¹James M. D. Day, ^2,3Lin Qiu, ²Richard D. Ash, ²William F. McDonough, ⁴Fang-Zhen Teng, ²Roberta L. Rudnick,⁵Lawrence A. Taylor
¹Geosciences Research Division, Scripps Institution of Oceanography, University of California San Diego, La Jolla, California, USA
²Department of Geology, University of Maryland, College Park, Maryland, USA
³Department of Geology and Geophysics, Yale University, New Haven, Connecticut, USA
⁴Department of Earth and Space Sciences, University of Washington, Seattle, Washington, USA
⁵Department of Earth and Planetary Sciences, University of Tennessee, Knoxville, Tennessee, USA

Lithium isotope and abundance data are reported for Apollo 15 and 17 mare basalts and the LaPaz low-Ti mare basalt meteorites, along with lithium isotope data for carbonaceous, ordinary, and enstatite chondrites, and chondrules from the Allende CV3 meteorite. Apollo 15 low-Ti mare basalts have lower Li contents and lower δ7Li (3.8 ± 1.2‰; all uncertainties are 2 standard deviations) than Apollo 17 high-Ti mare basalts (δ7Li = 5.2 ± 1.2‰), with evolved LaPaz mare basalts having high Li contents, but similar low δ7Li (3.7 ± 0.5‰) to Apollo 15 mare basalts. In low-Ti mare basalt 15555, the highest concentrations of Li occur in late-stage tridymite (>20 ppm) and plagioclase (11 ± 3 ppm), with olivine (6.1 ± 3.8 ppm), pyroxene (4.2 ± 1.6 ppm), and ilmenite (0.8 ± 0.7 ppm) having lower Li concentrations. Values of δ7Li in low- and high-Ti mare basalt sources broadly correlate negatively with 18O/16O and positively with 56Fe/54Fe (low-Ti: δ7Li ≤4‰; δ56Fe ≤0.04‰; δ18O ≥5.7‰; high-Ti: δ7Li >6‰; δ56Fe >0.18‰; δ18O <5.4‰). Lithium does not appear to have acted as a volatile element during planetary formation, with subequal Li contents in mare basalts compared with terrestrial, martian, or vestan basaltic rocks. Observed Li isotopic fractionations in mare basalts can potentially be explained through large-degree, high-temperature igneous differentiation of their source regions. Progressive magma ocean crystallization led to enrichment in Li and δ7Li in late-stage liquids, probably as a consequence of preferential retention of 7Li and Li in the melt relative to crystallizing solids. Lithium isotopic fractionation has not been observed during extensive differentiation in terrestrial magmatic systems and may only be recognizable during extensive planetary magmatic differentiation under volatile-poor conditions, as expected for the lunar magma ocean. Our new analyses of chondrites show that they have δ7Li ranging between −2.5‰ and 4‰. The higher δ7Li in planetary basalts than in the compilation of chondrites (2.1 ± 1.3‰) demonstrates that differentiated planetary basalts are, on average, isotopically heavier than most chondrites.

Reference
Day JMD, Qiu L, Ash RD, McDonough WF, Teng F-Z, Rudnick RL, Taylor LA (2016) Evidence for high-temperature fractionation of lithium isotopes during differentiation of the Moon. Meteoritics & Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12643]
Published by arrangement with John Wiley & Sons

¹Mikhail Yu. Zolotov, ²Mikhail V. Mironenko
¹School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287-1404, USA
²Vernadsky Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences, 19 Kosygin Str., Moscow 119991, Russia

