Signatures of the Martian regolith components entrained in some impact‐melt glasses in shergottites

1M. N. Rao, 2L. E. Nyquist, 3,4D. K. Ross, 5,6S. R. Sutton, 7P. Hoppe, 8C. Y. Shih, 9S. J. Wentworth, 10D. H. Garrison
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13177]
1SCI, Johnson Space CenterHouston, Texas, USA
2XI/NASA Johnson Space CenterHouston, Texas, USA
3Jacob JETS, NASA Johnson Space CenterHouston, Texas, USA
4UTEP–CASSMAREl Paso, Texas, USA
5Department of Geophysical Sciences, University of ChicagoChicago, Illinois, USA
6CARS, Argonne National LaboratoryArgonne, Illinois, USA
7Max‐Planck Institute für Chemie, Mainz, Germany
8Jacobs, Johnson Space CenterHouston, Texas, USA
9HEPCO, Jacobs Engineering, Johnson Space CenterHouston, Texas, USA
10Barios Technology, NASA, Johnson Space CenterHouston, Texas, USA
Published by arrangement with John Wiley & Sons

Martian regolith components are found in some impact melts (IM) containing Martian atmospheric gases in the shergottites Elephant Moraine (EET) 79001, Tissint, Zagami, and Shergotty. Excess sulfur abundances provide strong indicators for the presence of an exogenous component. High sulfur abundances and the SO3‐SiO2 correlation in polished thin section (PTS) EET 79001,507 (here #507) are comparable to those in Martian soils. Correlations of SO3 with FeO in #507 from Lithology B and of CaO and Al2O3 in EET 79001,506 (here #506) from Lithology A suggest the possible occurrence of two varieties of sulfate‐bearing phases in impact‐melt precursors. Fe/S (atomic) ratios of 1.02–1.34 determined in several sulfide blebs in #507 differ from those determined in igneous sulfides (Fe/S = 0.92), and suggest that most sulfide blebs in #507 are not related to igneous sulfides. Fe/S (atomic) ratios in a Tissint glass range from ~0.5 (pyrite) to >1.1 suggesting a mixture of sulfur‐bearing phases. S K‐XANES spectra of the blebs in EET 79001 and Tissint glasses show that sulfur occurs as mixed amorphous sulfide and sulfite. The δ34S values and the 87Sr/86Sr (I) ratios determined in EET 79001 impact melts are consistent with the proposition that the sulfide blebs result from decomposition of secondary sulfates into sulfites during shock heating followed by reduction to sulfides by isentropic cooling. These results suggest the presence in some shergottites of extraneous regolith components containing oxidized S‐bearing species resembling sulfur species present in Martian soils.

Impact cratering: The South American record – Part 1

1A.P.Crósta, 2,3W.U.Reimold,4M.A.R.Vasconcelos, 2N.Hauser, 1G.J.G.Oliveira,1M.V.Maziviero,5A.M.Góes
Chemie der Erde (in Press) Link to Article [https://doi.org/10.1016/j.chemer.2018.06.001]
1State University of Campinas, Brazil
2University of Brasília, Brazil
3Natural History Museum – Leibniz Institute for Evolution and Biodiversity Science, Berlin, Germany
4Federal University of Bahia, Brazil
5University of São Paulo, Brazil
Copyright Elsevier

The Earth’s impact record is known to be rather limited in both time and space. There are ca. 190 impact structures currently known on Earth, representing a minor fraction of all the impact events that contributed to the initial formation of our protoplanet, and then to formation and modification of the surface of the planet. Moreover, the distribution of impact structures on Earth is manifestly uneven. One continent that stands out for its relatively small number of confirmed impact structures and impact ejecta occurrences is South America. The limited impact record for this large continent makes a robust case that there is a significant potential for further discoveries. Significant information on the impact record of South America is dispersed in different types of publications (journal articles, books, conferences abstracts, etc.), and in several languages, making it difficult to access and disseminate it among the geoscientific community. We aim to present a summary of the current knowledge of the impact record of this continent, encompassing the existing literature on the subject. It is published in two parts, with the first one covering an up-to-date introduction to impact cratering processes and to the criteria to identify/confirm an impact structure and related deposits. This is followed by a comprehensive analysis of the Brazilian impact structures. The Brazilian impact record accounts for the totality of the large structures of this kind currently confirmed in South America. The second part will examine the impact record of other countries in South America, provide information about a number of proposed impact structures, and review those that already have been discarded as not being formed by impact.

