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

• Main Authors: K A Farley, P Martin, P D Archer Jr, S K Atreya, P G Conrad, J L Eigenbrode, A G Fairén, H B Franz, C Freissinet, D P Glavin, P R Mahaffy, C Malespin, D W Ming, R Navarro-Gonzalez, B Sutter
• Other Authors: The Pennsylvania State University CiteSeerX Archives
• Format: Text
• Language: English
• Published: 2016
• Subjects: Yellowknife; Yellowknife Bay
• Online Access: http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.1078.9706; http://www-personal.umich.edu/%7Eatreya/Articles/2016EPSL_Mars_Cl_Isotopes_Farley_etal.pdf

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 δ 37 Cl values ranging from −1 ± 25h to −51 ± 5h. Five analyses from two samples of the Sheepbed mudstone (Yellowknife Bay study area) are analytically indistinguishable with a mean δ 37 Cl of −11 ± 7h (1σ ). In contrast, four mudstones/sandstones from the Kimberley and Pahrump study areas also yielded indistinguishable ratios, but with a mean δ 37 Cl of −43 ± 6h. The Rocknest sand deposit gave a highly uncertain δ 37 Cl value of −7 ± 44h. These light and highly variable δ 37 Cl 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 δ 37 Cl 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 δ 37 Cl 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 37 Cl 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 ...