The most popular papers on Cosmochemistry Papers in January were:

1-Chan QHS, Chikaraishi Y, Takano Y, Ogawa NO, Ohkouchi N (2016) Amino acid compositions in heated carbonaceous chondrites and their compound-specific nitrogen isotopic ratios. Earth, Planets and Space 68, 7
Link to Article [doi:10.1186/s40623-016-0382-8]

2-Steenstra ES, Knibbe JS, Rai N, van Westrenen W (2016) Constraints on core formation in Vesta from metal–silicate partitioning of siderophile elements. Geochimica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2016.01.002]

3-Heck PR, Schmitz B, Rout SS, Tenner T, Villalon K, Cronholm A, Terfelt F,Kitae NT (2016) A search for H-chondritic chromite grains in sediments that formed immediately after the breakup of the L-chondrite parent body 470 Ma ago. Geochimica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2015.11.042]

4-Friedrich JM,Glavin DP,Rivers ML, Dworkin JP (2016) Effect of a synchrotron X-ray microtomography imaging experiment on the amino acid content of a CM chondrite. Meteoritics & Planetary Sciences (in Press)
Link to Article [DOI: 10.1111/maps.12595]

5-Haggerty SE (2016) Spinel in planetary systems. American Mineralogist 101,5-6
Link to Article[doi:10.2138/am-2016-5554]

¹Edward D. Young, ¹Issaku E. Kohl, ¹Paul H. Warren, ²David C. Rubie, ^2,3Seth A. Jacobson, ³Alessandro Morbidelli
¹Department of Earth, Planetary, and Space Sciences, University of California Los Angeles, Los Angeles, CA, USA.
²Bayerisches Geoinstitut, University of Bayreuth, D-95490 Bayreuth, Germany.
³Laboratoire Lagrange, Université de Nice–Sophia Antipolis, Observatoire de la Cote d’Azur, CNRS, 06304 Nice, France.

Earth and the Moon are shown here to have indistinguishable oxygen isotope ratios, with a difference in Δ′17O of −1 ± 5 parts per million (2 standard error). On the basis of these data and our new planet formation simulations that include a realistic model for primordial oxygen isotopic reservoirs, our results favor vigorous mixing during the giant impact and therefore a high-energy, high-angular-momentum impact. The results indicate that the late veneer impactors had an average Δ′17O within approximately 1 per mil of the terrestrial value, limiting possible sources for this late addition of mass to the Earth-Moon system.

Reference
Young ED,Kohl IE,Warren PH,Rubie DC,Jacobson SA,Morbidelli A (2016) Oxygen isotopic evidence for vigorous mixing during the Moon-forming giant impact. Science 351, 6272, 493-496
Link to Article [DOI: 10.1126/science.aad0525]
Reprinted with permission from AAAS

¹Y. Langevin et al. (>10)*
¹Institut d’Astrophysique Spatiale, CNRS/Univ. Paris-Sud, Orsay, France
*Find the extensive, full author and affiliation list on the publishers website

The COSIMA mass spectrometer on board the ROSETTA orbiter has collected dust in the near coma of comet 67P/Churyumov-Gerasimenko since August 11, 2014. The collected dust particles are identified by taking images with a microscope (COSISCOPE) under grazing incidence illumination before and after exposure of the target to cometary dust. More than 10,000 dust particles > 14 µm in size collected from August 11, 2014 to April 3, 2015 have been detected on three distinct target assemblies, including ∼ 500 dust particles with sizes ranging from 50 to more than 500 µm, that can be resolved by COSISCOPE (pixel size 14 µm). During this period, the heliocentric distance decreased from 3.5 AU to less than 2 AU. The collection efficiency on targets covered with “metal black” has been very high, due to the low relative velocity of incoming dust. Therefore, the COSISCOPE observations provide the first optical characterization of an unbiased sample of particles collected in the inner coma of a comet. The typology of particles > 100 µm in size is dominated by clusters with a wide range of structure and strength, most originating from the disruption of large aggregates (> 1 mm in size) shortly before collection. A generic relationship between these clusters and IDPs / Antarctic meteorites is likely in the framework of accretion models. About 15% of particles larger than 100 µm are compact particles with two likely contributions, one being linked to clusters and another leaving the cometary nucleus as single compact particles.