X‐ray computed tomography of extraterrestrial rocks eradicates their natural radiation record and the information it contains

1Derek W. G. Sears, 1Alexander Sehlke, 2,3Jon M. Friedrich, 4Mark L. Rivers, 21Denton S. Ebel
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13183]
1NASA Ames Research Center/BAER Institute, Mountain View, California, USA
2American Museum of Natural History, New York, New York, USA
3Department of Chemistry, Fordham University, Bronx, New York, USA
4Center for Advanced Radiation Sources, University of Chicago, Argonne, Illinois, USA
Published by arrangement with John Wiley & Sons

The radiation record of extraterrestrial rocks provides important insights into their thermal and radiation history. For meteorites this relates to their orbits, thermal history, terrestrial age, preatmospheric size and shape, and possibly cosmic ray exposure age. For meteorites from the Moon and Mars, the radiation record allows insights into transit times. For Martian surface samples, the radiation record enables estimates of their sedimentary age. Despite this, there is a growing tendency to artificially expose these samples to large radiation doses by the use of X‐ray computed tomography (CT) imaging, often as part of their initial examination. In order to understand the effect of synchrotron microCT on meteorites, we placed samples of the Bruderheim L6 chondrite in the CT imaging port of the Advanced Photon Source at the Argonne National Laboratory, Argonne, Illinois. Monoenergetic X‐ray beams of 25 and 46 keV and a high flux broad spectrum beam were used. The synchrotron CT procedure exposed the samples to radiation doses significantly higher than the natural doses observed for meteorites (1670 to ~10,000 Gyr, compared to ~1000 Gyr for natural samples). It is clear that CT imaging, whether using a laboratory system as in our previous report or using the synchrotron source, makes measurement of the natural radiation record of the samples impossible. Samples should not be placed in a CT scanner without due consideration of the loss of unique information for these valuable extraterrestrial samples.

Insights into Ceres’s evolution from surface composition

1Julie Castillo‐Rogez, 2,3Marc Neveu, 4Harry Y. McSween, 5Roger R. Fu, 6Michael J. Toplis, 7Thomas Prettyman
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13181]
1Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USA
2School of Earth and Space Exploration, Arizona State University, Tempe, Arizona, USA
3NASA Postdoctoral Management Program Fellow, NASA Headquarters, Washington, District of Columbia, USA
4Department of Earth and Planetary Sciences, The University of Tennessee in Knoxville, Knoxville, Tennessee, USA
5Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts, USA
6IRAP, Université de Toulouse, CNRS, UPS, Toulouse, France
7Planetary Science Institute, Tucson, Arizona, USA
Published by arrangement with John Wiley & Sons

Inspired by the recent results of the Dawn mission, thermodynamic models of rock alteration and brine evaporation have been used to help understand the conditions under which water–rock interaction took place within the dwarf planet Ceres. This analysis constrains Ceres’s early history and offers a framework within which future observations may be interpreted. A broad range of alteration conditions have been simulated using the Geochemist’s Workbench and PHREEQC software, associated with the FREZCHEM model that constrains the consequences of freezing the liquid phase in equilibrium with the observed mineralogical assemblage. Comparison of the modeling results with observed surface mineralogy at Ceres indicates advanced alteration under a relatively high fugacity of hydrogen, a conclusion that is consistent with predictions for, and observations of, large ice‐rich bodies. The simulations suggest production of methane that could help regulate the redox environment and possibly form clathrate hydrates upon freezing of the early ocean. The detection of localized occurrences of natrite (sodium carbonate) at the surface of Ceres provides key constraints on the composition of fluids that are necessarily alkaline. In addition, the combined hydrothermal and freezing simulations suggest that hydrohalite may be abundant in Ceres’s subsurface, similar to Earth’s polar regions. The global homogeneity of Ceres’s surface, made of material formed at depth, suggests a large‐scale formation mechanism, while local heterogeneities associated with impact craters and landslides suggest that some form of sodium carbonate and other salts are accessible in the shallow subsurface.

Variable distribution of s-process Hf and W isotope carriers in chondritic meteorites – evidence from 174Hf and 180W

1,2Bo-Magnus Elfers, 1,2Peter Sprung, 1,2,3Markus Pfeifer, 1,2Frank Wombacher, 4Stefan T.M.Peters, 1,2CarstenMünker
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2018.08.009]
1Institut für Geologie und Mineralogie, Universität zu Köln, Zülpicher Straße 49b, 50674 Köln, Germany
2Steinmann-Institut, Poppelsdorfer Schloss, Meckenheimer Allee 169, 53115 Bonn, Germany
3School of Earth Sciences, University of Bristol, Wills Memorial Building, Queens Road, Bristol, BS8 1RJ, United Kingdom
4Geowissenschaftliches Zentrum der Georg-August-Universität Göttingen, Goldschmidtstraße 1, 37077 Göttingen, Germany
Copyright Elsevier

The stepwise acid digestion of primitive chondritic meteorites allows the identification of nucleosynthetic isotope anomalies that are otherwise hidden on the bulk rock scale. Here, we present combined Hf and W isotope data for acid leachates, residues, and bulk rock aliquots of several primitive chondrites that include highly precise analyses of the heavy p-process isotopes 174Hf and 180W. Including data for these two p-process isotopes enables, for the first time, the clear-cut discrimination between s- and r-process contributions to the Hf and W isotope inventory. Our analyses reveal Hf and W isotopic homogeneity at the bulk rock scale, but significant Hf and W isotope anomalies that are complementary between acid leachates and residues. Since both r- to p-process isotope ratios are invariant in leachates and residues, the observed anomalies can unambiguously be tied to variable contributions of carrier phases enriched in s-process nuclides, as previously inferred for, i.e., Mo and Ru in leaching experiments. Hafnium and W isotope anomalies co-vary in leachate and residue fractions from CM chondrites, whereas CO and CV chondrites are characterized by distinctly larger Hf isotope anomalies compared to W. This observation is most likely explained by more efficient homogenization of s-process W carrier(s) or, alternatively, by local redistribution of anomalous W into secondary less resistant phases during parent body and/or nebular processing. This implies the presence of different s-nuclide carrier phases for Hf and W. Several carriers of s-process-material appear to have been selectively dissolved by our leaching protocol, while contributions from r- and p-process Hf and W carrier phases appear invariant, possibly due to the generally more labile nature of their carrier phases during solar nebula and/or parent body processing.

