JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 113, E12S43, doi: /2007je003049, 2008

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 113,, doi: /2007je003049, 2008 Visible, near-infrared, and middle infrared spectroscopy of altered basaltic tephras: Spectral signatures of phyllosilicates, sulfates, and other aqueous alteration products with application to the mineralogy of the Columbia Hills of Gusev Crater, Mars Victoria E. Hamilton, 1,2 Richard V. Morris, 3 John E. Gruener, 3 and Stanley A. Mertzman 4 Received 26 November 2007; revised 25 July 2008; accepted 22 October 2008; published 27 December [1] We studied visible to near-infrared (VNIR, mm) and middle infrared (MIR, cm 1, also called thermal infrared and vibrational) spectra of basaltic tephras from Mauna Kea volcano that were altered under ambient, hydrothermal (hydrolytic and acid sulfate), and dry heat conditions. Although models of MIR spectra of altered tephras generally produce fits whose quality is reduced compared to deconvolutions of primary lithologies, they successfully identify major alteration phases (cristobalite, oxide, phyllosilicate, and sulfate) except in palagonites. MIR spectra of the <45 mm fraction of all altered tephra samples exhibit an H 2 O peak at 1640 cm 1, but it cannot be used as an indicator of H 2 O content. This feature is present with band strengths >1% in spectra of the mm fraction only if phyllosilicates are present. Although Mauna Kea palagonitic tephra is considered a VNIR analog to Martian dust, comparison of MIR altered tephra spectra (<45 mm fraction) to dust spectra retrieved from Mars Global Surveyor and Mars Exploration Rover instruments do not provide good spectral matches. The best MIR match is a tephra that has a strong plagioclase feldspar transparency feature and was altered under dry, high-temperature, oxidizing conditions. This sample is not a VNIR analog and is not a process analog, but it emphasizes the mineralogical importance of plagioclase feldspar in Martian dust. No single tephra is a good spectral analog across the VNIR and MIR. We found no evidence for substantial sulfates or phyllosilicates in Mini-Thermal Emission Spectrometer (Mini-TES) spectra from Gusev Crater. Citation: Hamilton, V. E., R. V. Morris, J. E. Gruener, and S. A. Mertzman (2008), Visible, near-infrared, and middle infrared spectroscopy of altered basaltic tephras: Spectral signatures of phyllosilicates, sulfates, and other aqueous alteration products with application to the mineralogy of the Columbia Hills of Gusev Crater, Mars, J. Geophys. Res., 113,, doi: /2007je Introduction [2] Many investigators [e.g., Evans and Adams, 1979; Singer, 1982; Adams et al., 1986; Morris et al., 1990; Bell et al., 1993; Morris et al., 1993; Bishop et al., 1998] have suggested that certain samples of palagonitic tephra from Mauna Kea volcano (Hawai i) are spectral analogs for surface dust and soil in Martian bright regions on the basis of similarities at visible to near infrared wavelengths (VNIR, mm). The term palagonite refers to a yellow or orange isotropic mineraloid formed by hydration, hydroxylation, and devitrification of basaltic glass 1 Hawai i Institute of Geophysics and Planetology, University of Hawai i at Manoa, Honolulu, Hawaii, USA. 2 Now at Southwest Research Institute, Boulder, Colorado, USA. 3 NASA Johnson Space Center, Houston, Texas, USA. 4 Department of Earth and Environment, Franklin and Marshall College, Lancaster, Pennsylvania, USA. Copyright 2008 by the American Geophysical Union /08/2007JE (modified after Bates and Jackson [1984]). Palagonitic tephra from Mauna Kea does not have XRD-detectable phyllosilicates and owes its yellow to orange to brown color to a ferric absorption edge (0.40 to 0.75 mm) from nanophase ferric oxide (npox) particles embedded as a pigment in a spectrally neutral matrix [Morris et al., 1993, 2001]. The spectral/compositional properties of palagonitic tephra and other altered tephras from several locations also have been investigated previously at middle infrared wavelengths (MIR; 5 50mm or cm 1 ) [e.g., Bishop and Pieters, 1995; Roush and Bell, 1995; Esposito et al., 2000; Bishop et al., 2007]. To date, none of the palagonitic soils examined by these investigators are ideal matches to Martian MIR spectra, but spectral features attributable to H 2 O are visible, spectral features were observed to vary with decreased temperature and pressure, and palagonites from different localities may have different spectral signatures. [3] Mauna Kea palagonitic tephra also is an analog for the magnetic properties of Martian surface dust and soil 1of30

