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Characterizing the Mechanisms for the Preservation of Organics at the Painted Desert: Lessons for Msl, Exomars, and Mars 2020

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Characterizing the Mechanisms for the Preservation of Organics at the Painted Desert: Lessons for Msl, Exomars, and Mars 2020 47th Lunar and Planetary Science Conference (2016) 2796.pdf CHARACTERIZING THE MECHANISMS FOR THE PRESERVATION OF ORGANICS AT THE PAINTED DESERT: LESSONS FOR MSL, EXOMARS, AND MARS 2020. E.Z. Noe Dobrea1,2, A.C. McAdam3, C. Freissinet3, H. Franz3, I. Belmahdi3, M.R. Hamersley4, C.R. Stoker2, B. Parker5, K. Ja Kim6, D.P. Glavin3, F. Calef7, and A.D. Aubrey7. 1Planetary Science Institute ([email protected]), 2NASA/ARC, 3NASA/GSFC, 4 SOKA University, 5Petrified Forest National Park, 6Korea Inst. of Geoscience and Mineral Resources, 7NASA/JPL. Introduction: The Painted Desert of northern peared to be present wherever enhanced organic con- Arizona forms part of the ca. 215 My old Triassic centrations were found. Chinle formation, and consists of thick (ca. 100 m) In addition to the in-situ identification of jasorite layers of colorful bentonitic mudstones interbeded with in organic-rich units, we also detected jarosite in re- sandstones and limestone formed by fluvial and lacus- mote sensing hyperspectral imagery of the region using trine deposition [1]. Trapped in some of these layers is AVIRIS. The jarosite was found to be correlated to organic carbon derived from Late Triassic flora and dark-toned layers of the Purple Grey Formation (PGF). fauna that were rapidly buried in the prograding beds In this follow-up study, we sought to obtain a and floodplains of the fluvio-lacustrine system present more complete sampling of the units exposed at the at the time [2,3]. PFNP to better establish the correlation between or- The spectral and morphological character of the ganic preservation and jarosite. We measure the con- Painted Desert appear analogous in many ways to that centration of organics in different layers of the Chinle of the Al-phyllosilicate bearing units identified on the formation using techniques available to MSL in order plains units at Mawrth Vallis and Western Arabia to establish a framework for the interpretation of MSL Terra, Mars. Both regions present evidence for fluvial observations. Here, we report on findings from our activity and have thick Al-phyllosilicate sequences latest field activity, performed in the spring of 2015. containing Al-smectites, kaolins, hydrated silica, and Field Work and sample collection: Fieldwork jarosite [4-8]. These similarities make the Painted De- was performed at the PFNP with the intent of sampling sert a potential geological and compositional martian many of the members exposed within the park’s analog. Here, we are interested in understanding how boundaries. We targeted sequences of dark/light the chemistry and textural charater of these units may bentonite as well as units where jarosite had been iden- play a role in the preservation of organic compounds. tified in aerial hyperspectral imagery. Organic compounds show high affinities to clay Field work focused on site characterization and minerals, where interlayer binding acts as a sequestra- sample collection. Site characterization involved a tion mechanism for the sorption of organics onto phyl- study of the rock textures, grain size, cementation, and losilicates. Amino acids in particular demonstrate ex- mineralogy for each facies, with emphasis on compari- tremely strong sorptive properties with clay minerals son of the units that preserve organics to the units [9,10]. Additionally, low O2 concentrations at the time where organics have been lost due to silicification. of burial are conducive to preservation over long Where possible, we identified sedimentary and diage- timescales [11,12,13]. Smectites from the Painted De- netic features. Grain size distribution was assessed to sert experienced rapid burial after deposition, and O2 constrain mode of emplacement. concentrations are thought to have been low. Hence, Organics-clean sample collection was performed the preservation in these layers is expected to be high. using protocols developed during a previous field ex- Past work: A past pilot study, performed at the pedition. To minimize organic contamination, the glass Petrified Forest National Park (PFNP), focused on vials and all materials used to store and handle the identifying correlations between organic content and samples were previously pyrolyzed at 500 ºC over- rock-type and tonality of the host rock. The PFNP is night. Samples were collected from 30 cm within out- found in the Painted Desert and its members belong to crops exposed on steep-sided slopes (>60º) to reduce the Chinle Formation. Analyses of samples showed the chance of modern contamination by downward that the preservation of organic compounds varied water percolation and root penetration from plants. strongy from layer to layer [14,15]. Although some of Nitrile gloves were worn and replaced often and sam- the dark mudstone layers of the Blue Mesa Member pling instruments were rinsed in acetone and wiped (BMM) did show a much stronger propensity for pres- down with clean fiberglass