¹C.D. Cino, ¹E. Dehouck, ¹S.M. McLennan
Icarus (in Press) Link to Article [http://dx.doi.org/10.1016/j.icarus.2016.08.029]
¹Department of Geosciences, State University of New York at Stony Brook, Stony Brook NY, 11794-2100, USA
Copyright Elsevier

Burns formation sandstones, deposited by aeolian processes and preserved at Meridiani Planum, Mars, contain abundant sulfate minerals. These sedimentary rocks are thought to be representative of a sulfate-rich geological epoch during late Noachian – early Hesperian time that followed an earlier clay-rich epoch. Twenty Burns formation targets, abraded by the Rock Abrasion Tool (RAT) and for which alpha-particle X-ray spectrometry (APXS) and Mössbauer spectroscopy data are available, were selected for geochemical modeling. A linear unmixing modeling approach was employed. Mineralogical constituents quantitatively constrained by Mössbauer and Mini-TES spectroscopy and interpreted to be chemically precipitated from aqueous fluids during deposition and/or early diagenesis were subtracted from the bulk chemistry. Resulting residual chemical compositions, interpreted to be dominated by detrital siliciclastic components and representing ∼21–35% of the rocks, were then geochemically evaluated to constrain the potential for the presence of clay minerals or their poorly-crystalline or non-crystalline precursors/chemical equivalents. Calculations incorporated a robust estimate of the uncertainties in mineral abundances. On Al2O3 – (CaO+Na2O) – K2O (A-CN-K) and Al2O3 – (CaO+Na2O+K2O) – (FeOtotal+MgO) (A-CNK-FM) molar ternary diagrams, removal of chemical constituents resulted in a shift from igneous–like compositions to compositions consistent with secondary mineral assemblages containing significant aluminous clay mineral components. All of the residual compositions are corundum-normative, further supportive of the presence of highly aluminous phases. On the A-CNK-FM diagram, clay minerals plotting closest to the residual field are natural montmorillonites but could also represent mixtures of various Mg/Fe-rich phyllosilicates, such as nontronite or saponite, and other more Al-rich minerals such as Al-montmorillonite, kaolinite or illite. Depending on the age of clay mineral formation, occurrence of clay minerals or their poorly crystalline precursors/chemical equivalents in the Burns formation could suggest that any global transition from clay-rich to sulfate-rich environments on early Mars was more complex than previously recognized. Results are also consistent with models for the Burns formation aqueous history in which acidic conditions were more restricted in time and/or space than previously thought and thus may also be consistent with growing evidence that changing redox conditions, rather than global pH variations, was an important factor in the environmental evolution of early Mars.

^1,2Joshua F. Einsle, ¹Richard J. Harrison, ³Takeshi Kasama, ⁴Pádraig Ó Conbhuí, ⁵Karl Fabian, ⁴Wyn Williams, ⁷Leonie Woodland, ⁸Roger R. Fu, ⁹Benjamin P. Weiss,²Paul A. Midgley
American Mineralogist 101,  2070-2084 Link to Article [http://dx.doi.org/10.2138/am-2016-5738CCBY]
¹Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, U.K.
²Department of Materials Science & Metallurgy, University of Cambridge, Charles Babbage Road, Cambridge CB3 0FS, U.K.
³Center for Electron Nanoscopy, Technical University of Denmark, Kongens Lyngby, Denmark
⁴Grant Institute of Earth Science, University of Edinburgh, Kings Buildings, West Mains Road, Edinburgh EH9 3JW, U.K.
⁵Geological Survey of Norway, Leiv Eirikssons vei 39, 7491 Trondheim, Norway
⁶Centre for Arctic Gas Hydrate, Environment and Climate; Department of Geology, University of Tromsø, NO-9037 Tromsø, Norway
⁷The Stephen Perse Foundation, Union Road, Cambridge CB2 1HF, U.K.
⁸Lamont-Doherty Earth Observatory, Columbia University, Palisades, New York 10964, U.S.A.
⁹Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts
Copyright: The Mineralogical Society of America

