¹Christian Vollmer, ²Demie Kepaptsoglou, ³Jan Leitner, ⁴Henner Busemann, ⁴Nicole H. Spring, ²Quentin M. Ramasse, ³Peter Hoppe, ⁵Larry R. Nittler
¹Institut für Mineralogie, Universität Münster, D-48149 Münster, Germany;
²SuperSTEM Laboratory, Science & Technology Facilities Council Daresbury Laboratories, Daresbury WA4 4AD, United Kingdom;
³Abteilung Partikelchemie, Max-Planck-Institut für Chemie, D-55128 Mainz, Germany;
⁴School of Earth, Atmospheric and Environmental Sciences, University of Manchester, Manchester M13 9PL, United Kingdom; and
⁵Department of Terrestrial Magnetism, Carnegie Institution of Washington, NW, Washington, DC 20015

Isotopically anomalous carbonaceous grains in extraterrestrial samples represent the most pristine organics that were delivered to the early Earth. Here we report on gentle aberration-corrected scanning transmission electron microscopy investigations of eight 15N-rich or D-rich organic grains within two carbonaceous Renazzo-type (CR) chondrites and two interplanetary dust particles (IDPs) originating from comets. Organic matter in the IDP samples is less aromatic than that in the CR chondrites, and its functional group chemistry is mainly characterized by C–O bonding and aliphatic C. Organic grains in CR chondrites are associated with carbonates and elemental Ca, which originate either from aqueous fluids or possibly an indigenous organic source. One distinct grain from the CR chondrite NWA 852 exhibits a rim structure only visible in chemical maps. The outer part is nanoglobular in shape, highly aromatic, and enriched in anomalous nitrogen. Functional group chemistry of the inner part is similar to spectra from IDP organic grains and less aromatic with nitrogen below the detection limit. The boundary between these two areas is very sharp. The direct association of both IDP-like organic matter with dominant C–O bonding environments and nanoglobular organics with dominant aromatic and C–N functionality within one unique grain provides for the first time to our knowledge strong evidence for organic synthesis in the early solar system activated by an anomalous nitrogen-containing parent body fluid.

Reference
Vollmer C, Demie Kepaptsoglou D, Leitner J, Busemann H, Spring NH, Ramasse QM, Hoppe P, Nittler LR (2014)
Fluid-induced organic synthesis in the solar nebula recorded in extraterrestrial dust from meteorites. Proceedings of the National Academy of Sciences 111, 43, 15338–15343
Link to Article [doi: 10.1073/pnas.1408206111]

¹Michael A. Velbel, ²Eric K. Tonui, ³Michael E. Zolensky
¹Department of Geological Sciences, 288 Farm Lane, 206 Natural Science Building, Michigan State University, East Lansing, Michigan 48824-1115
²Upstream Technology, BP America, Inc., Houston, TX 77079
³KT Astromaterials Research and Exploration Science, NASA Johnson Space Center, Houston, Texas 77058

Isovolumetric replacement of euhedral and anhedral olivine by serpentine produced both centripetal and meshwork textures in the CM2 chondrites ALH 81002 and Nogoya. The compositions of these textural varieties of serpentine are uniform within narrow limits within each previously studied meteorite, independent of the composition of olivine being replaced, and different between the two meteorites. In QUE 93005 (CM2), coarse olivines of widely varying compositions (Fo<76-99) are replaced in a texturally similar manner by compositionally uniform serpentine (Mg0.73±0.05Fe0.27±0.05)3Si2O5(OH)4. The narrow compositional range of serpentine replacing coarse olivine indicates that the aqueous solution from which the serpentine formed was compositionally uniform on scales at least as large as the meteorite (∼2.5 cm in longest dimension).Isovolumetric textures and compositional observations constrain elemental redistribution from coarse olivine to serpentine and to surrounding phases during serpentinization. Regardless of olivine’s composition, isovolumetric replacement of coarse olivines by serpentine of the observed composition released more Mg and Si from olivine than was required to form the serpentine. Excess Mg and Si released by olivine destruction and not retained in serpentine were exported from the replaced volume.Olivines with different Fa/Fo proportions contributed different amounts of Fe and Mg to the serpentine. Fayalitic olivines released more Fe than required to form the serpentines replacing them, so some of the Fe released from fayalitic olivine was exported from the replaced volumes. Forsteritic olivines released less Fe than required to form the serpentines replacing them, so some Fe was imported into the replaced volumes augmenting the small amount of Fe released from forsteritic olivine. In QUE 93005 Fo83.8 is the threshold composition between Fe-exporting and Fe-importing behavior in individual olivine-serpentine pairs, which released exactly the amount of Fe required to form serpentine of the observed uniform composition. Compositions of serpentines isovolumetrically replacing olivines, and threshold olivine compositions, in QUE 93005 differ from the corresponding values in Nogoya.Solvent and solute species diffused through the serpentine between the olivine-serpentine interface and the aqueous solution outside the isovolumetrically replaced volume. In QUE 93005, some of Fe released from fayalitic olivine in excess of the amount required to form serpentine reacted with S sourced from outside the pseudomorphs to form Fe-sulfide decorating the margins of the pseudomorphs of serpentine after fayalitic olivine. Such Fe-sulfide-decorated outlines after fayalitic olivine do not occur in ALH 81002 or Nogoya, indicating different Fe and S mass transfer regimes in different CM2 chondrites. Mg, Fe, Si, and S in the aqueous solution, including the excess Mg and Si exported from all serpentine pseudomorphs after olivine of any composition, were available to be incorporated into other phases spatially separate from the pseudomorphs after olivine, including regularly interstratified serpentine-tochilinite. Serpentines that replaced coarse olivines in QUE 93005 and ALH 81002 are less magnesian than those in Nogoya, indicating that the Nogoya aqueous-alteration environment was more evolved toward Mg-rich solutions. This easily located and characterized phase assemblage may be potentially useful for characterizing clasts of varying degrees of alteration in brecciated and heterogeneous CM chondrites, and future returned samples from mineralogically similar asteroids.

Reference
Velbel MA, Tonui EK, Zolensky ME (2014) Replacement of olivine by serpentine in the Queen Alexandra Range 93005 carbonaceous chondrite (CM2): Reactant-product compositional relations, and isovolumetric constraints on reaction stoichiometry and elemental mobility during aqueous Alteration. Geochimica et Cosmochimica Acta (in Press)
Link to Article [DOI: 10.1016/j.gca.2014.10.007]

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