¹Maïa Kuga, ²Guy Cernogora, ¹Yves Marrocchi, ¹Laurent Tissandier,¹ Bernard Marty
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2017.08.031]
¹CRPG-CNRS, Université de Lorraine, 15 rue Notre Dame des Pauvres, 54500 Vandoeuvre-les-Nancy, France
²LATMOS, Université Versailles St. Quentin, UPMC Univ. Paris 06, CNRS, 11 Bvd. d’Alembert, 78280 Guyancourt, France
Copyright Elsevier

The main carrier of primordial heavy noble gases in chondrites is thought to be an organic phase, known as phase Q, whose precise characterization has resisted decades of investigation. The Q noble gas component shows elemental and isotopic fractionation relative to the Solar, in favor of heavy elements and isotopes. These noble gas characteristics were experimentally simulated using a plasma device called the “Nebulotron”. In this study, we synthesized thirteen solid organic samples by electron-dissociation of CO, in which a noble gas mixture was added. The analysis of their heavy noble gas (Ar, Kr and Xe) contents and isotopic compositions reveals enrichment in the heavy noble gas isotopes and elements relative to the light ones. The isotope fractionation is mass-dependent and is consistent with a mn- type law, where n≥1. Based on a plasma model, we propose that the ambipolar diffusion of ions in the ionized CO gas medium is at the origin of the noble gas isotopic fractionation. In addition, the elemental fractionation of experimental and chondritic samples can be accounted for by the Saha law of plasma equilibrium, which does not depend on the respective noble gas masses but rather on their ionization potentials. Our results suggest that the Q noble gases were trapped into growing organic particles starting from solar gases that were fractionated in an ionized medium by ambipolar diffusion and Saha processes. This would imply that both the formation of chondritic organic matter and the trapping of noble gases took place simultaneously in the ionized areas of the protoplanetary disk.

^1,2Alberto G. Fairén,¹Carolina Gil-Lozano,³Esther R. Uceda,⁴Elisabeth Losa-Adams,⁵Alfonso F. Davila,⁴Luis Gago-Duport
Journal of Geophysical Research, Planets (in Press) Link to Article [DOI: 10.1002/2016JE005229]
¹Centro de Astrobiología (CSIC-INTA), Madrid, Spain
²Department of Astronomy, Cornell University, Ithaca, NY, USA
³Facultad de Ciencias, Universidad Autónoma de Madrid, Cantoblanco Madrid, Spain
⁴Departamento de Geociencias Marinas, Universidad de Vigo, Lagoas Marcosende, Vigo, Spain
⁵NASA Ames Research Center, Moffett Field, CA, USA
Published by arrangement with John Wiley & Sons

Geochemical models of secondary mineral precipitation on Mars generally assume semi-open systems (open to the atmosphere but closed at the water-sediment interface) and equilibrium conditions. However, in natural multicomponent systems, the reactive surface area of primary minerals controls the dissolution rate and affects the precipitation sequences of secondary phases; and simultaneously the transport of dissolved species may occur through the atmosphere-water and water-sediment interfaces. Here we present a suite of geochemical models designed to analyze the formation of secondary minerals in basaltic sediments on Mars, evaluating the role of (i) reactive surface areas and (ii) the transport of ions through a basalt sediment column. We consider fully open conditions, both to the atmosphere and to the sediment, and a kinetic approach for mineral dissolution and precipitation. Our models consider a geochemical scenario constituted by a basin (i.e., a shallow lake) where supersaturation is generated by evaporation/cooling, and the starting point is a solution in equilibrium with basaltic sediments. Our results show that cation removal by diffusion, along with the input of atmospheric volatiles and the influence of the reactive surface area of primary minerals, play a central role in the evolution of the secondary mineral sequences formed. We conclude that precipitation of evaporites finds more restrictions in basaltic sediments of small grain size than in basaltic sediments of greater grain size.

¹Lu Pan, ^1,2Bethany L. Ehlmann, ³John Carter, ⁴Carolyn M. Ernst
Journal of Geophysical Research Planets (in Press) Link to Article [DOI: 10.1002/2017JE005276]
¹Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California, USA
²Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USA
³Institut d’Astrophysique Spatiale, Orsay, France
⁴The John Hopkins University Applied Physics Laboratory, Laurel, Maryland, USA
Published by arrangement with John Wiley & Sons

