Inventory of H2O in the Ancient Martian Regolith from Northwest Africa 7034: the Important Role of Fe Oxides, Geophys

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RESEARCH LETTER Inventory of H2O in the ancient Martian regolith 10.1002/2014GL062533 from Northwest Africa 7034: The important Key Points: role of Fe oxides • H2O in NWA 7034 is hosted by Fe oxyhydroxides, phyllosilicates, Nele Muttik1, Francis M. McCubbin1,2, Lindsay P. Keller3, Alison R. Santos1, Whitney A. McCutcheon1, and phosphates Paula P. Provencio1, Zia Rahman3, Charles K. Shearer1, Jeremy W. Boyce4, and Carl B. Agee1,2 • H2O is evenly distributed between

hydrous Fe oxides and phyllosilicates 1 2 • Fe oxide/hydroxides could be impor- Institute of Meteoritics, University of New Mexico, Albuquerque, New Mexico, USA, Department of Earth and Planetary tant hosts for water on the Sciences, University of New Mexico, Albuquerque, New Mexico, USA, 3Laboratory for Space Sciences, Mail Code KR, ARES, Martian surface NASA Johnson Space Center, Houston, Texas, USA, 4Department of Earth, Planetary, and Space Sciences, University of California, Los Angeles, California, USA Supporting Information: • Readme • Table S1 Abstract Water-rich Martian regolith breccia Northwest Africa (NWA) 7034 was analyzed by Fourier transform infrared spectroscopy and transmission electron microscopy to determine the inventory and phase distribution À Correspondence to: of H2O (used herein to refer to both molecular H2OandOH structural components in hydrous minerals). N. Muttik, Hydrous Fe oxide phases (hydromaghemite and an unidentiﬁed nanocrystalline Fe-bearing oxide phase [email protected] observed with hydromaghemite) and phyllosilicates (saponite) were identiﬁed as the primary mineralogic hosts

for H2O with a minor contribution from Cl-rich apatite. Based on mass balance calculations and modal Citation: abundances of minerals constrained by powder X-ray diffraction and petrography, we can account for the Muttik, N., F. M. McCubbin, L. P. Keller, A. R. Santos, W. A. McCutcheon, entire 6000 ppm H2O measured in bulk rock analyses of NWA 7034. This H2O is distributed evenly between P. P. Provencio, Z. Rahman, C. K. Shearer, hydrous Fe-rich oxides and phyllosilicates, indicating that Fe oxides could be as important as phyllosilicates for J. W. Boyce, and C. B. Agee (2014), H2O storage in Martian surface material. Inventory of H2O in the ancient Martian regolith from Northwest Africa 7034: The important role of Fe oxides, Geophys. Res. Lett., 41, 8235–8244, doi:10.1002/2014GL062533. 1. Introduction Substantial efforts have been made over the last several decades to identify and characterize (both spatially Received 14 NOV 2014 Accepted 19 NOV 2014 and temporally) the presence of aqueous activity at or near the surface of Mars. Aqueous activity on Mars Accepted article online 25 NOV 2014 has been inferred from orbit through detection of hydrous minerals (e.g., sulfates, phyllosilicates, and Published online 4 DEC 2014 chlorides) [Gendrin et al., 2005; Bibring et al., 2006; Ehlmann et al., 2008, 2009; Mustard et al., 2008; Osterloo et al., 2008; Milliken et al., 2010] and studies of fluvial geomorphic features (e.g., valley networks, slope linea, and paleoshorelines) [Irwin et al., 2002; Head et al., 2003; Perron et al., 2007; Carr and Head, 2010; McEwen et al., 2011; Carr, 2012]. Aqueous activity has also been inferred from in situ rover analyses and subsequent experimental and geochemical modeling studies using rover data [Golden et al., 2005; Grotzinger et al., 2005, 2014; Haskin et al., 2005; Tosca et al., 2005, 2008; Hurowitz et al., 2006; Knoll et al., 2008; Williams et al., 2013]. However, without Mars sample return, many of the details regarding Martian aqueous processes remain quite limited. Although some Martian meteorites have minor secondary aqueous phases (carbonates, iddingsite, sulfates, and clay minerals) [Bridges and Grady, 2000; Bridges et al., 2001; McCubbin et al., 2009; Niles et al., 2009; Changela and Bridges, 2010; Stopar et al., 2013; Hallis et al., 2014] that have been used to infer aqueous activity at or near the Martian surface, these samples are all relatively unaltered igneous rocks that provide little context for near-surface aqueous processes on Mars. However, the first brecciated sample from Mars has recently been identified (Martian meteorite Northwest Africa 7034) [Agee et al., 2013], and the elevated levels of hydration, as well as secondary mineral products formed in the presence of water, provide important information regarding secondary aqueous processes on Mars. Martian basaltic breccia Northwest Africa (NWA) 7034 and its pairings (e.g., NWA 7533, NWA 7475, NWA 7906, NWA 7907, NWA 8171, and NWA 8114) represent the first brecciated materials to be sampled from Mars. Furthermore, NWA 7034 contains components as old as 4.4 Ga [Humayun et al., 2013], and its bulk composition is similar to the estimates of the average Martian surface as measured by the gamma ray spectrometer on the Mars Odyssey Orbiter and to the crustal rocks and soils in Gusev Crater as measured by the Alpha Proton X-ray Spectrometer on the Mars Exploration Rover Spirit [Agee et al., 2013; McSween et al., 2009]. Consequently, NWA 7034 and pairings provide our first opportunity to directly investigate the