Numerical chemical models for water-basalt interaction have been used to constrain the formation of stratified mineralogical sequences of Noachian clay-bearing rocks exposed in the Mawrth Vallis region and in other places on cratered martian highlands. The numerical approaches are based on calculations of water-rock type chemical equilibria and models which include rates of mineral dissolution. Results show that the observed clay-bearing sequences could have formed through downward percolation and neutralization of acidic H2SO4-HCl solutions. A formation of weathering profiles by slightly acidic fluids equilibrated with current atmospheric CO2 requires large volumes of water and is inconsistent with observations. Weathering by solutions equilibrated with putative dense CO2 atmospheres leads to consumption of CO2 to abundant carbonates which are not observed in clay stratigraphies. Weathering by H2SO4-HCl solutions leads to formation of amorphous silica, Al-rich clays, ferric oxides/oxyhydroxides, and minor titanium oxide and alunite at the top of weathering profiles. Mg-Fe phyllosilicates, Ca sulfates, zeolites, and minor carbonates precipitate from neutral and alkaline solutions at depth. Acidic weathering causes leaching of Na, Mg, and Ca, from upper layers and accumulation of Mg-Na-Ca sulfate-chloride solutions at depth. Neutral MgSO4 type solutions dominate in middle parts of weathering profiles and could occur in deeper layers owing to incomplete alteration of Ca minerals and a limited trapping of Ca to sulfates. Although salts are not abundant in the Noachian geological formations, the results suggest the formation of Noachian salty solutions and their accumulation at depth. A partial freezing and migration of alteration solutions could have separated sulfate-rich compositions from low-temperature chloride brines and contributed to the observed diversity of salt deposits. A Hesperian remobilization and release of subsurface MgSO4 type solutions into newly-formed depressions could account for formation of massive layered sulfate deposits through freezing or evaporation. This scenario explains the observed deficiency of salts in Noachian formations, a paucity of Hesperian phyllosilicates, and the occurrence of sulfate deposits in Valles Marineris troughs, chaotic terrains, and some craters of the Hesperian age.

Reference
Zolotov MY, Mironenko MV (2016) Chemical models for martian weathering profiles: Insights into formation of layered phyllosilicate and sulfate deposits. Icarus (in Press)
Link to Article [doi:10.1016/j.icarus.2016.04.011]
Copyright Elsevier

^1,2Martin Schmieder, ^1,2David A. Kring, ³Timothy D. Swindle, ⁴Jade C. Bond,⁵Carleton B. Moore
¹Lunar and Planetary Institute, Houston, Texas, USA
²NASA Solar System Exploration Research Virtual Institute
³Lunar and Planetary Laboratory, University of Arizona, Tucson, Arizona, USA
⁴Department of Astrophysics, School of Physics, University of New South Wales, Sydney, New South Wales, Australia
⁵Center for Meteorite Studies, Arizona State University, Tempe, Arizona, USA

The Gao-Guenie H5 chondrite that fell on Burkina Faso (March 1960) has portions that were impact-melted on an H chondrite asteroid at ~300 Ma and, through later impact events in space, sent into an Earth-crossing orbit. This article presents a petrographic and electron microprobe analysis of a representative sample of the Gao-Guenie impact melt breccia consisting of a chondritic clast domain, quenched melt in contact with chondritic clasts, and an igneous-textured impact melt domain. Olivine is predominantly Fo80–82. The clast domain contains low-Ca pyroxene. Impact melt-grown pyroxene is commonly zoned from low-Ca pyroxene in cores to pigeonite and augite in rims. Metal–troilite orbs in the impact melt domain measure up to ~2 mm across. The cores of metal orbs in the impact melt domain contain ~7.9 wt% of Ni and are typically surrounded by taenite and Ni-rich troilite. The metallography of metal–troilite droplets suggest a stage I cooling rate of order 10 °C s−1 for the superheated impact melt. The subsolidus stage II cooling rate for the impact melt breccia could not be determined directly, but was presumably fast. An analogy between the Ni rim gradients in metal of the Gao-Guenie impact melt breccia and the impact-melted H6 chondrite Orvinio suggests similar cooling rates, probably on the order of ~5000–40,000 °C yr−1. A simple model of conductive heat transfer shows that the Gao-Guenie impact melt breccia may have formed in a melt injection dike ~0.5–5 m in width, generated during a sizeable impact event on the H chondrite parent asteroid.