Reference
Langevin Y et al. (2016) Typology of dust particles collected by the COSIMA mass spectrometer in the inner coma of 67P/Churyumov Gerasimenko. Icarus (in Press)
Link to Article [doi:10.1016/j.icarus.2016.01.027]
Copyright Elsevier

¹O. Pravdivtseva, ¹A. Meshik, ¹C.M. Hohenberg,²A.N. Krot
¹Physics Department, Washington University, St. Louis MO 63130, USA
²Hawai‘i Institute of Geophysics and Planetology, School of Ocean, Earth Science and Technology, University of Hawai’i at Mānoa, Honolulu, HI 96822, USA

It is inferred that magnesian non-porphyritic chondrules in the CB (Bencubbin-type) carbonaceous chondrites formed in an impact generated plume of gas and melt 4562.49 ± 0.21 Ma (Bollard et al., 2015) and could be suitable for the absolute age normalization of relative chronometers. Here xenon isotopic compositions of neutron irradiated chondrules from the CB chondrites Gujba and Hammadah al Hamra (HH) 237 have been analyzed in an attempt to determine closure time of their I-Xe isotope systematics. One of the HH 237 chondrules, #1, yielded a well-defined I-Xe isochron that corresponds to a closure time of 0.29 ± 0.16 Ma after the Shallowater aubrite standard. Release profiles and diffusion properties of radiogenic 129∗Xe and 128∗Xe extracted from this chondrule by step-wise pyrolysis, indicate presence of two iodine host phases with distinct activation energies of 73 and 120 kcal/mol. In spite of the activation energy differences, the I-Xe isotope systematics of these two phases closed simultaneously, suggesting rapid heating and cooling (possibly quenching) of the CB chondrules. The release profiles of U-fission Xe and I-derived Xe correlate in the high temperature host phase supporting simultaneous closure of 129I-129Xe and 207Pb-206Pb systematics.

The absolute I-Xe age of Shallowater standard is derived from the observed correlation between I-Xe and Pb-Pb ages in a number of samples. It is re-evaluated here using Pb-Pb ages adjusted for an updated 238U/235U ratio of 137.794 and meteorite specific U-isotope ratios. With the addition of the new data for HH 237 chondrule #1, the re-evaluated absolute I-Xe age of Shallowater is 4562.4 ± 0.2 Ma. The absolute I-Xe age of the HH 237 chondrule #1 is 4562.1 ± 0.3 Ma, in good agreement with U-corrected Pb-Pb ages of the Gujba chondrules (Bollard et al., 2015) and HH 237 silicates (Krot et al., 2005).

All I-Xe data used here, and in previous estimates of the absolute age of Shallowater, are calculated using 15.7 ± 0.6 Ma value for 129I half-life. The slopes of I-Xe – Pb-Pb correlation lines plotted for different sets of samples for Shallowater normalization are always ⩽ 1. Assuming uranium half-life values are correct; this restricts the half-life of 129I to ⩽ 15.7 Ma.

Reference
Pravdivtseva O, Meshik A, Hohenberg CM, Krot AN (2016) I-Xe systematics of the impact plume produced chondrules from the CB carbonaceous chondrites: Implications for the half-life value of 129I and absolute age normalization of 129I-129Xe chronometer. Geochimica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2016.01.012]
Copyright Elsevier

¹Noriyuki Kawasaki, ²Shoichi Itoh, ³Naoya Sakamoto, ¹Hisayoshi Yurimoto
¹Department of Natural History Sciences, Hokkaido University, Sapporo 060-0810, Japan
²Department of Earth and Planetary Sciences, Kyoto University, Kyoto 606-8502, Japan
³Creative Research Institution, Hokkaido University, Sapporo 001-0021, Japan