The basin-filling materials of the northern lowlands, which cover ~1/3 of Mars’ surface, record the long-term evolution of Mars’ geology and climate. The buried stratigraphy was inferred through analyses of impact crater mineralogy, detected using data acquired by the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM). Examining 1045 impact craters across the northern lowlands, we find widespread olivine and pyroxene and diverse hydrated/hydroxylated minerals, including Fe/Mg smectite, chlorite, prehnite, and hydrated silica. The distribution of mafic minerals is consistent with infilling volcanic materials across the entire lowlands (~1–4⋅107 km3), indicating a significant volume of volatile release by volcanic outgassing. Hydrated/hydroxylated minerals are detected more frequently in large craters, consistent with the scenario that the hydrated minerals are being excavated from deep basement rocks, beneath 1-2 km thick mafic lava flows or volcaniclastic materials. The prevalences of different types of hydrated minerals are similar to statistics from the southern highlands. No evidence of concentrated salt deposits has been found, which would indicate a long-lived global ocean. We also find significant geographical variations of local mineralogy and stratigraphy in different basins (geological provinces), independent of dust cover. For example, many hydrated and mafic minerals are newly discovered within the polar Scandia region (> 60°N), and Chryse Planitia has more mafic mineral detections than other basins, possibly due to a previously unrecognized volcanic source.

^1,2P.M. Ranjith, ^1,2Huaiyu He, ³Bingkui Miao, ¹Fei Su, ^3,4Chuantong Zhang, ³Zhipeng Xia, ³Lanfang Xie, ¹Rixiang Zhu
Planetary and Space Science (in Press) Link to Article [https://doi.org/10.1016/j.pss.2017.08.009]
¹Key Laboratory of Earth and Planetary Physics, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, PR China
²University of Chinese Academy of Sciences, Beijing 100049, PR China
³Institute of Meteorites and Planetary Research, Guilin University of Technology, Guilin 541004, PR China
⁴Key Laboratory of Planetray Geological Evolution, Guilin University of Technology, Guilin 541004, PR China

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¹Richard D. Starr et al. (>10)
Planetary and Space Science (in Press) Link to Article [https://doi.org/10.1016/j.pss.2017.08.001]
¹Physics Department, The Catholic University of America, Washington, DC 20064, USA

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¹M.E. Varela, ²S-L. Hwang, ³P. Shen, ⁴H-T. Chu, ⁵T-F. Yui, ⁵Y. Iizuka, ⁶F. Brandstätter, ⁷Y.A. Abdu
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2017.08.027]
¹Instituto de Ciencias Astronómicas de la Tierra y del Espacio (ICATE), Avenida España 1512 sur, J5402DSP, San Juan, Argentina
²Department of Materials Science and Engineering, National Dong Hwa University, Hualien, Taiwan, ROC
³Department of Materials and Optoelectronic Science, National Sun Yat-sen University, Kaohsiung, Taiwan, ROC
⁴Central Geological Survey, PO Box 968, Taipei, Taiwan, ROC
⁵Institute of Earth Sciences, Academia Sinica, Taipei, Taiwan, ROC
⁶Mineralogisch-Petrographische Abteilung, Naturhistorisches Museum, Burgring 7, 1010 Wien, Austria
⁷Department of Applied Physics and Astronomy, University of Sharjah, P.O.Box 27272, Sharjah, United Arab Emirate
Copyright Elsevier

Olivinites, together with olivine megacrysts, are the most magnesian phases found in angrites. Their chemical composition (mg# 90) is out of equilibrium with the groundmass and far away from that of possible precipitates from angrite parent melts. Therefore olivinites, as well as olivine megacrysts, were considered as xenoliths and xenocrysts. We report here a detailed study of five olivinites from the angrite D’Orbigny. Our results indicate that D’Orbigny experienced metasomatic alteration processes, which led to enrichments in FeO and MnO (relative to the original composition), changing the initial Mg-rich composition of the olivines to the one seen now. As this process took place in equilibrium with a chondritic reservoir (e.g., Fe/Mn ratios spreading around primitive values), the primitive (Mg-rich) olivine chemical composition was changed towards a more fayalitic one while preserving a chondritic signature. This chondritic signature was preserved in the Fe/Mn ratio of the olivinites, olivine megacrysts, augite grains in olivinites and groundmass olivine of D’ Orbigny. Therefore the fayalite content of about 35 mol.% that characterizes the groundmass olivine of this rock – as well as other angrites- does not correspond to its original composition but may be the result of a late metasomatic process that affected these rocks. If so, olivinites and Mg-rich olivines might not be compositionally exotic phases but are an early constituent phase that retained the pristine more reducing conditions that have been preserved in some angrites, where they form either a small part of the rock (e.g., Asuka 881371 and D’Orbigny) or the majority of it (NWA 8535).

