Mineralogical and Spectral (Near-Infrared) Characterization of Fe-Rich Vermiculite-Bearing Terrestrial Deposits and Constraints for Mineralogy of Oxia Planum, ExoMars 2022 Landing Site

Abstract

Oxia Planum is a Noachian plain on Mars. It was chosen as the final landing site for in situ studies by ExoMars 2022 rover. The main scientific objectives of the mission are to understand the mineralogy and aqueous evolution of ancient Mars with relevance to habitability. Oxia is covered by vast deposits of Fe,Mgphyllosilicates, but the exact nature of these deposits is not yet fully understood. We performed a survey of potential terrestrial analog rocks, and here we show combined mineralogical characterization of these rocks with their near-infrared spectral analysis. Samples from two terrestrial sites were studied: (1) vermiculitized chlorite-schists from Otago, New Zealand, which underwent an alteration process without significant oxidation; and (2) basaltic tuffs from Granby, Massachusetts, USA, with Fe-rich clays filling amygdales of supposedly hydrothermal origin. Both analogues are incorporated into the newly built Planetary Terrestrial Analogue Library (PTAL) collection. Oxia bedrock clay-rich deposits are spectrally matched best by a well-crystallized trioctahedral vermiculite/saponite mixture from the basaltic tuff, although the contribution of saponite must be minor. Otago vermiculite is a good analogue to Oxia vermiculite in terms of overall mineralogy and Fe content. However, spectral inconsistencies related to the Al content in the Otago clays indicate that illitization of vermiculite, which results from postalteration oxidation, did not occur at Oxia. This implies limited water/rock interactions and reducing conditions during deposition of sediments now constituting the bedrock at Oxia. Whereas the spectral match does not conclusively imply the mineralogy, trioctahedral vermiculite should be considered a likely mineral component of the bedrock unit at Oxia Planum. Vermiculite has great potential to store organic matter, and the postdeposition geological context of Oxia Planum derived from understanding of environmental conditions in analog sites is promising for organic matter preservation.

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“…Finally, chlorite-smectite (C-S) intermediates seem to be the major alteration products of the vitreous component in both studied lithotypes. While chlorite-rich C-S is clearly of hydrothermal origin (ŠEGVIĆ et al, 2023), relative intensities of the 002 reflection at ~7.1 Å and the 001 reflection at 14.1-14.3 Å of such C-S (WEAVER, 1956), are taken as evidence of chlorite vermiculitization which took place during oxidative dissolution (GU et al, 2020;KRZESIŃSKA et al, 2021), following the emplacement of the analyzed rocks. Further weathering likely led to C-S dissolution and emergence of illite and illitesmectite interlayers as documented in sample PB-1 (Table 2).…”

Spectral Analysis of Clay-bearing Outcrops in Northern Xanthe Terra, Mars: Comparison with Oxia Planum, the Landing Site for the ExoMars Rover Mission

“…These two lab spectra are available through a spectral library (i.e., Swayze et al 2018), whose samples have been verified and identified as such (see also Brossier et al 2022a). The only spectra displayed in the literature so far and available in the RELAB repository (e.g., Carter et al 2013;Krzesińska et al 2021;Mandon et al 2021) are not considered as pure, but are rather vermiculite-rich mixtures with biotite, hydrobiotite, and vermiculite components (Cloutis 2015). 4(c), for comparison purposes.…”

“…Fe-rich saponites have a very weak overtone near 2.4 μm, only recognizable after continuum removal (Figure 4(c); see, e.g., Treiman et al 2014;Chemtob et al 2015;Fox et al 2021;Noda et al 2021). Although the large variety of spectra for vermiculite ores implies diverse band depths and centers in the near-infrared spectral range (e.g., Swayze et al 2018;Krzesińska et al 2021;Cuadros et al 2022), pure vermiculite also has a 2.4 μm absorption relatively weaker than in hydrobiotite and other clay minerals (saponites and nontronites; Figure 4(c)). Although being close to noise level, this absorption is clearly identified in the FRT0605 cube (Figure 4(a)), which could favor the presence of Fe-saponite and/or pure vermiculite in Xanthe's outcrops.…”