wipes to remove organics ervation of organics than the overlying sandstones of and any adhered material from previous sites. Pyrol- the Petrified Forest Member (PFM), other equivalently ized silica powder was brought to the field and taken dark mudstones in the BMM did not exhibit any en- through as many of the sample processing steps as hancement in organics beyond those found in the PFM possible to serve as a procedural blank. sandstones. Correlations between organic content and Sample analysis: mudstone tonality were also found to be weak. How- Composition and chemistry: Mineralogy was as- ever, it was noted during the study that jarosite ap- sessed using VNIR spectroscopy and x-ray diffraction 47th Lunar and Planetary Science Conference (2016) 2796.pdf (XRD), and chemistry was assessed using x-ray fluo- correlation between TOC and rock tonality, suggesting rescence (XRF) techniques in order to establish a geo- that organic carbon does not necessarily play a role in chemical setting for the samples, to distinguish be- the darkening of the rocks. The highest concentrations tween different types of alteration, and to allow the of organic carbon are found in association with the development of a coherent geological interpretation. jarosite-bearing samples and the calcite nodules, irre- Total Organic Carbon, Nitrogen, and Stable Iso- spective of rock type. In particular, jarosite-rich rinds tope Measurements: Total carbon (and, following car- that encased dark, calcite-rich nodules were found to bonate acidification, organic carbon), nitrogen content be rich in organics relative to the surrounding sand- and stable isotopic composition was quantified by mi- stone or mudstone matrix. cro-Dumas combustion followed by isotope ratio mass Preliminary EGA analysis of our samples show spectrometry. Carbonates were removed by two-stage CO2, SO2, H2O evolutions, as well as peaks in EGA acidification with 2 N HCl, followed by desiccation in traces associated with organic fragments. Some of the a vacuum centrifuge. Samples were weighed into tin mudstone samples produce H2O evolutions consistent capsules for analysis. with dehydration and dehydroxylation of a smectite, EGA and GCMS analysis: SAM-like evolved gas which demonstrates the SAM EGA’s ability to help analysis mass spectrometry (EGA) analyses were per- constrain sample phyllosilicate compositions. GCMS formed using a Setaram LabSys Evo instrument cou- analysis shows that samples which evolved a lot of pled to a Pfieffer OmniStar mass spectrometer. Pow- SO2 were richer in organics variety, indicating a strong dered rock samples were loaded into cleaned sample correlation between organic preservation and sulfur cups and placed into the oven. SAM-like helium pres- minerals (e.g., sulfates). Extraction via water-propanol sures (~25 mb) and gas flow conditions (~0.8 sccm) + MTBSTFA derivatization was found to be the most were used. Evolved gases were monitored for com- efficient procedure for analysis of organics content of pounds between 2-300 AMU as samples were heated the Painted Desert samples. at 35oC/min from ~50oC to ~1000oC. The organics Discussion: Although the identification of or- content analyses were performed on a gas chromato- ganics in the ancient fluvial sediments of the PFNP is graph coupled to a mass spectrometer (GCMS), by not particularly surprising, the apparent lack of correla- direct pyrolysis of the sample and by extraction- tion in organic concentration and rock type is some- derivatization to reach the more complex and refrac- what perplexing, as one expects organics to be better tory organics. GCMS experiments were run using a preserved in mudstones than in sandstones. This sug- commercial pyrolysis unit coupled to the injector inlet gests that additional parameters must be taken ino ac- of a GCMS instrument. Powdered samples from the count when considering the preservation of organics. Painted Desert were heated inside a quartz boat at tem- The detection of organics in association with jarosite perature steps of 300ºC, 450ºC, 600ºC, and 800ºC un- precipitates points to a mechanism of preservation that der helium carrier gas flow. Volatile organics released is of high interest to MSL, Exomars, and Mars 2020. from the samples were pre-concentrated using the These results are especially compelling, particularly in built-in CDS pyroprobe Tenax and glass bead trap held light of the recent identification of organics by cooled to <10°C. Hydrocarbons adsorbed by the trap MSL/SAM in the jarosite-bearing Murray mudstone were then carried to the GC column inlet under He samples [16,17]. Additional analysis of the jarosite- carrier gas flow after rapid heating of the trap to rich samples will hopefully allow us to establish a 300ºC, followed by detection by electron impact quad- genesis for the sulfate as well as a mechanisms for rupole mass spectrometry. Some of the organic-rich entrapment and preservation of these ancient organics. samples were also extracted by heating directly in a References: [1] Allen (1930). Amer. J. Sci. mixture of MTBSTFA and dimethylformamide (DMF) doi:10.2475/ajs.s5-19.112.283 [2] Demko et al. (1998) prior to pyrolysis GCMS analysis to target amino ac- Geology.
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