¹Jean-Christophe Viennet,¹Benjamin Bultel,²Lucie Riu,¹Stephanie C. Werner
Journal of Geophysical Research, Planets (in Press) Link to Article [DOI: 10.1002/2017JE005343]
¹Centre for Earth Evolution and Dynamics, Department for Geosciences, University of Oslo, Norway
²Institut d’Astrophysique Spatiale, Université Paris-Sud, Orsay, France
Published by arrangement with John Wiley & Sons

Widespread occurrence of Fe,Mg-phyllosilicates have been observed on Noachian Martian terrains. Therefore, the study of Fe,Mg-phyllosilicates formation, in order to characterize early Martian environmental conditions, is of particular interest to the Martian community. Previous studies have shown that the investigation of Fe,Mg-smectite formation alone helps to describe early Mars environmental conditions, but there are still large uncertainties in terms of pH range, oxic/anoxic conditions, etc… Interestingly, carbonates and/or zeolites have also been observed on Noachian surfaces in association with the Fe,Mg-phyllosilicates.

Consequently, the present study focuses on the di/trioctahedral phyllosilicate/carbonate/zeolite formation as a function of various CO2 contents (100% N2, 10% CO2 / 90% N2, 100% CO2), from a combined approach including closed system laboratory experiments for 3 weeks at 120°C and geochemical simulations. The experimental results show that as the CO2 content decreases, the amount of dioctahedral clay minerals decreases in favour of trioctahedral minerals. Carbonates and dioctahedral clay minerals are formed during the experiments with CO2. When Ca-zeolites are formed, no carbonates and dioctahedral minerals are observed. Geochemical simulation aided in establishing pH as a key parameter in determining mineral formation patterns. Indeed, under acidic conditions dioctahedral clay minerals and carbonate minerals are formed, while trioctahedral clay minerals are formed in basic conditions with a neutral pH value of 5.98 at 120°C. Zeolites are favoured from pH >~7.2. The results obtained shed new light on the importance of dioctahedral clay minerals versus zeolites and carbonates versus zeolites competitions, to better define the aqueous alteration processes throughout early Mars history.

¹Elizabeth C. Sklute, ²A. Deanne Rogers, ²Jason C. Gregerson, ²Heidi B. Jensen, ²Richard J. Reeder, ¹M. Darby Dyar
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2017.11.018]
¹Department of Astronomy, Mount Holyoke College, 50 College St., South Hadley, Massachusetts, 01075, U.S.A.
²Department of Geoscience, Stony Brook University, 255 Earth and Space Science Building, Stony Brook, NY 11794-2100, U.S.A.
Copyright Elsevier

¹L. Folco, ²B.P. Glass, ¹M. D’Orazio, ³P. Rochette,
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2017.11.017]
¹Dipartimento di Scienze della Terra, Università di Pisa, Via S. Maria 53, 56126 Pisa, Italy
²Department of Geosciences, University of Delaware 19716, USA
³CEREGE, Aix-Marseille Université CNRS, France
Copyright Elsevier

Impactor identification is one of the challenges of large-scale impact cratering studies due to the dilution of meteoritic material in impactites (typically < 1 wt%). The nature of the impactor that generated the Australasian tektite/microtektite strewn field, i.e., the largest Cenozoic strewn field (∼15% of the Earth’s surface), the youngest (∼0.78 Myr old) on Earth, and the only one without an associated impact crater so far, is an outstanding issue. We identify a chondritic impactor signature in 77 Australasian microtektites (size range: ∼200 to 700 µm) from within 3000 km from the hypothetical impact location in Indochina (∼17°N, 107°E) based on variations of Cr, Co and Ni interelement ratios in a Co/Ni vs Cr/Ni space (46 microtektites analyzed in this work by Laser Ablation-Inductively Coupled Plasma -Mass Spectrometry and 31 from literature by means of Instrumental Neutron Activation Analyses with Cr, Co and Ni concentrations up to ∼370, 50 and 680 µg/g, respectively). Despite substantial overlap in Cr/Ni versus Co/Ni composition for several meteorite types with chondritic composition (chondrites and primitive achondrites), regression calculation based on ∼85% of the studied microtektites best fit a mixing line between crustal compositions and an LL chondrite. However, due to some scatter mainly in the Cr versus Ni ratios in the considered dataset, an LL chondrite may not be the best fit to the data amongst impactors of primitive compositions. Eight high Ni/Cr and five low Ni/Cr outlier microtektites (∼15% in total) deviate from the above mixing trend, perhaps resulting from incomplete homogenization of heterogeneous impactor and target precursor materials at the microtektite scale, respectively.