Reference
Schmieder M, Kring DA, Swindle TD, Bond JC, Moore CB (2016) The Gao-Guenie impact melt breccia—Sampling a rapidly cooled impact melt dike on an H chondrite asteroid? Meteoritics & Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12642]
Published by arrangement with John Wiley & Sons

^1,2Francis M. McCubbin, ³Jeremy W. Boyce, ²Poorna Srinivasan, ²Alison R. Santos, ⁴Stephen M. Elardo, ⁵Justin Filiberto, ⁴Andrew Steele,²Charles K. Shearer
¹NASA Johnson Space Center, Houston, Texas, USA
²Institute of Meteoritics, Department of Earth & Planetary Sciences, University of New Mexico, Albuquerque, New Mexico, USA
³Department of Earth & Space Sciences, University of California Los Angeles, Los Angeles, California, USA
⁴Geophysical Laboratory, Carnegie Institution of Washington, Washington, District of Columbia, USA
⁵Department of Geology, Southern Illinois University, Carbondale, Illinois, USA

We conducted a petrologic study of apatite within 12 Martian meteorites, including 11 shergottites and one basaltic regolith breccia. These data were combined with previously published data to gain a better understanding of the abundance and distribution of volatiles in the Martian interior. Apatites in individual Martian meteorites span a wide range of compositions, indicating they did not form by equilibrium crystallization. In fact, the intrasample variation in apatite is best described by either fractional crystallization or crustal contamination with a Cl-rich crustal component. We determined that most Martian meteorites investigated here have been affected by crustal contamination and hence cannot be used to estimate volatile abundances of the Martian mantle. Using the subset of samples that did not exhibit crustal contamination, we determined that the enriched shergottite source has 36–73 ppm H2O and the depleted source has 14–23 ppm H2O. This result is consistent with other observed geochemical differences between enriched and depleted shergottites and supports the idea that there are at least two geochemically distinct reservoirs in the Martian mantle. We also estimated the H2O, Cl, and F content of the Martian crust using known crust-mantle distributions for incompatible lithophile elements. We determined that the bulk Martian crust has ~1410 ppm H2O, 450 ppm Cl, and 106 ppm F, and Cl and H2O are preferentially distributed toward the Martian surface. The estimate of crustal H2O results in a global equivalent surface layer (GEL) of ~229 m, which can account for at least some of the surface features on Mars attributed to flowing water and may be sufficient to support the past presence of a shallow sea on Mars’ surface.

Reference
McCubbin FM, Boyce JW, Srinivasan P, Santos AR, Elardo SM, FilibertoJ, Steele A, Shearer CK (2016) Heterogeneous distribution of H2O in the Martian interior: Implications for the abundance of H2O in depleted and enriched mantle sources. Meteoritics & Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12639]
Published by arrangement with John Wiley & Sons

¹Hugues Leroux, ¹Damien Jacob, ²Maya Marinova, ^3,4Roger H. Hewins, ^3,4,5Brigitte Zanda, ³Sylvain Pont, ⁶Jean-Pierre Lorand, ⁷Munir Humayun
¹Unité Matériaux et Transformations, University of Lille & CNRS, Villeneuve d’Ascq, France
²Institut Chevreul, University of Lille & CNRS, Villeneuve d’Ascq, France
³Institut de Minéralogie, de Physique des Matériaux, et de Cosmochimie (IMPMC), Sorbonne Université, Muséum National d’Histoire Naturelle, UPMC Université Paris 06, IRD & CNRS, Paris, France
⁴Department of Earth and Planetary Sciences, Rutgers University, Piscataway, New Jersey, USA
⁵Institut de Mécanique Céleste et de Calcul des Ephémérides, Observatoire de Paris, Paris Cedex, France
⁶Laboratoire de Planétologie et Géodynamique, Université de Nantes, Nantes, France
⁷Department of Earth, Ocean & Atmospheric Science and National High Magnetic Field Laboratory, Florida State University, Tallahassee, Florida, USA