^1,2Stepan M. Chernonozhkin, ³Mona Weyrauch, ^1,2Steven Goderis, ³Martin Oeser, ²Seann J. McKibbin, ³Ingo Horn, ⁴Lutz Hecht, ³Stefan Weyer, ²Philippe Claeys, ¹Frank Vanhaecke
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2017.08.022]
¹Ghent University, Department of Analytical Chemistry, Campus Sterre, Krijgslaan, 281 – S12, 9000 Ghent, Belgium
²Vrije Universiteit Brussel, Analytical, Environmental, and Geo- Chemistry, Pleinlaan 2, 1050 Brussels, Belgium
³Leibniz Universität Hannover, Institute of Mineralogy, Callinstrasse 3, 30167 Hannover, Germany
⁴Museum für Naturkunde, Leibniz Institute for Evolution and Biodiversity Science, Invalidenstrasse 43, 10115 Berlin, Germany
Copyright Elsevier

In this work, a femtosecond laser ablation (LA) system coupled to a multi-collector inductively coupled plasma-mass spectrometer (fs-LA-MC-ICP-MS) was used to obtain laterally resolved (30-80 μm), high-precision combined Ni and Fe stable isotope ratio data for a variety of mineral phases (olivine, kamacite, taenite, schreibersite and troilite) composing main group pallasites (PMG) and iron meteorites. The stable isotopic signatures of Fe and Ni at the mineral scale, in combination with the factors governing the kinetic or equilibrium isotope fractionation processes, are used to interpret the thermal histories of small differentiated asteroidal bodies. As Fe isotopic zoning is only barely resolvable within the internal precision level of the isotope ratio measurements within a single olivine in Esquel PMG, the isotopically lighter olivine core relative to the rim (Δ56/54Ferim-core = 0.059 ‰) suggests that the olivines were largely thermally equilibrated. The observed hint of an isotopic and concentration gradient for Fe of crudely similar width is interpreted here to reflect Fe loss from olivine in the process of partial reduction of the olivine rim. The ranges of the determined Fe and Ni isotopic signatures of troilite (δ56/54Fe of -0.66 to -0.09 ‰) and schreibersite (δ56/54Fe of -0.48 to -0.09 ‰, and δ62/60Ni of -0.64 to +0.29 ‰) may result from thermal equilibration. Schreibersite and troilite likely remained in equilibrium with their enclosing metal to temperatures significantly below their point of crystallization. The Ni isotopic signatures of bulk metal and schreibersite correlate negatively, with isotopically lighter Ni in the metal of PMGs and isotopically heavier Ni in the metal of the iron meteorites analyzed. As such, the light Ni isotopic signatures previously observed in PMG metal relative to chondrites may not result from heterogeneity in the Solar Nebula, but rather reflect fractionation in the metal-schreibersite system. Comparison between the isotope ratio profiles of Fe and Ni determined across kamacite-taenite interfaces (Δ56/54Fekam-tae = -0.51 to -0.69 ‰ and Δ62/60Nikam-tae = +1.59 to +2.50 ‰) and theoretical taenite sub-solidus diffusive isotopic zoning broadly constrain the cooling rates of Esquel, CMS 04071 PMGs and Udei Station IAB to between ∼25 and 500 °C/Myr.

¹A.A. Nemchin, ²H. Jeon, ³J.J. Bellucci, ¹N.E. Timms, ³J.F. Snape, ²M.R. Kilburn, ³M.J. Whitehouse
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2017.08.024]
¹Department of Applied Geology, Curtin University, Perth, WA 6845, Australia
²Centre for Microscopy Characterisation and Analysis, University of Western Australia, 35 Stirling Highway, Perth, WA 6009, Australia
³Department of Geosciences, Swedish Museum of Natural History, SE-104 05 Stockholm, Sweden
Copyright Elsevier

Pb-Pb isochron ages of ca. 3.92 Ga for three K-feldspar-rich clasts from Apollo 14 breccias 14303 and 14083 were determined using Secondary Ion Mass Spectrometry (SIMS). These ages are interpreted to represent the resetting of the U-Pb system in the clasts as a result of brecciation during the Imbrium impact. One of the clasts contains zircon grains that record a significantly older crystallization age (ca. 4.33-4.35 Ga) for the rock represented by that clast. Initial Pb compositions determined for the clasts, combined with the previously measured Pb isotopic compositions of K-feldspar grains from several Apollo 14 breccia samples, constrain a range of initial Pb compositions in the ca. 3.9 Ga Fra Mauro formation at the Apollo 14 landing site. This range in initial Pb compositions indicates that the rocks represented by these clasts, or the sources of those rocks, evolved with a high 238U/204Pb (μ-value) for substantial periods of time, although the precise crystallization ages of the rocks represented by at least two of the clasts investigated here are unknown.