Together with previous evidence from the ∼35 Myr old Popigai impact spherules and the ∼1 Myr old Ivory Coast microtektites, our finding suggests that at least three of the five known Cenozoic distal impact ejecta were generated by the impacts of large stony asteroids of chondritic composition, and possibly of ordinary chondritic composition. The impactor signature found in Australasian microtektites documents mixing of target and impactor melts upon impact cratering. This requires target-impactor mixing in both the two competing microtektite formation models in literature proposed for the Australasian: the impact cratering and low-altitude airburst plume models.

¹Dennis Harries, ^1,2Falko Langenhorst
Geochimica et Cosmochimica Acta 222, 53-73, Link to Article [https://doi.org/10.1016/j.gca.2017.10.019]
¹Institute of Geoscience, Friedrich Schiller University Jena, Carl-Zeiss-Promenade 10, 07745 Jena, Germany
²Hawai’i Institute of Geophysics and Planetology, School of Ocean and EarthScience and Technology, University of Hawai’i at Manoa, HI 96822, USA
Copyright Elsevier

We found that the particle RA-QD02-0115 returned by the Hayabusa spacecraft from near-Earth asteroid 25143 Itokawa contains the iron carbide haxonite (Fe_21.9-22.7Co_0.2-0.3Ni_0.2-0.8)C₆ and several Fe,Ni alloys, including multi-domain tetrataenite and spinodally decomposed taenite. Ellipsoidal to nearly spherical voids occur throughout the particle and suggest the presence of a fluid phase during textural and chemical equilibration of the host rock within the parent asteroid of 25143 Itokawa. The calculated solubility of carbon in Fe,Ni metal indicates that the carbide formed at temperatures larger than 600 °C during thermal metamorphism of the LL-chondritic mineral assemblage. Haxonite formed metastably with respect to graphite and cohenite, probably due to its high degree of lattice match with neighboring taenite, a low cooling rate at peak metamorphic temperatures, and the hindered nucleation of graphite. Thermodynamic equilibrium calculations indicate that the fluid present was dry (H₂O-poor) and dominated by methane. The reactive fluid most plausibly had an atomic H/C ratio of 4–5 and was derived from the reduction of macromolecular, insoluble organic matter (IOM) that initially co-accreted with water ice. The initial presence of water is a necessary assumption to provide sufficient hydrogen for the formation of methane from hydrolyzed IOM. Metallic iron was in turn partially oxidized and incorporated into the ferromagnesian silicates during the high-temperature stage of metamorphism. An exemplary bulk reaction from unequilibrated material on the left to an equilibrated assemblage on the right may be written as:

330 CH_0.8O_0.2(IOM) + 500 H₂O_(ice/g) + 681 Fe_{(in alloy)} + 566 FeSiO_{3(in Opx)} → 300 CH_4(g) + 32 H_2(g) + 5 Fe₂₃C_{6(in Hx)} + 566 Fe₂SiO_{4(in Ol)}

(Opx = orthopyroxene, Hx = haxonite, Ol = olivine, g = fluid species).

The best estimate of the fluid/rock ratio in the region of the LL parent body where RA-QD02-0115 formed is about 3 × 10⁻³ and corresponds to an initial ice/rock ratio of about 7 × 10⁻³(both by mass).

¹Dimitri D. Badyukov,^2,3Natalia S. Bezaeva,⁴Pierre Rochette,⁴Jérôme Gattacceca,⁵Joshua M. Feinberg,⁶Myriam Kars,⁷Ramon Egli,⁸Jouko Raitala,⁹Dilyara M. Kuzina
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.13006]
¹V.I. Vernadsky Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences, Moscow, Russia
²Institute of Physics and Technology, Ural Federal University, Ekaterinburg, Russia
³Institute of Geology and Petroleum Technologies, Kazan Federal University, Kazan, Russia
⁴Aix-Marseille Université, CNRS, IRD, Coll. France, CEREGE, Aix en Provence, France
⁵Institute for Rock Magnetism, University of Minnesota, Minneapolis, Minnesota, USA
⁶Center for Advanced Marine Core Research, Kochi University, Nankoku, Japan
⁷Central Institute for Meteorology and Geodynamics, Vienna, Austria
⁸Astronomy Department, University of Oulu, Oulu, Finland
⁹Institute of Geology and Petroleum Technologies, Kazan Federal University, Kazan, Russia
Published by arrangement with John Wiley & Sons