Northwest Africa (NWA) 7533 is a Martian regolith breccia. This meteorite (and its pairings) offers a good opportunity to study (near-) surface processes that occurred on early Mars. Here, we have conducted a transmission electron microscope study of medium- and coarse-grained (a few tens to hundreds of micrometers) Ca-rich pyroxene clasts in order to define their thermal and shock histories. The pyroxene grains have a high-temperature (magmatic) origin as revealed by the well-developed pigeonite–augite exsolution microstructure. Exsolution lamella characteristics (composition, thickness, and spacing) indicate a moderately slow cooling. Some of the pyroxene clasts display evidence for local decomposition into magnetite and silica at the submicron scale. This phase decomposition may have occurred at high temperature and occurred at high oxygen fugacity at least 2–3 log units above the QFM buffer, after the formation of the exsolution lamellae. This corresponds to oxidizing conditions well above typical Martian magmatic conditions. These oxidizing conditions seem to have prevailed early and throughout most of the history of NWA 7533. The shock microstructure consists of (100) mechanical twins which have accommodated plastic deformation. Other pyroxene shock indicators are absent. Compared with SNC meteorites that all suffered significant shock metamorphism, NWA 7533 appears only mildly shocked. The twin microstructure is similar from one clast to another, suggesting that the impact which generated the (100) twins involved the compacted breccia and that the pyroxene clasts were unshocked when they were incorporated into the NWA 7533 breccia.

Reference
Leroux H, Jacob D, Marinova M, Hewins RH, Zanda B, Pont S, Lorand J-P, Humayun M (2016) Exsolution and shock microstructures of igneous pyroxene clasts in the Northwest Africa 7533 Martian meteorite. Meteoritics & Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12637]
Published by arrangement with John Wiley & Sons

^1,2Arshad Ali, ¹Iffat Jabeen, ³David Gregory, ⁴Robert Verish,¹Neil R. Banerjee
¹Department of Earth Sciences & Centre for Planetary Science and Exploration, Western University, London, Ontario, Canada
²Earth Sciences Research Centre (ESRC), Sultan Qaboos University, Muscat, Sultanate of Oman
³St. Thomas, Ontario, Canada
⁴Meteorite-Recovery Lab, Escondido, California, USA

We report precise triple oxygen isotope data of bulk materials and separated fractions of several Shergotty–Nakhla–Chassigny (SNC) meteorites using enhanced laser-assisted fluorination technique. This study shows that SNCs have remarkably identical Δ17O and a narrow range in δ18O values suggesting that these meteorites have assimilated negligibly small surface materials (<5%), which is undetectable in the oxygen isotope compositions reported here. Also, fractionation factors in coexisting silicate mineral pairs (px-ol and mask-ol) further demonstrate isotopic equilibrium at magmatic temperatures. We present a mass-dependent fractionation line for bulk materials with a slope of 0.526 ± 0.016 (1SE) comparable to the slope obtained in an earlier study (0.526 ± 0.013; Franchi et al. 1999). We also present a new Martian fractionation line for SNCs constructed from separated fractions (i.e., pyroxene, olivine, and maskelynite) with a slope of 0.532 ± 0.009 (1SE). The identical fractionation lines run above and parallel to our terrestrial fractionation line with Δ17O = 0.318 ± 0.016‰ (SD) for bulk materials and 0.316 ± 0.009‰ (SD) for separated fractions. The conformity in slopes and Δ17O between bulk materials and separated fractions confirm oxygen isotope homogeneity in the Martian mantle though recent studies suggest that the Martian lithosphere may potentially have multiple oxygen isotope reservoirs.

