Evaluating the role of sulfide-weathering in the formation of sulfates or carbonates on Mars

Research areas:
Year:
2012
Authors:
Journal:
GEOCHIMICA ET COSMOCHIMICA ACTA
Volume:
90
Pages:
47-63
Month:
AUG 1
ISSN:
0016-7037
Abstract:
Orbital and landed missions to Mars have revealed complex sulfate-and
Fe-oxide-bearing mineralogical assemblages, which have been interpreted
as the result of a late planet-wide acidic period due to large SO2
emissions. However, this global scenario does not explain the recent
discovery of Hesperian and Noachian-aged carbonates in several regions,
because they should have been dissolved by the acidic conditions, nor
the restriction of sulfate occurrences in preferential regions. Here we
present the results of a 4-year-long experimental study designed to test
the impact of Fe-sulfides (pyrrhotite Fe0.9S) on the weathering of
basaltic silicates (olivine, clino- and orthopyroxene) under an
early-Martian-like, CO2-rich atmosphere. Our weathered silicate/sulfide
mixtures showed complex parageneses containing elemental sulfur,
hydrated sulfates (gypsum, hexahydrite, jarosite) and Fe-(oxy)
hydroxides. Olivine-only samples produced nesquehonite, an Mg-carbonate
precursor of hydromagnesite and magnesite. These secondary phases are
similar to those observed in the Martian sulfate- and carbonate-bearing
deposits. Therefore, we propose a geochemical model in which, on the one
hand, Martian sulfates mainly formed from weathering of sulfide-enriched
basalts producing locally acidic environments and, on the other hand,
carbonates were preserved and could even have formed in regions
initially devoid of sulfides, thereby resolving the apparent paradox
arising from the possible coeval formation of the two types of minerals.
These results raise doubts on the need for a global acidic event to
produce the sulfate-bearing assemblages, suggest that regional
sequestration of sulfate deposits is due to regional differences in
sulfide content of the bedrock, and pave the way for reevaluating the
likelihood that early sediments preserved biosignatures from the
earliest times. (C) 2012 Elsevier Ltd. All rights reserved.