Hypervelocity impacts occur on bodies throughout our solar system, and play an important role in altering the mineralogy, texture, and magnetic properties in target rocks at nanometer to planetary scales. Here we present the results of hypervelocity impact experiments conducted using a two-stage light-gas gun with 5 mm spherical copper projectiles accelerated toward basalt targets with ~6 km s⁻¹ impact velocities. Four different types of magnetite- and titanomagnetite-bearing basalts were used as targets for seven independent experiments. These laboratory impacts resulted in the formation of agglutinate-like particles similar in texture to lunar agglutinates, which are an important fraction of lunar soil. Materials recovered from the impacts were examined using a suite of complementary techniques, including optical and scanning electron microscopy, micro-Raman spectroscopy, and high- and low-temperature magnetometry, to investigate the texture, chemistry, and magnetic properties of newly formed agglutinate-like particles and were compared to unshocked basaltic parent materials. The use of Cu-projectiles, rather than Fe- and Ni-projectiles, avoids magnetic contamination in the final shock products and enables a clearer view of the magnetic properties of impact-generated agglutinates. Agglutinate-like particles show shock features, such as melting and planar deformation features, and demonstrate shock-induced magnetic hardening (two- to seven-fold increases in the coercivity of remanence B_cr compared to the initial target materials) and decreases in low-field magnetic susceptibility and saturation magnetization.

¹Geoffrey H. Howarth,²Arya Udry,³James M. D. Day
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12999]
¹Department of Geological Sciences, University of Cape Town, Rondebosch, South Africa
²Department of Geoscience, University of Nevada Las Vegas, Las Vegas, Nevada, USA
³Geosciences Research Division, Scripps Institution of Oceanography, University of California San Diego, La Jolla, California, USA
Published by arrangement with John Wiley & Sons

Northwest Africa (NWA) 8657 is an incompatible trace element-enriched, low-Al basaltic shergottite, similar in texture and chemistry to Shergotty, Zagami, and NWA 5298. It is composed of zoned pyroxene, maskelynite, merrillite, and Ti-oxide minerals with minor apatite, silica, and pyrrhotite. Pyroxene grains are characterized by patchy zoning, with pigeonite or augite cores zoned to Fe-rich pigeonite mantles. The cores have rounded morphologies and irregular margins. Combined with the low Ti/Al of the cores, the morphology and chemistry of the pyroxene grains are consistent with initial crystallization at depth (30–70 km) followed by partial resorption en route to the surface. Enriched rare earth element (REE) equilibrium melt compositions and calculated oxygen fugacities (fO2) conditions for pigeonite cores indicate that the original parent melts were enriched shergottite magmas that staged in chambers at depth within the Martian crust. NWA 8657 does not represent a liquid but rather entrained a proportion of pyroxene crystals from magma chambers where fractional crystallization was occurring at depth. Variation between fO2 and bulk-rock (La/Yb)N of the enriched and intermediate shergottites suggests that oxidation conditions and degree of incompatible element enrichment in the source may not be correlated, as thought previously. Shock melt pockets are characterized by an absence of phosphates and oxide minerals. It is likely that these phases were melted during shock. REEs were redistributed during this process into maskelynite and to a lesser extent the shock melt; however, the overall normalized REE profile of the shock melt is like that of the bulk-rock, but at lower absolute concentrations. Overall, shock melting has had a significant effect on the mineralogy of NWA 8657, especially the distribution of phosphates, which may be significant for geochronological applications of this meteorite and other Martian meteorites with extensive shock melt.

¹Jean-Cristophe Viennet,¹Benjamin Bultel,²Lucie Riu,¹Stephanie C. Werner
Journal of Geophysical Research, Planets (in Press) Link to Article [DOI: 10.1002/2017JE005343]
¹Centre for Earth Evolution and Dynamics, Department for Geosciences, University of Oslo, Norway
²Institut d’Astrophysique Spatiale, Université Paris-Sud, Orsay, France
Published by arrangement with John Wiley & Sons

Widespread occurrence of Fe,Mg-phyllosilicates have been observed on Noachian Martian terrains. Therefore, the study of Fe,Mg-phyllosilicates formation, in order to characterize early Martian environmental conditions, is of particular interest to the Martian community. Previous studies have shown that the investigation of Fe,Mg-smectite formation alone helps to describe early Mars environmental conditions, but there are still large uncertainties in terms of pH range, oxic/anoxic conditions, etc… Interestingly, carbonates and/or zeolites have also been observed on Noachian surfaces in association with the Fe,Mg-phyllosilicates.

