Nanoscale Variations In Natural Amorphous And Nanocrystalline Weathering Products In Mafic To Intermediate Volcanic Terrains On Earth: Implications For Amorphous Detections On Mars

Sedimentary amorphous and nanocrystalline materials are abundant on Mars, but their formation conditions are poorly understood due in part to instrument limitations and the lack of knowledge about terrestrial sedimentary amorphous and nanocrystalline material compositions and structures. Additionally, it is surprising that the Curiosity rover finds these metastable phases preserved in ancient (3-4 Gya) rocks in Gale crater. Here, we use X-ray diffraction and high-resolution transmission electron microscopy to investigate the X-ray amorphous weathering products found in soils, sediments, and paleosols from Mars-relevant mafic to intermediate volcanic terrains altered under different climates. The X-ray amorphous components of our samples are often complex mixtures of amorphous and nanocrystalline materials with compositions that vary at the nanometer-scale, but are dominated by iron-oxide, Al2O3, and SiO2, the proportions of which reflect large-scale weathering conditions, such that those formed in warm and wet climates are deficient in more mobile cations, and those formed under cooler climates reflect restricted cation leaching. We also find high abundances (similar to 40 wt.%) of X-ray amorphous weathering products in lithified and diagenetically altered volcanic soils that have been preserved over time scales of similar to 10(7) years. These results have wide-reaching implications for soil property and paleoclimate investigations on Earth and Mars, and suggest that amorphous and nanocrystalline materials are important, but overlooked components of some sedimentary rocks on Earth. Future investigations are needed to better understand the kinetics behind the transformation from amorphous and nanocrystalline to crystalline materials and the factors that allow their preservation on Earth and Mars.Plain Language Summary Amorphous and nanocrystalline weathering products are abundant across the surface of Mars. Like crystalline weathering products, amorphous and nanocrystalline weathering products can help inform about ancient weathering environments and conditions. Unfortunately, even on Earth, little is known about how formation conditions affect the chemical compositions and atomic structures of amorphous and nanocrystalline weathering products. Additionally, on Earth amorphous and nanocrystalline weathering products are thought to quickly convert to more stable crystalline minerals, and so it is surprising to find abundant amorphous and nanocrystalline materials in 3-4 billion year old rocks on Mars. This study attempts to address these knowledge gaps by examining the abundance, composition, and structure of amorphous and nanocrystalline weathering products in Mars analog soils, sediments, and 30-50 million year old sedimentary rocks from different weathering environments. We find that the compositions of the amorphous and nanocrystalline weathering products reflect large-scale weathering conditions, and that amorphous and nanocrystalline weathering products are preserved in high abundances in our similar to 10(7) year old sedimentary rocks. These findings indicate that amorphous and nanocrystalline weathering products can be used to help constrain past environments on Mars, and suggest that amorphous and nanocrystalline weathering products are overlooked components of some sedimentary rocks on Earth.