Fluffy Type A Ca-Al-rich inclusions (CAIs) containing reversely zoned melilite crystals are suggested to be direct condensates from solar nebular gas. We conducted an investigation of 26Al−26Mg systematics of a fluffy Type A CAI from Vigarano, named V2-01, with known oxygen isotopic distributions of reversely zoned melilite crystals; we also conducted oxygen isotope measurements of coexisting minerals. Two of six reversely zoned melilite crystals show continuous variations in magnesium isotopic composition, with δ25Mg becoming small along the inferred direction of crystal growth, which supports the idea that they originated through condensation. Petrography suggests that the constituent minerals of V2-01 formed in the following order: first spinel and fassaite enclosed by melilite, then reversely zoned melilite crystals, and spinel and diopside in the Wark-Lovering rim. The spinel enclosed by melilite has 16O-rich compositions (Δ17O ∼ −24‰) and an initial value of (26Al/27Al)0 = (5.6 ± 0.2) × 10−5. The fassaite enclosed by melilite crystals shows variable oxygen isotopic compositions (Δ17O ∼ −12‰ and −17‰) and plots on an isochron with (26Al/27Al)0 = (5.6 ± 0.2) × 10−5. The oxygen isotopic compositions of reversely zoned melilite showed continuous variations in Δ17O along the inferred direction of crystal growth, suggesting that surrounding nebular gas, during the formation of the reversely zoned melilite, changed from 16O-poor (Δ17O values larger than −10‰) to 16O-rich (Δ17O ∼ −25‰). The six reversely zoned melilite crystals show indistinguishable initial 26Al/27Al values with an average (26Al/27Al)0 of (4.7 ± 0.3) × 10−5, which is clearly distinguishable from the value of enclosed spinel and fassaite, indicating a younger formation age than the enclosed spinel and fassaite. The spinel and diopside from the Wark-Lovering rim shows 16O-rich compositions (Δ17O ∼ −23‰) with (26Al/27Al)0 = (4.5 ± 0.4) × 10−5. The values of (26Al/27Al)0 are consistent with the formation sequence inferred from petrography. The formation period for the V2-01 CAI is estimated to be 0.18 ± 0.07 Myr from the difference in initial 26Al/27Al values. These data suggest that the oxygen isotopic composition of solar nebular gas surrounding the CAI changed from 16O-rich to 16O-poor and back to 16O-rich during the first ∼0.2 Myr of Solar System formation.

Reference
Kawasaki N,Itoh S,Sakamoto N, Yurimoto H (2016) Chronological study of oxygen isotope composition for the solar protoplanetary disk recorded in a fluffy Type A CAI from Vigarano. Geochimica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2015.12.031]
Copyright Elsevier

¹J. P. Das, ¹S. L. Baldwin, ²J. W. Delano
¹Department of Earth Sciences, Syracuse University, Syracuse 13244, NY, USA
²Department of Atmospheric and Environmental Sciences, University of Albany (SUNY), Albany 12222, NY, USA

We currently do not have a copyright agreement with this publisher and cannot display the abstract here

Reference
Das JP, Baldwin SL, Delano JW (2016) 40 Ar/ 39 Ar and cosmic ray exposure ages of plagioclase-rich lithic fragments from Apollo 17 regolith, 78461. Earth, Planets and Space 68, 11
Link to Article [doi:10.1186/s40623-016-0381-9]

¹K.A.Farley et al. (>10)*
¹Division of Geological and Planetary Sciences, Caltech, Pasadena, CA, USA
*Find the extensive, full author and affiliation list on the publishers website

Cl isotope ratios measured on HCl thermally evolved from as-yet-unknown phases in sedimentary rocks and sand in Gale Crater provide unexpected insights to the Martian surficial Cl cycle. The seven samples yield δ37Cl values ranging from −1±25‰−1±25‰ to −51±5‰−51±5‰. Five analyses from two samples of the Sheepbed mudstone (Yellowknife Bay study area) are analytically indistinguishable with a mean δ37Cl of −11±7‰−11±7‰(1σ)(1σ). In contrast, four mudstones/sandstones from the Kimberley and Pahrump study areas also yielded indistinguishable ratios, but with a mean δ37Cl of −43±6‰−43±6‰. The Rocknest sand deposit gave a highly uncertain δ37Cl value of −7±44‰−7±44‰.

These light and highly variable δ37Cl values are unique among known solar system materials. Two endmember models are offered to account for these observations, and in both, perchlorate, with its extreme ability to fractionate Cl isotopes, is critical. In the first model, SAM is detecting HCl from an oxychlorine compound (e.g., perchlorate) produced from volcanic gas emissions by atmospheric chemical reactions. Similar reactions in Earth’s atmosphere may be responsible for the isotopically lightest known Cl outside of this study, in perchlorate from the Atacama Desert. Some of the Gale Crater δ37Cl values are more negative than those in Atacama perchlorate, but because reaction mechanisms and associated fractionation factors are unknown, it is impossible to assess whether this difference is prohibitive. If the negative δ37Cl signal is produced in this fashion, the isotopic variability among samples could arise either from variations in the relative size of the reactant chloride and product perchlorate reservoirs, or from variations in the fraction of perchlorate reduced back to chloride after deposition. Such reduction strongly enriches 37Cl in the residual perchlorate.