Dusty olivine (olivine containing multiple sub-micrometer inclusions of metallic iron) in chondritic meteorites is considered an ideal carrier of paleomagnetic remanence, capable of maintaining a faithful record of pre-accretionary magnetization acquired during chondrule formation. Here we show how the magnetic architecture of a single dusty olivine grain from the Semarkona LL3.0 ordinary chondrite meteorite can be fully characterized in three dimensions, using a combination of focused ion beam nanotomography (FIB-nT), electron tomography, and finite-element micromagnetic modeling. We present a three-dimensional (3D) volume reconstruction of a dusty olivine grain, obtained by selective milling through a region of interest in a series of sequential 20 nm slices, which are then imaged using scanning electron microscopy. The data provide a quantitative description of the iron particle ensemble, including the distribution of particle sizes, shapes, interparticle spacings and orientations. Iron particles are predominantly oblate ellipsoids with average radii 242 ± 94 × 199 ± 80 × 123 ± 58 nm. Using analytical TEM we observe that the particles nucleate on sub-grain boundaries and are loosely arranged in a series of sheets parallel to (001) of the olivine host. This is in agreement with the orientation data collected using the FIB-nT and highlights how the underlying texture of the dusty olivine is crystallographically constrained by the olivine host. The shortest dimension of the particles is oriented normal to the sheets and their longest dimension is preferentially aligned within the sheets. Individual particle geometries are converted to a finite-element mesh and used to perform micromagnetic simulations. The majority of particles adopt a single vortex state, with “bulk” spins that rotate around a central vortex core. We observed no particles that are in a true single domain state. The results of the micromagnetic simulations challenge some preconceived ideas about the remanence-carrying properties of vortex states. There is often not a simple predictive relationship between the major, intermediate, and minor axes of the particles and the remanence vector imparted in different fields. Although the orientation of the vortex core is determined largely by the ellipsoidal geometry (i.e., parallel to the major axis for prolate ellipsoids and parallel to the minor axis for oblate ellipsoids), the core and remanence vectors can sometimes lie at very large (tens of degrees) angles to the principal axes. The subtle details of the morphology can control the overall remanence state, leading in some cases to a dominant contribution from the bulk spins to the net remanence, with profound implications for predicting the anisotropy of the sample. The particles have very high switching fields (several hundred millitesla), demonstrating their high stability and suitability for paleointensity studies.

¹Queenie H.S. Chan, ¹Michael E. Zolensky, ²James E. Martinez, ³Akira Tsuchiyama, ³Akira Miyake
American Mineralogist 101, 2041-2050  Link to Article [http://dx.doi.org/10.2138/am-2016-5604]
¹ARES, NASA Johnson Space Center, Houston, Texas 77058, U.S.A.
²Jacobs Engineering, Houston, Texas 77058, U.S.A.
³Graduate School of Science, Kyoto University, Kitashirakawa Oiwake-cho, Sakyo-ku, Kyoto 606-8502, Japan
Copyright: The Mineralogical Society of America

Life on Earth shows preference toward the set of organics with particular spatial configurations. Enantiomeric excesses have been observed for α-methyl amino acids in meteorites, which suggests that chiral asymmetry might have an abiotic origin. A possible abiotic mechanism that could produce chiral asymmetry in meteoritic amino acids is their formation under the influence of asymmetric catalysts, as mineral crystallization can produce spatially asymmetric structures. Although magnetite plaquettes have been proposed to be a possible candidate for an asymmetric catalyst, based on the suggestion that they have a spiral structure, a comprehensive description of their morphology and interpretation of the mechanism associated with symmetry-breaking in biomolecules remain elusive. Here we report observations of magnetite plaquettes in carbonaceous chondrites (CC) that were made with scanning electron microscopy and synchrotron X-ray computed microtomography (SXRCT). We obtained the crystal orientation of the plaquettes using electron backscatter diffraction (EBSD) analysis. SXRCT permits visualization of the internal features of the plaquettes. It provides an unambiguous conclusion that the plaquettes are devoid of a spiral feature and, rather that they are stacks of individual magnetite disks that do not join to form a continuous spiral. Despite the lack of spiral features, our EBSD data show significant changes in crystal orientation between adjacent magnetite disks. The magnetite disks are displaced in a consistent relative direction that lead to an overall crystallographic rotational mechanism. This work offers an explicit understanding of the structures of magnetite plaquettes in CC, which provides a fundamental basis for future interpretation of the proposed symmetry-breaking mechanism.

