Hyperspectral mapping of alteration assemblages within a hydrothermal … · 2019-04-19 ·...

Hyperspectral mapping of alteration assemblages within a hydrothermal vug at the

Haughton impact structure, Canada

Rebecca N. GREENBERGER1,9,*, John F. MUSTARD1, Gordon R. OSINSKI2,3,4,Livio L. TORNABENE2,3,5, Alexandra J. PONTEFRACT2,3,6, Cassandra L. MARION2,3,

Roberta L. FLEMMING2,3, Janette H. WILSON7, and Edward A. CLOUTIS8

1Department of Earth, Environmental, and Planetary Sciences, Brown University, 324 Brook St, Box 1846, Providence, Rhode

Island 02912, USA2Centre for Planetary Science and Exploration, University of Western Ontario, 1151 Richmond Street, London,

Ontario N6A 5B7, Canada3Department of Earth Sciences, University of Western Ontario, 1151 Richmond Street, London, Ontario N6A 5B7, Canada4Department of Physics and Astronomy, University of Western Ontario, 1151 Richmond Street, London, Ontario N6A 5B7,

Canada5SETI Institute, 189 Bernardo Ave, Mountain View, California 94043, USA

6Planetary Science Institute, 1700 East Fort Lowell, Suite 106, Tucson, Arizona 85719, USA7Headwall Photonics, Inc., 601 River Street, Fitchburg, Massachusetts 01420, USA

8Department of Geography, University of Winnipeg, 515 Portage Avenue, Winnipeg, Manitoba R3B 2E9, Canada9Present address: Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, M/S 171-310, Pasadena,

California 91109, USA*Corresponding author. E-mail: [email protected]

(Received 30 March 2015; revision accepted 12 July 2016)

Abstract–Meteorite impacts on Earth and Mars can generate hydrothermal systems thatalter the primary mineralogies of rocks and provide suitable environments for microbialcolonization. We investigate a calcite–marcasite-bearing vug at the ~23 km diameterHaughton impact structure, Devon Island, Nunavut, Canada, using imaging spectroscopy ofthe outcrop in the field (0.65–1.1 lm) and samples in the laboratory (0.4–2.5 lm), pointspectroscopy (0.35–2.5 lm), major element chemistry, and X-ray diffraction analyses. Themineral assemblages mapped at the outcrop include marcasite; marcasite with minorgypsum and jarosite; fibroferrite and copiapite with minor gypsum and melanterite; gypsum,Fe3+ oxides, and jarosite; and calcite, gypsum, clay, microcline, and quartz. Hyperspectralmapping of alteration phases shows spatial patterns that illuminate changes in alterationconditions and formation of specific mineral phases. Marcasite formed from the postimpacthydrothermal system under reducing conditions, while subsequent weathering oxidized themarcasite at low temperatures and water/rock ratios. The acidic fluids resulting from theoxidation collected on flat-lying portions of the outcrop, precipitatingfibroferrite + copiapite. That assemblage then likely dissolved, and the changing chemistryand pH resulting from interaction with the calcite-rich host rock formed gypsum-bearingred coatings. These results have implications for understanding water–rock interactions andhabitabilities at this site and on Mars.

INTRODUCTION

The heat resulting from impact processes generateshydrothermal systems when water or ice is present inthe target rocks (e.g., Newsom 1980; Naumov 2005;Osinski et al. 2013). Evidence of hydrothermal systems

is seen at most complex craters on Earth that have beenstudied in any detail, with hydrothermal mineralizationidentified in impact melt-bearing units, the centraluplift, ejecta, the crater rim, and sediments from lakesfilling the craters (Osinski et al. 2013). There is reportedpostimpact hydrothermal alteration at structures as

Meteoritics & Planetary Science 51, Nr 12, 2274–2292 (2016)

doi: 10.1111/maps.12716

2274© The Meteoritical Society, 2016.

small as Lonar crater (1.8 km diameter; Hagerty andNewsom 2003). A range of secondary minerals havebeen identified at terrestrial impact structures, includingsilicates (clays, zeolites, feldspars, quartz), carbonates,sulfides, sulfates, native elements, oxides/hydroxides,and halides (Naumov 2005; Osinski et al. 2013), andmodels have placed constraints on the mineralassemblages expected through similar processes on Mars(Schwenzer and Kring 2009, 2013; Schwenzer et al.2012).

Key to understanding these impact-generatedhydrothermal systems are the mineral assemblages thatform through low-temperature weathering of originalhydrothermal mineralization after hydrothermalcirculation ceased because these are the assemblagesobserved today (Izawa et al. 2011; Osinski et al. 2013).Both the hydrothermal mineralization and theweathering products elucidate the conditions of water–rock interactions since the impact event and shed lighton environments that are promising for microbialcolonization (Newsom et al. 1996, 2001; Kring 2000;Abramov and Kring 2005; Schulze-Makuch et al. 2007;Parnell et al. 2010; Izawa et al. 2011; Osinski et al.2013; Sapers et al. 2014). Furthermore, studies ofaqueous processes within terrestrial impact structurescan aid in our understanding of similar processes onMars, where such systems likely occurred (e.g., Newsom1980; Newsom et al. 2001; Osinski et al. 2013;Tornabene et al. 2013).

Specific morphologies and special characteristics onMars have been attributed to impact-drivenhydrothermal systems, such as at Toro crater (Marzoet al. 2010) and Holden crater (Tornabene et al. 2009;Osinski et al. 2013), and secondary minerals throughouthighly cratered regions of Mars have been suggested toform from a variety of impact-induced alterationpathways, including hydrothermal systems (Tornabeneet al. 2013). In situ evidence from the Opportunityrover includes Ca-sulfate veins and deposits enriched inZn that may have formed from impact-drivenhydrothermal processes at the ~22 km diameterEndeavour Crater (Squyres et al. 2012; Arvidson et al.2014).

Studies of mineral deposits formed through impact-generated hydrothermal processes on Earth usingremote sensing technologies can help us to betterunderstand such systems and lead to development ofexploration criteria for investigating similar deposits onMars. Here, we investigate a vug lined withhydrothermal calcite and marcasite formed throughimpact processes at the Haughton impact structure,Devon Island, Nunavut, Canada, using noveltechniques. Previous work by Izawa et al. (2011)characterized individual samples from the vug and the

habitability of the weathering products, and their worksuggested that the marcasite altered and weathered atlow temperatures to sulfates and oxides. What remainsunexamined is the spatial relationships between thedifferent mineral assemblages, which provide newinsights into the water–rock interactions andhabitability of this site. We expand on work by Izawaet al. (2011) by using imaging spectroscopy in the fieldand laboratory to map the spatial distributions of theprimary hydrothermal mineralization and later low-temperature weathering products. We report the resultsof spectral mapping of the deposit; the insights into theevolution of fluids as temperatures, redox states, andchemistries changed; and the implications for detectingand understanding the habitability of such systems onMars.

