^1,2M. E. I. RIEBE,¹H. BUSEMANN,²C. M. O’D. ALEXANDER,²L. R. NITTLER,³C. D. K. HERD,¹C. MADEN,²J. WANG,¹R. WIELER
Meteoritics & Planetary Science (in Press) Link to Article [doi: 10.1111/maps.13383]
¹Institute of Geochemistry and Petrology, ETH Zurich, CH-8092, Zurich, Switzerland
²DTM, Carnegie Institution of Washington, 5241 Broad Branch Road, Washington, District of Columbia 20015, USA
³Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta T6G 2E3, Canada
Published by arrangement with John Wiley & Sons

Effects of aqueous alteration on primordial noble gas carriers were investigated by analyzing noble gases and determining presolar SiC abundances in insoluble organic matter (IOM) from four Tagish Lake meteorite (C2-ung.) samples that experienced different degrees of aqueous alteration. The samples contained a mixture of primordial noble gases from phase Q and presolar nanodiamonds (HL, P3), SiC (Ne-E[H]), and graphite (Ne-E[L]). The second most altered sample (11i) had a ~2–3 times higher Ne-E concentration than the other samples. The presolar SiC abundances in the samples were determined from NanoSIMS ion images and 11i had a SiC abundance twice that of the other samples. The heterogeneous distribution of SiC grains could be inherited from heterogeneous accretion or parent body alteration could have redistributed SiC grains. Closed system step etching (CSSE) was used to study noble gases in HNO3-susceptible phases in the most and least altered samples. All Ne-E carried by presolar SiC grains in the most altered sample was released during CSSE, while only a fraction of the Ne-E was released from the least altered sample. This increased susceptibility to HNO3 likely represents a step toward degassing. Presolar graphite appears to have been partially degassed during aqueous alteration. Differences in the 4He/36Ar and 20Ne/36Ar ratios in gases released during CSSE could be due to gas release from presolar nanodiamonds, with more He and Ne being released in the more aqueously altered sample. Aqueous alteration changes the properties of presolar grains so that they react similar to phase Q in the laboratory, thereby altering the perceived composition of Q.

¹A. GARENNE,^1,2P. BECK,³G. MONTES-HERNANDEZ,¹L. BONAL,¹E. QUIRICO,⁴O. PROUX,⁵J.L. HAZEMANN
Meteoritics & Planetary Science (In Press) Link to Article [doi: 10.1111/maps.13377]
¹CNRS, IPAG, Universite Grenoble Alpes, F-38000 Grenoble, France ²Institut Universitaire de France, Paris, France
³Institut des Sciences de la Terre (IsTERRE), Universite Grenoble Alpes/CNRS-INSU, Grenoble, France
⁴Observatoire des Sciences de l’Univers de Grenoble (OSUG) CNRS UMS 832, 414 rue de la piscine, 38400 Saint Martin d’Heres, France
⁵CNRS, Institut Neel, Universite Grenoble Alpes, 25 av. des Martyrs, 38042 Grenoble, France
Published by arrangement with John Wiley & Sons

The valence of iron has been used in terrestrial studies to trace the hydrolysis of primary silicate rocks. Here, we use a similar approach to characterize the secondary processes, namely thermal metamorphism and aqueous alteration, that have affected carbonaceous chondrites. X-ray absorption near-edge structure spectroscopy at the Fe-Kedge was performed on a series of 36 CM, 9 CR, 10 CV, and 2 CI chondrites. While previous studies have focused on the relative distribution of Fe0 with respect to oxidized iron (Feox = Fe2+ + Fe3+) or the iron distribution in some specific phases (e.g., Urey–Craig diagram; Urey and Craig 1953), our measurements enable us to assess the fractions of iron in each of its three oxidation states: Fe0, Fe2+, and Fe3+. Among the four carbonaceous chondrites groups studied, a correlation between the iron oxidation index (IOI = [2 (Fe2+) + 3(Fe3+)]/[FeTOT]) and the hydrogen content is observed. However, within the CM group, for which a progressive alteration sequence has been defined, a conversion of Fe3+ to Fe2+ is observed with increasing degree of aqueous alteration. This reduction of iron can be explained by an evolution in the mineralogy of the secondary phases. In the case of the few CM chondrites that experienced some thermal metamorphism, in addition to aqueous alteration, a redox memory of the aqueous alteration is present: a significant fraction of Fe3+ is present, together with Fe2+ and sometimes Fe0. From our data set, the CR chondrites show a wider range of IOI from 1.5 to 2.5. In all considered CR chondrites, the three oxidation states of iron coexist. Even in the least-altered CR chondrites, the fraction of Fe3+ can be high (30% for MET 00426). This observation confirms that oxidized iron has been integrated during formation of fine-grained amorphous material in the matrix (Le Guillou and Brearley 2014; Le Guillou et al. 2015; Hopp and Vollmer 2018). Last, the IOI of CV chondrites does not reflect the reduced/oxidized classification based on metal and magnetite proportions, but is strongly correlated with petrographic types. The valence of iron in CV chondrites therefore appears to be most closely related to thermal history, rather than aqueous alteration, even if these processes can occur together (Krot et al. 2004; Brearley and Krot 2013).

^1,2Celia Dalou,²Marc M.Hirschmann,³Steven D.Jacobsen,⁴CharlesLe Losq
Geochimica et Cosmochimcia Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2019.08.029]
¹Centre de Recherches Pétrographiques et Géochimiques, 15 rue Notre-Dame des Pauvres, BP20, 54501 Vandoeuvre-lès-Nancy Cedex, France
²Dept. of Earth Sciences, 108 Pillsbury Hall, University of Minnesota, Minneapolis, MN 55455, USA
³Department of Earth and Planetary Sciences, Northwestern University, Evanston, IL 60208, United States
⁴Research School of Earth Sciences, The Australian National University, Building 142, Mills Road, Canberra, ACT 2601, Australia
Copyright Elsevier

To better understand the solution of volatile species in a reduced magma ocean, we identify via Raman spectroscopy the nature of C-O-H-N volatile species dissolved in a series of reduced basaltic glasses. The oxygen fugacity (ƒ_O2) during synthesis varied from highly reduced at two log units below the iron-wustite buffer (IW-2.1) to moderately reduced (IW-0.4), spanning much of the magmatic ƒ_O2 conditions during late stages of terrestrial accretion. Raman vibrational modes for H₂, NH₂^–, NH₃, CH₄, CO, CN^–, N₂, and OH^– species are inferred from band assignments in all reduced glasses. The integrated area of Raman bands assigned to N₂, CH₄, NH₃ and H₂ vibrations in glasses increases with increasing molar volume of the melt, whereas that of CO decreases. Additionally, with increasing ƒ_O2, CO band areas increase while those of N₂ decrease, suggesting that the solubility of these neutral molecules is not solely determined by the melt molar volume under reduced conditions. Coexisting with these neutral molecules, other species as CN^–, NH₂^– and OH^– are chemically bonded within the silicate network. The observations indicate that, under reduced conditions, 1) H₂, NH₂^–, NH₃, CH₄, CO, CN^–, N₂, and OH^– species coexist in silicate glasses representative of silicate liquids in a magma ocean 2) their relative abundances dissolved in a magma ocean depend on melt composition, ƒ_O2 and the availability of H and, 3) metal-silicate partitioning or degassing reactions of those magmatic volatile species must involve changes in melt and vapor speciation, which in turn may influence isotopic fractionation.