Perchlorate reduction alone offers an alternative endmember model that can explain the observed data if SAM measured HCl derived from chloride. In this model isotopically normal perchlorate produced by an unspecified mechanism is reduced to chloride. Depending on the relative size of the reduced reservoir, the integrated product chloride can vary in isotopic composition from −70‰−70‰ in the first increment all the way to the starting composition if the perchlorate is fully reduced. Thus, variable degrees of perchlorate reduction can produce chloride with the appropriate δ37Cl range. Combination of the two endmember models, in which the perchlorate subject to post-deposition reduction is isotopically negative from atmospheric reactions, is also possible.

Determination of the phase hosting the Cl measured by SAM, an oxychlorine compound or chloride, is critical for selecting between these models, and for developing implications of the results for the Mars surficial Cl cycle. At present it is not possible to conclusively establish which phase is responsible (possibly both), but limited evidence favors the conclusion that the measured Cl derives mostly from an oxychlorine compound.

Reference
Farley KA et al. (2016) Light and variable 37Cl/35Cl ratios in rocks from Gale Crater, Mars: Possible signature of perchlorate. Earth and Planetary Science Letters 438, 14–24.
Link to Article [doi:10.1016/j.epsl.2015.12.013]
Copyright Elsevier

¹Ewa Słaby, ²Monika Koch-Müller, ²Hans-Jürgen Förster, ²Richard Wirth, ²Dieter Rhede, ²Anja Schreiber, ³Ulrich Schade
¹Institute of Geological Sciences, Polish Academy of Sciences, Research Centre in Warsaw, Warsaw, Poland
²Helmholtz-Zentrum Potsdam, Deutsches GeoForschungsZentrum GFZ, Telegrafenberg, Potsdam, Germany
³Helmholtz-Zentrum Berlin, Elektronenspeicherring BESSY II, Berlin, Germany

We combined the focused ion beam sample preparation technique with polarized synchrotron-based FTIR (Fourier transform infrared) spectroscopy, laser-Raman spectroscopy, electron microprobe analysis (EMPA), and transmission electron microscope (TEM) analysis to identify and quantify structurally bound OH, F, Cl, and CO3 groups in fluorapatite from the Northwest Africa 2975 (NWA 2975) shergottite. In this study, the first FTIR spectra of the OH-stretching region from a Martian apatite are presented that show characteristic OH-bands of a F-rich, hydroxyl-bearing apatite. Depending on the method of apatite-formula calculation and whether charge balance is assumed or not, the FTIR-based quantification of the incorporated OH, expressed as wt% H2O, is in variably good agreement with the H2O concentration calculated from electron microprobe data. EMP analyses yielded between 0.35 and 0.54 wt% H2O, and IR data yielded an average H2O content of 0.31 ± 0.03 wt%, consistent with the lower range determined from EMP analyses. The TEM observations implied that the volatiles budget of fluorapatite is magmatic. The water content and the relative volatile ratios calculated for the NWA 2975 magma are similar to those established for other enriched or intermediate shergottites. It is difficult to define the source of enrichment: either Martian wet mantle or crustal assimilation. Comparing the environment of parental magma generation for NWA 2975 with the terrestrial mantle in terms of water content, it displays a composition intermediate between enriched and depleted MORB.

Reference
Słaby E, Monika Koch-Müller, Förster H-J, Wirth R, Rhede D, Schreiber A, Schade U (2016) Determination of volatile concentrations in fluorapatite of Martian shergottite NWA 2975 by combining synchrotron FTIR, Raman spectroscopy, EMPA, and TEM, and inferences on the volatile budget of the apatite host-magma. Meteoritics & Planetary Sciences (in Press)
Link to Article [DOI: 10.1111/maps.12598]
Published by arrangement with John Wiley & Sons

¹Knut Metzler, ²Andreas Pack
¹Institut für Planetologie, Westfälische Wilhelms-Universität Münster, Münster, Germany
²Geowissenschaftliches Zentrum, Abteilung Isotopengeologie, Georg-August-Universität Göttingen, Göttingen, Germany