GEOLOGICAL SETTING

The calcite–marcasite-bearing vug within theHaughton impact structure is located in the High Arcticof Canada at UTM 16N 0428046 E, 8364683 N(Fig. 1). The Haughton impact structure has a ~23 kmapparent diameter (Osinski et al. 2005b) and is dated to23.5 � 2.0 Ma (Young et al. 2013). This compleximpact structure formed from a meteorite impact into~1880 m of Lower Paleozoic sedimentary rocks and theunderlying Precambrian crystalline basement rocks(Osinski et al. 2005b). The clast-rich impact melt sheetoriginally covered an area approximately 12 km indiameter (125 km2) and today covers ~54 km2 of thecrater interior (Osinski et al. 2005b), containing lithicclasts within a matrix of quenched calcite melt, silicateglass, and anhydrite (Osinski and Spray 2001, 2003;Osinski et al. 2005b). The geologic units at the impactstructure have been mapped on the ground throughtraditional geologic methods by Osinski et al. (2005b),and these units are in excellent agreement withlithologic mapping using orbital spectroscopic remotesensing data by Tornabene et al. (2005).

There is evidence for structurally controlledhydrothermal mineralization throughout the Haughtonimpact structure, with hydrothermal mineralizationidentified in the impact melt rocks, central uplift, andfaulted crater rim (Osinski et al. 2001, 2005a). Mineralsthat formed in these hydrothermal systems includecalcite, selenite, marcasite, quartz, celestite, barite,fluorite, and Fe oxides/hydroxides (Osinski et al.2005a). We focus here on the largest singlehydrothermal deposit at Haughton, a calcite- andmarcasite-bearing vug within the impact melt rocks(Fig. 1). Osinski et al. (2005a) suggest that themarcasite here formed at temperatures 100–240 °C andpH <5 during the main stage of the postimpact

Hyperspectral mapping of alteration at Haughton 2275

hydrothermal system. Homogenization temperatures offluid inclusions within the late-stage calcite from thesame vug give similar formation temperatures of 118–210 °C (Osinski et al. 2005a). These minerals formedduring the main stages of hydrothermal activity as thesystem cooled and both liquid and vapor were present

(Osinski et al. 2001, 2005a). The sulfides weathered tosulfates and oxides/hydroxides, including copiapite,goethite, jarosite, rozenite, fibroferrite, gypsum,melanterite, and szomolnokite (Izawa et al. 2011). Thechemical and redox gradients present at this deposit arethought to be good energy sources for microbes (Izawa

Fig. 1. Geologic map of the Haughton impact structure modified from Osinski et al. (2005a) with the location of the calcite- andmarcasite-bearing vug studied here starred. The vug is located within the clast-rich impact melt rocks. The inset in the upper leftshows the approximate location of the impact structure.

2276 R. N. Greenberger et al.

et al. 2011). In addition, isotopic fractionations betweensedimentary gypsum and hydrothermal marcasite andpyrite at Haughton are too high to have formed fromabiotic processes and rather are likely the result ofmicrobial activity during the hydrothermal system thatwas active within 10,000 yr of the impact event (Parnellet al. 2010). These microbial signatures are alsopreserved in the sulfates that formed throughweathering of the sulfides (Parnell et al. 2012).

METHODS

Overview

The calcite–marcasite-bearing vug is several meterswide and located within the clast-rich impact meltrocks. This deposit was exposed as the adjacent riversystematically eroded into the near-vertical cliff ofimpact melt rock. The outcrop was imaged in the fieldwith a hyperspectral imager, and five samplesrepresentative of the different mineral assemblagesvisibly identified at the site and located within images ofthe outcrop were collected. Sample descriptions aregiven in Table 1. These samples were analyzed with apoint spectrometer in the field, and other mineralogicaland chemical analyses were completed in the laboratoryupon return.

Point Spectroscopy

Point spectra were acquired on various coatingsand surfaces of each sample with an AnalyticalSpectral Devices, Inc. (ASD) Fieldspec 3 from 0.35–2.5 lm with spectral resolutions of 3 nm at 0.7 lmand 10 nm at 1.4 and 2.1 lm. This instrument’ssampling intervals are 1.4 nm for wavelengths 0.35–1.0 lm and 2 nm for wavelengths 1.0–2.5 lm, and thesoftware automatically resamples spectra to 1 nmspacing for output. Samples collected from the outcropwere stored on ice within a small cave while in thefield to minimize changes in mineralogy or hydrationstate before ASD spectra were obtained. The

measurements were performed in the field using theASD contact probe attachment, which has its owninternal light source and a viewing geometry ofapproximately 0° incidence and 30° emission angles.Measurements were made relative to Spectralon� andcorrected for the reflectance properties of Spectralon�

by multiplying by spectrum HL-JFM-012 of SRT-99-050 measured in the Reflectance ExperimentLaboratory (RELAB, Brown University, Providence,RI, USA; Pieters 1983) that was resampled to thewavelengths of output ASD spectra.

Hyperspectral Imaging in the Laboratory

All samples collected at the outcrop except APDI-13-14j were imaged at visible-shortwave infrared(VSWIR) wavelengths in the laboratory at HeadwallPhotonics, Inc. (Fitchburg, MA, USA) using theirpushbroom High Efficiency Hyperspec� visible–near-infrared (VNIR) E-series imager (338 spectral channelsfrom 0.4 to 1.0 lm, 7 nm spectral resolution defined asthe full width at half maximum of the slit imageintensity, 1.785 nm sampling interval) and HighEfficiency Hyperspec� shortwave infrared (SWIR) X-series imager (138 spectral channels from 1.0 to2.5 lm, 12 nm spectral resolution, and 12.0656 nmspectral sampling). Samples were moved through theimaging spectrometers’ fields of view on a motorizedscanning table that is part of the Hyperspec� StarterKitTM and illuminated with a quartz tungsten halogenbulb. The lens on the VNIR imager has aninstantaneous field of view (IFOV) of 0.382 mrad,corresponding to spatial resolutions up to ~43.5 lm/pixel. The lens on the SWIR imager has an IFOV of1.2 mrad with achievable spatial resolutions up to~133 lm/pixel. The imagers viewed the samples fromnadir. Where possible, SWIR images were warped toVNIR images using manually selected control points, afirst degree polynomial, and nearest neighborresampling. The VNIR and SWIR images were thenjoined at 0.997 lm (VNIR) and 0.998 lm (SWIR) andstacked to create a single image cube, deletingoverlapping bands and removing any offset betweenthe detectors by multiplying the SWIR image by afactor that assumed no change in reflectance between0.997 and 0.998 lm. It was not possible to join imagesof samples with significant surface topography becausethe changes in distance from the different lenses of theVNIR and SWIR systems led to spatial offsets thatcould not be removed with simple polynomials.Imaging and calibration methods are described byGreenberger et al. (2015a). Analyses of these imageswere done through use of spectral (e.g., Clark andRoush 1984; Pelkey et al. 2007; Viviano-Beck et al.

Table 1. Sample descriptions.

Sample name Sample description

APDI-13-14a Yellow, popcorn-textured mixed sulfate

APDI-13-14b Dark red coating on calcite host rockAPDI-13-14c Small green marcasite crystals with

minor pyrite on calcite host rock

APDI-13-14h Friable blue-gray phaseAPDI-13-14j Oxidized, dark red, weathered marcasite

with the primary textures and morphologiesof the marcasite still visible


2014) and mineral indicator parameters given inTable S1 in supporting information.