mineralogy and secondary chemical processes that occur in Martian regolith materials and how these processes may have changed through time [e.g., Mustard et al., 2008]. In particular, NWA 7034 has elevated

abundances of Martian H2O in the bulk rock (6000 ppm) [Agee et al., 2013], so it is a prime candidate for examining the action of aqueous ﬂuids at or near the Martian surface/crust.

The oxygen isotopic composition of H2O from NWA 7034 is within the range exhibited by the SNC meteorites [Agee et al., 2013; Nunn et al., 2013] and is consistent with a Martian origin. Furthermore, much of the H2O in NWA 7034 is released at low temperatures (65% H2O released by 300°C), suggesting that the H2Ois primarily hosted by secondary phases rather than primary igneous minerals. The H isotopic composition of

H2O in NWA 7034 is much lighter (À100 to +300‰) than typical H isotopic compositions reported for the Martian atmosphere and hydrous phases in Martian meteorites (i.e., >2500‰)[Boctor et al., 2003; Watson et al., 1994; Leshin et al., 1996; Greenwood et al., 2008; Webster et al., 2013]; however, there is evidence that a light isotopic reservoir of H does exist on Mars [Leshin et al., 1996; Hallis et al., 2012; Usui et al., 2012]. An isotopically light hydrogen component has been interpreted as representing a Martian mantle component of hydrogen, especially the geochemically depleted mantle source [Usui et al., 2012]. It has been suggested that Mars and Earth accreted water from the same source material with similar carbonaceous chondrite-like δD values early in the solar system’shistory[Hallis et al., 2012; McCubbin et al., 2012; Usui et al., 2012; Saraﬁan et al., 2014].

In the present study, we examine the mineralogical hosts of H2O (both as OH and H2O) in NWA 7034 using electron probe microanalysis (EPMA), micro-Fourier transform infrared spectroscopy (micro-FTIR), and transmission electron microscopy (TEM). Through our analyses, we can account for all (4800 to 7400 ppm

H2O) of the H2O in the bulk rock, and we subsequently use that information to glean new insights into the mineralogical hosts for H2O in Martian surface materials.

2. Analytical Techniques 2.1. Electron Microprobe Analysis/Imaging Backscattered electron images of NWA 7034 secondary phases were collected from two thin sections: University of New Mexico (UNM) Section 3A,3 and UNM Section 1B,2 at the University of New Mexico using a JEOL 8200 Electron Probe Microanalyzer, with an accelerating voltage of 15 kV and a beam current of 20 nA. Both thin sections were from the main mass of NWA 7034 housed at the University of New Mexico (UNM). Phase identiﬁcation and qualitative chemical determination were conducted using energy dispersive spectroscopy (EDS).

