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

Farley, K. A. and Martin, P. and Archer, P. D. and Atreya, S. K. and Conrad, P. G. and Eigenbrode, J. L. and Fairén, A. G. and Franz, H. B. and Freissinet, C. and Glavin, D. P. and Mahaffy, P. R. and Malespin, C. and Ming, D. W. and Navarro-González, R. and Sutter, B. (2016) Light and variable ^(37)Cl/^(35)Cl ratios in rocks from Gale Crater, Mars: Possible signature of perchlorate. Earth and Planetary Science Letters, 438 . pp. 14-24. ISSN 0012-821X. https://resolver.caltech.edu/CaltechAUTHORS:20160216-085105769

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Abstract

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±25‰ to −51±5‰. Five analyses from two samples of the Sheepbed mudstone (Yellowknife Bay study area) are analytically indistinguishable with a mean δ^(37)Cl of −11±7‰ (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±6‰. The Rocknest sand deposit gave a highly uncertain δ^(37)Cl value of −7±44‰. 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 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‰ 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 δ^(37)Cl 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.


Item Type:Article
Related URLs:
URLURL TypeDescription
http://dx.doi.org/10.1016/j.epsl.2015.12.013DOIArticle
http://www.sciencedirect.com/science/article/pii/S0012821X15007761PublisherArticle
ORCID:
AuthorORCID
Farley, K. A.0000-0002-7846-7546
Atreya, S. K.0000-0002-1972-1815
Eigenbrode, J. L.0000-0003-3089-1986
Glavin, D. P.0000-0001-7779-7765
Mahaffy, P. R.0000-0003-1896-1726
Malespin, C.0000-0002-5852-5016
Ming, D. W.0000-0003-0567-8876
Sutter, B.0000-0002-3036-170X
Alternate Title:Light and variable 37Cl/35Cl ratios in rocks from Gale Crater, Mars: Possible signature of perchlorate
Additional Information:© 2015 Elsevier B.V. Received 8 August 2015; Received in revised form 13 December 2015; Accepted 16 December 2015; Available online 22 January 2016. This work was supported by the National Aeronautics and Space Administration. This work benefited from several anonymous re-views.
Funders:
Funding AgencyGrant Number
NASAUNSPECIFIED
Subject Keywords:Mars; Cl isotopes; perchlorate; evolved gas analysis
Record Number:CaltechAUTHORS:20160216-085105769
Persistent URL:https://resolver.caltech.edu/CaltechAUTHORS:20160216-085105769
Official Citation: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, Light and variable 37Cl/35Cl ratios in rocks from Gale Crater, Mars: Possible signature of perchlorate, Earth and Planetary Science Letters, Volume 438, 15 March 2016, Pages 14-24, ISSN 0012-821X, http://dx.doi.org/10.1016/j.epsl.2015.12.013. (http://www.sciencedirect.com/science/article/pii/S0012821X15007761)
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:64489
Collection:CaltechAUTHORS
Deposited By: Tony Diaz
Deposited On:17 Feb 2016 22:02
Last Modified:09 Mar 2020 13:19

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