Near-Infrared (NIR) Hyperspectral Imaging in the Field

The outcrop was imaged in the field during the2013 field season using a Channel Systems near-infrared(NIR) hyperspectral imager. This imaging system is anacousto-optic tunable filter with 10 nm spectralresolution and sampling that measures from 0.65 to1.1 lm. The dark current signal of the instrument wasmeasured by acquiring an image with the lens cap on,and this image was subtracted from the image of theoutcrop before atmospheric correction to removeinstrumental effects. Exposure times at each wavelengthwere determined using the imager’s autoexpose functionthat maximizes the exposure time at each wavelengthwithout saturating. The exposure times were saved, andthe images where light reached the sensor (i.e., all butthe dark current images) were normalized to accountfor the varying exposure times as the signal receivedscales linearly with exposure time for images where lightreaches the sensor. Dark current measurements areindependent of exposure time, and the dark imagestherefore were not normalized for exposure times (i.e.,the raw signal from the dark measurements wassubtracted from the raw signal of the outcrop and whiteimages; see equation below). Images were calibratedthrough a dark object subtraction using the minimummeasurement of any pixel within the image at eachwavelength, which was assumed to be the additivecontribution to the signal from atmospheric scattering(Vincent 1972; Chavez 1988, 1996, and referencestherein). Then, the image was calibrated to reflectanceby dividing by the spatially averaged signal receivedfrom a Spectralon� panel within an image taken next tothe outcrop and multiplied by the same spectrum ofSpectralon� used in the ASD calibration but resampledto the wavelengths sampled by this imager. Thecalibration to reflectance (R) was done using thefollowing equation:

R ¼Sds�Do

toWds�Dds

ts

� Rs;

where Sds is the signal in the image of the outcropafter subtracting the signal from the dark image ateach pixel, Do is the dark object spectrum describedabove for the outcrop image, to are the exposure timesat each wavelength for the image of the outcrop, Wds

is the average signal from the image of theSpectralon� panel after subtracting the dark current ateach pixel, Dds is the dark object spectrum from the

image of the Spectralon� panel, ts are the exposuretimes at each wavelength for the image of theSpectralon� panel, and Rs is the reflectance spectrumof Spectralon� measured in RELAB. These methodsare similar to calibration of AVIRIS data using themethods of Clark et al. (2002). In total, five images ofthe outcrop were acquired.

Lithologies were mapped within the hyperspectralimages using spectral parameters (e.g., Clark and Roush1984; Pelkey et al. 2007; Viviano-Beck et al. 2014)and lithological indicators, which are combinationsof spectral parameters (Table S2 in supportinginformation). These parameters and indicators werederived using spectra from the NIR image of the samplesthat were later collected from within the scene andanalyzed in the laboratory, and the parameters werefurther refined to ensure that obviously differentlithologies (i.e., different color or morphology inphotographs, different appearance in NIR false colorcomposites, or different spectral characteristics) were notmapped by the same parameters. The equations weredeveloped using spectra of rock surfaces, not powders. Inthe wavelength range sampled by the NIR imager,spectra at this outcrop show electronic transitions andintervalence charge transfers of transition metal cations,Fe2+ and Fe3+ (e.g., Burns 1993). Here, Fe sulfides,sulfates, and oxides are known to be present (Izawa et al.2011), and variations in the shape, center wavelength,width, and slope of these features at the wavelengthssampled by the imager are due to the differences in Femineralogy. Different mapped units correspond with thesamples that were collected and, using the ASD spectraand spectra from the images of the known samplinglocations, spectrally similar materials were mappedacross the outcrop. Spectral parameters given inTable S2 calculate depths of absorption features andslopes. Parameters mapping lithologies consistent withthe samples collected, also given in Table S2, weredetermined through an iterative process wherecombinations of spectral parameters are used torelatively uniquely map spectrally similar materials.

Major Element Chemistry Analyses

Major element chemistries for samples collected atthe outcrop were determined through inductivelycoupled plasma–atomic emission spectroscopy (ICP-AES) analyses at the Brown University EnvironmentalChemistry Facilities following a flux fusion samplepreparation for Al, Ca, Cr, Fe, K, Mg, Mn, Na, Ni, P,Si, and Ti content using methods modified from Murrayet al. (2000), with complete details given by Greenbergeret al. (2015a). Standard reference materials (NIST2711,MAG-1, DTS-2B, NIST1646a, BIR-1, SGR-1, BHVO-


2, NIST2702, BCR-2, G-2, and NIST1c) and blankswere run with the same methods as unknown samples.Solutions of fused beads quenched in 10% nitric acidwere analyzed on a JY2000 Ultrace ICP-AES usingintensities for peaks given in Murray et al. (2000), and asolution with intermediate composition was run initiallyand periodically throughout the analysis to correct forinstrumental drift with time. Concentration data weredetermined after correcting for instrument drift andsubtracting blanks using linear regression curves fromthe standard reference materials. One sample was run induplicate, and two were run in triplicate. Average errorsas determined by standards that were run as unknownswere typically <5% as a percentage of the actualconcentrations or <0.03 wt% in actual concentration.Standard deviations reported here are in some caseshigher likely due to sample heterogeneity for smallvolume samples (i.e., coatings). Analysis results arereported in oxide weight percentages to be consistentwith geochemical convention. However, some Fe is insulfides and is coordinated by S, not O.

The carbon content of the samples was measuredwith elemental analyses on a CE Instruments ModelNC2100 Elemental Analyzer. Samples were prepared byweighing 8–10 mg of each into tin capsules, which werethen folded and crushed. Two blanks were run, andvarying amounts of four standards (acetanilide,cyclohexane, pine–SRM 1575, and Montana soil–SRM2711 for all samples but APDI-13-14j and sulfanilamidefor APDI-13-14j) were analyzed to calculate regressioncurves and to check the quality and consistency of themeasurements. Typical errors on these measurementsare <10%.

X-bay Diffraction (XRD)

Bulk powdered samples were prepared for powderX-ray diffraction (pXRD) analyses using a back-packing method and measured on a Rigaku rotating-anode XRD with Co Ka radiation operated at 45 kVand 160 mA. With this instrument, monochromation isachieved with a curved crystal, diffracted beam, andgraphite monochromator, and X-rays were collimatedwith 1° divergent and scatter slits and a 0.15 mmreceiving slit. Samples were scanned from 2 to 82° 2hwith 0.020 steps at a rate of 10° 2h/min. Mineralogies ofcoatings were determined using micro-X-ray diffraction(lXRD; Flemming 2007) with no sample preparationon a Bruker D8 Discover lXRD. A 60 mm Co GobelMirror with a 300 lm snout provides a nominal 300 lmbeam diameter. This instrument has an area detector,and each sample was scanned in omega (x) scan modewith two frames. The parameters for the first framewere h1 = 15°, h2 = 20.5°, and x = 10, and this scan ran

for 90 minutes. The second frame was acquired over145 minutes with the following parameters: h1 = 34.5°,h2 = 40°, and x = 19. x is the degree of rotation ofboth source and detector in a clockwise direction duringanalysis. The two frames were converted to two-dimensional XRD patterns and combined to obtaincontinuous measurements from 14° to 95° 2h inDiffrac.Suite EVA software (https://www.bruker.com/products/x-ray-diffraction-and-elemental-analysis/x-ray-diffraction/xrd-software/overview/eva.html). Mineralogiesfrom pXRD and lXRD patterns were determined bycomparing peak positions of the unknowns withdiffraction patterns from the International Centre ForDiffraction Data (ICDD) database through an interfacein the software package EVA.

