Constraining the Ancient Geochemical Environments that formed the Complex Phyllosilicate and Sulfate Assemblages at Mawrth Vallis Through Comparison with Field Sites

The Mawrth Vallis region of Mars features abundant phyllosilicate outcrops as well as smaller areas where sulfates are present. This study evaluates field sites where phyllosilicates and sulfates co-occur to assist in constraining the ancient geochemical environments on Mars. Mineral transitions at Mawrth Vallis indicate changing geochemical conditions over time - from the lower Fe-rich smectite horizon about 200 meters thick to thinner units of sulfates, Al-phyllosilicates including halloysite in some regions, then Si-rich phases at the top of the profile [1] (Fig. 1a-c). Smectites form through wet/dry cycling in generally arid environments, while halloysite requires more humid conditions. Ca-sulfates and Al-clays represent a decrease in pH from the environment supporting formation of Fe/Mg-smectite, and the jarosite outcrops indicate an even lower pH. Sulfuric gases released from Syrtis may have produced sulfates in the groundwater [2] that flowed downhill from Meridiani towards Mawrth Vallis (Fig. 1d). This could have produced acidic brines that altered the expansive Fe/Mg-smectites at Mawrth Vallis and formed pockets of sulfates.

 

Specific phyllosilicate and sulfate minerals are mapped using vibrational bands in CRISM images (Fig. 1a). Spectral features in the region ~1.4-2.6 µm are most useful for identifying and characterizing these minerals, including the H₂O combination band near 1.91-1.92 µm. The Fe-rich smectite outcrops also exhibit Fe-OH bands near 1.42 and 2.29-2.30 µm. The Al-smectite units include Al-OH bands at 1.41-1.42 and 2.20-2.21 µm, while small locations containing halloysite/kaolinite and alunite have additional bands near 1.39-1.47, 1.75, 2.17, and 2.32 µm (Fig. 2). Areas containing jarosite have spectral bands near 1.47, 1.85, 2.22, and 2.26 µm (Fig. 3). Sites including hydrated sulfates include a drop in reflectance near 2.4-2.5 µm and Ca sulfates have a band near 1.75-1.78 µm.

 

Phyllosilicate - sulfate assemblages were investigated at field sites to assist in constraining the environments where these minerals form. The Painted Desert in Arizona features expansive outcrops of clay-bearing horizons (Fig. 4a), similar to Mawrth Vallis. Coordinated analyses of spectra from the field, lab, and aerial instruments of the light-toned and reddish horizons show the presence of clays and carbonates [3] (Fig. 4b-d). Sulfates are present in regions with polygonally-cracked surfaces where combinations of gypsum, jarosite, and montmorillonite are observed [4] (Fig 4ef). The south sulfur bank inside the Kilauea caldera (Fig. 5) contains a mixture of nontronite, saponite, montmorillonite, opal-A, gypsum, jarosite, and ferrihydrite due to hydrothermal alteration of ash and basalt from volcanic gases [5]. Lighter-toned outcrops are dominated by opal with some gypsum, saponite and jarosite (Fig. 5d), while the darker orange-tan layers include nontronite, ferrihydrite, and jarosite in addition to opal and gypsum (Fig. 5e). Analogs from the rainy Waimea Canyon region of Kauai include goethite, halloysite, ferrihydrite, and allophane in altered rinds on the rocks (Fig. 6). Additional sites altered under lower pH conditions contain hematite and jarosite [6].

 

Pedogenic alteration at the Painted Desert produced wide horizons of clay-bearing units interspersed with units of iron oxides/hydroxides and carbonates. Sulfates are observed together with phyllosilicates in regions with polygonally-cracked terrain. Jarosite and gypsum are present in a hydrothermal setting at Kilauea, while halloysite and goethite or jarosite and hematite are observed in the rainy and highly leached environment of Kauai. Observations of alteration minerals at these field sites suggest the Mawrth Vallis region of Mars experienced a largely arid environment with wet/dry cycling to produce the thick smectite profiles, with short-term periods of acidic fluids to form sulfates and strong leaching to form halloysite. Allophane likely formed on Mars when water was less abundant or colder [7].

 

Acknowledgements: The authors are grateful for support from NASA MDAP # 80NSSC19K1230 and NASA SSW #80NSSC23K0032.

 

References: [1] Bishop et al. (2020) Multiple mineral horizons in layered outcrops at Mawrth Vallis, Mars, signify changing geochemical environments on early Mars, Icarus, 341, 113634. [2] Moore & Szynkiewicz (2023) Aqueous sulfate contributions in terrestrial basaltic catchments: Implications for understanding sulfate sources and transport in Meridiani Planum, Mars, Icarus, 391, 115342. [3] McKeown et al. (2009) Coordinated lab, field, and aerial study of the Painted Desert, AZ, as a potential analog site for phyllosilicates at Mawrth Vallis, Mars, 40th LPSC, #2509. [4] Perrin et al. (2018) Mars evaporite analog site containing jarosite and gypsum at Sulfate Hill, Painted Desert, AZ, 49th LPSC, #1801. [5] Bishop et al. (2024) Solfataric alteration at the South Sulfur Bank, Kilauea, Hawaii, as a mechanism for formation of sulfates, phyllosilicates, and silica on Mars American Miner., 109, 1871–1887. [6] Gruendler et al. (2023) Characterizing Altered Volcanic Rocks from Waimea Canyon, Kauai, 54th LPSC, #1892. [7] Bishop et al. (2018) Surface clay formation during short-term warmer and wetter conditions on a largely cold ancient Mars, Nature Astronomy, 2, 206-213.

 

Figures:

 

Fig. 1 a-c) Alteration at Mawrth Vallis, Mars [1]. a) CRISM spectra of 5 distinct units. b) Diagram of altered stratigraphy. c) View of CRISM over HRSC. d) Potential formation mechanism for sulfate formation in groundwater at Mawrth Vallis, after [2].

 

Fig. 2 a) HRSC view of light-toned phyllosilicate-rich units. b) CRISM image FRT0000B141. c) Mineral parameter maps for Fe-smectite, halloysite, Al-smectite. d) CRISM spectra of selected outcrops compared to spectra of minerals.

 

Fig. 3 a) HRSC view of light-toned phyllosilicate-rich units. b) CRISM image FRT0000A425 with region containing jarosite marked by blue oval. c) Mineral parameter maps including jarosite. d) CRISM spectra of selected outcrops compared to spectra of minerals.

 

Fig. 4   Painted Desert, Arizona. a) Phyllosilicate-rich horizons. b-c) Close-up views of changing mineralogy. d) Comparison of HyMap aerial spectra with field and lab spectra. e) Gypsum and jarosite outcrops under polygonally-cracked terrain. f) Spectra of Painted Desert materials and lab mixtures compared to CRISM spectrum.

 

Fig. 5  Kilauea south sulfur bank, Hawaii. a) LG with field spectrometer. b) View of light-toned material. c) JLB sampling orange layered material. d-e) VNIR spectra of light-toned material, orange layers, and minerals.

 

Fig. 6  a) Waimea Canyon, Kauai. b-c) Close-up views of altered rocks. d) Spectra of selected samples compared to spectra of minerals.