Mars.

We experimentally weathered basaltic tephras of different compositions in neutral pH water for 10 days to create weathering solutions. The solutions were separated from the weathered tephra, and were then rapidly evaporated (~0.01-0.1 mL/min) onto substrates to collect precipitates for further analysis. Precipitates were characterized using visible/near-infrared (VNIR) and mid-infrared (MIR) spectroscopy and X-ray diffraction (XRD).

VNIR spectra of precipitates have a smooth ferric absorption edge from 0.5 to 1.0 µm indicative of poorly crystalline ferric oxides. Hydration features at 1.4 and 1.9 µm, and a metal-OH absorption at 2.3 µm are consistent with a Mg/Fe phyllosilicate. However, the broad absorption features between 800 - 1150 cm^-1 in MIR spectra indicate that the precipitates are amorphous or poorly crystalline, which is supported by preliminary XRD results. Precipitates formed from different tephra starting compositions have distinct MIR spectra—one having allophane-like characteristics the other mimicking basaltic glass—indicating that solution chemistry strongly influences the precipitate mineralogy. Combined, these datasets indicate that low-temperature, neutral-pH weathering with rapid evaporation can produce poorly-crystalline to amorphous aluminosilicate phases whose mineralogy can vary depending on initial basalt composition. Thus, we might also expect the mineralogy of amorphous materials on Mars to vary regionally.