Subrata Chakraborty^*, Petia Yanchulova, Mark H. Thiemens

Department of Chemistry and Biochemistry, University of California, San Diego, La Jolla, CA 92093–0356, USA.

We currently seek a copyright agreement with Science to display abstracts of their cosmochemistry related publications.

Reference
Chakraborty S, Yanchulova P and Thiemens MH (in press) Mass-Independent Oxygen Isotopic Partitioning During Gas-Phase SiO2 Formation. Science 342:463-466.
[doi:10.1126/science.1242237]

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Florian J. Zurfluh^1,*, Beda A. Hofmann², Edwin Gnos³, Urs Eggenberger¹

¹Institut für Geologie, Universität Bern, Bern, Switzerland
²Naturhistorisches Museum der Burgergemeinde Bern, Bern, Switzerland
³Muséum d’histoire naturelle de la Ville de Genève, Genève 6, Switzerland

The common appearance of hygroscopic brine (“sweating”) on ordinary chondrites (OCs) from Oman during storage under room conditions initiated a study on the role of water-soluble salts on the weathering of OCs. Analyses of leachates from OCs and soils, combined with petrography of alteration features and a 11-month record of in situ meteorite and soil temperatures, are used to evaluate the role of salts in OC weathering. Main soluble ions in soils are Ca²⁺, SO₄²⁻, HCO₃⁻, Na⁺, and Cl⁻, while OC leachates are dominated by Mg²⁺ (from meteoritic olivine), Ca²⁺ (from soil), Cl⁻ (from soil), SO₄²⁻ (from meteoritic troilite and soil), and iron (meteoritic). “Sweating meteorites” mainly contain Mg²⁺ and Cl⁻. The median Na/Cl mass ratio of leachates changes from 0.65 in soils to 0.07 in meteorites, indicating the precipitation of a Na-rich phase or loss of an efflorescent Na-salt. The total concentrations of water-soluble ions in bulk OCs ranges from 600 to 9000 μg g⁻¹ (median 2500 μg g⁻¹) as compared to 187–14140 μg g⁻¹ in soils (median 1148 μg g⁻¹). Soil salts dissolved by rain water are soaked up by meteorites by capillary forces. Daily heating (up to 66.3 °C) and cooling of the meteorites cause a pumping effect, resulting in a strong concentration of soluble ions in meteorites over time. The concentrations of water-soluble ions in meteorites, which are complex mixtures of ions from the soil and from oxidation and hydrolysis of meteoritic material, depend on the degree of weathering and are highest at W3. Input of soil contaminants generally dominates over the ions mobilized from meteorites. Silicate hydrolysis preferentially affects olivine and is enhanced by sulfide oxidation, producing local acidic conditions as evidenced by jarosite. Plagioclase weathering is negligible. After completion of troilite oxidation, the rate of chemical weathering slows down with continuing Ca-sulfate contamination.

Reference
Zurfluh FJ, Hofmann BA, Gnos E and Eggenberger U (in press) “Sweating meteorites”—Water-soluble salts and temperature variation in ordinary chondrites and soil from the hot desert of Oman. Meteoritics & Planetary Science
[doi:10.1111/maps.12211]
Published by arrangement with John Wiley & Sons

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M. Jadhav¹, M. Pignatari^2,6, F. Herwig^3,6,7, E. Zinner⁴, R. Gallino⁵ and G. R. Huss¹

¹Hawai’i Institute of Geophysics and Planetology, University of Hawai’i at Mānoa, Honolulu, HI 96822, USA
²Department of Physics, University of Basel, CH-4056 Basel, Switzerland
³Department of Physics & Astronomy, University of Victoria, Victoria, BC V8P5C2, Canada
⁴Laboratory for Space Sciences & the Physics Department, Washington University, St. Louis, MO 63130, USA
⁵Dipartimento di Fisica Generale, Università di Torino & INAF, Via Pietro Giuria 1, 10125 Torino, Italy
⁶NuGrid Collaboration: http://www.nugridstars.org.
⁷Also at The Joint Institute for Nuclear Astrophysics, Notre Dame, IN 46556, USA.

