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A possible hydrothermal origin of Martian phyllosilicates: Exploring the talc/saponite-nontrinte system

Final Report Summary - HYDRO-MARS (A possible hydrothermal origin of Martian phyllosilicates: Exploring the talc / saponite-nontrinte system)

The overarching objective of the HYDROMARS project was to test the hypothesis that clay minerals on Mars formed through hydrothermal processes through a rigorous spectroscopic and mineralogical investigation of hydrothermally formed clays on Earth and the application to infrared remote sensing data of Mars. In order to achieve this goal, the plan was to investigate the detailed mineralogy and crystal chemistry of approximately 20 - 30 clay samples collected from the sea floor because this environment contains a range of clay compositions over short spatial distances, all of which are relatively rich in Fe and Mg and therefore of relevance to Fe-Mg-clays on Mars of varied spectroscopic character.

Since the beginning of the project, we have ultimately analysed approximately 70 samples of clays collected from environments such as the Atlantis II Deep Basin, the Grimsely Graben (south of Iceland), the East Pacific Rise, Gulf of California, and various continental environments (for comparison to submarine clays). Through an initial phase of basic characterisation, this group of samples was down-selected to a group of 35 clay-rich samples. We also supplemented this sample group with a group of 21 Al-rich (kaolinite and montmorillonite-bearing) clays in order expand our study to address the geology of aluminous deposits on Mars. Fifty six samples have now been investigated using 10 different techniques.

The techniques employed have included several types of x-ray analysis (including theoretical modelling of X-ray data), Mossbauer spectroscopy, infrared transmission spectroscopy, visible / near-infrared spectroscopy (VNIR), geochemical analyses, electron microscopy, thermal gravimetry (TG), evolved gas analysis (EGA) and isotope geochemistry.

The results show interesting trends in the infrared character of Fe-Mg-rich and Al-rich smectitic clays related to variation in the structures and crystal-chemical properties of the materials. These results have led to major conclusions in two main areas:

1) We have gained new insight into the fundamental clay mineralogy of sea floor clays and the nature of mixed-layering in the Fe-Mg system ('mixed layering' is a scenario where different types of clays grow together at nanometer scale).
2) We have provided new insights into the nature of clays on Mars, which argues for hydrothermal processes in some deposits.

One of the papers already published includes the first strong spectral evidence for mixed-layer clays on Mars, in particular kaolinite-smectite mixed layer clays. Through detailed analyses, we were able to show that physically mixed kaolinite and smectite have a different spectroscopic character from that of mixed layer kaolinite-smectite. We created spectroscopic indices that can be used to distinguish between the two and applied those indices to infrared data returned from Mars. The paper, which was published in Icarus in 2012 identifies deposits on Mars that contain mixed-layer kaolinite smectite, which in certain deposits, appear to have formed in a hydrothermal context.

Another paper that has been published is focused on the formation of Fe-Mg clays from Fe-rich groundwater on Mars in environments that were likely habitable to simple microorganisms in the Martian subsurface. We compared spectra from our laboratory studies of clays to infrared data of clays on Mars that likely formed in an alkaline lake setting where groundwater would have been the fluid source. Using these data, we were able to demonstrate the presence of Fe-rich clays that likely formed from emergent alkaline, Fe-rich fluids. This paper, published in Nature Geoscience in 2013, reinvigorates arguments that hydrothermal environments in the Martian subsurface should be the top candidates for future exobiological exploration activities.

Two additional papers are in preparation now. The first reports fundamental breakthroughs in our understanding of the nature of mixed layering and crystal chemical variation in seafloor clays. We show that glauconitic clays, which are commonly thought to form in pelagic settings, are in fact a key component of hydrothermal assemblages in sea floor settings. In addition, we recognize a number of mixed layer dioctahedral-trioctahedral clays (Di-Tri clays). Di-Tri clays are special because they represent mixed layering between two end-member types of clays - those that are dioctahedral (Di) and contain trivalent cations (Al3+ or Fe3+) and those that are trioctahedral (Tri) and contain divalent cations (Mg2+ and Fe2+). We find that Di-Tri clays are common in hydrothermal settings, but are not commonly recognized because of their structural complexities. Lastly, we are able to determine how the chemistry and ordering of these clays is related to water content and the nature of hydroxyl ions in the structure. A manuscript reporting these results and others will be submitted in late spring of 2013.

The next manuscript will focus on the infrared properties of the Fe-Mg clays and how spectral characteristics are related to cation substitution, mixed layering and order / disorder. We will then apply the results to make a comparison with infrared data of well-known Martian clay deposits in order to establish trends in the spectra that are related to hydrothermal processes.

The final results of this project will represent a major breakthrough in the understanding of the aqueous history of Mars as evidenced by the presence of ancient clay minerals in the Martian crust. The new results from this project will fundamentally change our interpretation of the mineralogy of clays identified previously on Mars from infrared data, and will enable a renewed evaluation of the past habitability of the red planet.