Cluster chondrites are characterized by close-fit textures of deformed and indented chondrules, taken as evidence for hot chondrule accretion (Metzler 2012). We investigated seven cluster chondrite clasts from six brecciated LL3 chondrites and measured their bulk oxygen isotopic and chemical composition, including REE, Zr, and Hf. The same parameters were measured in situ on 93 chondrules and 4 interchondrule matrix areas. The CI-normalized REE patterns of the clasts are flat, showing LL-chondritic concentrations. The mean chemical compositions of chondrules in clasts and other LL chondrites are indistinguishable and we conclude that cluster chondrite chondrules are representative of the normal LL chondrule population. Type II chondrules are depleted in MgO, Al2O3 and refractory lithophiles (REE, Zr, Hf) by factors between 0.65 and 0.79 compared to type I chondrules. The chondrule REE patterns are basically flat with slight LREE < HREE fractionations. Many chondrules exhibit negative Eu anomalies while matrix shows a complementary pattern. Chondrules scatter along a correlation line with a slope of 0.63 in the oxygen 3-isotope diagram, interpreted as the result of O-isotope exchange between chondrule melts and 18O-rich nebular components. In one clast, a distinct anticorrelation between chondrule size and δ18O is found, which may indicate a more intense oxygen isotope exchange by smaller chondrules. In some clasts the δ18O values of type I chondrules are correlated with concentrations of SiO2 and MnO and anticorrelated with MgO, possibly due to the admixture of a SiO2- and MnO-rich component to chondrule melts during oxygen isotope exchange. Two chondrules with negative anomalies in Sm, Eu, and Yb were found and may relate their precursors to refractory material known from group III CAIs. Furthermore, three chondrules with strong LREE > HREE and Zr/Hf fractionations were detected, whose formation history remains to be explained.

Reference
Metzler K, Pack A (2016) Chemistry and oxygen isotopic composition of cluster chondrite clasts and their components in LL3 chondrites. Meteoritics & Planetary Sciences (in Press)
Link to Article [DOI: 10.1111/maps.12592]
Published by arrangement with John Wiley & Sons

R.Wieler et al. (>10)*
¹Department of Earth Sciences, ETH Zürich, Zürich, Switzerland
*Find the extensive, full author and affiliation list on the publishers website

We present noble gas data for 16 shergottites, 2 nakhlites (NWA 5790, NWA 10153), and 1 angrite (NWA 7812). Noble gas exposure ages of the shergottites fall in the 1–6 Ma range found in previous studies. Three depleted olivine-phyric shergottites (Tissint, NWA 6162, NWA 7635) have exposure ages of ~1 Ma, in agreement with published data for similar specimens. The exposure age of NWA 10153 (~12.2 Ma) falls in the range of 9–13 Ma reported for other nakhlites. Our preferred age of ~7.3 Ma for NWA 5790 is lower than this range, and it is possible that NWA 5790 represents a distinct ejection event. A Tissint glass sample contains Xe from the Martian atmosphere. Several samples show a remarkably low (21Ne/22Ne)cos ratio < 0.80, as previously observed in a many shergottites and in various other rare achondrites. This was explained by solar cosmic ray-produced Ne (SCR Ne) in addition to the commonly found galactic cosmic ray-produced Ne, implying very low preatmospheric shielding and ablation loss. We revisit this by comparing measured (21Ne/22Ne)cos ratios with predictions by cosmogenic nuclide production models. Indeed, several shergottites, acalpulcoites/lodranites, angrites (including NWA 7812), and the Brachina-like meteorite LEW 88763 likely contain SCR Ne, as previously postulated for many of them. The SCR contribution may influence the calculation of exposure ages. One likely reason that SCR nuclides are predominantly detected in meteorites from rare classes is because they usually are analyzed for cosmogenic nuclides even if they had a very small (preatmospheric) mass and hence low ablation loss.

Reference
Wieler R et al. (2016) Noble gases in 18 Martian meteorites and angrite Northwest Africa 7812—Exposure ages, trapped gases, and a re-evaluation of the evidence for solar cosmic ray-produced neon in shergottites and other achondrites. Meteoritics & Planetary Sciences (in Press)
Link to Article [DOI: 10.1111/maps.12600]
Published by arrangement with John Wiley & Sons