Light and variable 37Cl/35Cl ratios in rocks from Gale Crater, Mars: Possible signature of perchlorate

Cl isotope ratios measured on HCl thermally evolved from as-yet-unknown phases in sedimentary rocks and sand in Gale Crater provide unexpected insights to the Martian surficial Cl cycle. The seven samples yield delta Cl-37 values ranging from -1 +/- 25 parts per thousand to -51 +/- 5 parts per thousand. Five analyses from two samples of the Sheepbed mudstone (Yellowknife Bay study area) are analytically indistinguishable with a mean delta Cl-37 of -11 +/- 7 parts per thousand (1 sigma). In contrast, four mudstones/sandstones from the Kimberley and Pahrump study areas also yielded indistinguishable ratios, but with a mean delta Cl-37 of -43 +/- 6 parts per thousand. The Rocknest sand deposit gave a highly uncertain delta Cl-37 value of -7 +/- 44 parts per thousand. These light and highly variable delta Cl-37 values are unique among known solar system materials. Two endmember models are offered to account for these observations, and in both, perchlorate, with its extreme ability to fractionate Cl isotopes, is critical. In the first model, SAM is detecting HCl from an oxychlorine compound (e.g., perchlorate) produced from volcanic gas emissions by atmospheric chemical reactions. Similar reactions in Earth's atmosphere may be responsible for the isotopically lightest known Cl outside of this study, in perchlorate from the Atacama Desert. Some of the Gale Crater delta Cl-37 values are more negative than those in Atacama perchlorate, but because reaction mechanisms and associated fractionation factors are unknown, it is impossible to assess whether this difference is prohibitive. If the negative delta Cl-37 signal is produced in this fashion, the isotopic variability among samples could arise either from variations in the relative size of the reactant chloride and product perchlorate reservoirs, or from variations in the fraction of perchlorate reduced back to chloride after deposition. Such reduction strongly enriches Cl-37 in the residual perchlorate. Perchlorate reduction alone offers an alternative endmember model that can explain the observed data if SAM measured HCl derived from chloride. In this model isotopically normal perchlorate produced by an unspecified mechanism is reduced to chloride. Depending on the relative size of the reduced reservoir, the integrated product chloride can vary in isotopic composition from -70 parts per thousand in the first increment all the way to the starting composition if the perchlorate is fully reduced. Thus, variable degrees of perchlorate reduction can produce chloride with the appropriate delta Cl-37 range. Combination of the two endmember models, in which the perchlorate subject to post-deposition reduction is isotopically negative from atmospheric reactions, is also possible. Determination of the phase hosting the Cl measured by SAM, an oxychlorine compound or chloride, is critical for selecting between these models, and for developing implications of the results for the Mars surficial Cl cycle. At present it is not possible to conclusively establish which phase is responsible (possibly both), but limited evidence favors the conclusion that the measured Cl derives mostly from an oxychlorine compound. (C) 2015 Elsevier B.V. All rights reserved.