Reference
Ali A, Jabeen I, Gregory D, Verish R, Banerjee NR (2016) New triple oxygen isotope data of bulk and separated fractions from SNC meteorites: Evidence for mantle homogeneity of Mars. Meteoritics & Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12640]
Published by arrangement with John Wiley & Sons

¹Lee, M. R., ¹Lindgren, P.
¹School of Geographical and Earth Sciences, University of Glasgow, Glasgow, UK

Forsterite and clinoenstatite in type IAB chondrules from the Murchison CM carbonaceous chondrite have been partially serpentinized, and the mechanisms of their alteration reveal crystallographic and microstructural controls on the reaction of silicate minerals with parent body aqueous solutions. Grains of forsterite were altered in two stages. Narrow veinlets of Fe-rich serpentine formed first and by the filling of sheet pores. Most of these pores were oriented parallel to (010) and (001) and had been produced by earlier fracturing and/or congruent dissolution. In the second stage, the subset of veinlets that were oriented parallel to (001) was widened accompanying the replacement of forsterite by Mg-Fe serpentine. This reaction proceeded most rapidly parallel to [001], and crystallographic controls on the trajectory of retreating vein walls created fine-scale serrations. Murchison clinoenstatite grains have a skeletal appearance due to the presence of abundant veinlets and patches of phyllosilicate. Two alteration stages can again be recognized, with initial water–mineral interaction producing tochilinite-rich veinlets by the filling of (001)-parallel contraction cracks. Pores then formed by congruent dissolution that was guided principally by orthopyroxene lamellae, and they were subsequently filled by submicrometer-sized crystals of polyhedral serpentine. This finding that Murchison forsterite and clinoenstatite grains have been altered demonstrates that aqueous processing of magnesium silicate minerals started much earlier in CM parent body history than previously believed. Our results also show that the occurrence of polyhedral serpentine can be used to locate former pore spaces within the parent body.

Reference
Lee MR, Lindgren P. (2016) Aqueous alteration of chondrules from the Murchison CM carbonaceous chondrite: Replacement, pore filling, and the genesis of polyhedral serpentine. Meteoritics & Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12644]
Published by arrangement with John Wiley & Sons

¹Elena S. Amador, ²Joshua L. Bandfield
¹Department of Earth and Space Sciences, University of Washington
²Space Science Institute

The Nili Fossae region of Mars contains some of the most mineralogically diverse bedrock on the planet. Previous studies have established three main stratigraphic units in the region: a phyllosilicate-bearing basement rock, a variably altered olivine-rich basalt, and a capping rock. Here, we present evidence for the localized alteration of the northeast Nili Fossae capping unit, previously considered to be unaltered. Both near-infrared and thermal-infrared spectral datasets were analyzed, including the application of a method for determining the relative abundance of bulk-silica (SiO2) over surfaces using Thermal Emission Imaging System (THEMIS) images. Elevated bulk-silica exposures are present on surfaces previously defined as unaltered capping rock. Given the lack of spectral evidence for phyllosilicate, hydrated silica, or quartz phases coincident with the newly detected exposures – the elevated bulk-silica may have formed under a number of aqueous scenarios, including as a product of the carbonation of the underlying olivine-rich basalt under moderate water:rock scenarios and temperatures. Regardless of formation mechanism, the detection of elevated bulk-silica exposures in the Nili Fossae capping unit extends the history of aqueous activity in the region to include all three of the main stratigraphic units.

Reference
Amador ES, Bandfield JL (2016) Elevated bulk-silica exposures and evidence for multiple aqueous alteration episodes in Nili Fossae, Mars. Icarus (in Press)
Link to Article [doi:10.1016/j.icarus.2016.04.015]
Copyright Elsevier

^1,2,3Levke Kööp, ^4,5Daisuke Nakashima, ^1,2,3Philipp R. Heck, ⁴Noriko T. Kita, ⁴Travis J. Tenner, ⁶Alexander N. Krot, ⁶Kazuhide Nagashima, ^6,7Changkun Park, ^1,2,3,8Andrew M. Davis
¹Department of the Geophysical Sciences, The University of Chicago, Chicago, IL 60637, USA
²Chicago Center for Cosmochemistry, The University of Chicago, Chicago, IL 60637, USA
³Robert A. Pritzker Center for Meteoritics and Polar Studies, Field Museum of Natural History, Chicago, IL, USA
⁴Department of Geoscience, University of Wisconsin, Madison, WI 53706, USA
⁵Division of Earth and Planetary Material Sciences, Faculty of Science, Tohoku University, Aoba, Sendai, Miyagi 980-8578 Japan
⁶Hawai‘i Institute of Geophysics and Planetology, School of Ocean and Earth Science and Technology, University of Hawai‘i at Mānoa, Honolulu, HI
⁷Korea Polar Research Institute, Incheon 406-840, Korea
⁸Enrico Fermi Institute, The University of Chicago, Chicago, IL 60637, USA.