^1,2A.P. Jordan, ^2,3T.J. Stubbs, ^1,2J.K. Wilson, ^1,2N.A. Schwadron, ^1,2H.E. Spence
Icarus (in Press) Link to Article [http://dx.doi.org/10.1016/j.icarus.2016.08.027]
¹Institute for the Study of Earth, Oceans, and Space, University of New Hampshire, Durham, New Hampshire, USA
²Solar System Exploration Research Virtual Institute, NASA Ames Research Center, Moffett Field, California, USA
³NASA Goddard Space Flight Center, Greenbelt, Maryland, USA
Copyright Elsevier

Large solar energetic particle events may cause dielectric breakdown in the upper 1 mm of regolith in permanently shadowed regions (PSRs). We estimate how the resulting breakdown weathering compares to meteoroid impact weathering. Although the SEP event rates measured by the Cosmic Ray Telescope for the Effects of Radiation (CRaTER) on the Lunar Reconnaissance Orbiter (LRO) are too low for breakdown to have significantly affected the regolith over the duration of the LRO mission, regolith gardened by meteoroid impacts has been exposed to SEPs for ∼106 yr. Therefore, we estimate that breakdown weathering’s production rate of vapor and melt in the coldest PSRs is up to 1.8−3.5×10−71.8−3.5×10−7 kg m−2−2 yr−1,−1, which is comparable to that produced by meteoroid impacts. Thus, in PSRs, up to 10–25% of the regolith may have been melted or vaporized by dielectric breakdown. Breakdown weathering could also be consistent with observations of the increased porosity (“fairy castles”) of PSR regolith. We also show that it is conceivable that breakdown-weathered material is present in Apollo soil samples. Consequently, breakdown weathering could be an important process within PSRs, and it warrants further investigation.

^1,2Rebecca N. Greenberger, ¹John F. Mustard, ^3,4,5Gordon R. Osinski, ^3,4,6Livio L. Tornabene, ^3,4,7Alexandra J. Pontefract, ^3,4Cassandra L. Marion, ^3,4Roberta L. Flemming, ⁸Janette H. Wilson, ⁹Edward A. Cloutis
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12716]
¹Department of Earth, Environmental, and Planetary Sciences, Brown University, Providence, Rhode Island, USA
²Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USA
³Centre for Planetary Science and Exploration, University of Western Ontario, London, Ontario, Canada
⁴Department of Earth Sciences, University of Western Ontario, London, Ontario, Canada
⁵Department of Physics and Astronomy, University of Western Ontario, London, Ontario, Canada
⁶SETI Institute, Mountain View, California, USA
⁷Planetary Science Institute, Tucson, Arizona, USA
⁸Headwall Photonics, Inc., Fitchburg, Massachusetts, USA
⁹Department of Geography, University of Winnipeg, Winnipeg, Manitoba, Canada
Published by arrangement with John Wiley & Sons

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

¹Doris Breuer, ¹Ana-Catalina Plesa, ^1,2Nicola Tosi, ¹Matthias Grott
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12727]
¹DLR, Institute of Planetary Research, Berlin, Germany
²Department of Astronomy and Astrophysics, Technische Universität Berlin, Berlin, Germany
Published by arrangement with John Wiley & Sons

Petrological analysis of the Martian meteorites suggests that rheologically significant amounts of water are present in the Martian mantle. A bulk mantle water content of at least a few tens of ppm is thus expected to be present despite the potentially efficient degassing during accretion, magma ocean solidification, and subsequent volcanism. We examine the dynamical consequences of different thermochemical evolution scenarios testing whether they can lead to the formation and preservation of mantle reservoirs, and compare model predictions with available data. First, the simplest scenario of a homogenous mantle that emerges when ignoring density changes caused by the extraction of partial melt is found to be inconsistent with the isotopic evidence for distinct reservoirs provided by the analysis of the Martian meteorites. In a second scenario, reservoirs can form as a result of partial melting that induces a density change in the depleted mantle with respect to its primordial composition. However, efficient mantle mixing prevents these reservoirs from being preserved until present unless they are located in the stagnant lid. Finally, reservoirs could be formed during fractional crystallization of a magma ocean. In this case, however, the mantle would likely end up being stably stratified as a result of the global overturn expected to accompany the fractional crystallization. Depending on the assumed density contrast, little secondary crust would be produced and the lithosphere would be extremely cool and dry, in contrast to observations. In summary, it is very challenging to obtain a self-consistent evolution scenario that satisfies all available constraints.

