Nanoscale Variations in Natural Amorphous and Nanocrystalline Weathering Products in Mafic to Intermediate Volcanic Terrains on Earth: Implications for Amorphous Detections on Mars

Abstract

AbstractSedimentary 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 (∼40 wt.%) of X‐ray amorphous weathering products in lithified and diagenetically altered volcanic soils that have been preserved over time scales of ∼107 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.