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

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Inventory of H2O in the Ancient Martian Regolith from Northwest Africa 7034: the Important Role of Fe Oxides, Geophys PUBLICATIONS Geophysical Research Letters 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 MUTTIK ET AL. ©2014. American Geophysical Union. All Rights Reserved. 8235 Geophysical Research Letters 10.1002/2014GL062533 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 MUTTIK ET AL. ©2014. American Geophysical Union.
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