¹William H. Farrand, ²Jeffrey R. Johnson, ³Melissa S. Rice, ⁴Alian Wang, ⁵James F. Bell III
American Mineralogist 101, 2005-2019  Link to Article [DOI: 10.2138/am-2016-5627]
¹Space Science Institute, 4750 Walnut Street, number 205, Boulder, Colorado 80301, U.S.A.
²Johns Hopkins University Applied Physics Laboratory, 11100 Johns Hopkins Road, Laurel, Maryland 20723, U.S.A.
³Department of Geology, Western Washington University, 516 High Street, Bellingham, Washington 98225, U.S.A.
⁴Department of Earth and Planetary Sciences, Washington University, Campus Box 1169, One Brookings Drive, St. Louis, Missouri 63130, U.S.A.
⁵School of Earth and Space Exploration, Arizona State University, P.O. Box 87104, Tempe, Arizona 85287, U.S.A.
Copyright: The Mineralogical Society of America

Multispectral visible and near infrared (VNIR) observations from the Mars Exploration Rover Pancam multispectral stereo camera systems are consistent with materials having been subjected to various aqueous processes. Ferric oxides in the form of hematite in the Burns and Grasberg formations of Meridiani Planum have been well characterized by Opportunity on the basis of strong 535 and 864 nm absorptions and positive 754–1009 nm and 934–1009 nm slopes. On the rim of Noachian-aged Endeavour crater, Opportunity has observed light-toned veins with high Ca and S, as determined by the rover’s Alpha Particle X-ray Spectrometer (APXS), and a negative 934–1009 nm slope in VNIR spectra extracted from Pancam data, indicative of a 1000 nm H2O overtone absorption. Together these observations indicate that the veins are composed of gypsum. Rocks overturned by Opportunity on the Murray Ridge portion of the Endeavour crater rim display dark- and light-toned coatings. The dark-toned coatings have a red, featureless slope that is consistent with the slope observed in laboratory spectra of high-valence manganese oxide minerals. Potential Mn oxide coatings may also be associated with some exposures of the Grasberg formation. APXS results for high Mg and S in the light-toned coatings of the Murray Ridge overturned rocks and a negative 934–1009 nm slope are consistent with hydrated Mg-sulfates. Opportunity has also observed spectral features in rocks that are consistent with orbital observations of Fe-smectites, as well as Al-smectites and possible hydrated silica in light-toned fracture-fill materials. The Spirit rover observed sulfate-rich light-toned soils exposed by the rover’s wheels. Several of these soil observations contained spectral features, such as a broad absorption centered near 800 nm, consistent with ferric sulfate minerals, a finding confirmed by the rover’s Mössbauer spectrometer. Spirit also excavated light-toned Si-rich soils. These soils have a flat near-infrared spectrum with a drop in reflectance from 934–1009 nm that is consistent with free water contained in voids or adsorbed onto the surface of the silica.

^1,2P. Mane, ¹R. Hervig, ^1,2M. Wadhwa, ^1,2L. A. J. Garvie, ^3,4J. B. Balta, ³H. Y. McSween Jr
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12717]
¹School of Earth and Space Exploration, Arizona State University, Tempe, Arizona, USA
²Center for Meteorite Studies, Arizona State University, Tempe, Arizona, USA
³Department of Earth and Planetary Sciences and Planetary Geosciences Institute, University of Tennessee, Knoxville, Tennessee, USA
⁴Department of Geology and Planetary Science, University of Pittsburgh, Pittsburgh, Pennsylvania, USA
Published by arrangement with John Wiley & Sons