2.2. Fourier Transform Infrared Spectrometry Infrared (IR) analysis of the NWA 7034 hydrous phases and matrix areas was collected from thin sections UNM Section 3A,3 and UNM Section 1B,2 in reflectance mode using a Nicolet Nexus 670 Fourier transform IR spectrometer housed in the Institute of Meteoritics at the University of New Mexico. The FTIR spectrometer is equipped with a Globar source, XT-KBr beamsplitter, and a Continuμm microscope attachment with a liquid nitrogen-cooled MCT-A detector. All analyses were performed by running a dry air purge into an ~100 cm3 À volume around the sample. Each spectrum was collected in a spectral range of 4500–400 cm 1 over 1024 scans À with 4 cm 1 resolution using a 25 × 25 to 100 × 100 μm sampling area. For each analysis, a background spectrum was collected on a gold-coated glass slide as it has a reflection coefficient of ~100% over the wavelength region measured. The raw spectra were collected in units of percent reflectance (%R). Data were plotted using Omnic software for analysis by visual identification and comparison of absorption features to known organic and inorganic materials.

2.3. Transmission Electron Microscopy Samples for TEM analysis included both powdered samples and focused ion beam (FIB) sections from thin section UNM Section 3A,3. Splits from sieved samples of NWA 7034 were powdered and then analyzed at UNM using TEM/STEM (transmission electron microscopy for nanometer-scale imaging and scanning TEM for chemical contrast) and energy ﬁltered TEM for imaging speciﬁc chemical species. FIB sections of iron-rich alteration and matrix areas were strategically cut from thin sections of NWA 7034, in order to characterize the texture, structure, and chemistry of the alteration phases at the micrometer- to nanometer-scale using TEM/EDS. TEM sample preparation was carried out using the focused ion beam (FIB) technique with a FEI

Quanta 3-D ﬁeld emission gun scanning electron microscope/FIB instrument at UNM and FEI Quanta 3D600 FIB at NASA Johnson Space Center (JSC). TEM images, quantitative EDS X-ray analyses, and selected area electron diffraction (SAED) were performed using a JEOL 2010 F FEGTEM/scanning TEM (STEM) at 200 kV at UNM and using the JEOL 2500SE 200 keV ﬁeld emission–scanning transmission electron microscope (FE-STEM) equipped with a Noran thin window energy-dispersive X-ray spectrometer at JSC. In order to minimize terrestrial alteration after cutting FIB sections, all samples were placed immediately in the TEM and/or stored under vacuum until analysis.

2.4. Obstacles to H2O Quantiﬁcation in NWA 7034 Oxides The methods described above were primarily used as a qualitative tool to identify the molecular H-bearing

species present within NWA 7034 oxide phases. However, the quantiﬁcation of H2O abundances in individual phases would allow us to constrain the absolute abundance of H2O in the sample and the distribution of H2O among the hydrous phases identiﬁed in the present study. Previous studies have demonstrated that secondary ion mass spectrometry (SIMS) and FTIR spectroscopy are two powerful techniques for measuring

volatile species in geologic materials, so we assessed their applicability for determining the H2O abundances of the nanophase oxides in NWA 7034. We determined that quantifying the hydrogen abundances in hydrated nanophases in NWA 7034 using these methods faces several seemingly insurmountable tasks, and we begin ﬁrst with SIMS. Given the small sizes of the nanophase oxide patches (<10 μm), the nanoSIMS is the only SIMS instrument with the spatial resolution needed to analyze these materials. Past efforts to construct calibration curves for hydrogen with nanoSIMS techniques have been very successful in silicates and phosphates [Saal et al., 2008; Hauri et al., 2011; Liu et al., 2012; Barnes et al., 2013, 2014; Tartese et al., 2013, 2014], although this method is highly matrix dependent [Ihinger et al., 1994; Hauri et al., 2002]. There are currently no standards that are matrix matched for the nanophase oxides in NWA 7034, which makes the