2 owing to the presence strongly magnetic, igneous titanomagnetite [Morris et al., 1990, 2001]. The volcano also has produced analogs to the hematite spherules found on Mars at Meridiani Planum by the process of hydrothermal alteration of basaltic tephra under acid sulfate conditions [Morris et al., 2005]. Acid sulfate, hydrothermal alteration of basaltic tephra on Mauna Kea also produced jarosite [Morris et al., 1996], which is found at Meridiani Planum [e.g., Morris et al., 2006a]. Mauna Kea itself and the basaltic tephra and ash it produced are process and compositional analogs for Home Plate at Gusev Crater, which is interpreted as a volcanic structure with an accumulation of basaltic pyroclastic deposits [Squyres et al., 2007]. Arguably, any alteration process on Mauna Kea and the associated alteration products are relevant analogs to Martian processes and mineralogy and provide a means, by laboratory characterization of the alteration products, to identify Martian alteration products and constrain alteration pathways. [4] In this paper, we extend previous VNIR and MIR studies of Martian analog samples to a suite of Mauna Kea tephra samples that have a variety of alteration products (palagonite, phyllosilicates, cristoabalite sulfates, and oxides) and whose formation pathways are known. The presence or absence of palagonite and other products of aqueous alteration on the Martian surface and their stratigraphic context constrain the nature and timing of alteration processes. Our focus is comparison of MIR emissivity spectra of the tephras to the spectra of Martian surface dust and soils acquired by the Thermal Emission Spectrometer (TES) on the Mars Global Surveyor (MGS) orbiter ( cm 1 ) [Bandfield and Smith, 2003] and the Mini-Thermal Emission Spectrometer (Mini-TES) instruments on the Mars Exploration Rovers (MER) ( cm 1 )[Christensen et al., 2004a]. Our data also provide points of comparison for ongoing analyses of data from the VNIR hyperspectral imagers Observatoire pour la Minéralogie l Eau, les Glaces et l Activité (OMEGA; mm) on the Mars Express Orbiter [e.g., Bibring et al., 2005; Poulet et al., 2005; Bibring et al., 2006] and the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM; mm) on the Mars Reconnaissance Orbiter (MRO) [Mustard et al., 2008]. The OMEGA and CRISM hyperspectral imagers provide important new mineralogical constraints as to the presence or absence of palagonite-like material and other H 2 O/OH /SO 4 2 -bearing phases on the Martian surface, in large part because of the cation-oh spectral features that occur between 2.0 and 2.6 mm. [5] The measurement objective of our study is to collect laboratory VNIR reflectance ( mm) and MIR emissivity (5 40 mm or cm 1 ) spectra of unaltered and altered basaltic tephra from Mauna Kea volcano (Hawai i) that are well characterized with respect to chemical and mineralogical composition as determined in prior studies [Golden et al., 1993; Morris et al., 1997, 2000a, 2001] or as a part of this study. As noted above, our primary focus is the MIR emissivity data set. VNIR spectra for similar samples and an equivalent spectral range, acquired in situ on Mauna Kea volcano and in the laboratory on collected samples, are reported by Swayze et al. [2002] and Guinness et al. [2007]. They used the spectral data as ground truth for Airborne Visible/Infrared Imaging Spectrometer (AVIRIS) measurements as an analog for VNIR measurements of the Martian surface by OMEGA and CRISM. [6] Several questions relevant to the interpretation of the MIR spectra of Mars are addressed by this study: [7] 1. What is the spectral variability among tephras subjected to different alteration pathways and can this variability be related to specific alteration products? [8] 2. Does the 1640 cm 1 (6.1 mm) fundamental bending vibration of the H 2 O molecule appear in spectra of all H 2 O-containing tephras and is the appearance of this band dependent on particle size? [9] 3. Which tephra sample(s) best match the MIR spectra of Martian surface soil and dust, and do they match the VNIR spectra of Martian surface dust? [10] 4. How do such comparisons constrain the mineralogical composition of alteration products on Mars? Specifically, are phyllosilicates and/or sulfates present at the MER Gusev Crater landing site based on MIR emissivity spectra? [11] 5. Can MIR emissivity spectra of altered tephra samples be linearly deconvolved to identify their primary alteration phases? 2. Samples and Data Acquisition [12] We studied 18 tephra samples collected as the <1 mm size fraction (dry sieving) on Mauna Kea volcano, Hawai i, between 1980 and 1999 (Figure 1 and Table 1). The sample suite was selected to include unaltered tephra and tephra altered under a variety of environmental conditions, including palagonitic alteration of basaltic glass under ambient conditions, thermal alteration at high temperatures, and hydrothermal alteration under both acid sulfate and neutral-to-alkaline conditions [Ugolini, 1974; Golden et al., 1993; Wolfe et al., 1997; Morris et al., 2000a]. The primary alteration products are palagonite (altered basaltic glass), hematite, phyllosilicates (smectite and kaolinite), sulfates (alunite and jarosite), and cristobalite. We previously reported chemical and mineralogical data for the unaltered and palagonitic tephra samples and a few of the sulfateand low-cristobalite-bearing samples [Golden et al., 1993; Morris et al., 1997, 2000a, 2001] (Table 1). For tephra samples where we did not have mm and <45 mm size fractions, we obtained those size fractions from bulk tephra samples (<1 mm size fraction) by wet sieving (ethanol) using precision sieves (Stork-VECO). The samples were not crushed prior to sieving. In a few cases, sample availability limited us to other size fractions (Table 1). For previously unanalyzed samples, major and selected minor element chemistry, weight loss upon ignition in air at 900 C (LOI), titration for Fe 2+, and powder X-ray diffraction (XRD) data for mineralogical composition were obtained using the methods described and referenced by Morris et al. [2001]. [13] Middle infrared emissivity spectra ( cm 1 at 4 cm 1 resolution) were acquired in the Thermal Emission Spectroscopy laboratory at Arizona State University using a Nicolet Nexus 670 FTIR spectrometer or at the University of Hawai i using a Nicolet 470 FTIR spectrometer. Details of instrument configurations and data calibration are essentially the same and are described by Ruff et al. [1997] and Hamilton and Lucey [2005]. Samples were poured into 3 cm diameter copper cups painted IR-black and the cups were tapped gently on a table to settle the samples. Samples were heated 2of30

