Nanoscale Analyses of X-ray Amorphous Material from Terrestrial
Ultramafic Soils Record Signatures of Environmental Conditions Useful
for Interpreting Past Martian Conditions
Abstract
X-ray amorphous material that is variably Mg/Fe/Si-rich and Al-poor and
that likely contains secondary alteration products is prevalent in Gale
crater sediments and rocks (15-73 wt.%). However, the structure and
origin of these materials and their implications for past environmental
conditions remain unknown. In this study, we use transmission electron
microscopy and synchrotron microprobe analyses to examine Mg/Fe/Si-rich
and Al-poor ultramafic soils from the warm Mediterranean climate Klamath
Mountains of California and cold subarctic climate Tablelands of
Newfoundland, Canada to help interpret environmental conditions during
the formation of chemically similar X-ray amorphous material in Gale
crater, Mars. Primary glass is absent from the Klamath Mountains and
Tablelands materials; secondary X-ray amorphous material includes
globular amorphous silica and chemically heterogeneous nanospherical
amorphous material and nanocrystalline phases. Globular amorphous silica
is only present in soils that undergo extensive periods of cyclic
freezing. Fe-containing X-ray amorphous material from the subarctic
Tablelands is significantly richer in Mg and Si than X-ray amorphous
material from the warmer Klamath Mountains. Fe-rich nanocrystallites
contain more Mg and Si in the subarctic Tablelands but are more highly
Fe-enriched in the warmer Klamath Mountains. Potential secondary
nanocrystalline phyllosilicates are only observed in the warmest
examined soil in the Klamath Mountains. These characteristics – the
presence or absence of amorphous silica, the chemical composition of
X-ray amorphous material, the abundance and composition of Fe-rich
nanocrystallites, and the presence or absence of secondary
phyllosilicates - provide helpful identifiers to interpret past
environmental conditions during the formation of X-ray amorphous
material on Mars.