quantification of H2O by nanoSIMS on the NWA 7034 nanophase oxides impossible until such standards are developed. Additionally, nanoSIMS analyses are sensitive to the local electric and magnetic fields at the sample surface. The local electric field is in large part controlled by the sample topography, which is difficult to control during polishing due to the variable hardness of nanophase oxides and other phases, and the friability of the nanophase oxide clusters. The nanophase oxides also have variable magnetic properties— and possibly orientations—potentially further compounding problems of quantification even if standards were available. Finally, fine-grained materials have abundant grain boundaries, which are likely sites hosting excess hydrogen, making any measured value maximums [Greenwood et al., 2011]. Transmission micro-FTIR has been used to quantify dissolved volatiles (e.g., O–H) in different geologic materials (glasses and minerals) using the Beer–Lambert law, where an IR beam is passed through a doubly polished wafer of sample with known thickness and the absorption of radiation is observed in the frequency region corresponding to the characteristic vibration of the dissolved O–H species [Stolper, 1982; Dixon et al., 1995; Libowitzky and Rossman, 1996, 1997; Mandeville et al., 2002]. The quantification requires a known molar absorption coefficient, and for anisotropic media, spectra must be collected along all three orthogonal polarization directions [Libowitzky and Rossman, 1996, 1997]. However, this is not possible for the nanophase oxides in NWA 7034 because (1) we cannot make doubly polished wafers of the nanophase oxides in NWA 7034, (2) molar absorption coefficients for the oxides in our sample are not known, (3) the materials are too fine grained to separate out and measure individually along each of the three orthogonal polarization directions, and (4) the spot size for transmission FTIR is typically >10 μm, which will result in phase overlap during the analysis, which would further complicate the quantification. Consequently, we used stoichiometric constraints

to determine the relative distributions of H2O among the identiﬁed hydrous phases within NWA 7034.

3. Results 3.1. Mineralogy of the Bulk Matrix The bulk matrix domain of NWA 7034 has been described as the interconnected ﬁne-grained (0.01–5 μm) groundmass that holds the meteorite together [Santos et al., 2014]. The ﬁne-grained portion of the groundmass consists of 0.1–1 μm sized pyroxene, plagioclase, phosphate (typically Cl-rich apatite), and various Fe- and FeTi-oxide phases (Figure 1a). This material is holocrystalline (i.e., no glass) and shows evidence of mild thermal annealing based on common equilibrium grain boundaries that meet at 120° angle

(Figure 3a) [Muttik et al., 2014]. Of the phases identiﬁed by EPMA, only the apatite [Ca5(PO4)3(F,Cl,OH)] is

Figure 1. (a and b) Backscattered electron (BSE) images of ﬁne-grained crystalline matrix and different clasts. (c and d) High- contrast BSE images of Fe-rich phases in NWA 7034. Dark gray/black = plagioclase, light gray = pyroxene, white = oxides, Mt = magnetite, and Mh = maghemite.

known to contain OH in its crystalline structure. Hydroxyl cannot be measured directly by the EPMA technique; however, a missing component in the X site of apatite can be calculated on the basis of stoichiometry. If both F and Cl are analyzed with sufﬁcient accuracy [McCubbin et al., 2010, 2011], this missing À 2À 2À 2À À À component can be attributed to some combination of the anions OH ,O ,CO3 ,S ,Br , and I and/or structural vacancies [Pan and Fleet, 2002] or structural H2O[Mason et al., 2009]. The most likely constituent for this missing component in Martian apatite À is OH [Leshin,2000;Boctor et al., 2003; Greenwood et al., 2008; McCubbin et al., 2012, 2014]. EPMA analysis of F and Cl in apatite from NWA 7034 suggests that it contains the equivalent of 3000 ppm

(0.3 wt %) H2O in its structure. 3.2. FTIR FTIR spectra of the hydrated Fe oxide minerals in the NWA 7034 bulk matrix (Figure 2) are similar and exhibit a broad À spectral band around 3550 cm 1 due to the O–H stretching vibration [Socrates, 1980]. The vibrational water OH modes can be conﬁrmed by a very weak

fundamental bending vibration of H2Oat À around ~1640 cm 1 [Socrates, 1980; Figure 2. Representative FTIR spectra of measured hydrated Fe oxide Cornell and Schwertmann, 1996]. In some phases and groundmass in NWA 7034. The band assignment of Fe samples, an additional absorption band is oxide spactra gives the best ﬁt to maghemite. Note that the strong À1 Si-O band is due to the pyroxene and plagioclase surrounding the superimposed at 3720 cm due to metal Fe-rich phases. hydroxyl species [Socrates, 1980; Cornell

Figure 3. (a) Bright field TEM image of nanophase magnetite and maghemite in subcrystalline plagioclase and pyroxene matrix. (b) Bright field TEM image of banded maghemite surrounding sulfide mineral. (c) High-resolution TEM image of the banded maghemite nanoparticles. (d) High-resolution TEM image of saponite fringes along pyroxene grain boundaries. Mt = magnetite, Mh = maghemite, and Sf = sulfide.