Consequently, the present study focuses on the di/trioctahedral phyllosilicate/carbonate/zeolite formation as a function of various CO₂ contents (100% N₂, 10% CO₂ / 90% N₂, 100% CO₂), from a combined approach including closed system laboratory experiments for 3 weeks at 120°C and geochemical simulations. The experimental results show that as the CO₂ content decreases, the amount of dioctahedral clay minerals decreases in favour of trioctahedral minerals. Carbonates and dioctahedral clay minerals are formed during the experiments with CO₂. When Ca-zeolites are formed, no carbonates and dioctahedral minerals are observed. Geochemical simulation aided in establishing pH as a key parameter in determining mineral formation patterns. Indeed, under acidic conditions dioctahedral clay minerals and carbonate minerals are formed, while trioctahedral clay minerals are formed in basic conditions with a neutral pH value of 5.98 at 120°C. Zeolites are favoured from pH >~7.2. The results obtained shed new light on the importance of dioctahedral clay minerals versus zeolites and carbonates versus zeolites competitions, to better define the aqueous alteration processes throughout early Mars history.

^1,2Jon K.Hillier, ³Z.Sternovsky,³S.Kempf, ²M.Trieloff, ²M.Guglielmino, ²F.Postberg, ¹M.C.Price
Planetary and Space Science (in Press) Link to Article [https://doi.org/10.1016/j.pss.2017.10.002]
¹Centre for Astrophysics and Planetary Science, University of Kent, Canterbury, Kent CT2 7NH, UK
²Klaus-Tschira-Labor für Kosmochemie, Institut für Geowissenschaften, Im Neuenheimer Feld 234-236, Universität Heidelberg, 69120 Heidelberg, Germany
³Laboratory for Atmospheric and Space Physics, 1234 Innovation Drive, Boulder, CO 80303, USA

We currently do not have a copyright agreement with this publisher and cannot display the abstract here

¹Charity M.Phillips-Lander, ²S.R.Parnell, ³L.E.McGraw, ⁴M.E.Elwood Madden
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2017.10.024]
¹School of Geology and Geophysics, University of Oklahoma, 100 E. Boyd St., Norman, OK 73071, USA
Copyright Elsevier

A diverse suite of carbonate minerals including calcite (CaCO3) and magnesite (MgCO3) have been observed on the martian surface and in meteorites. Terrestrial carbonates usually form via aqueous processes and often record information about the environment in which they formed, including chemical and textural biosignatures. In addition, terrestrial carbonates are often found in association with evaporite deposits on Earth. Similar high salinity environments and processes were likely active on Mars and some areas may contain active high salinity brines today. In this study, we directly compare calcite and magnesite dissolution in ultrapure water, dilute sulfate and chloride solutions, as well as near-saturated sulfate and chloride brines with known activity of water (ɑH2O) to determine how dissolution rates vary with mineralogy and ɑH2O, as well as aqueous cation and anion chemistry to better understand how high salinity fluids may have altered carbonate deposits on Mars. We measured both calcite and magnesite initial dissolution rates at 298 K and near neutral pH (6–8) in unbuffered solutions containing ultrapure water (18 MΩ cm−1 UPW; ɑH2O = 1), dilute (0.1 mol kg−1; ɑH2O = 1) and near-saturated Na2SO4 (2.5 mol kg−1, ɑH2O = 0.92), dilute (0.1 mol kg−1, ɑH2O = 1) and near-saturated NaCl (5.7 mol kg−1, ɑH2O = 0.75). Calcite dissolution rates were also measured in dilute and near-saturated MgSO4 (0.1 mol kg−1, ɑH2O = 1 and 2.7 mol kg−1, ɑH2O = 0.92, respectively) and MgCl2 (0.1 mol kg−1, ɑH2O = 1 and 3 mol kg−1, ɑH2O = 0.73, respectively), while magnesite dissolution rates were measured in dilute and near-saturated CaCl2 (0.1 mol kg−1, ɑH2O = 1 and 9 mol kg−1, ɑH2O = 0.35).
Initial calcite dissolution rates were fastest in near-saturated MgCl2 brine, while magnesite dissolution rates were fastest in dilute (0.1 mol kg−1) NaCl and CaCl2 solutions. Calcite dissolution rates in near-saturated Na2SO4 were similar to those observed in the dilute solutions (−8.00 ± 0.12 log mol m−2 s−1), while dissolution slowed in both NaCl solutions (0.1 mol kg−1; −8.23 ± 0.10 log mol m−2 s−1 and (5.7 mol kg−1; −8.44 ± 0.11 log mol m−2 s−1), as well as near-saturated MgSO4 brine (2.7 mol kg−1; −8.35 ± 0.05 log mol m−2 s−1). The slowest calcite dissolution rates observed in the near-saturated NaCl brine. Magnesite dissolution rates were ∼5 times faster in the dilute salt solutions relative to UPW, but similar to UPW (−8.47 ± 0.06 log mol m−2 s−1) in near-saturated Na2SO4 brines (−8.41 ± 0.18 log mol m−2 s−1). Magnesite dissolution slowed significantly in near-saturated CaCl2 brine (−9.78 ± 0.10 log mol m−2 s−1), likely due to the significantly lower water activity in these experiments. Overall, magnesite dissolution rates are slower than calcite dissolution rates and follow the trend: All dilute salt solutions >2.5 mol kg−1 Na2SO4 ≈ UPW > 5.7 mol kg−1 NaCl >> 9 mol kg−1 CaCl2. Calcite rates follow the trend 3 mol kg−1 MgCl2 > 2.5 mol kg−1 Na2SO4 ≈ UPW ≈ all dilute salt solutions >2.7 mol kg−1 MgSO4 ≈ 5.7 mol kg−1 NaCl. Magnesite dissolution rates in salt solutions generally decrease with decreasing ɑH2O in both chloride and sulfate brines, which indicates water molecules act as ligands and participate in the rate-limiting magnesite dissolution step. However, there is no general trend associated with water activity observed in the calcite dissolution rates. Calcite dissolution accelerates in near-saturated MgCl2, but slows in near-saturated NaCl brine despite both brines having similar water activities (ɑH2O = 0.73 and 0.75, respectively). High Mg calcite was observed as a reaction product in the near-saturated MgCl2, indicating Mg2+ from solution likely substituted for Ca2+ in the initial calcite, releasing additional Ca2+ into solution and increasing the observed calcite dissolution rate. Calcite dissolution rates also increase slightly as Na2SO4 concentration increases, while calcite dissolution rates slow slightly with increasing concentration of MgSO4 and NaCl. However, all of the carbonate rates vary by less than 0.5 log units and are within or near the standard deviation observed for each set of replicate experiments.
Carbonate mineral lifetimes in high salinity brines indicate magnesite may be preferentially preserved compared to calcite on Mars. Therefore, Mg-carbonates that have experienced post-depositional aqueous alteration are more likely to preserve paleoenvironmental indicators and potential biosignatures. Rapid weathering of carbonates in circum-neutral pH sulfate brines may provide a potential source of cations for abundant sulfate minerals observed on Mars, Ceres, and other planetary bodies.

