Olivine dissolution by acidic fl uids in Argyre Planitia, Mars: Evidence for a widespread process? Joshua L. Bandfi eld* Department of Earth and Space Sciences, University of Washington, Seattle, Washington 98195-1310, USA A. Deanne Rogers* Department of Geosciences, State University of New York, 255 Earth and Space Sciences Building, Stony Brook, New York 11794-2100, USA ABSTRACT and shape of spectral absorption features present Compositional and morphological analyses of basin rim units in Argyre Planitia are con- from near-infrared through thermal-infrared sistent with local olivine-rich materials that become relatively olivine-poor in the process of wavelengths (0.9–50 μm). These data sets were being transformed from rocky materials to fi ner particulate regolith. Where high-thermal- calibrated, atmospherically corrected, and ana- inertia rocky surfaces are observed, the composition is more mafi c than the surrounding fi nes. lyzed using methods described by Smith et al. This relationship between rocks and soils appears to be common on Mars as a similar pattern (2000), Bandfi eld et al. (2004), and Pelkey is observed in other locations, such as Gusev Crater, Isidis Planitia, and Nili Fossae. Despite et al. (2007). Specifi cally, low-spatial -resolution the lack of local detectable alteration products, these trends may be an indication that most (~3 by 8 km sampling) but high-spectral- Martian dark regions are not similar in composition to the primary igneous composition from resolution (143 spectral channels from ~7 to which they are derived. The Martian crust may be signifi cantly more mafi c, and alteration of 50 μm) TES data were used to obtain the detailed these surfaces may be more pervasive than has been inferred from the bulk surface mineral- bulk mineralogy of large-scale compositional ogy derived from orbital observations. units. Higher-spatial-resolution (100 m/pixel) multispectral (nine spectral channels from Keywords: Mars, alteration, infrared spectroscopy, regolith formation, surface composition. 7 to 15 μm) THEMIS imaging data were used to de fine surface compositional units and INTRODUCTION 2000; Bibring et al., 2005; Mustard et al., 2005; establish their spatial relationships. OMEGA The Martian meteorites and many geo- Rogers and Christensen, 2007). Composi- near-infrared spectra (0.9–2.5 μm at >400 m chemical and spectroscopic data sets have been tional and morphological analyses presented sampling) were used to identify iron-bearing used to investigate the compositional diversity here demonstrate that this presumption is minerals, sulfates, and phyllosilicates that may of Mars. These investigations have led to the incorrect in isolated locations and may be be present in minor quantities. In addition, identifi cation of a number of geochemical and generally incorrect planetwide. This is sup- results were incorporated from other studies mineralogical signatures that have provided ported by previous laboratory experiments that used data from the MER Miniature clues about the igneous and water-related his- and Mars Exploration Rover (MER) investi- Thermal Emission Spectrometer (Mini-TES) tory of Mars. This includes identifi cation of gations within Gusev Crater (e.g., Tosca et al., and Panoramic Camera (Pancam). minor carbonates, isolated regions that contain 2004; Hurowitz et al., 2006; McSween concentrations of sulfates, hematite, amorphous et al., 2006a). Soil formation on Mars may RESULTS silica, and phyllosilicates, and the ubiqui- involve signifi cant alteration of the source Morphological and Thermophysical Analyses tous presence of oxidized iron (e.g., Singer lithology, dominated by the dissolution of Figure 1 displays THEMIS infrared data et al., 1979; Bell et al., 1990; Gooding , 1992; olivine by acidic fl uids. “Soil,” as used here, acquired over the northeast portion of Argyre Christensen et al., 2000; Bandfi eld et al., 2003; is the fi ner fraction of the regolith and does Basin, near 330°E, 45°S. The region consists Poulet et al., 2005; Bibring et al., 2005; Ming not imply organic content. Confi dence in the of isolated hills surrounded by fl at plains. The et al., 2006; Morris et al., 2006; Glotch et al., interpretation of surface formation processes albedo of relatively dark and bright surfaces 2006). These compositions have been used to is bolstered by the ability to examine spatial within the region is 0.15 and 0.16, respectively. defi ne the limited extent of aqueous altera- and morphologic relationships between com- All surfaces are considered dark and relatively tion of the Martian crust. The identifi cation positions at high resolution. free of obscuring dust cover. of other minerals, such as quartz, plagioclase, Several hills in the region appear rela- pyroxene , and olivine have been used to charac- DATA SETS tively warm in nighttime THEMIS infrared terize Martian igneous processes and potential Several data sets were examined, including imagery, indicating a higher thermal inertia complexities