Graphite is one of the many presolar circumstellar condensate species found in primitive meteorites. While the isotopic compositions of low-density graphite grains indicate an origin in core-collapse supernovae, some high-density grains have extreme isotopic anomalies in C, Ca, and Ti, which cannot be explained by envelope predictions of asymptotic giant branch (AGB) stars or theoretical supernova models. The Ca and Ti isotopic anomalies, however, match the predictions of He-shell abundances in AGB stars. In this study, we show that the C, Ca, and Ti isotopic anomalies are consistent with nucleosynthesis predictions of the H-ingestion phase during a very late thermal pulse (VLTP) event in post-AGB stars. The low ¹²C/¹³C isotopic ratios in these grains are a result of abundant ¹²C efficiently capturing the protons that are being ingested during the VLTP. Very high neutron densities of ~10¹⁵ cm⁻³, typical of the i-process, are achieved during this phase in post-AGB stars. The large ^42,43,44Ca excesses in some graphite grains are indicative of neutron capture nucleosynthesis during VLTP. The comparison of VLTP nucleosynthesis calculations to the graphite data also indicate that apparent anomalies in the Ti isotopic ratios are due to large contributions from ^46,48Ca, which cannot be resolved from the isobars ^46,48Ti during the measurements. We conclude that presolar graphite grains with moderate to extreme Ca and Ti isotopic anomalies originate in post-AGB stars that suffer a VLTP.

Reference
Jadhav M, Pignatari M, Herwig F, Zinner E, Gallino R, and Huss GR (in press) Relics of Ancient Post-AGB Stars in a Primitive Meteorite. The Astrophysical Journal – Letters
[doi:10.1088/2041-8205/777/2/L27]

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Emma C. Marcucci^1,2, Brian M. Hynek^1,2,*, Kathryn S. Kierein-Young², K. L. Rogers^3,4

¹Department of Geological Sciences, University of Colorado Boulder, Boulder, Colorado, USA
²Laboratory for Atmospheric and Space Physics, University of Colorado Boulder, Boulder, Colorado, USA
³Geophysical Laboratory, Carnegie Institution of Washington, Washington, DC, USA
⁴Now at Department of Earth and Environmental Sciences, Rensselaer Polytechnic Institute, Troy, New York, USA

Acid-sulfate weathering at Nicaraguan hydrothermal sites Cerro Negro, Momotombo, and Telica volcanoes and Hervidores de San Jacinto mudpots was characterized as an analog for similar processes that likely operated on early Mars. In situ mineralogical analyses were conducted with a field portable visible near-infrared spectrometer for comparison to similar Martian data sets. Three classes of alteration minerals were identified: sulfates (gypsum and natroalunite), oxides/hydroxides (hematite and goethite), and phyllosilicates (kaolinite/halloysite, montmorillonite, and saponite), as well as elemental sulfur and hydrated silica phases. Our sites had similar suites of minerals, but frequencies varied with location. The results of this field campaign allow inferences regarding the paleo-environmental conditions that were likely present at similar relic hydrothermal sites identified on Mars. In particular, sulfates and phyllosilicates could have coevolved under hydrothermal conditions at Noctis Labyrinthus as is seen in Nicaragua. Fe/Mg smectites were detected in areas with pH of 3–4. Alunite spectra at Terra Sirenum demonstrated mineral mixing effects on spectroscopy. Mineral mixing can cause uncertainties in spectral identification due to a dominant spectrum, such as iron minerals, masking another or the suppression of weaker bands. When viewed from orbit, our field sites would likely be dominated by hydrated silica and Mars sites, such as one in Syrtis Major, could have a more diverse mineralogy than the data reveal. Concentrated amorphous silica, such as at Gusev crater, can result from acidic fumarolic activity, while Mg sulfates may indicate a lack of reworking by water. This field spectroscopy study helps confirm and provide insight into hydrothermal processes on ancient Mars.

Reference
Marcucci EC, Hynek BM, Kierein-Young KS and Rogers KL (in press) Visible-near-infrared reflectance spectroscopy of volcanic acid-sulfate alteration in Nicaragua: Analogs for early Mars. Journal of Geophysical Research – Planets
[doi:10.1002/jgre.20159]
Published by arrangement with John Wiley & Sons

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