¹J. Moreau,^1,2T. Kohout,³K. Wünnemann
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12935]
¹Department of Physics, University of Helsinki, Helsinki, Finland
²Institute of Geology, The Czech Academy of Sciences, Prague, Czech Republic
³Museum für Naturkunde, Leibniz Institute for Evolution and Biodiversity Science, Berlin, Germany
Published by arrangement with John Wiley & Sons

We determined the shock-darkening pressure range in ordinary chondrites using the iSALE shock physics code. We simulated planar shock waves on a mesoscale in a sample layer at different nominal pressures. Iron and troilite grains were resolved in a porous olivine matrix in the sample layer. We used equations of state (Tillotson EoS and ANEOS) and basic strength and thermal properties to describe the material phases. We used Lagrangian tracers to record the peak shock pressures in each material unit. The post-shock temperatures (and the fractions of the tracers experiencing temperatures above the melting point) for each material were estimated after the passage of the shock wave and after the reflections of the shock at grain boundaries in the heterogeneous materials. The results showed that shock-darkening, associated with troilite melt and the onset of olivine melt, happened between 40 and 50 GPa with 52 GPa being the pressure at which all tracers in the troilite material reach the melting point. We demonstrate the difficulties of shock heating in iron and also the importance of porosity. Material impedances, grain shapes, and the porosity models available in the iSALE code are discussed. We also discuss possible not-shock-related triggers for iron melt.

¹Klaus Keil, ²Timothy J.McCoy
Chemie der Erde (in Press) Link to Article [https://doi.org/10.1016/j.chemer.2017.04.004]
¹Hawai’i Institute of Geophysics and Planetology, School of Ocean and Earth Science and Technology, University of Hawai’i at Manoa, Honolulu, HI 96822, USA
²Department of Mineral Sciences, National Museum of Natural History, Smithsonian Institution, Washington, DC 20560-0119, USA
Copyright Elsevier

Acapulcoites (most ancient Hf-W ages are 4,563.1 ± 0.8 Ma), lodranites (most ancient Hf-W ages are 4,562.6 ± 0.9 Ma) and rocks transitional between them are ancient residues of different degrees of partial melting of a chondritic source lithology (e.g., as indicated by the occurrence of relict chondrules in 9 acapulcoites), although the precise chondrite type is unknown. Acapulcoites are relatively fine- grained (∼150–230 μm) rocks with equigranular, achondritic textures and consist of olivine, orthopyroxene, Ca-rich clinopyroxene, plagioclase, metallic Fe,Ni, troilite, chromite and phosphates. Lodranites are coarser grained (540–700 μm), with similar equigranular, recrystallized textures, mineral compositions and contents, although some are significantly depleted in eutectic Fe,Ni-FeS and plagioclase- clinopyroxene partial melts. The acapulcoite-lodranite clan is most readily distinguished from other groups of primitive achondrites (e.g., winoanites/IAB irons) by oxygen isotopic compositions, although more than 50% of meteorites classified as acapulcoites currently lack supporting oxygen isotopic data. The heat source for melting of acapulcoites-lodranites was internal to the parent body, most likely 26Al, although some authors suggest it was shock melting. Acapulcoites experienced lower temperatures of ∼980–1170 °C and lower degrees of partial melting (∼1–4 vol.%) and lodranites higher temperatures of ∼1150–1200 °C and higher degrees (∼5 ≥ 10 vol.%) of partial melting. Hand-specimen and thin section observations indicate movement of Fe,Ni-FeS, basaltic, and phosphate melts in veins over micrometer to centimeter distances. Mineralogical, chemical and isotopic properties, Cosmic Ray Exposure (CRE) ages which cluster around 4–6 Ma and the occurrence of some meteorites consisting of both acapulcoite and lodranite material, indicate that these meteorites come from one parent body and were most likely ejected in one impact event. Whereas the precise parent asteroid of these meteorites is unknown, there is general agreement that it was an S-type object. There is nearly total agreement that the acapulcoite-lodranite parent body was <∼100 km in radius and, based on the precise Pb–Pb age for Acapulco of 4555.9 ± 0.6 Ma, combined with the Hf/W and U/Pb records and cooling rates deduced from mineralogical and other investigations, that the parent body was fragmented during its cooling which the U/Pb system dates at precisely 4556 ± 1 Ma. Hf-W chronometry suggests that the parent body of the acapulcoites-lodranites and, in fact, the parent bodies of all “primitive achondrites” accreted slightly later than those of the differentiated achondrites and, thus, had lower contents of 26Al, the heat producing radionuclide largely responsible for heating of both primitive and differentiated achondrites. Thus, the acapulcoite-lodranite parent body never experienced the high degrees of melting responsible for the formation of the differentiated meteorites, but arrested its melting history at relatively low degrees of ∼15 vol.%.