3 Figure 1. Photograph of unaltered and altered tephra samples analyzed in this work. All size fractions are <1 mm except for PH-13 which is mm. The samples were photographed together, so the lighting conditions are constant. to 80 C throughout the data acquisition period; 270 scans were acquired on spot sizes 1 cm in diameter over 5.35 min and were averaged together by the spectrometer. The spectrometer and sample environment were continuously purged during the measurement with scrubbed air and nitrogen to minimize atmospheric water vapor and CO 2. Measured radiance spectra were converted to emissivity by the methods outlined by Christensen and Harrison [1993] and Ruff et al. [1997]. [14] Visible to near infrared ( mm) reflectance spectra were acquired at room temperature and ambient atmosphere at the NASA Johnson Space Center using either a FieldSpec RS 3 spectrometer with the Muglight option or a FieldSpec Pro spectrometer with the Flashlight option; both are Analytical Spectral Devices, Inc. instruments, with the latter on loan from Washington University at St. Louis. According to the manufacturer, the spectral resolution is 3 nm at 0.70 mm and 10 nm at 1.4 and 2.1 mm, and the wavelength accuracy is±1 nm. The Muglight and Flashlight have internal light sources, and the angle between the axis of the light source and the axis of the fiber optic cable to the detector is 23. Spectral data were acquired with the samples oriented in a horizontal position with no window in the optical path, and the surface area of the sample analyzed by the instrument is 1 cm. Spectralon was used as the white reflectance standard, with the spectral data output in units of absolute reflectance. A Scintag XDS 2000 X-ray diffractometer using CuKa radiation and a step size of q was employed to obtain X-ray diffraction powder patterns (293 K). 3. Results 3.1. Chemistry, Oxidation State, and XRD Mineralogical Composition [15] Chemical data for the <1 mm size fraction are given in Table 2 for the 8 tephra samples not previously analyzed by Morris et al. [2000a]. Mineralogical compositions according to XRD are compiled in Table 1. Representative XRD spectra of the finest size fractions (<20 or <45 mm) are shown in Figure 2, except in the cases of HWMK513 and PH-13, in which the spectrum represents a fine powder of the <1 mm size fraction. [16] The XRD pattern for a fine powder of the <1 mm size fraction of unaltered tephra HWMK513 (Figure 2a) shows a very broad peak (20 to 40 2q) from the basaltic glass and sharp peaks primarily from phenocrysts of plagioclase feldspar. The pattern for the <45 mm size fraction of HWMK600 (Figure 2b) is typical for palagonitic tephra [Morris et al., 2001]. Sharp peaks from plagioclase feldspar dominate the pattern. The npox and allophanelike alteration products are XRD amorphous and not detectable even as broad peaks at the y axis scaling in Figure 2b. The npox alteration product dominates the Mössbauer spectra of the fine size fractions of palagonitic tephra from Mauna Kea volcano [Morris et al., 2000a, 2001]. The phyllosilicate-free palagonitic tephras are enriched in Fe 2 O 3 T (total concentration of Fe expressed as Fe 2 O 3 ) and depleted in SiO 2 relative to unaltered glassy precursor tephra [Morris et al., 2001]. [17] XRD spectra of phyllosilicate-bearing tephras like HWMK904, HWMK741, and HWMK742 (Figures 2c, 2d, and 2e) also are characterized by sharp peaks from plagioclase feldspar. The clearest marker for the presence of phyllosilicates is the prominent peak near 6 2q. The asymmetric peak near 20 2q also is a phyllosilicate peak. Together with minor alunite, HWMK742 is the most phyllosilicate-rich sample examined in this study. Hydrothermal formation of phyllosilicates on Mauna Kea is discussed by Ugolini [1974], and the presence of phyllosilicates along with the feldspar is evidence that the phyllosilicates are primarily a product of hydrothermal alteration of the basaltic glass. 3of30

4 Table 1. Collection Location, Size Fractions, Fe 3+ /Fe T Ratio, Weight Loss on Ignition, XRD Mineralogy, and Alteration Conditions for Samples of Unaltered and Altered Basaltic Tephra From Mauna Kea Volcano, Hawaii Sample Location a Fraction b Fe 3+ c /Fe T LOI c (%) SO 3 (%) XRD Mineralogy d Alteration Conditions Ref e Size Unaltered Tephra HWMK513 GT-Exc A, B Gl, F, Ol, Mt Unaltered 3 PH-13 PHu B, D Gl, F, Ol, Mt, Hm, tr-mica Unaltered 3 Palagonitic (Phyllosilicate-Free) Tephra HWMK530 V-Ctr A, B, C F, Mt, Ol, tr-hm, (npox) f Ambient, ±neutral to basic 2, 3, 4 HWMK600 VLBA A, B, C F, Ol, Hm, Mt, (npox) f Ambient, ±neutral to basic 3, 4 HWMK919 VLBA A, B, C F, Mt, Ol, tr-hm, tr-gl, Ambient, ±neutral to basic 3, 4 tr-mica, (npox) f JSC Mars-1 PNe A,. B, C F, Mt, Ol, Gl, (npox) f Ambient, ±neutral to basic 3, 4, 5 Phyllosilicate-Bearing Tephra HWMK720 PWa A, B, C F, Mt, Ol, Kaol, Smec Hydrothermal, ±neutral to basic HWMK741 PPl A, B, C F, Smec, Mt, Kaol Hydrothermal, ±neutral to basic HWMK904 PWa A, B, C F, Smec, Hm, Mt, Ol, tr-mica Hydrothermal, ±neutral to basic HWMK911 PWa A, B, C F, Smec, Ol, Mt, tr-hm Hydrothermal, ±neutral to basic Hematite-Bearing (Calcined) Tephra HWMK11 SBk A, B, E, F F, Hm, tr-ol, tr-mt Dry, high temperature 1, 2, 3 Cristobalite- and Phyllosilicate-Bearing Tephra HWMK30 PPh A, B, C F, Hm, Low-Cri, Smec, Zeo Hydrothermal, mild acid sulfate 2 HWMK31 PPh A, B, C Low-Cri, F, Hm, Smec Hydrothermal, mild acid sulfate Sulfate-Bearing Tephra HWMK508 GT-Exc A, B, C F, Na-Alu, Mt, Ol, Gl Hydrothermal, acid sulfate 3 HWMK515 GT-Exc A, B, C Na-Jar, F, Mt, Gl Hydrothermal, acid sulfate 3 HWMK940 g GT A, B, C F, Na-Jar, Mt, Hm Hydrothermal, acid sulfate 3 Sulfate- and Phyllosilicate-Bearing Tephra HWMK740 PPl A, B, C F, Na-Alu, Smec, Hm Mt, Hydrothermal, acid sulfate Kaol, tr-mica HWMK742 PPl A, B, C F, Smec, Na-Alu, Mt, Hm, Hydrothermal, acid sulfate tr-kaol a LOI, loss on ignition. Mauna Kea summit locations: GT, surface tephra collected down slope from the Gemini telescope; GT-Exc, subsurface tephra collected from exposure produced during basement excavation for the Gemini telescope; SBk, surface tephra collected at switchback to Gemini telescope; PPl, surface tephra collected in gully on south side of Puu Poliahu cone; PWa, surface tephra collected on Puu Waiau cone; PPh, surface tephra collectedon the west side of the Puu Pohaku cone. Other Mauna Kea locations: V-Ctr, surface tephra collected on west side of summit road opposite Mauna Kea Visitor s Center; VLBA, surface tephra collected along side road near the Very Long Baseline Array telescope.; PNe, subsurface tephra collected on Puu Nene cone (west of Saddle Road and Summit Road junction); PHu, subsurface tephra from Puu Huluhulu cone (at Saddle Road and Summit Road junction). See references for additional location information. Also see Swayze et al. [2002] and Guinness et al. [2007] for locale descriptions. b A = <1000 mm, B = mm, C = <45 mm, D = mm, E = mm, and F = <20 mm. c Pertains to the <1 mm size fraction. d From XRD analyses of C or F size fraction, except HWMK513 and PH-13, where fine powders of the <1 mm size fraction were used. F, plagioclase feldspar; Ol, olivine; Gl, glass; Mt, magnetite/titanomagnetite; Low-Cri, low crystobalite; NpOx, nanophase ferric oxide; Na-Jar, natrojarosite; Na-Alu, natroalunite; Hm, hematite; Kaol, kaolinite; Smec, smectite. Ordered in decreasing abundance. e 1, Golden et al. [1993]; 2, Morris et al. [1997]; 3, Morris et al. [2000a]; 4, Morris et al. [2001]. f NpOx alteration product is amorphous to XRD. NpOx is detected by Mössbauer spectroscopy. g HWMK940 equivalent to HWMK620 described by Morris et al. [2000a]; collected in different years. [18] Tephra HWMK11 (Figure 2f) is characterized by XRD peaks from hematite (a-fe 2 O 3 ) which, based on the experiments of Golden et al. [1993], formed when unaltered basalt tephra was calcined at high temperatures under oxidizing temperatures. Ti-bearing hematite is present as large grains as the thermal oxidation product of primary titanomagnetite, and a red hematite pigment is present as an oxidative exsolution product from primary Fe 2+ -bearing phases (e.g., olivine). Feldspar also is present. [19] Cristobalite-bearing samples like HWMK31 (Figure 2g) are characterized by sharp XRD lines from well crystalline low cristobalite, which suggests hydrothermal formation conditions. HWMK31 also has peaks from feldspar, hematite, and a phyllosilicate. HWMK30 is similar, except feldspar and hematite are more abundant than low cristobalite. Minor elevation in SO 3 concentration for this sample compared to unaltered tephra suggests mild acid sulfate conditions during alteration. The cristobalite-bearing samples have the highest SiO 2 concentrations and are markedly depleted in MnO compared to the other tephra samples (Table 2 and Morris et al. [2000a]). [20] The XRD patterns for the sulfate-bearing samples have jarosite ((K,Na,H 3 O)Fe 3 (SO 4 ) 2 (OH) 6 ) and alunite ((K,Na,H 3 O)Al 3 (SO 4 ) 2 (OH) 6 ) as the sulfate bearing phases (Figures 2h, 2i, and 2j). All sulfate-bearing samples have minor feldspar, and HWMK740 has in addition a phyllosilicate (Figure 2j). The positions of the sulfate XRD peaks are consistent with Na + K > H 3 O for the monovalent cations. The particularly high Al 2 O 3,K 2 O, and SO 3 concentrations (26.1, 3.06, and 7.82 wt %, respectively) for HWMK740 results from the high concentration of alunite for the <1 mm size fraction (Table 2). 4of30