À and Schwertmann, 1996], and additional Fe–OH bending modes can be observed at ~985 and ~890 cm 1 [Cornell and Schwertmann, 1996]. The exact wavelength of the fundamental OH stretching vibration depends on the particular cations (e.g., Al, Mg, and Fe) bonded to OH in the crystal structure and shifts depending on the mineralogy and mineral composition [e.g., Bishop et al., 2008; Socrates, 1980]. If Fe3+ is present in the structure, the fundamental OH stretching vibrations are broader and occur at longer wavelengths [Cornell and Schwertmann, 1996]. Because of the ﬁne grain size of NWA7034 matrix, FTIR spectra from Fe oxide/hydroxide minerals also contain spectral features due to the presence of silicates (e.g., pyroxene and plagioclase). À1 À1 However, characteristic absorption bands of maghemite (γFe2O3) can be observed near 720 cm ,650cm , À and 590 cm 1 (Table S1 in the supporting information), which correspond to the vibrational frequencies associated with Fe–O bonds in the tetrahedral and octahedral sites [Cornell and Schwertmann, 1996].

Maghemite can incorporate up to 2 wt % H2O in its structure (as protons on the vacant octahedral sites) [Cornell and Schwertmann, 1996] and is likely the primary OH-hosting oxide phase. We also observe spectral features À1 near 1400 and 1500 cm , which are consistent with spectral bands observed in ferrihydrite (Fe8.2O8.5(OH) 7.4x3H2O) [Bishop and Murad, 2002], although there was no evidence of ferrihydrite in the Mössbauer analysis of À NWA 7034 [Gattacceca et al., 2014]. Carbonates also exhibit spectral features near 1400 cm 1; however, our data exhibit no other band characteristic of carbonate minerals.

3.3. TEM It is difﬁcult to distinguish among various hydrous Fe oxide phases based on EPMA and FTIR spectral features alone, so we also employed chemical and structural analyses using transmission electron microscopy (TEM), interfaced with energy dispersive spectroscopy and selected area electron diffraction. In addition, TEM analyses also provide better understanding of the textural relationship among the Fe oxides and hydroxides. These

techniques provide additional evidence for the presence and identiﬁcation of hydrous phases in NWA 7034. We

determined based on textural, chemical, and structural analysis that both magnetite (Fe3O4) and maghemite

(γFe2O3) are the dominant Fe-rich phases in the fine-grained bulk matrix of NWA 7034 (Figure 3a), which is consistent with the bulk rock X-ray diffraction (XRD) analysis by Agee et al. [2013] and magnetic analysis by Gattacceca et al. [2014]. The selected area electron diffraction (SAED) patterns of the sample indicate the presence of crystalline magnetite and maghemite embedded in subcrystalline matrix (Figure 3a). The SAED patterns show intense d spacings of 0.48, 0.34, 0.36–0.33, 0.25, 0.22, and 0.18 nm, which are consistent with magnetite and maghemite phases [Cornell and Schwertmann, 1996]. The TEM high-magnification micrographs of alteration rinds around sulfide phases (Figure 3b) clearly show concentric bands of maghemite that are composed of closely packed small nanoparticles (Figure 3c). In addition, nanometer-sized Fe oxide/hydroxide occurs between the coarser-grained maghemite particles based on high-resolution TEM (HRTEM) analysis. This phase may be related to the superparamagnetic göethite phase identified by Mössbauer analysis of NWA 7034 [Gattacceca et al., 2014]. However, overlapping electron diffraction d values of different Fe oxides/hydroxides make it difficult to distinguish the exact nature of this phase. Diagnostic d spacings of hematite or göethite were not observed in the electron diffraction data or in the Fourier transforms of lattice fringe images. The estimated amount of this nanophase Fe oxide/hydroxide in NWA7034 based on its spatial distribution in high-resolution TEM images is about 1–2 wt %, which is consistent with previous Mössbauer analysis of NWA 7034, indicating that about 8% of the total iron in NWA 7034 exists as nanophase Fe3+ [Gattacceca et al., 2014]. However, none of the techniques employed in our study have definitively identified göethite or hematite in the bulk matrix of NWA 7034. High-resolution TEM (HRTEM) analysis of the bulk matrix area also revealed minor amounts of phyllosilicates