Insights into the origin of carbonaceous chondrite organics from their triple oxygen isotope composition

1Romain Tartèse, 2Marc Chaussidon, 3Andrey Gurenko, 4Frédéric Delarue, 5François Robert
Proceedings of the National Academy of Sciences of the United States of America (in Press) Link to Article [DOI:https://doi.org/10.1073/pnas.1808101115]
1School of Earth and Environmental Sciences, The University of Manchester, Manchester M13 9PL, United Kingdom;
2Institut de Physique du Globe de Paris, Université Sorbonne-Paris-Cité, Université Paris Diderot, CNRS UMR 7154, F-75238 Paris, France;
3Centre de Recherches Pétrographiques et Géochimiques, UMR 7358, Université de Lorraine, F-54501 Vandoeuvre-lès-Nancy, France;
4Sorbonne Université, Université Pierre-et-Marie-Curie, CNRS, École Pratique des Hautes Etudes, Paris Sciences et Lettres, UMR 7619 Milieux Environnementaux, Transferts et Interactions dans les Hydrosystèmes et les Sols, F-75005 Paris, France;
5Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie, Muséum National d’Histoire Naturelle, Sorbonne Universités, CNRS, Université Pierre-et-Marie-Curie, and Institut de Recherche pour le Développement, F-75005 Paris, France

Dust grains of organic matter were the main reservoir of C and N in the forming Solar System and are thus considered to be an essential ingredient for the emergence of life. However, the physical environment and the chemical mechanisms at the origin of these organic grains are still highly debated. In this study, we report high-precision triple oxygen isotope composition for insoluble organic matter isolated from three emblematic carbonaceous chondrites, Orgueil, Murchison, and Cold Bokkeveld. These results suggest that the O isotope composition of carbonaceous chondrite insoluble organic matter falls on a slope 1 correlation line in the triple oxygen isotope diagram. The lack of detectable mass-dependent O isotopic fractionation, indicated by the slope 1 line, suggests that the bulk of carbonaceous chondrite organics did not form on asteroidal parent bodies during low-temperature hydrothermal events. On the other hand, these O isotope data, together with the H and N isotope characteristics of insoluble organic matter, may indicate that parent bodies of different carbonaceous chondrite types largely accreted organics formed locally in the protosolar nebula, possibly by photochemical dissociation of C-rich precursors.

Volatile element evolution of chondrules through time

1Brandon Mahan, 1,2Frédéric Moynier, 1,2Julien Siebert, 3,4Bleuenn Gueguen, 3Arnaud Agranier, 1,5Emily A. Pringle, 6Jean Bollard, 6James N. Connelly, 1,6Martin Bizzarro
Proceedings of the National Academy of Sciences of the United States of America (in Press) Link to Article [https://doi.org/10.1073/pnas.1807263115]
1Institut de Physique du Globe de Paris, Université Paris Diderot, Sorbonne Paris Cité, CNRS UMR 7154, 75238 Paris Cedex 05, France;
2Institut Universitaire de France, 75005 Paris, France;
3Laboratoire Géosciences Océan, UMR CNRS 6538, Université de Bretagne Occidentale et Institut Universitaire Européen de la Mer, 29280 Plouzané, France;
4UMS CNRS 3113, Institut Universitaire Européen de la Mer, 29280 Plouzané, France;
5Scripps Institution of Oceanography, University of California, San Diego, La Jolla CA 92093;
6Center for Star and Planet Formation, University of Copenhagen, DK-1350 Copenhagen, Denmark

Chondrites and their main components, chondrules, are our guides into the evolution of the Solar System. Investigating the history of chondrules, including their volatile element history and the prevailing conditions of their formation, has implications not only for the understanding of chondrule formation and evolution but for that of larger bodies such as the terrestrial planets. Here we have determined the bulk chemical composition—rare earth, refractory, main group, and volatile element contents—of a suite of chondrules previously dated using the Pb−Pb system. The volatile element contents of chondrules increase with time from ∼1 My after Solar System formation, likely the result of mixing with a volatile-enriched component during chondrule recycling. Variations in the Mn/Na ratios signify changes in redox conditions over time, suggestive of decoupled oxygen and volatile element fugacities, and indicating a decrease in oxygen fugacity and a relative increase in the fugacities of in-fluxing volatiles with time. Within the context of terrestrial planet formation via pebble accretion, these observations corroborate the early formation of Mars under relatively oxidizing conditions and the protracted growth of Earth under more reducing conditions, and further suggest that water and volatile elements in the inner Solar System may not have arrived pairwise.