5 Table 2. Major Element Compositions, Weight Loss on Ignition, and Fe 3+ /Fe T Ratio for Average Unaltered Tephra and Selected Altered Tephras (<1 mm Size Fractions) From Mauna Kea Volcano, Hawaii a Average Unaltered b HWMK11 HWMK30 HWMK31 HWMK720 HWMK740 HWMK741 HWMK742 HWMK904 HWMK911 HWMK940 SiO 2 (%) TiO 2 (%) Al 2 O 3 (%) Fe 2 O 3 T (%) MnO (%) MgO (%) CaO (%) Na 2 O (%) K 2 O (%) P 2 O 5 (%) V(mg/g) Cr (mg/g) Total (%) LOI (%) SO 3 (%) FeO (%) Fe 2 O 3 (%) Fe 3+ /Fe T a LOI, loss on ignition. Major element analyses were made on the powders remaining after LOI measurements (after heating to 900 C in air). Oxidation state and SO 3 measurements were made on unheated samples. b From Morris et al. [2000a]. Major element compositions for all other tephra samples in Table 1 are reported in the same reference. [21] Unaltered tephras HWMK513 and PH-13 and calcined tephra HWMK11 have low concentrations of volatiles (LOI < 1.3 wt %) that reflect their unaltered state and high-temperature alteration, respectively (Table 1). The remaining tephra samples have LOI concentrations between 4.8 and 21.4%, which reflect their uptake of volatiles during aqueous alteration. Because the LOI measurement is weight loss at 900 C under oxidizing conditions, the volatile evolved for palagonitic, phyllosilicate-rich, and cristobalite-bearing tephras is mostly H 2 O (from dehydration and dehydroxylation), and the volatiles evolved for the sulfatebearing tephras are H 2 O and SO 3. With the exception of tephras HWMK508 and HWMK740, for which alunite is the alteration product, aqueous alteration occurred under oxidizing conditions as shown by the generally high values of Fe 3+ /Fe T ( ) relative to unaltered tephra ( ) and by the Fe 3+ -bearing alteration products (npox, hematite, and jarosite) (Table 1) Classification According to Mineralogical Composition [22] Our tephra samples are divided into seven groups according to the mineralogical composition of the major alteration product (Table 1 and Figure 2): (1) unaltered tephras HWMK513 and PH-13; (2) palagonitic (phyllosilicate-free) tephras HWMK530, HWMK600, HWMK919, and JSC Mars-1; (3) phyllosilicate-bearing tephras HWMK720, HWMK741, HWMK904, and HWMK911; (4) a hematite-rich tephra HWMK11; (5) low-cristobaliteand phyllosilicate-bearing tephras HWMK30 and HWMK31; (6) and sulfate-bearing tephras HWMK508, HWMK515, and HWMK940; and (7) sulfate- and phyllosilicate-bearing tephras HWMK740 and HWMK742. For discussion purposes, tephras HWMK740 and HWMK742, which have both phyllosilicates and the sulfate alunite, are considered with sulfate-bearing and phyllosilicate-bearing tephras in accordance with their dominant alteration product. We attribute the trace levels of mica detected in some samples to eolian mica from China [Syers et al., 1969] Visible and Near-Infrared Spectral Region Overview [23] The primary absorptions observable at the VNIR wavelengths covered by this study (0.35 to 2.5 mm) arise from Fe 2+ and Fe 3+ electronic transitions and combination tones and overtones of fundamental vibrational modes of the H 2 O molecule and M-OH groups (M = Fe 3+,Fe 2+, Mg, Al, and Si). The shapes and positions of spectral features provide information about the speciation of Fe according to coordination, oxidation, and mineralogical state, the nature Figure 2. Powder XRD diffraction patterns between 5 and 50 2q (Cu Ka radiation) for representative unaltered and altered tephra samples. (a) Unaltered tephra sample (fine powder of the <1 mm size fraction of HWMK513) showing very broad peak from basaltic glass (20 to 40 2q) and sharp peaks from plagioclase feldspar (F). (b) Palagonitic tephra (HWMK600, <45 mm) showing sharp peaks from plagioclase feldspar. The alteration products for palagonitic tephra (nanophase ferric oxide (npox) and an allophane-like material) are XRD amorphous [see Morris et al., 2001]. (c, d, and e) Phyllosilicate-bearing tephra (<45 mm size fractions of HWMK904, HWMK741, and HWMK742) showing broad phyllosilicate peaks (P) at 6 and 20 2q and sharp peaks from plagioclase feldspar. (f) Calcined tephra (HWMK11, <20 mm) showing sharp peaks from feldspar and hematite (H). (g) Low-cristobalite-bearing tephra (HWMK31, <45 mm) showing sharp peaks from that phase (LC) and a broad peak from minor phyllosilicate. (h) Sulfate-bearing tephra (HWMK515, <45 mm) showing sharp peaks from jarosite (J) and minor plagioclase feldspar. (i) Sulfate-bearing tephra (HWMK508, <45 mm) showing sharp peaks from alunite (A) and minor plagioclase feldspar. (j) Sulfate bearing tephra (HWMK740, <45 mm) showing sharp peaks for alunite and feldspar and broad peaks from phyllosilicate. 5of30

