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

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.