of their formation mechanisms the Mars Global Surveyor Thermal Emission consistent with rockier surfaces or limited (e.g., Adams and McCord, 1969; McSween, Spectrometer (TES) (Christensen et al., 2001) exposures of bedrock. The warmer surfaces 1994; Bandfi eld et al., 2000; Hamilton et al., and Mars Orbiter Camera (MOC) (Malin and have a thermal inertia of 550 J m–2 K–1 s–1/2, 2003; Hoefen et al., 2003; Bibring et al., 2005; Edgett, 2001), Mars Odyssey Thermal Emis- and the surrounding regions have values of Mustard et al., 2005; McSween et al., 2006a, sion Imaging System (THEMIS) (Christensen ~300–350 J m–2 K–1 s–1/2. These observations 2006b; Ruff et al., 2006; Bandfi eld, 2006; et al., 2004), and the Mars Express Observa- are consistent with rocky surfaces surrounded Rogers and Christensen, 2007). toire pour la Mineralogie, l’Eau, les Glaces et by a regolith dominated by fi ner particulates. There has been vigorous debate regarding the l’Activité (OMEGA) (Bibring et al., 2005). The Several high-resolution MOC images are formation mechanism of some Martian surface visible-wavelength imaging data sets (MOC and available within the region. These images dis- compositions. However, low-albedo regions THEMIS VIS) were used to characterize sur- play a prevalence of gullies (Malin and Edgett, composed primarily of plagioclase, pyroxene, face morphology at 1.5–18 m/pixel. Nighttime 2000) on the northern and eastern slopes of the and olivine have been widely presumed to THEMIS surface temperature images were used higher-thermal-inertia surfaces of the isolated be unaltered basalt (e.g., Bandfi eld et al., to derive thermal inertia, which is an indicator of hills. The gullies have incised into the hills, and surface cover particle size (e.g., Fergason et al., the sediment transport paths lead from the rocky *E-mails: [email protected]; adrogers@ 2006). Spectroscopic data sets were used to surfaces to the surrounding relatively fl at terrain notes.cc.sunysb.edu. derive surface mineralogy based on the position that is dominated by fi nes (Fig. 2). © 2008 The Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. GEOLOGY,Geology, July July 2008; 2008 v. 36; no. 7; p. 579–582; doi: 10.1130/G24724A.1; 4 fi gures. 579 44°S N N 20 km 47.5°S 328.75°E Nighttime temperatures 332.5°E Colder Warmer 3 km 400 m Figure 1. Infrared mosaic of the region in northeast Argyre Planitia discussed in the Figure 2. Portion of Mars Orbiter Camera (MOC) image E1104337 for the area denoted in text. Daytime infrared measurements are Figure 3. The dotted box in the left image shows the region of detail on the right. Olivine- used for shading, and nighttime infrared rich high-inertia regions are coincident with the textured relatively high-albedo surfaces. measurements are displayed in the color Smoother surfaces and incised gullies are coincident with olivine-poor surfaces. Although scale. Nighttime temperatures indicate the albedo differences appear to be large, especially in the areas with gullies, they are only relative values of thermal inertia within the ~0.01 and are similar to other regions, such as Nili Patera where sediments are slightly region. The white box indicates the region darker than bedrock surfaces. shown in Figure 3. more spatially extensive olivine-rich surfaces unit show a slight 2.1 μm spectral absorption Spectral and Mineralogical Analyses present in other Martian regions, such as Nili feature characteristic of monohydrated sulfate False-color THEMIS images show indigo, Fossae and Ares Valles (Hoefen et al., 2003; (Gendrin et al., 2005). However, the lack of a orange/red, and green/yellow colors that corre- Hamilton et al., 2003; Rogers et al., 2005; corresponding 1.6 μm hydration feature, along spond to three primary spectral units within the McSween et al., 2006a). Deconvolution of the with the fact that the image was acquired under region (Fig. 3). The indigo spectral unit is coinci- measured spectra to fi nd the relative weight- nonideal solar incidence angles (>70°), make dent with the rockier surfaces and has spectral ing of mineral component spectral signatures this feature suspect. Olivine, high-Ca pyroxene, absorption features at relatively long wave- indicates that those surfaces have up to ~35% low-Ca pyroxene, and phyllosilicates have been lengths (10–12 μm). The indigo unit is not spa- olivine by volume with associated pyroxene identifi ed in unreleased OMEGA data covering tially extensive enough to be resolved with TES and plagioclase. The shorter-wavelength spec- the northern rim of Argyre Basin (Buczkowski data. The orange/red spectral unit is commonly tral absorption features characteristic of the et al., 2007). However, phyllosilicates or other located within local depressions in the hilly ter- orange/red spectral unit are consistent with secondary alteration products were not identi- rain and on the slopes and bases of the hills. This surfaces of higher SiO2 contents relative to the fi ed in the northeast portion discussed here.