6 of the cation in the M-OH functional group, and speciation of OH- and H 2 O according to mineralogical state. VNIR reflectance spectra over our full wavelength range are shown in Figures 3 6. The wavelength region between 1.7 and 2.5 mm, where the vibrational bands from the H 2 O molecule and the M-OH functional groups occur, is expanded in Figure 7 for the smallest size fraction (usually <45 mm) of each sample. Spectra for the smallest size fractions have the highest spectral contrast for the spectral features. Representative library spectra for the region between 1.7 and 2.5 mm are shown in Figure 8. Our assignment of bands to specific overtone and combination tone vibrations is based on extensive literature on the subject [e.g., Hunt and Salisbury, 1970; Hunt et al., 1971; Hunt, 1979; Clark et al., 1990; Grauby et al., 1994]. [24] The observation of increased reflectivity with decreasing particle diameter for wavelengths longer than 0.75 mm is attributed both to the increase in scattering Figure 2 6of30

7 Figure 3. VNIR and MIR spectra for unaltered tephra samples (a and b) PH-13 and (c and d) HWMK513 and for palagonitic tephra samples (e and f) HWMK600, (g and h) HWMK919, (i and j) JSC Mars-1, and (k and l) HWMK 530. Palagonitic tephra were altered under ambient, neutral ph conditions. Size fractions are A = <1000 mm; B = mm; C = <45 mm; and D = mm. Vertical lines in these and subsequent figures denote spectral feature locations discussed in text. 7of30

8 Figure 4. VNIR and MIR spectra for altered spectra samples (a and b) HWMK911, (c and d) HWMK720, (e and f) HWMK741, (g and h) HWMK904, and (i and j) HWMK742. Phyllosilicates are the dominant alteration products, with alteration occurring under hydrothermal, neutral ph conditions. Size fractions are A = <1000 mm; B = mm; and C = <45 mm. with decreasing particle size and to the increase in the proportion high albedo alteration products (palagonite, jarosite, alunite hematite, and phyllosilicates) with decreasing particle diameter. Evidence for higher proportions of alteration products in the smallest size fractions includes the Fe 3+ /Fe T ratio (e.g., 0.65 and 0.75 for the mm and <2 mm size fractions of HWMK600 and 0.80 and 0.99 for the <1 mm and <5 mm size fractions of HWMK919), the 8of30

9 Figure 5. VNIR and MIR spectra for altered spectra samples (a and b) HWMK11, (c and d) HWMK30, and (e and f) HWMK31. HWMK11 was altered at high temperatures under dry conditions, and HWMK30 and HWMK31 were altered under hydrothermal, acidic conditions. Size fractions are A = <1000 mm; B = mm; C = <45 mm; E = mm; F = <20 mm. SO 3 concentrations (e.g., 6.5% and 28.6% for the <1 mm and <5 mm size fractions of HWMK515), and values of LOI (e.g., 12.7% and 19.7% for the <1 mm and <5 mm size fractions of HWMK919) [Morris et al., 2000b, 2001]. As expected, the albedo of the <1 mm size fraction for each sample is intermediate to those for the corresponding coarse and fine size fractions Unaltered Tephra [25] The VNIR spectra of unaltered tephras HWMK513 and PH-13 (Figures 3a, 3c, and 7a) are distinctive from those of all other tephras in this study in that they do not have a spectral fingerprint from Fe 3+ alteration products and have virtually undetectable vibrational features from OH and H 2 O near 1.4, from H 2 O near 1.9 mm, and from M OH between 2.0 and 2.6 mm. We attribute the virtually undetectable hydration features to the low volatile content of the <1 mm size fractions (<1.3 wt %; Table 1) and to the very low VNIR reflectance at 1.4 and 1.9 mm (<0.04; Figures 3a and 3c). The broad minimum centered near 1.1 mm and the inflection near 2.0 mm result from the so-called 1 mm and 2 mm bandsforfe 2+ in the glass. The very weak inflections near 2.21 mm are consistent with minor hydroxylation of the basaltic glass. Compared to the altered samples, the unaltered samples have very low albedo throughout the VNIR (<0.06) Palagonitic Tephra [26] As discussed above, palagonite is the yellow-toorange-to-brown alteration product of basaltic glass by way of hydration, hydroxylation, and devitrification. For the palagonitic tephra included this study (HWMK530, HWMK600, HWMK919, and JSC Mars-1), the color is produced by the ferric absorption edge extending from 0.35 to 0.80 mm (Figures 3e, 3g, 3i, and 3k). On the basis of VNIR, Mössbauer, transmission electron microscope, and differential leaching studies, the ferric pigment in these samples is nanophase ferric oxide particles [Morris et al., 1993, 2001]. On the basis of XRD data of size fractions <5 mm, phyllosilicates are not detected in these tephras [Morris et al., 2001]. Phyllosilicate-free palagonitic tephra is a widely cited a spectral analog material for Martian 9of30

