Geochemical Trends in the Burns Formation Layered Sulfate Deposits at Meridiani Planum, Mars, and Implications for Their Origin. Issue 9 (22nd September 2018) - McCollom, Thomas M.
Abstract: The layered deposits that comprise the Burns formation at Meridiani Planum have a chemical composition that closely resembles Martian basalts with addition of an oxidized sulfur component. Alternative scenarios proposed to explain this composition differ over whether the sulfur component was added by itself ( sulfur only ) or was accompanied by transport of other elements, primarily Mg, Fe, and Ca ( sulfur‐plus‐cations ). Analysis of trends in elemental abundance relative to SO3 and TiO2 indicates that many elements, including Si, Al, K, Na, P, Cr, and Ti, were essentially immobile during the addition of the sulfur component and throughout all subsequent diagenetic processes. Conversely, the divalent cations (Fe, Mg, Ca, Mn, and Zn) were mobilized during diagenesis. This mobilization obscures any trends in composition that were established prior to diagenesis, making it difficult to determine whether these elements were transported into the materials along with sulfur or not. Mass balance models indicate that the current compositions can plausibly be accounted for by either sulfur‐only or sulfur‐plus‐cations scenarios. However, the sulfur‐plus‐cations scenario requires that the bedrocks had a much more complex geochemical history in order to arrive at their present chemical composition, including highly variable compositions prior to diagenetic alteration and extensive mobilization of elements during diagenesis. Conversely, sulfur‐only scenarios are consistent with the evidence for immobility of many elements and require only limited mobilization of cations during diagenesis. Overall, sulfur‐only scenarios provide a much simpler and more straightforward means of accounting for the chemical composition of the Burns formation than scenarios that involve cation mobility. Plain Language Summary: The bedrocks at the landing site of the Opportunity rover on Meridiani Panum, Mars, are layered sandstones composed of a mixture of sulfate mineral and altered basalt components. Several different scenarios have been proposed to explain the chemical composition of the rocks, each of which implies very different processes and environmental conditions during formation. One key difference among these scenarios is whether the sulfate minerals initially formed through addition of sulfuric acid alone or whether other chemical elements were transported into the deposits along with dissolved sulfate by evaporation of infiltrating fluids. Here detailed analysis of the chemical compositions indicates that most chemical elements were immobile throughout the formation of the rocks, while others were mobilized by secondary diagenetic processes after the sulfate‐bearing sediments were already deposited at their current location. For the latter elements, secondary mobilization obscures evidence concerning whether or not they were transported into the deposits together with sulfate. The chemistry of the rocks can be plausibly explained with or without the addition of other elements along with sulfate, but addition of sulfate alone without transporting other elements into the deposits provides the simplest explanation for their chemical composition. Key Points: Geochemical trends indicate that several elements were immobile throughout the formation of the deposits including Si, Al, K, Na, P, Cr, and Ti Divalent cations including Mg, Fe, and Ca were mobilized during diagenesis, obscuring trends from earlier stages in the deposits' history Addition of sulfate to the deposits without mobilization of other elements offers the simplest explanation for their chemical composition