Hydrothermal Mineral Signatures in Icelandic Outflood Deposits and the Search for Ancient Hydrothermal Systems on Mars

The discovery of hydrothermal minerals on planetary surfaces can provide clues for assessing the potential for past habitable environments. In this study we describe the nature and abundance of subglacial, hydrothermally altered material identified on the surface of catastrophic outflood plains in southern Iceland. The Icelandic sandur plains are sites of high discharge catastrophic outflows induced by subglacial volcanic melting that transport and deposit basaltic material in various states of alteration. These systems are potential process and morphologic analogs to Martian catastrophic outflow systems. The sandur surfaces are dominated by boulder to sand-sized clasts of dense to moderately vesicular basaltic lavas, palagonitized tuffs, and palagonitized breccias. Basaltic glass, vesicular basaltic pumice, and crystals of quartz, zeolites, and plagioclase are common in the sand matrix. Palagonite breccia and palagonite tuffs represent the dominant basaltic alteration material and comprise 1 – 20% (by volume) of clasts on the sandur. Random and oriented (< 20 μm clay) powder x-ray diffraction analysis of palagonite breccia clasts indicate authigenic mineral assemblages suggestive of subglacial hydrothermal alteration at temperatures of 100 - 120 degrees C. These include randomly ordered (R < 1) smectite-illite (30 – 40% illite), analcime, heulandite, stilbite, laumontite and low-quartz. VIS/NIR lab spectroscopy of palagonite breccia clasts and sandur mixtures containing >5% palagonite clasts show H₂O and OH absorptions at 1.40 μm and 1.90 μm. Clay absorptions at 2.20 μm, 2.29 μm, and 2.31 μm, consistent with illite/montmorillonite, nontronite, and saponite respectively, were also identified. VIS/NIR remote sensing data from the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) reveal diagnostic clay absorptions between 2.20 and 2.30 μm in bedrock proximal localities on the sandur surfaces corresponding with the regions of highest palagonite abundance and coarsest grain size.