10 Figure 6. VNIR and MIR spectra for altered spectra samples (a and b) HWMK508, (c and d) HWMK740, (e and f) HWMK515, and (g and h) HWMK940. Sulfates (alunite and jarosite) are the dominant alteration products, with alteration occurring under hydrothermal, acid sulfate conditions. Size fractions are A = <1 mm; B = mm; and C = <45 mm. surface dust [Singer, 1982; Morris et al., 1990; Bell et al., 1993; Morris et al., 1993, 2000a, 2001]. The features centered near 1.42 mm and 1.92 mm are assigned to the combination and overtone bands of the fundamental vibrations of the H 2 O molecule (1.42 and 1.92 mm) and hydroxyl (OH;1.42 mm only). Absorption by (Al,Si,Fe,Mg) -OH produces the weak, asymmetric feature (Figure 7a; most evident in the spectra for HWMK919 and JSC Mars-1) with a minimum position at 2.21 mm (from (Al,Si) -OH; OH stretch and M-OH combination band) and asymmetry to longer wavelengths. The relative strength of the absorption near 2.21 mm is a result of the high concentration of Al relative to Fe and Mg in the precursor basaltic glass (Table 2), although there is a weak feature near 2.31 mm, presumably from Mg-OH. In addition, potential contributions from Fe 3+ -OH are reduced because the cation is tied up as npox. The spectral features for H 2 O/OH are deepest for JSC Mars-1, which among the palagonitic tephras has the highest value of LOI (18 wt %). [27] As can be seen most clearly for sample HWMK919, there are two weak bands centered near and mm that overlap the band centered near 1.91 mm from the H 2 O bending plus stretching combination band. These spectral features are shown clearly in Figure 9 along with data for a synthetic SiO 2 glass and a calculated transmission spectrum for H 2 O vapor. Comparison of the transmission spectrum to the reflectance spectra shows that the spectral features at and mm are a measurement artifact and are 10 of 30

11 Figure 7. VNIR spectral features in the region between 1.7 and 2.5 mm where combination tones and overtones of vibrations resulting from H 2 O and M-OH occur. (a) Unaltered tephra (HWMK513 and PH-13), palagonitic tephra (HWMK600, HWMK530, HWMK919, and JSC Mars-1), phyllosilicatebearing tephra (HWMK720, HWMK911, HWMK741, and HWMK904), and tephra HWMK11. (b) Phyllosilicate- and/or cristobalite-bearing tephra (HWMK30, HWMK31, HWMK742) and sulfatebearing tephra (HWMK940, HWMK525, HWMK508, and HWMK740). Spectra are offset for comparison and labeled using the numeric digits of the sample name and a letter (A, B, C, or E) corresponding to the particle size fraction. Size fractions are A = <1 mm; B = mm; C = <45 mm; and E = mm. not inherent to the samples. This happened because the reflectance axis of our ASD instrument is calibrated under ambient laboratory conditions against Spectralon, which is inherently not an H 2 O/OH-bearing material. Our samples, however, can contain significant adsorbed H 2 O. For example, palagonitic tephra, depending on individual sample and on size fraction, have 2 8% adsorbed water under ambient laboratory conditions [Bell et al., 1993; Morris et 11 of 30

12 Figure 8. VNIR spectral features in the region between 1.7 and 2.5 mm where combination tones and overtones of vibrations resulting from H 2 O and M-OH occur for representative phyllosilicate and sulfate minerals. (a) Spectra that have a strong band at 1.91 mm, indicating the presence of molecular water. (b) Spectra with bands from between 1.7 and 2.5 um resulting from the M-OH group (M = Fe 2+,Fe 3+,Al, Mg, and Si) and a weak to absent band at 1.91 mm. al., 1993]. During measurement of a sample with significant adsorbed H 2 O, heat from the ASD light source desorbs H 2 O producing an atmosphere enriched in H 2 O vapor relative to the atmosphere present during the Spectralon measurement Phyllosilicate-Bearing Tephra [28] Our phyllosilicate-bearing altered tephras display a variety of VNIR spectral characteristics (Figure 4). All 5 samples have a ferric absorption edge extending from 0.35 to 0.80 mm, implying the presence of npox and/or crystalline ferric oxides. The presence of an inflection near 0.56 mm is an indicator for the presence of crystalline ferric oxides, particularly hematite [e.g., Morris et al., 2000a]. On this basis, HWMK904 (Figure 4g) and particularly HWMK742 (Figure 4i) have higher proportions of a crystalline ferric oxide than the other samples in Figure 4. This observation is in agreement with XRD data (Table 1), which show hematite detections. [29] All phyllosilicate-bearing tephra samples have spectral features at both 1.42 and 1.92 mm that are indicative of molecular H 2 O (e.g., Figure 4i). Because they all have smectite as the phyllosilicate according to XRD, the two bands originate from the interlayer H 2 O plus unknown contributions from 12 of 30