RESULTS

Individual Sample Analyses

ASD spectra of sample APDI-13-14a (Table 1;Fig. 2) have electronic transitions of Fe3+ at 0.42, 0.55,and 0.82 lm consistent with the Fe3+-bearing sulfateminerals fibroferrite and copiapite/ferricopiapite(Crowley et al. 2003; Cloutis et al. 2006). Copiapite andferricopiapite have spectra that are nearly identical inthis wavelength range and cannot be distinguished withVSWIR spectroscopy alone (Cloutis et al. 2006);however, the pXRD results are more consistent withcopiapite. A deep 0.55 lm electronic transition suggeststhat fibroferrite is also present (Crowley et al. 2003).The shape and positions of deep absorption featuresfrom OH and H2O are generally consistent with thesemineral identifications and include (1) a broad, complexfeature around ~1.4–1.6 lm composed of overlappingabsorptions at ~1.44, 1.49, and 1.55 lm (due to the OHstretching overtone of OH and H2O [Crowley et al.2003; Cloutis et al. 2006]); (2) a broad, weak featurearound 1.77 lm consistent with a H2O-bearing sulfate(Cloutis et al. 2006); and (3) a strong feature due to aH-O-H combination band at 1.93 and 1.99 lm (Huntet al. 1971; Clark et al. 1990; Cloutis et al. 2006).Flattening of the spectra from 2.1 to 2.3 lm may bedue to weak combination bands related to metal-OHbonds and SO4 (e.g., Cloutis et al. 2006), but individualfeatures are too poorly defined to determine the exactband centers and assignments. Overall, these overtonesand combination tones of vibrational absorptionfeatures in our reflectance spectra are consistent withcopiapite and fibroferrite (e.g., Crowley et al. 2003;Cloutis et al. 2006). Bearing in mind that the ASDmeasures a larger area, sometimes averaging multiplecomponents within a single spectrum, hyperspectralimaging allows us to obtain spectra of much smaller


https://www.bruker.com/products/x-ray-diffraction-and-elemental-analysis/x-ray-diffraction/xrd-software/overview/eva.html



areas (100s of micrometers) within single pixels andthereby identify additional phases. A VSWIRhyperspectral image of this sample (Fig. 3a) showssmall areas of spectrally distinct gypsum, which isreadily identified by its characteristic triplet absorptionfeature at 1.4 lm (e.g., Hunt et al. 1971; Clark et al.1990). Mineral identifications of copiapite, fibroferrite,and gypsum are confirmed by pXRD. pXRD alsoidentifies melanterite, which was not identified withspectroscopy, perhaps due its low abundance. Anotherpossibility is that the melanterite may be finelydisseminated within the sample, and its spectralsignature could be masked by the other phases that arepresent and have strong spectral features. Cull-Hearthet al. (2016) identified shifts in the positions of copiapiteabsorption features when mixed with melanterite,particularly a longward shift of the 0.866 lm feature.The spectra measured here do not show evidence forthose shifts likely due to the presence of fibroferrite aswell, which should cause a shift in the oppositedirection for the 0.866 lm feature. The sample cation

chemistry is dominated by Fe with minor Ca, consistentwith the identification of Fe-bearing sulfates (copiapite/ferricopiapite and fibroferrite) with minor gypsum.

ASD spectra of sample APDI-13-14b (Table 1;Fig. 2) exhibit absorptions from electronic transitionsof Fe3+ centered around 0.93–0.95 lm and 0.51 lmthat may arise from a Fe3+-oxide or -sulfate, such asgoethite or jarosite (e.g., Burns 1993; Scheinost et al.1998; Cloutis et al. 2006), though spectra lack otherSWIR features due to jarosite. Based on lXRD results,these spectral characteristics are consistent with acombination of both goethite and jarosite. In theSWIR, these spectra have a well-expressed triplet at1.4–1.7 lm from OH stretching overtones that matchgypsum (e.g., Hunt et al. 1971; Clark et al. 1990).Additional absorption features at 1.75/1.77 lm, 1.94/1.98 lm (H2O combination band), 2.17–2.18 (S-Oovertone or OH/H2O combination or overtone), 2.22(S-O overtone or OH/H2O combination or overtone),2.27 (S-O overtone or OH/H2O combination orovertone), 2.43 (S-O overtone), and 2.48 lm (S-O

0.5 1.0 1.5 2.0 2.5

Wavelength (μm)

Refle

ctan

ce (o

ffset

and

sca

led

for c

larit

y)APDI-13-14a

APDI-13-14b

APDI-13-14c

APDI-13-14h

APDI-13-14j

NIR imager

0.8 1.0

ASD spectra of samples NIR spectra of similar units

0.5 1.0 1.5 2.0 2.5

Library spectra

Calcite

Montmorillonite

Gypsum

Hematite

Copiapite

Marcasite

Fig. 2. left) Reflectance spectra of samples measured in the field with an ASD with contact probe attachment. middle)Reflectance spectra from the NIR images of similar units to those sampled and measured with the ASD. The colors match thecorresponding ASD spectra and the units mapped in Fig. 5. (right) Library spectra of some minerals present in the samplesanalyzed here. Copiapite (GDS21), hematite (GDS27), gypsum (SU2202), montmorillonite (STx-1), and calcite (CO2004) spectraare from the USGS spectral library (Clark et al. 2007). The marcasite spectrum (MAR101) is from the RELAB database (Pieters1983) and was originally reported by Cloutis and Gaffey (1994). Spectra in all plots are offset and scaled for clarity.


overtone), all confirm the identification of gypsum (e.g.,Hunt et al. 1971; Clark et al. 1990; Cloutis et al. 2006).Hyperspectral imaging also shows that this coating iscomposed of fine-grained Fe3+-oxides, Fe3+-sulfates,and gypsum, with variations in the relative strengths ofabsorption features likely due to variations in therelative abundances of these phases (Fig. 3b). Thechemical composition of this sample is more complexthan APDI-13-14a, with a large component of Ca inaddition to Fe (Table 2). Carbon is detected in thissample and is likely due to contributions from thecalcite on which this coating precipitated. lXRDidentifies gypsum, jarosite, akaganeite, and goethite in

these coatings (Table 2). pXRD also identifies pyrite,quartz, calcite, and marcasite. Some interior faces notin the field of view of the NIR hyperspectral data havevisually identified marcasite and likely also pyrite, arenot the coatings mapped in the NIR hyperspectraldata, and were not targeted for measurement bylXRD.

The reflectance spectrum of APDI-13-14c (Table 1;Fig. 2) exhibits a crystal field absorption featurecentered around 0.83–0.84 lm due to Fe2+ and isroughly consistent with marcasite (Hunt et al. 1971;Cloutis and Gaffey 1994). ASD spectra of otherportions of the sample indicate the presence of calcite

Fig. 3. Hyperspectral images of three samples measured in the laboratory. A–C) Color composite images with bands selected toapproximate a true color image for samples (A) APDI-13-14a, (B) APDI-13-14b, and (C) APDI-13-14h. Red: 0.666 lm, green:0.534 lm, blue: 0.427 lm. D) Mineral indicator map of sample APDI-13-14a. Magenta: fibroferrite + copiapite, green: gypsum2.E) Spectral parameter and mineral indicator map of sample APDI-13-14b. Cyan: gypsum2. Red: BD950 (due to Fe3+). F)Mineral indicator map of sample APDI-13-14h. Magenta: calcite. Green: gypsum1. See Table S1 for descriptions of eachparameter. The parameters shown in (E) and (F) have been median filtered to reduce noise.


on which the marcasite precipitated and gypsum. Thehigh Fe concentration measured through chemicalanalyses is consistent with the spectral interpretation ofmarcasite (Table 2). There is also significant Ca in thissample, which is likely from the calcite-rich host rockthat was not completely removed during samplepreparation. lXRD data of two points on the surfaceof the sample identify only marcasite. The powderedsample for pXRD indicated other phases in the largerrock, including calcite, celestine, pyrite, gypsum, andquartz.

