Geochemical Consequences of Widespread Clay Mineral Formation in Mars' Ancient Crust

Research areas:
Year:
2013
Authors:
  • Bethany L. Ehlmann
  • Gilles Berger
  • Nicolas Mangold
  • Joseph R. Michalski
  • David C. Catling
  • Steven W. Ruff
  • Eric Chassefiere
  • Paul B. Niles
  • Vincent Chevrier
  • Francois Poulet
Journal:
SPACE SCIENCE REVIEWS
Volume:
174
Number:
1-4
Pages:
329-364
Month:
January
ISSN:
0038-6308
Abstract:
Clays form on Earth by near-surface weathering, precipitation in water
bodies within basins, hydrothermal alteration (volcanic- or
impact-induced), diagenesis, metamorphism, and magmatic precipitation.
Diverse clay minerals have been detected from orbital investigation of
terrains on Mars and are globally distributed, indicating geographically
widespread aqueous alteration. Clay assemblages within deep
stratigraphic units in the Martian crust include Fe/Mg smectites,
chlorites and higher temperature hydrated silicates. Sedimentary clay
mineral assemblages include Fe/Mg smectites, kaolinite, and sulfate,
carbonate, and chloride salts. Stratigraphic sequences with multiple
clay-bearing units have an upper unit with Al-clays and a lower unit
with Fe/Mg-clays. The typical restriction of clay minerals to the
oldest, Noachian terrains indicates a distinctive set of processes
involving water-rock interaction that was prevalent early in Mars
history and may have profoundly influenced the evolution of Martian
geochemical systems. Current analyses of orbital data have led to the
proposition of multiple clay-formation mechanisms, varying in space and
time in their relative importance. These include near-surface
weathering, formation in ice-dominated near-surface groundwaters, and
formation by subsurface hydrothermal fluids. Near-surface, open system
formation of clays would lead to fractionation of Mars' crustal
reservoir into an altered crustal reservoir and a sedimentary reservoir,
potentially involving changes in the composition of Mars' atmosphere. In
contrast, formation of clays in the subsurface by either aqueous
alteration or magmatic cooling would result in comparatively little
geochemical fractionation or interaction of Mars' atmospheric, crustal,
and magmatic reservoirs, with the exception of long-term sequestration
of water. Formation of clays within ice would have geochemical
consequences intermediate between these endmembers. We outline the
future analyses of orbital data, in situ measurements acquired within
clay-bearing terrains, and analyses of Mars samples that are needed to
more fully elucidate the mechanisms of martian clay formation and to
determine the consequences for the geochemical evolution of the planet.