13 Figure 9. (a) Calculated transmission VNIR spectrum in the region between 1.7 and 2.1 mm for 30 ppr mm of H 2 O vapor with a 1 m path length (R. E. Arvidson and P. C McGuire, personal communication, 2007). (b) VNIR reflectance spectra for five samples that show minor spectral features superimposed on the sample 1.91 mm feature that is an H 2 O bending plus stretching combination band. Comparison of Figures 9a and 9b shows that the features at and mm are artifacts resulting from transmission through H 2 O vapor in the atmosphere above the sample that was not present in the atmosphere above the standard white reference sample (Spectralon). The H 2 O vapor bands at and mm are not detected in the presence of the relatively more intense band at 1.91 mm that is inherent to the samples. adsorbed water. Evidence for adsorbed H 2 O that was partially desorbed during measurement is the presence of the and mm H 2 O-vapor bands (Figure 7). [30] The spectral region between 1.7 and 2.5 mm provides information on the mineralogical composition of the smectite (Figures 7 and 8). Spectral features are located near 2.21, 2.24, and 2.30 mm. The band at 2.21 mm results from Al-OH. It is the only notable M-OH feature detected in HWMK742 (Figure 7b), so that the smectite present is montmorillonite (generalized chemical formula (Na,Ca 0.5 ) 0.33 (Al,Fe 3+ ) 1.67,Mg 0.33 Si 4 O 10 (OH) 2 nh 2 O with Al > Fe 3+ ; compare with STX-1 in Figure 8). Samples HWMK720, HWMK741, HWMK904, and HWMK911 have bands with variable intensity near 2.21 (Al-OH), 2.24 ((Al,Fe) -OH), and 2.29 (Fe-OH) mm (see Gates et al. [2002] for band assignments). These samples are probably mixtures in variable proportions of an Al-rich smectite like montmorillonite plus one or more smectite with both Fe and Al in the octahedral sites, for example a ferruginous smectite (generalized chemical formula (Na,Ca 0.5 ) 0.33 (Al,- Fe 3+ ) 2 Si 4 O 10 (OH) 2 nh 2 O with Fe 3+ > Al; compare with SWA-1 in Figure 8). We do not have a sample with a Fe-OH band near 2.29 mm and no bands near 2.21 and 2.24 mm, which would indicate the presence of only nontronite, the Fe 3+ end-member smectite (generalized formula (Na,Ca 0.5 ) 0.33 (Fe 3+ ) 2 (Al 0.33 Si 3.67 )O 10 (OH) 2 nh 2 O; compare with API33A in Figure 8). We also do not have samples where the phyllosilicate present is predominantly a Mg-rich phyllosilicate (e.g., saponite, with generalized chemical formula (Na,Ca 0.5 ) 0.33 (Mg,Fe 2+,Fe 3+ ) 3 (Si,Al) 4 O 10 (OH) 2 nh 2 O, and serpentine, with chemical formula (Mg,Fe 2+ )Si 2 O 5 (OH) 4 ), because of the absence of a strong Mg-OH band at 2.31 mm (compare with SAPCA-1, SRPSL01, and CA9SRP1 in Figure 8). This result is consistent with the low concentration of Mg in these samples in comparison to Al and Fe (Table 2 and Morris et al. [2000a]). We do not exclude that Mg is present at minor levels in the ferruginous smectite discussed above. The ferruginous smectite sample SWA-1 has Fe 2 O 3 T, Al 2 O 3, and MgO concentrations equal to 11.4%, 25.5%, and 2.3%, respectively, on a water-free basis (R. V. Morris, unpublished data, 2008). Although kaolinite is indicated in HWMK720 and HWMK741 by XRD (Table 1), we do not have a clear VNIR detection (compare with KGA-1 in Figure 8) Calcined and Cristobalite-Bearing Tephra [31] The next two groups of altered tephras (HWMK11, HWMK30, and HWMK31) are all highly oxidized (Fe 3+ / Fe T > 0.96; Table 1) and have spectral features resulting from hematite Fe 3+ electronic transitions in the region between 0.35 and 1.0 mm (Figure 5). The hematite Fe 3+ features are the inflections near 0.52 and 0.62 mm, the relative reflectivity maximum near 0.78 mm, and the plateau to minimum centered near 0.88 mm [e.g., Morris et al., 2000a]. Hematite is indicated in the XRD data (Table 1), and available Mössbauer data show that Fe mineralogy for HWMK11 and HWMK30 is predominantly hematite [Golden et al., 1993; Morris et al., 1997]. The weak bands near 1.42 and 1.92 mm for HWMK11 are consistent with its low volatile content (LOI = 1.15 wt %; Table 1) from having been calcined under dry conditions [Golden et al., 1993]. [32] In the spectral region between 1.7 and 2.5 mm, HWMK11 (Figure 7a) has weak features near 2.21, 2.24, and 2.30 mm indicating an assemblage of Al-rich smectite (montmorillonite) and Al-Fe smectite, as discussed above. The smectite may be a dust contamination coating larger particles or it may be smectite that survived calcination. [33] Similar to HWMK742, HWMK30 has a prominent M-OH spectral feature (Figure 7b) located at 2.21 mm that is consistent with Al- (OH) in Al-rich smectite (e.g., montmorillonite). Because this sample has low-cristobalite according to XRD (Table 1), it is possible that the feature 13 of 30