APDI-13-14h spectra (Table 1; Fig. 2) have weak,broad absorption features arising from electronictransitions of Fe2+ centered at ~0.9 and 1.1 lm, whichis potentially in an Fe2+-bearing phyllosilicate or asulfate such as glauconite or rozenite (Crowley et al.2003; Cloutis et al. 2006). Spectra also indicate somegypsum identified by a weak triplet from OH stretchingovertones near 1.4–1.5 lm and the shape of the 1.9 lmabsorption band (e.g., Hunt et al. 1971; Clark et al.1990). The absorption feature at 2.21 lm may be theresult of an Al-OH combination in a clay such as

montmorillonite or illite, from an OH/H2O combinationor S-O overtone in gypsum, or both (e.g., Clark et al.1990; Cloutis et al. 2006) or could also result fromminor water in quartz (Aines and Rossman 1984;Goryniuk et al. 2004). Finally, the deeper 2.34 lmabsorption feature is a C-O combination in calcite(Hunt and Salisbury 1971). Interpretations ofspectroscopy suggest that this sample is a mixture ofFe2+- and Al-bearing clays, calcite, and gypsum withpossible hydrated silica. While most of the sample isdominated spectrally by calcite, VSWIR imagingspectroscopy identifies small areas with more gypsum(Fig. 3c). Chemical analyses show high Ca andrelatively high Si compared with other samples fromthis site along with some Mg and K (Table 2). The Fecontent of this sample is quite low relative to othersfrom this site. pXRD analyses identify quartz,microcline, and hematite in addition to phases identifiedthrough spectroscopy (Table 2). There are likely bothsmectite and illite clays present, and a clay such asglauconite is consistent with the pXRD pattern andcould explain the Fe2+ feature in the spectra.

Table 2. Chemical compositions (ICP-AES and elemental analyses) in wt% with standard deviations given formeasurements with multiple replicates, tentative identifications with VSWIR spectroscopy, and mineralogy frompXRD and/or lXRD for samples discussed in the text. Reported compositions are averages for samples withreplicate analyses.

APDI-13-14a APDI-13-14b APDI-13-14c APDI-13-14h APDI-13-14j

Replicates 2 1 3 3 1Al2O3 0.15 � 0.02 0.31 0.14 � 0.01 2.97 � 0.12 0.07CaO 2.40 � 0.01 18.78 18.60 � 0.61 38.60 � 0.94 3.93

Cr2O3 0.001 � 0.000 0.001 n.d. 0.002 � 0.000 0.001FeO 25.86 � 0.06 28.08 25.97 � 3.85 2.54 � 0.26 38.79K2O 0.08 � 0.01 0.14 0.06 � 0.01 1.78 � 0.09 0.03

MgO 0.84 � 0.00 0.47 0.72 � 0.06 2.13 � 0.08 0.13MnO 0.001 � 0.00 0.005 0.005 � 0.000 0.012 � 0.000 0.002Na2O 0.018 � 0.027 n.d. 0.002 � 0.011 0.022 � 0.008 n.d.NiO 0.001 � 0.000 n.d. n.d. 0.001 � 0.000 0.002

P2O5 0.01 � 0.02 0.01 0.01 � 0.01 0.05 � 0.01 0.01SiO2 0.99 � 0.00 2.01 1.15 � 0.03 18.52 � 1.14 0.69TiO2 0.005 � 0.000 0.009 0.004 � 0.001 0.128 � 0.010 n.d.

C 0.06 � 0.02 2.99 3.05 � 0.12 8.11 � 0.10 1.77Mineralogy VSWIR: Fft,

Cop, minor Gp

pXRD: Fft,Cop, Gp, Mlt

VSWIR: Gp, Fe3+-oxideor sulfate

lXRD (coating):Gp, Jrs, Gth, AkgpXRD: Cal,

Gp, Mrc, Py, Qz

VSWIR: MrclXRD: Mrc

pXRD: Mrc,Cal, Py, Clt, Gp, Qz

VSWIR: Fe2+-clay/sulfate,Al-clay, Gp, Cal

pXRD: Qz, Cal, Mc,Gp, Hem, Ilt, Mnt

VSWIR: Mrc,Jrs or Gth

lXRD: Gp,Jrs, Mrc

n.d. = not detected, Mineral abbreviations (from Whitney and Evans [2010] where possible), endmember chemical formulas, and best matching

ICDD card numbers: Akg = akaganeite (Fe3+O(OH,Cl); 00-042-1315), Cal = calcite (CaCO3; 01-083-1762), Clt = celestine (SrSO4; 00-005-

0593), Cop = copiapite (Fe2+Fe3+4(SO4)6(OH)2�20H2O; 00-035-0583), Fft = fibroferrite (Fe3+(SO4)(OH)�5H2O; 01-083-1803), Gp = gypsum

(CaSO4�2H2O; 00-036-0432), Gth = goethite (Fe3+OOH; 00-029-0713); Hem = hematite (Fe2O3; 00-003-0800); Ilt = illite (KAl2(Si3Al)

O10(OH)2; 00-010-0490), Jrs = jarosite (KFe3+3(SO4)2(OH)6; 00-002-0602), Mc = microcline (KAlSi3O8; 00-022-0675), Mlt = melanterite

(Fe2+SO4�7H2O; 00-022-0633), Mnt = montmorillonite ((Na,Ca)0.3(Al,Mg)2Si4O10(OH)2�nH2O; 00-007-0051), Mrc = marcasite (FeS2; 00-

037 = 0475), Py = pyrite (FeS2; 01-073-8127), Qz = quartz (SiO2; 00-005-0490).


Spectroscopy of APDI-13-14j (Table 1) shows apeak in reflectance (0.65 lm) at longer wavelengths thanthe marcasite sample APDI-13-14c (0.54 lm) attributedto partial oxidation (Fig. 2). There is also a weak Fe3+

electronic transition absorption feature at 0.48 lm inAPDI-13-14j and a stronger crystal field absorptionfeature centered at 0.87–0.90 lm that may result fromthe convolution of the underlying marcasite with aFe3+-oxide such as goethite, which has a Fe3+

electronic transition at 0.95 lm (Scheinost et al. 1998),or a Fe3+-sulfate such as jarosite. The spectrum has anegative slope into the infrared that is often observedwith fine-grained coatings, where the bright coatingdominates the spectrum at the shortest wavelengths butis more transparent at longer wavelengths (Singer andRoush 1983; Fischer and Pieters 1993; Johnson andGrundy 2001). The negative slope may also result fromRayleigh scattering of extremely fine particles (Clarket al. 2008; Brown 2014). H-O-H combination bandsare seen at 1.9 lm along with a very weak, broadfeature at ~2.20–2.27 lm from S-O or OH/H2Oovertones or combination bands that are too weak andpoorly defined to assign to a specific mineralogy (e.g.,Clark et al. 1990; Cloutis et al. 2006). This sample ischemically similar to APDI-13-14c, though with higherFe and lower Ca, likely due to a lower proportion ofthe calcite host rock powdered with the sample(Table 2). lXRD analyses of the sample surface identifygypsum and jarosite in addition to marcasite, thoughlow abundances of Ca and K suggest that the gypsumand jarosite are minor phases.