The hydrogen isotopic composition of planetary reservoirs can provide key constraints on the origin and history of water on planets. The sources of water and the hydrological evolution of Mars may be inferred from the hydrogen isotopic compositions of mineral phases in Martian meteorites, which are currently the only samples of Mars available for Earth-based laboratory investigations. Previous studies have shown that δD values in minerals in the Martian meteorites span a large range of −250 to +6000‰. The highest hydrogen isotope ratios likely represent a Martian atmospheric component: either interaction with a reservoir in equilibrium with the Martian atmosphere (such as crustal water), or direct incorporation of the Martian atmosphere due to shock processes. The lowest δD values may represent those of the Martian mantle, but it has also been suggested that these values may represent terrestrial contamination in Martian meteorites. Here we report the hydrogen isotopic compositions and water contents of a variety of phases (merrillites, maskelynites, olivines, and an olivine-hosted melt inclusion) in Tissint, the latest Martian meteorite fall that was minimally exposed to the terrestrial environment. We compared traditional sample preparation techniques with anhydrous sample preparation methods, to evaluate their effects on hydrogen isotopes, and find that for severely shocked meteorites like Tissint, the traditional sample preparation techniques increase water content and alter the D/H ratios toward more terrestrial-like values. In the anhydrously prepared Tissint sample, we see a large range of δD values, most likely resulting from a combination of processes including magmatic degassing, secondary alteration by crustal fluids, shock-related fractionation, and implantation of Martian atmosphere. Based on these data, our best estimate of the δD value for the Martian depleted mantle is −116 ± 94‰, which is the lowest value measured in a phase in the anhydrously prepared section of Tissint. This value is similar to that of the terrestrial upper mantle, suggesting that water on Mars and Earth was derived from similar sources. The water contents of phases in Tissint are highly variable, and have been affected by secondary processes. Considering the H2O abundances reported here in the driest phases (most likely representing primary igneous compositions) and appropriate partition coefficients, we estimate the H2O content of the Tissint parent magma to be ≤0.2 wt%.

¹Walter S. Kiefer, ^1,2Qingsong Li
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12720]
¹Lunar and Planetary Institute, Houston, Texas, USA
²BP, Houston, Texas, USA
Published by arrangement with John Wiley & Sons

Based on meteorite evidence, the present-day Martian mantle has a combined abundance of up to a few hundred ppm of H2O, Cl, and F, which lowers the solidus and enhances the magma production rate. Adiabatic decompression melting in upwelling mantle plumes is the best explanation for young (last 200 Myr) volcanism on Mars. We explore water undersaturated mantle plume volcanism using a finite element mantle convection model coupled to a model of hydrous peridotite melting. Relative to a dry mantle, the reduction in solidus temperature due to water increases the magma production rate by a factor of 1.3–1.7 at 50 ppm water and by a factor of 1.9–3.2 at 200 ppm water. Mantle water also decreases the viscosity and increases the vigor of convection, which indirectly increases the magma production rate by thinning the thermal boundary layer and increasing the flow velocity. At conditions relevant to Mars, these indirect effects can cause an order of magnitude increase in the magma production rate. Using geologic and geophysical observations of the Late Amazonian magma production rate and geochemical observations of melt fractions in shergottite meteorites, present-day Mars is constrained to have a core–mantle boundary temperature of ~1750 to 1800 °C and a volume-averaged thermal Rayleigh number of 2 × 106 to 107, indicating that moderately vigorous mantle convection has persisted to the present day. Melting occurs at depths of 2.5–6 GPa and is controlled by the Rayleigh number at the low pressure end and by the mantle water concentration at high pressure.

^1,2,3Eve L. Berger, ³Dante S. Lauretta, ^3,4Thomas J. Zega, ⁵Lindsay P. Keller
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12721]
¹GeoControl Systems, Inc.—Jacobs JETS contract—NASA Johnson Space Center, Houston, Texas, USA
²NASA Postdoctoral Program, Oak Ridge, Tennessee, USA
³Lunar and Planetary Laboratory, University of Arizona, Tucson, Arizona, USA
⁴Naval Research Laboratory, Washington, District of Columbia, USA
⁵NASA Johnson Space Center, Houston, Texas, USA
Published by arrangement with John Wiley & Sons