(~1–2 wt %), which is an additional mineralogical host for H2O in NWA 7034. The phyllosilicates are ﬁne grained (<50 nm), poorly crystalline, and occur along pyroxene grain boundaries (Figure 3d). The phyllosilicates show wavy lattice fringes with basal spacings of ~1 nm, consistent with collapsed smectite-type phyllosilicates. Measured lattice fringes consist of repeating three layer units, which can be explained as 2:1 tetrahedral-octahedral-tetrahedral (T-O-T)-structured silicate clay, indicating that one octahedral sheet (O) is linked to two tetrahedral sheets (T), one on each side. This three-layered structure 2:1 T–O–T is a characteristic of smectite with a variable interlayer spacing of exchangeable cations [Solin, 1997]. The 0.7 and 1.4 nm fringe characteristics of serpentine and chlorite group minerals, respectively, were not observed. The chemical analysis and microstructures are consistent with a partially collapsed Fe-saponite type of

smectite (Ca0.25(Mg, Fe)3((Si, Al)4O10)(OH)2xnH2O). In general, Fe-saponite smectites can hold up to 19 wt % H2O[Meunier, 2005]. Under ambient conditions, saponite shows basal spacings up to 1.5 nm depending on the extent of hydration and the amount of interlayer H2O that is present. In the high vacuum of the TEM, however, the interlayer water is lost, and the layers collapse to ~1 nm. Additionally, phyllosilicate layers are electron beam sensitive and are beam damaged and degraded during prolonged electron beam exposure. Consequently, we cannot determine with certainty the original hydration state of the saponite in our samples, but the material we did analyze is consistent with a minimum of partial hydration. TEM investigations of ﬁne-grained matrix also indicate the presence of submicrometer-sized Cl-rich apatite that appears to be similar in composition to the larger apatite grains that occur in the matrix.

4. Discussion

4.1. H2O Inventory of NWA 7034

NWA 7034 has approximately 6000 ppm H2O, and based on the O-isotopic composition of this H2O, it appears to be entirely Martian in origin [Agee et al., 2013]. Our results suggest that H2O in NWA 7034 is hosted by Fe oxyhydroxides, phyllosilicates, and phosphates. Most of the H2O in NWA 7034 is released between approximately 150–500°C [Agee et al., 2013], which also provides insights into the nature of the H2O-hosting phase. Thermogravimetric analyses of apatites indicate that they do not dehydrate until high temperature

(>600°C) [Mason et al., 2009]. However, maghemite loses a substantial amount of H2O over the temperature range of 150–400°C [Barron and Torrent, 2002]. Furthermore, most phyllosilicates start to lose absorbed and

interlayer H2O at temperatures of ~100°C and exhibit signiﬁcant degradations in their crystal structures between 400 and 800°C due to the loss of structural OH [Grim and Kulbicki, 1961; Bishop et al., 1994; Frost et al., 2000;

Milliken and Mustard,2005;Fairen et al., 2010; Milliken et al., 2010; Che and Glotch, 2012]. Therefore, previous mineral dehydration studies implicate secondary alteration products (Fe oxyhydroxides and phyllosilicates) as

the main mineralogical hosts for H2OinNWA7034.

In an attempt to quantify the H2O inventory of NWA 7034, we combined our results with estimates of modal abundances of minerals derived from previous XRD and Mössbauer analyses of NWA 7034. Each phase is

discussed, and the total contribution of H2O to the bulk rock is determined. In our calculations, we used the higher end of the H2O estimates for each of the hydrous phases. The Cl-rich apatite has a stoichiometric missing component in the X site that can be attributed to 0.17 ± 0.06

structural formula units of OH, which equates to 0.3 ± 0.1 wt % H2O. The modal abundance of apatite in NWA

7034 was estimated to be up to 5%, indicating the maximum contribution of Martian H2O from apatite to be approximately 150 ppm ± 50 ppm in NWA 7034. These observations rule out apatite as the primary host of