Outcrop Imaging and Mapping

The outcrop units spectrally similar to the samplesdescribed above were mapped with the NIR imagingspectroscopy. The spectra from the NIR imager (Fig. 2)are relatively consistent with ASD spectra ofcorresponding units, with slight differences in overallslopes probably due to artifacts from the atmosphericcorrection, differences in viewing geometry between theSpectralon� panel used for calibration and the outcrop,or differences in the precise spatial area sampled.However, these differences occur in all spectra, andthere is remarkable consistency between spectra of thesame materials present across the images. In these data,variations in the positions of Fe2+ and Fe3+ electronictransitions and slopes result from differences inmineralogy between the units.

Spectral parameters (Table S2) were used to mapspectrally similar units, an example of which is shownin Fig. 4. These units were drawn onto a photograph ofthe outcrop (Fig. 5). Several key relationships betweenphases are identified. Large areas of marcasite (green)

and oxidized marcasite (brown) occur on the roof of thevug. The oxidized marcasite sometimes occurs entirelywithin the unaltered marcasite and sometimes occursalong the edges of the marcasite. The mixed sulfatephase (yellow) is often found on flat areas below themarcasite, and the red coating (red) generally is presentimmediately below the mixed sulfates. Only one smallpatch of the blue-gray phase (blue) is identified.

DISCUSSION

The focus of our investigations is a vug in whichthere has been significant hydrothermal mineralizationwithin the clast-rich impact melt rocks of the Haughtonimpact structure. Using imaging spectroscopy of thisoutcrop, we map the spatial distributions of primaryand secondary assemblages to understand the formationpathways and the habitability of the site. The primaryhydrothermal mineralogies identified here are calciteand marcasite. Secondary weathering products include amixture of fibroferrite and copiapite; a gypsum,akaganeite, goethite, and jarosite coating on the calcitehost rock; oxidized marcasite with formation of a thinFe3+-oxide and sulfate coating; and a blue-gray phasethat is a mixture of calcite, gypsum, clay minerals,microcline, quartz, and hematite. The mineralogical andchemical analyses show remarkable agreement: thecation composition of samples APDI-13-14a, APDI-13-14c, and APDI-13-14j are dominated by Fe, consistentwith mineral identifications; sample APDI-13-14bcontains Fe- and Ca-bearing minerals and chemicallycontains significant Fe and Ca; and both the mineralogyand chemistry of APDI-13-14h are more complex. Themineral identifications are similar to those reported forthe same site by Izawa et al. (2011) with the exceptionof jarosite, which was only identified as a minorcomponent in the coatings of APDI-13-14b and APDI-13-14j, likely because the locations within the outcropwhere jarosite has previously been found as a majorcomponent were not sampled and/or they had mostlybeen removed by previous sampling. Our work extendsthe work of Izawa et al. (2011) by measuring thespectroscopic properties of these samples and usingimaging spectroscopy in the field to map thedistributions of these mineral assemblages in order togain insight into the alteration processes.

Inferred Water–Rock Interactions

The results of mapping of secondary phases usingNIR imaging spectroscopy (Fig. 5) show spatialrelationships between different sulfur-bearingassemblages that illuminate changing conditions duringthe postimpact hydrothermal alteration and subsequent


low temperature weathering (summarized in Fig. 6).Marcasite formed from the impact-generatedhydrothermal system (Osinski et al. 2001, 2005a). Thesulfur within the marcasite was likely sourced from thegypsum- and anhydrite-rich Bay Fiord Formation in thetarget rocks during postimpact fluid circulation (Osinskiet al. 2001). This formation, which is part of the impactstructure’s central uplift, outcrops nearby and underliesthe impact melt-rich breccia at this location based onfield and orbital mapping (Osinski et al. 2005b;Tornabene et al. 2005). The marcasite at this site wasinterpreted by Osinski et al. (2001, 2005a) to haveformed at temperatures of ~100–240 °C and pH <5. Thepresence of both Fe2+ and sulfide cations in the initialfluids from which the marcasite precipitated require

reducing conditions. The marcasite that has beenpreserved and unweathered is mostly located onoverhanging and sloped surfaces such as the roof of theoutcrop where contact with altering fluids is relativelylimited. Nevertheless, regions with enhanced fluid flowshow distinct mineralogic and spectroscopic signatures(mapped in brown as the oxidized marcasite unitcontaining gypsum and jarosite in Fig. 5). We interpretthis assemblage as forming in situ at low temperaturesand under more oxidizing conditions, allowing forpartial oxidation of Fe and S. An assemblage ofgypsum and jarosite is predicted to form at pH ~2–4(Elwood Madden et al. 2004), and these coatings likelyformed from fluids in that pH range with acidityderived from oxidation of the primary marcasite. The

Fig. 4. A) Example NIR false color composite image of one portion of the outcrop. Red: 0.99 lm, green: 0.80 lm, blue:0.71 lm. B) Example lithological indicator parameter image of the outcrop. Red: red coating, green: marcasite, blue: mixedsulfate. Dark/shadowed areas have been masked in both images. See Table S2 for descriptions of each parameter. The locationof this image within the outcrop is shown in Fig. 5A.


very thin nature of the coating and limited extent ofalteration suggest that volumes of water involved in thealteration were likely very low, and we note that thesesmall amounts of fluids may have quickly reachedequilibrium with the surrounding marcasite, impedingfurther alteration. The location of this oxidized materialnearly exclusively on overhanging portions of theoutcrop is consistent with the interpreted low water/rock ratios because there is nowhere for water to pooland remain in contact with the marcasite for extendedperiods of time. Weathering of the marcasite may befurther hampered by kinetic effects if reaction rates areslow.

The efflorescent popcorn-textured mixed sulfatephase (yellow in Fig. 5) often occurs on flatter surfacesbelow the overhanging rocks coated with marcasite.

This physical location suggests fluids dripped onto thesesurfaces after interacting with the marcasite resulting inprecipitation of these mixed sulfates. The chemicalcomposition of this phase is dominated by Fe, SO4, andOH/H2O. The sulfur in these samples is entirelyoxidized, and Fe occurs in both +2 (copiapite andmelanterite) and +3 (copiapite and fibroferrite) valencestates. The chemical components of this sample likelyderive mostly from the marcasite with little interactionwith the surrounding calcite-rich rock, which wouldhave resulted in incorporation of significant Ca into thesample mineralogy. There is only 2.4 wt% CaOidentified, and laboratory imaging spectroscopy maps aminor component of gypsum. However, the dominantmineralogies in this sample are fibroferrite andcopiapite, neither of which contains Ca. The oxidationof the primary marcasite produces sulfuric acid (e.g.,Jambor et al. 2000), and pH values of approximately�1 are reported in the literature for waters associatedwith copiapite (Jambor et al. 2000; Jamieson et al.2005). These low pH values may result from evaporativeconcentration of H+ and other cations (Nordstromet al. 2000), particularly given the overall low volumesof fluids at the site. In summary, this mixed sulfatephase likely formed at ambient temperatures fromextremely low pH fluids that resulted from oxidation ofmarcasite (Fig. 6). These fluids are more oxidizing thanthe fluids that formed the marcasite, containing Fe2+,Fe3+, and SO4