14 Figure 10. Normalized midinfrared emissivity spectra of unaltered tephras, quenched basalt, and glasses. Vertical lines denote locations of fundamental band minima in unaltered tephras. Spectra offset for clarity. is not associated with a phyllosilicate and results instead from Si-OH in a SiO 2 -rich phase. The presence of Si-OH requires that a band centered near 1.39 mm also be present. Its absence (Figure 5c) implies the spectral feature at 2.21 mm results from Al-OH. [34] Sample HWMK31 is characterized by bands at 2.25 and 2.30 mm from (Fe,Al) -OH and Fe-OH, respectively. The pattern is very similar to that for the ferruginous smectite SWA-1 (Figure 8a). The absence of a band at 2.21 mm in HWMK31, which has more low cristobalite by XRD than HWMK30 (Table 1), is indirect evidence that the 2.21 mm feature in HWMK30 is associated with Al-OH and not Si-OH Sulfate-Bearing Tephra [35] The final group of samples (Figure 6), those altered under hydrothermal, acid sulfate conditions, consists of two samples whose primary alteration phase is alunite (HWMK508 and HWMK740) and two samples whose primary alteration phase is jarosite (HWMK515 and HWMK940). The spectrum of HWMK508 has a very weak absorption edge between 0.40 and 0.60 mm which is presumably associated with minor Fe 3+ for Al 3+ substitution in alunite. The spectrum of HWMK740, because of the inflection near 0.56 mm, the relative reflectivity maximum near 0.75 mm, and the minimum near 0.88 mm, is consistent with the presence of minor hematite. HWMK740 has the highest concentration of both Al and K, consistent with the mineralogical dominance of alunite in this sample (Tables 1 and 2). The spectra of HWMK515 and HWMK940 have distinct spectral features from Fe 3+ electronic absorptions in jarosite (e.g., the sharp band centered near 0.44 mm and the minimum near 0.95 mm) as discussed previously for Mauna Kea jarosite-bearing tephra [Morris et al., 1997, 2001]. [36] All four sulfate-bearing samples exhibit H 2 O and M-OH features that are most distinct in the <45 mm size fraction. The alunite-bearing samples (HWMK508 and HWMK740) have the characteristic alunite absorption bands at 1.48, 1.77, and 2.18 um (Figures 7 and 8). The feature at 2.21 mm HWMK740 results from kaolinite (Al-OH) as detected by XRD. Note that the reference spectrum for alunite (WD151; Figure 8b) also has kaolinite, but at a level spectrally intermediate to those for HWMK508 and HWMK740. The presence of bands near 1.42 and 1.92 mm for HWMK740 in particular are consistent with the presence of the smectite shown in XRD data. [37] The jarosite-bearing samples (HWMK515 and HWMK940) have the characteristic jarosite absorption bands at 1.85 and 2.27 mm. Phyllosilicates are not spectrally indicated for these two samples, although minor amounts of serpentine (Figure 8) would not be detected in the presence of jarosite. The low intensity of the 1.9 mm and especially the 1.4 mm band also shows that H 2 O bearing phyllosilicates are not spectrally important relative to the alunitebearing tephra. Phyllosilicates were also not detected in XRD data for the jarosite-bearing tephra Middle Infrared Spectral Region Overview [38] The primary features observable in the MIR region ( cm 1 ) in these samples arise from fundamental vibrational modes of oxides, silicates, and sulfate anions, the H 2 O molecule, and from scattering in fine particulates. The shapes and positions of spectral features provide information about the composition and structure of phases in the tephra samples. MIR emissivity spectra are shown in Figures 3 6. The assignment of features to specific phases as discussed below is based on direct comparison (e.g., Figures 10 and 11) to spectra we provide or that are found in the literature [e.g., Salisbury et al., 1991b; Ramsey and Christensen, 1998; Christensen et al., 2000; Wyatt et al., 2001; Michalski et al., 2003]. Relative to the spectra of pure minerals and well crystalline rocks, the spectral contrast of unaltered and altered tephras in the cm 1 range generally is low. Reduced emissivity (leading to greater spectral contrast) in the spectra of the finer (<45 mm) size fractions, particularly between 1800 and 1300 cm 1,isa result of volume scattering as the particle size approaches the size of the wavelength of observation [e.g., Hunt and Vincent, 1968; Aronson and Emslie, 1973; Salisbury and Wald, 1992]. The magnitude of this effect varies inversely with particle size and the finest size fractions generally exhibit this effect most strongly. The emissivity peak at 1640 cm 1 is attributable to the fundamental bending mode of H 2 O in the samples. This feature (discussed for all samples in section 3.4.7) appears as a peak in emission in fine particulate samples because the H 2 O fundamental is strongly absorbing (lowering reflectivity/increasing emissivity, and not subject to scattering otherwise observed in this wave number region). The high-frequency features in this region and in the region between 400 and 250 cm 1 are attributable to water vapor (e.g., sample HWMK513A). 14 of 30

15 Figure 11. Comparison of midinfrared emissivity spectra of altered tephras and minerals that are the tephras primary alteration products: (a) hydrothermally altered tephra dominated by the phyllosilicate montmorillonite, (b) hydrothermally altered tephra dominated by cristobalite, and tephras altered under hydrothermal acid sulfate conditions, containing (c) alunite and (d) jarosite. Mineral spectra are offset by 0.05 for clarity and may be contrast reduced for comparison Unaltered Tephra [39] Between 1300 and 800 cm 1, the emissivity spectra of unaltered tephras (HWMK513 and PH-13 in Figures 3b and 3d) are characterized by a relatively featureless V-shaped absorption with a minimum near 1040 cm 1 (region for silicate stretching vibrations) and another absorption near 440 cm 1 (region for silicate bending vibrations). This shape typically is observed for glass and poorly crystalline materials [Ondrusek et al., 1993; Ramsey and Fink, 1999; Wyatt et al., 2001; Michalski et al., 2003], and is consistent with VNIR and XRD data for these samples. The wave number position of the V-shaped emissivity minimum is dependent on bulk chemical composition, as shown in Figure 10. The position ranges from 1130 cm 1 for pure SiO 2 glass to lower values as the SiO 2 concentration decreases in granitic composition glass and obsidian (1087 cm 1 ), and basaltic glass (960 cm 1 ). The position of the bending vibration also varies in the same sample order from 484 cm 1 to 440 cm 1. The Fe, Mg, Ca, etc. in a basaltic glass, relative to SiO 2 -rich glasses, lengthen the average Si-O bond distance, which results in a shift of the Si-O features near 1000 and 500 cm 1 to lower wave numbers. This behavior also is observed in XRD data, in which the broad envelope of the glass shifts to larger 2-Theta with increasing basaltic composition. The basaltic glass features in MIR spectra are less intense and broader than in SiO 2 glass spectra simply because the distribution of energies (bond lengths) is greater. [40] The MIR spectra of PH-13 and HWMK513 ( mm size fraction) are broadly similar to that of the quenched basalt (Figure 10), except that the silicate stretching feature has a minimum near 1040 cm 1 in addition to a shoulder that resembles the emissivity minimum at 960 cm 1 in the basaltic glass spectrum. A possible explanation for the shape of the tephra spectra is that they are chemically heterogeneous glasses whose range in composition is sufficient to produce a complex structure in the stretching region but not in the bending region Palagonitic Tephra [41] MIR emissivity spectra of palagonitic tephras (Figures 3f, 3h, 3j, and 3l) show that the relatively deep V shape observed in the spectra of unaltered basaltic tephras in the cm 1 region is not apparent, having been replaced by a broad, bowl-shaped feature of variable shape having a minimum nearer 1000 cm 1. The decreased emissivity (and shoulder) near 800 cm 1 in the C spectra (<45 mm) and some of the A spectra (<1 mm) is a transparency feature (scattering in fine particulates) attributable to feldspar [Ramsey and Christensen, 1998]. HWMK919C also shows a superimposed slope at <1300 cm 1, which may be an effect of a thermal gradient in the sample, which can occur in fluffy fine particulate samples [e.g., Henderson and Jakosky, 1994]. At low wave numbers (<600 cm 1 ), the pronounced minimum that was present in the unaltered samples is not visible, and the spectrum instead displays a generally negative slope and 15 of 30