| dcterms.abstract | Trace elements (Ni, Zn, and Cr) and halogens (Br and Cl) can be used as powerful tracers for evaluating Martian surficial processes, and are broadly relevant to intriguing questions such as aqueous chemistry and history, brine and mineral stability, and potential habitability of the Red Planet. Large volumes of geochemical data recently have become available for Ni, Zn, Cr, Br and Cl from Mars in the form of SNC meteorite analyses and in situ measurements by rovers, and, in the case of Cl, from orbital gamma-ray spectroscopy. In order to best interpret these geochemical data or utilize them to test models relevant to understanding the surface of Mars, a variety of simulation experiments are important to gain better understanding of the fundamental geochemistry of these elements. In this study, a series of laboratory simulation experiments were conducted to investigate the behavior of trace elements (Ni, Zn, and Cr) and halogens (Br and Cl) during (1) post-depositional diagenetic oxidation processes of ferrous sulfate to ferric oxide and (2) photochemical related processes during evaporation of halide saline systems. A model of post-depositional oxidation of ferrous sulfate (melanterite; FeSO4*7H2O) to ferric oxide (hematite; α -Fe2O3) via two major pathways is well-established as a possible diagenetic process affecting certain Martian sedimentary systems: (1) via schwertmannite (Fe8O8(OH)6(SO4)) and/or goethite (α -FeOOH) intermediaries; (2) via a jarosite ((H3O,K)Fe3(OH)6(SO4)2) intermediary. For initial solutions with equal concentrations of Cr3+, Ni2+ and Zn2+, uptake of Cr3+ by the precipitates was orders of magnitude greater than Ni2+ and Zn2+ for both pathways, due to probable substitution of Cr3+ for Fe3+ in mineral structures, in contrast to Ni2+ and Zn2+, which were mainly adsorbed onto mineral surfaces. Preferred uptake of Ni2+ and Zn2+ by schwertmannite and goethite was in the order Ni2+ > Zn2+, but was reversed in the case of jarosite (Zn2+ > Ni2+). By comparing these experimental results to in-situ rover measurements at Meridiani Planum, important constraints on the diagenetic pathways that produced the hematitic spherules (interpreted as sedimentary concretions) are obtained. The observed high Ni (≥ 2,000 ppm) content and high Ni/Zn mass ratios (2~4) in the hematitic spherule-bearing samples of the Burns formation, relative to Martian crust (Ni = 337 ppm and mass Ni/Zn = 1.05), can be readily explained by the evolution pathway involving goethite. Other diagenetic pathways, such as via jarosite, could be valid only with an additional source of Ni (e.g., meteoritic or especially high-Ni basalt provenance). The anions Br- and Cl-, present in such diagenetic solutions, also demonstrate different partitioning behavior via different mineralogical evolution pathways. For solutions with initial molar Cl-/Br- > 1, jarosite incorporated at least an order of magnitude more Br than Cl and greatly enriched Br over Cl in its structure. Such preferential Br-enrichment was not found in goethite. Incorporation of large amounts of Br- would greatly decrease jarosite stability during aqueous alteration. Accordingly, it is concluded that jarosite could be a plausible candidate holding elevated Br and fractionated Br/Cl ratios at the Martian surface. The changes in decomposition rates of jarosite, observed when incorporating halide anions, should be considered when attempting to interpret the aqueous history of Meridiani Planum by using jarosite as a " stopwatch" . Based on detailed statistical examination of Cl, Br, and S distributions of Martian soil profiles at Gusev Crater and Meridiani Planum, it is suggested that photochemical processes could play an important role in controlling halogen behavior at the Martian surface. Laboratory experiments demonstrate that photo-oxidation (ultraviolet λ = 254 nm) of evaporative saline systems containing Br- and Cl- are able of producing perchlorate and chlorate at up to ~2% levels of total chlorine under conditions relevant to Mars. In addition, significant Br/Cl fractionation was present in the resulting evaporites due to preferential volatilization of Br over Cl into the atmosphere. Experimental evaluation of variables such as brine compositions, pH, sediment grain size, atmospheric conditions, and length of UV irradiation indicate an efficient heterogeneous reaction pathway, rather than commonly proposed atmospheric gas-gas reactions, for perchlorate and chlorate to form. Bromine is strongly influenced by photochemical processes and at high abundance is capable of competing with Cl for available oxidants, reacting with Cl radicals, forming oxy-bromine species, and cycling at the Martian surface. Consequently, such processes could cause substantial local variations in Br concentrations, which is consistent with the highly variable Br abundances that have been detected on the Martian surface. | |