2- in solution.Next, we propose that some of the copiapite and

fibroferrite dissolved. The reaction between these acidicfluids and the calcite host rock dissolved some calcite,leading to addition of Ca2+ cations to the fluids andneutralizing the acid somewhat. These water–rockinteractions resulted in precipitation of gypsum andFe3+-oxide and sulfate-bearing coatings on the calciterock directly below the mixed sulfate phases from lessacidic fluids with Fe3+, SO4

2-, and Ca2+ in solution aswell as extremely minor K+. A phase diagrampresented by Elwood Madden et al. (2004) for the Fe-S-Ca-Na-HCO3-H2O system predicts an assemblage ofgypsum and jarosite at pH ~2–4, and akaganeitedominates at pH 3–7 (Bibi et al. 2011). Thus, a pH of~3–4 is likely for the fluids forming this red coating.While still acidic, the inferred pH is well above the -1value expected for copiapite (Jambor et al. 2000;Jamieson et al. 2005). The neutralization of acidic fluidsby calcite and formation of gypsum coatings are alsoobserved in acid mine drainage systems (e.g., Boothet al. 1997; Jamieson et al. 2005; Chou et al. 2013;Lindsay et al. 2015).

The blue-gray material is more difficult to fit intothe context of these other units, in part because it isconfined to one discrete patch at the outcrop. It may

Fig. 5. Map of different mineral assemblages within this vug.A) Photograph of the vug with locations from which sampleswere collected labeled. Boxes outline the regions shown inFig. 4 (light blue), 6B (dark gray), and 6C (light gray). B)Mineral assemblages that are spectrally similar to the collectedsamples. Yellow regions are mixed sulfates similar to APDI-13-14a, red regions are red coatings similar to APDI-13-14b, greenregions are unaltered marcasite similar to APDI-13-14c, brownregions are oxidized marcasite similar to APDI-13-14j, andblue is the blue-gray phase spectrally similar to APDI-13-14h.


have formed through alteration by sulfate-bearing fluidsthat resulted from interactions between the mixedsulfates and red coatings, the calcite host rock, andsilicates within the impact melt rock. The 18.5 wt%SiO2 in the sample and the identification of clays andmicrocline suggest a large contribution from thesurrounding impact melt rocks, and this lithology maybe a weathered clast of the silicate-bearing basementgneiss that is common in the impact melt rock (e.g.,Osinski and Spray 2001).

The trends in weathering at this site as evidencedfrom the imaging spectroscopy measurements are similarto reported reactions that have been documented, albeiton much larger scales, at gossans and sites of acid minedrainage (e.g., Bigham and Nordstrom 2000; Jamboret al. 2000; Nordstrom et al. 2000). For example,airborne imaging spectroscopy has mapped mine sites

with zones of sulfate and oxide mineralogies resultingfrom progressive neutralization of acid with increasingdistance from the source (Swayze et al. 2000; Roachet al. 2006). These studies map more mature weatheringproducts (i.e., jarosite, goethite, and hematite) than themain units at our site, where weathering products likelyare less mature due to the low water–rock ratios and lowweathering intensity in this polar desert. The vug studiedhere also differs from sites of acid mine drainage in thatthis calcite- and marcasite-bearing vug was formed in animpact-generated hydrothermal system (Osinski et al.2001, 2005a).

Implications for Mars

This work has implications for understandingsimilar mineralogies and aqueous environments in

Fig. 6. A) Alteration sequence inferred from the results of this paper. Of the minerals represented, only marcasite formed in thepostimpact hydrothermal system, but other assemblages would not be present if not for the primary hydrothermalmineralization. Low-temperature designations for most of the assemblages refer to ambient temperatures as opposed tohydrothermal. B, C) Photographs showing examples of fluid-flow pathways over short distances. Sample APDI-13-14b wascollected within (B), and sample APDI-13-14a was collected within (C). In (C), the area outlined in light brown is marcasite withoxidized coatings (intermediates in A). Locations of these photos within the outcrop are shown in Fig. 5A.


planetary exploration. While the vug itself is encasedwithin clast-rich impact melt rocks and is not clearlyvisible from orbit, gypsum has been detected in orbitalspectroscopic datasets covering the outcrop (Tornabeneet al. 2005, 2014). This and other outcrops with gypsumsignatures identified from orbit are thus promising sitesfor further exploration with field and groundinvestigations. The orbital detection of gypsum suggeststhat there was significant available sulfur for ahydrothermal system, should one develop. While thissite is not a perfect analog for known deposits on Marsand massive carbonates as are present here have notbeen found on Mars (e.g., Edwards and Ehlmann 2015),the lithologies present at this vug are similar to someminerals detected on Mars. Sulfate-bearing units havebeen identified on Mars from orbital spectroscopy (e.g.,Gendrin et al. 2005; Langevin et al. 2005; Bibring et al.2006, 2007; Mustard et al. 2008; Bishop et al. 2009) andin situ rover measurements (e.g., Squyres et al. 2004a,2004b, 2009; Vaniman et al. 2014), and there have beenlimited identifications of carbonates in situ (Boyntonet al. 2009; Morris et al. 2010) and from orbit (e.g.,Bandfield et al. 2003; Ehlmann et al. 2008). One lessonlearned from our investigations is that a large portionof the outcrop today is dominated spectrally by sulfatesand oxides that formed at ambient temperatures despitethe deposit’s origin in the impact-driven hydrothermalsystem. Similar processes could have occurred on Mars.Detection of sulfates that typically form at lowtemperatures on Mars does not preclude the existenceof higher temperature aqueous conditions previously;the sulfates could have formed through oxidation andweathering of sulfides that formed at highertemperatures. This formation mechanism is particularlyrelevant where sulfates are identified near a source ofheat, such as within an impact structure or in volcanicterrains. In addition, we note that, while this outcrop isvolumetrically dominated by carbonates, coatings ofiron sulfates, sulfides, and oxyhydroxides dominate thespectral signatures at the outcrop scale and masksignatures of the carbonates. Similar relationships couldbe present on Mars. The polar desert environment inthe High Arctic where the Haughton impact structure islocated is a good analog for the current climate ofMars, and the types of weathering seen here may wellhave occurred on Mars.