^1,2,3Rebecca A. Fischer, ²Francis Nimmo, ⁴David P. O’Brien
Earth and Planetary Science Letters 482, 105-114 Link to Article [https://doi.org/10.1016/j.epsl.2017.10.055]
¹Smithsonian Institution, National Museum of Natural History, Department of Mineral Sciences, United States
²University of California Santa Cruz, Department of Earth and Planetary Sciences, United States
³Harvard University, Department of Earth and Planetary Sciences, United States
⁴Planetary Science Institute, United States
Copyright Elsevier

The Ru–Mo isotopic compositions of inner Solar System bodies may reflect the provenance of accreted material and how it evolved with time, both of which are controlled by the accretion scenario these bodies experienced. Here we use a total of 116 N-body simulations of terrestrial planet accretion, run in the Eccentric Jupiter and Saturn (EJS), Circular Jupiter and Saturn (CJS), and Grand Tack scenarios, to model the Ru–Mo anomalies of Earth, Mars, and Theia analogues. This model starts by applying an initial step function in Ru–Mo isotopic composition, with compositions reflecting those in meteorites, and traces compositional evolution as planets accrete. The mass-weighted provenance of the resulting planets reveals more radial mixing in Grand Tack simulations than in EJS/CJS simulations, and more efficient mixing among late-accreted material than during the main phase of accretion in EJS/CJS simulations. We find that an extensive homogeneous inner disk region is required to reproduce Earth’s observed Ru–Mo composition. EJS/CJS simulations require a homogeneous reservoir in the inner disk extending to ≥3–4 AU (≥74–98% of initial mass) to reproduce Earth’s composition, while Grand Tack simulations require a homogeneous reservoir extending to ≥3–10 AU (≥97–99% of initial mass), and likely to ≥6–10 AU. In the Grand Tack model, Jupiter’s initial location (the most likely location for a discontinuity in isotopic composition) is ∼3.5 AU; however, this step location has only a 33% likelihood of producing an Earth with the correct Ru–Mo isotopic signature for the most plausible model conditions. Our results give the testable predictions that Mars has zero Ru anomaly and small or zero Mo anomaly, and the Moon has zero Mo anomaly. These predictions are insensitive to wide variations in parameter choices.