Preservation of organic compounds and constraints on diagenetic processes by reactive iron phases on Mars

Over 20% of organic carbon in sediments on Earth are bound to reactive Fe mineral phases [1]. These reactive Fe phases are generally Fe (oxyhydr)oxides, often associated with clay minerals. It is important to note that they occur as nanoparticulate and X-ray amorphous phases that are challenging to identify. On Earth, proxy methods such as chemical sequential extractions are often used but they can produce misleading results when used for mineral identification [2,3]. We develop and use Mössbauer spectroscopy applications to identify these phase [2-4] and compare these to Raman spectroscopy because the Mars 2020 Perseverance rover and the ExoMars 2022 Rosalind Franklin rover use Raman spectrometers for mineralogical identification.Reactive Fe phases are abundant on Mars. It is important to note that they are not the well-crystalline expression of Fe (oxyhydr)oxides such as hematite and goethite that have been observed from orbit and with a variety of rover-based instruments. Instead, reactive Fe phases are represented by as yet unidentified Fe phases: Aqueously altered rocks and soils in Gusev crater and at Meridiani Planum (including the Burns formation) contain large amounts of nanophase iron oxides (npOx and Fe3D3) [5]; and 20-60 wt% of minerals in fluvio-lacustrine deposits in Gale crater are X-ray amorphous and this amorphous phase is rich in iron [6]. Mineralogical interpretation of CRISM data of Rosalind Franklin's landing site at Oxia Planum also suggest the presence of these phase. These reactive Fe phases can be any combination of a number of minerals including ferrihydrite, lepidocrocite, akaganèite, hissingerite, schwertmannite, and superparamagnetic (i.e. nanoparticulate) hematite and goethite [5].The preservation of organic compounds by reactive Fe species is effective over hundreds of thousands of years in Earth sediments [1]. In return, organic compounds slow down the transformation of reactive Fe species such as ferrihydrite into the more crystalline and thermodynamically stable Fe (oxyhydr)oxides hematite or goethite during diagenetic processes. With temperature and pressure rising further during diagenesis, however, organic compounds are oxidized and destroyed through the reduction of Fe (resulting in the diagenetic formation of the Fe carbonate siderite, for example), and the non-reduced Fe species are transformed into thermodynamically stable minerals. Thus, the presence of reactive Fe species in Martian sediments/sedimentary rocks indicates only little diagenetic overprinting and therefore a high preservation potential of organic compounds. Such samples will be of high priority for analysis with MOMA. However, the presence of Fe species during pyrolysis can reduce the detectability of certain organic compounds. This effect depends on the specific Fe species present and is mitigated in the presence of clay minerals [7,8].We will present Mössbauer and Raman spectrocopy investigations of reactive Fe phases in various sedimenatry settings and compare these results into the context of rover landing sites on Mars.References:[1] Lalonde et al (2012) Nature 483, 198-200. [2] Schröder et al (2016) Hyperfine Interact 237, 85. [3] Hebpburn et al (2020) Chem Geol 543, 119584. [4] Klingelhöfer et al (2003) J Geophys Res 108(E12), 8067. [5] Morris et al (2019) in Remote Compositional Analysis: Techniques for Understanding Spectroscopy, Mineralogy, and Geochemistry of Planetary Surfaces, pp. 538-554, Cambridge University Press. [6] Rampe et al (2017) Earth Planet Sci Lett 471, 172–185. [7] Tan et al (2021) Astrobiology 21, 199-218. [8] Royle et al (2021) Astrobiology in press.