In addition, this site is relevant to astrobiologicalinvestigations of Mars. The existence of an activehydrothermal system postimpact led to present-daydeposits of both oxidized and reduced assemblages thatprovide a redox gradient that can be exploited bymicroorganisms, as discussed in detail by Izawa et al.(2011). Previously, sulfur isotopic fractionationsmeasured within the marcasite of the vug indicate active

biotic processes temporal with the formation of thesulfides (Parnell et al. 2010); the later low-temperatureweathering processes may have also had microbialcomponents (Izawa et al. 2011). In acid mine drainagesites with similar mineralogies, microbes are oftenpresent and can significantly increase rates of iron andsulfur oxidation (e.g., Nordstrom and Southam 1997; Alet al. 2000; Schippers et al. 2010), and Izawa et al.(2011) suggested that similar iron and sulfur oxidizerscould be present at this site. Our map (Fig. 5) extendsthe work of Izawa et al. (2011) by mapping thelocations of these redox and chemical gradients bothwithin single assemblages and at the borders betweendifferent units where Izawa et al. (2011) suggested thatmicrobes are most likely to be found. Based on ourresults (Fig. 5), Fe2+ within the marcasite, on theboundaries between the marcasite and oxidized coatingsof jarosite and gypsum, and in the popcorn-texturedphase of copiapite, fibroferrite, gypsum, and melanteritecould all host iron-oxidizing communities. Sulfuroxidizers might be expected at boundaries betweenmarcasite and sulfate-bearing assemblages, whileacidophiles could be present in fluids with low pHvalues such as are typically needed to form copiapite.Examples of boundaries where redox gradients arepresent are shown at higher resolution in photographsin Fig. 6; in particular, boundaries between themarcasite and oxidized marcasite as well as between themixed sulfates and red coating would be promisinglocations to search for evidence of microbial activity.Finally, there is potential for preservation ofthermophiles or hyperthermophiles within the marcasitein the vug if present during formation of the marcasitefrom hydrothermal fluids (Izawa et al. 2011). Theimaging spectroscopy technique utilized here allows forthe remote assessment of the mineralogies present,revealing areas of potential biological interest andhelping to identify ideal sampling locations forplanetary exploration missions. Future work aims atexploring the putative biology associated with thesedeposits and will strive to elucidate whether or notmicrobial activity correlates with the redox boundariesdefined in this study.

Finally, this study also serves in part as a proof-of-concept for the utility of imaging spectroscopy indetermining mineralogy, past aqueous conditions, andastrobiological potential in planetary exploration. Thespectral techniques used here are validated bylaboratory measurements and provide new insights intothe weathering processes at this vug. Spectralparameters are commonly used to highlight spectralvariance in imaging spectroscopy data (e.g., Pelkeyet al. 2007; Viviano-Beck et al. 2014) to aid in detectingminerals and mapping lithologies. Spectral parameters


mapping slopes or depths of single absorption featuresare nonunique in that a spectral feature at a givenwavelength often is present in more than one mineral orthe cause of a spectral slope may be due to manyfactors. To make parameters more reliable, wedeveloped mineral and lithologic indicator parametersto detect the presence (and absence) of specificabsorption features characteristic of minerals of interest.By combining spectral parameters, more robust mineraldetections and mineral assemblage determinations arepossible (e.g., Viviano-Beck et al. 2014). Validation ofthe accuracy of a particular parameter or suite ofparameters for mineral detection is typically approachedby detailed spectral analysis of a subset of thehighlighted pixels to assess the rate of false positives.Samples from key sites detected in the remotely senseddata are also analyzed using other laboratory or in situmethods to increase confidence in the mapping. Theparameters used in this work were finely tuned to theparticular lithologies present at this outcrop, making therate of false positives quite low.

Imaging spectroscopy is a powerful tool both toobtain an initial assessment of what compositional unitsare present and where there is variability and to scalemore limited and costly targeted laboratory analyses tolarger samples and outcrops (Greenberger et al. 2015b).Our results here show the evolution from a reducinghydrothermal system to low-temperature fluids thatbecome progressively more oxidizing, evolve from acidicto more neutral, and change from simpler chemistriesdominated by Fe and sulfate to more complex solutionsincorporating Ca after reacting with the calcite hostrock. With an imaging spectrometer extending into theshortwave infrared similar to the SWIR imager used inthe laboratory, mineralogies at the outcrop could bemapped directly by the overtones and combinationtones of vibrational absorption features, providingadditional constraints on the precise mineralogiespresent, the past aqueous environments, and thehabitability at this outcrop or on Mars.

CONCLUSIONS

Hydrothermal systems generated by meteoriteimpacts are likely to be important in the formation ofsecondary phases on both Earth and Mars and providehabitats for microbial colonization (e.g., Izawa et al.2011; Sapers et al. 2014). This work investigated ahydrothermal calcite- and marcasite-bearing vug at theHaughton impact structure that has undergone low-temperature weathering using hyperspectral imaging inthe field and laboratory as well as supporting reflectancespectroscopy, mineralogy, and chemistry measurements.The mineral assemblages identified and mapped at the

outcrop are as follows: unaltered marcasite; oxidizedmarcasite with a coating of gypsum and jarosite; amixed sulfate phase consisting of fibroferrite, copiapite,and minor gypsum and melanterite; a red coating ofgypsum, akaganeite, goethite, and jarosite; and a blue-gray phase with calcite, gypsum, clays, microcline,quartz, and hematite.

The aqueous conditions that altered this outcrophave been inferred from the mineral assemblages. Themarcasite likely formed at high temperatures during thepostimpact hydrothermal system, and the other phasesformed at ambient temperatures after the hydrothermalsystem cooled. The oxidized coatings on marcasiteprobably formed first in a system with low water/rockratios that partially oxidized the Fe and S. Extremelyacidic fluids that had interacted with the marcasite thenlikely dripped onto flatter surfaces below, resulting inprecipitation of the fibroferrite and copiapite/ferricopiapite mixed sulfate phase. We argue that thoseminerals then underwent some dissolution, and theresulting fluids interacted with the calcite host rock,dissolving some calcite while neutralizing the acidicfluids somewhat, leading to precipitation of gypsum andFe3+-oxide/sulfate-bearing coatings. The spectral unitsmapped and the inferred changes in fluid chemistry andoxidation state with progressive alteration are relevantto Mars and can be used to predict the locations ofmicrobial populations, with biosignatures most likely tobe found at boundaries between various mapped units.This study also shows the utility of hyperspectralimaging in exploring sites that have undergone aqueousalteration, including through impact-driven processes,and in assessing their astrobiological potential onanother planet.

Acknowledgments—We thank Paul Mann for his effortsin maintaining, packing, and shipping the equipmentthat made this work possible and David Bannon, KwokWong, and Headwall Photonics, Inc. for use of theirhyperspectral imagers in the laboratory. We thank DaveMurray and Joe Orchardo for assistance with fluxfusion, ICP-AES, and elemental analyses. We are alsograteful for help in the field from other members of thefield team: Jerome Gattacceca, Jeremy Hansen, YoannQuesnel, Pierre Rochette, and Scott Simpson. We thankBrown University for supporting this research. ThePolar Continental Shelf Program provided field supportfor this work. Funding from the Natural Sciences andEngineering Research Council (NSERC) DiscoveryGrant and Northern Supplement program fundedGRO, LLT, AP, and CLM. AP and CLM also thankthe Northern Scientific Training Program for funding.EAC thanks the University of Winnipeg, the CanadianSpace Agency, the Manitoba Research Innovations


Funds, and NSERC of Canada for supporting theestablishment of the University of Winnipeg’s PlanetarySpectrophotometer Facility and this study. Finally, wethank Carle Pieters for editorial handling and AdrianBrown and Matthew Izawa for constructive reviews thatsignificantly improved this paper. The authors confirmthat there is no conflict of interest to declare.

Editorial Handling—Dr. Carle Pieters

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SUPPORTING INFORMATION

Additional supporting information may be found inthe online version of this article:

Table S1. Equations and interpretations of spectraland mineral indicator parameters for laboratoryhyperspectral imaging data.

Table S2. Equations and interpretations for spectraland lithological indicator parameters for NIR fieldhyperspectral imaging data.


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