We report oxygen, calcium, titanium and 26Al-26Mg isotope systematics for spinel-hibonite inclusions (SHIBs), a class of calcium-aluminum-rich inclusions (CAI) common in CM chondrites. In contrast to previous studies, our analyses of 33 SHIBs and four SHIB-related objects obtained with high spatial resolution demonstrate that these CAIs have a uniform Δ17O value of approximately –23‰, similar to many other mineralogically pristine CAIs from unmetamorphosed chondrites (e.g., CR, CV, and Acfer 094). Five SHIBs studied for calcium and titanium isotopes have no resolvable anomalies beyond 3σ uncertainties. This suggests that nucleosynthetic anomalies in the refractory elements had been significantly diluted in the environment where SHIBs with uniform Δ17O formed. We established internal 26Al-26Mg isochrons for eight SHIBs and found that seven of these formed with uniformly high levels of 26Al (a multi-CAI mineral isochron yields an initial 26Al/27Al ratio of ∼4.8×10–5), but one SHIB has a smaller initial 26Al/27Al of ∼2.5×10–5, indicating variation in 26Al/27Al ratios when SHIBs formed. The uniform calcium, titanium and oxygen isotopic characteristics found in SHIBs with both high and low initial 26Al/27Al ratios allow for two interpretations. (1) If subcanonical initial 26Al/27Al ratios in SHIBs are due to early formation, as suggested by Liu et al. (2012), our data would indicate that the CAI formation region had achieved a high degree of isotopic homogeneity in oxygen and refractory elements before a homogeneous distribution of 26Al was achieved. (2) Alternatively, if subcanonical ratios were the result of 26Al-26Mg system resetting, the clustering of SHIBs at a Δ17O value of ∼ –23‰ would imply that a 16O-rich gaseous reservoir existed in the nebula until at least ∼0.7 Ma after the formation of the majority of CAIs.

Reference
Kööp L, Nakashima D, Heck PR, Kita NT, Tenner TJ, Krot AN, Nagashima K, Park C, Davis AM (2016) New constraints on the relationship between 26Al and oxygen, calcium, and titanium isotopic variation in the early Solar System from a multielement isotopic study of spinel-hibonite inclusions. Geochimica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2016.04.018]
Copyright Elsevier

¹Altobelli N. et al. (>10)*
¹European Space Agency, European Space Astronomy Centre, Madrid, Spain.
*Find the extensive, full author and affiliation list on the publishers website

Interstellar dust (ISD) is the condensed phase of the interstellar medium. In situ data from the Cosmic Dust Analyzer on board the Cassini spacecraft reveal that the Saturnian system is passed by ISD grains from our immediate interstellar neighborhood, the local interstellar cloud. We determine the mass distribution of 36 interstellar grains, their elemental composition, and a lower limit for the ISD flux at Saturn. Mass spectra and grain dynamics suggest the presence of magnesium-rich grains of silicate and oxide composition, partly with iron inclusions. Major rock-forming elements (magnesium, silicon, iron, and calcium) are present in cosmic abundances, with only small grain-to-grain variations, but sulfur and carbon are depleted. The ISD grains in the solar neighborhood appear to be homogenized, likely by repeated processing in the interstellar medium.

Reference
Altobelli N et al. (2016) Flux and composition of interstellar dust at Saturn from Cassini’s Cosmic Dust Analyzer. Science 352, 312-318
Link to Article [DOI: 10.1126/science.aac6397]
Reprinted with permission from AAAS