H2O in NWA 7034. According to Agee et al. [2013] and Gattacceca et al. [2014], about 4–7% of NWA 7034 is

composed of maghemite, which can contain approximately 2 wt % H2O in its structure (as protons on the vacant octahedral sites) [Cornell and Schwertmann, 1996]. Consequently, maghemite could account for

800–1400 ppm of the 6000 ppm bulk H2O. Additionally, Gattacceca et al. [2014] reported that NWA 7034 has up to 2 wt % superparamagnetic göethite, which may be the nanophase Fe hydroxide that could not be

unequivocally identiﬁed in the present study. Göethite contains approximately 10 wt % H2O (as OH in its

structure), so it may contribute up to 2000 ppm H2O to the NWA 7034 water budget. Based on our estimates,

Fe oxyhydroxides could potentially provide 2800–3400 ppm H2O to the bulk H2O budget of NWA 7034. Phyllosilicates, similar to Fe saponite, could make up as much as 2% of NWA 7034; however, it was not detected by bulk powder XRD, so phyllosilicates are unlikely to exceed 1–2% of the bulk. In a fully hydrated

state, saponite can hold up to 19 wt % H2O in its structure. Our investigations suggest incomplete hydration of this phase; however, its basal spacing measurements are in good agreement with the Lafayette phyllosilicate (saponite) lattice fringe measurements [Changela and Bridges, 2010; Hallis et al., 2014; Hicks et al., 2014], suggesting that the NWA 7034 phyllosilicates may be similarly hydrated to Lafayette saponite, which contains

an average of 12–13 wt % H2O[Hicks et al., 2014]. Based on mass balance calculations, phyllosilicates could

contribute as much as 1900–3800 ppm H2O to NWA 7034; however, if the hydration state we observe is the

pristine hydration state, the amount of H2O released from this phase is more likely in the range of 1200–

2600 ppm. Altogether, we have accounted for approximately 4800 to 7400 ppm H2O from the presently

identified H2O-bearing phases, which may account for the entire H2O inventory of NWA 7034. Distinguishing between a Martian origin of the aqueous activity and a possible terrestrial origin of the alteration at the nanoscale can be difficult, especially for a hot desert meteorite find. The oxygen isotope

signature of H2O released from NWA 7034, at even the lowest temperature of 50°C, is Martian in composition [Agee et al., 2013], which supports a Martian origin for the aqueous alteration. However, even if the H2Oin NWA 7034 released at lower temperatures is terrestrial, this study indicates that the H2O carrying capacity of phase in NWA 7034 is almost evenly distributed between oxide phases and phyllosilicates showing that Fe oxides can be just as important of a reservoir of water in the Martian regolith as phases like clay minerals and not all of the Fe oxides on Mars are anhydrous.

4.2. Implications for Mars The NWA 7034 bulk composition closely resembles Martian crustal rocks and soils measured by recent rover and orbital missions [Agee et al., 2013; McSween et al., 2009; Stolper et al., 2013; Schmidt et al., 2014]. Therefore, NWA 7034 is currently the best analogue material for the ancient hydrated and extensively brecciated

Martian crust. Our results suggest that most of the H2O released from NWA 7034 originates from Fe-rich oxide phases and phyllosilicates, with approximately equal contributions from both phases. Fe oxides and hydroxides are very common in weathered rocks and soils on Earth and Mars, and they are important components of terrestrial and Martian dust [Pollack et al., 1979]. In addition to phyllosilicates,

hydrated Fe oxides can contain substantial amounts of H2O in their structures due to their large speciﬁc surface areas [Cornell and Schwertmann, 1996], and therefore, maghemite and the ferric nanophase oxides (superparamagnetic göethite) may contribute substantially to the hydrogen budget of the Martian surface, as we observe here for NWA 7034.

Our investigation indicates that hydrous Fe oxide phases are at least as volumetrically important to the

inventory of H2O in NWA 7034 as are phyllosilicates. If NWA 7034 is an analogue of an ancient brecciated Martian regolith, it implies that Fe oxide phases could be important hosts for H2O over broad expanses of the Martian surface, adding to the amount of H2O that has been previously accounted for in other hydrous phases (e.g., clay and zeolites). Furthermore, hydrated Fe oxides could be one of the H2O-hosting phases in the global dust, which is hydrated and has abundant oxide minerals [ Milliken et al., 2007; Bish et al., 2013].

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