¹Sherry K. Fieber-Beyer, ¹Michael J. Gaffey
¹Department of Space Studies, University Stop 9008, University of North Dakota, 58202

The research is an integrated effort beginning with telescopic observations and extending through detailed mineralogical characterizations to provide constraints on the composition and meteorite affinities of a subset of fourteen asteroids in/near the 3:1 Kirkwood Gap. Eight asteroids were identified as having either one or two absorption features, while six were deemed featureless. The compositional analysis of asteroids (355) Gabriella and (1447) Utra reveal Fs and Fa values which are consistent with values for the L-type ordinary chondrites (Fs19-22 and Fa22-26). The location of these two bodies with respect to each other and to the previously identified L-chondrite parent body asteroid (1722) Goffin, suggests a small L-chondrite genetic family. These results support the model that the L-chondrites come from an asteroid family rather than from a single object. Asteroids (1368) Numidia, (1587) Kahrstadt, (1854) Skortsov, (2497) Kulikovskij, and (5676) Voltaire were analyzed and determined to have “basaltic” silicate mineralogies similar to those of the HED (howardite – eucrite – diogenite) meteorite group. In particular, we found that the compositions of (1368), (1587) and (1854) are consistent with olivine-orthopyroxenitic diogenites, while (2497) and (5676)’s compositions are consistent with harzburgitic diogenites. The Band I and Band II absorption feature depths are much shallower than seen in diogenite spectra, typically ∼70% depth ( Burbine et al. 2000). The nature of the weak features seen in the asteroid spectra when compared to measured band depths of in situ diogenite samples indicate an additional mechanism(s) acting to weaken the features, most likely space weathering. The aforementioned five asteroids are plausible sources for the olivine-orthopyroxenitic diogenites and harzburgitic diogenites, and very well may be fragments of Vesta. Asteroid (46) Hestia is an interesting object whose surface minerals may be consistent with a CR2 chondrite; however, the unique spectrum deserves further study in the future. Featureless asteroids (248) Lameia, (1960) Guisan, (3345) Tarkovskij and (6212) 1993 MS1 surface materials are likely organic assemblages consistent with the Type 1 or 2 carbonaceous chondrite meteorite class; however specific terrestrial meteorite analog could not be identified. The spectra of asteroids (3228) Pire and (3999) Aristarchus are consistent with each other and have been assigned to the Eulalia by Walsh et al. (2013). Spectrally they are similar to (495) in terms of blue-slope and albedo ( Fieber-Beyer et al. 2012), thus increasing our confidence the three bodies are truly related dynamically and genetically. By extrapolation and due to their location adjacent to the 3:1 Kirkwood Gap, (3228) and (3999) are plausible sources of the CV3OXB carbonaceous chondrites.

Reference
Fieber-Beyer SK, Gaffey MJ (2015) Near-infrared Spectroscopy of 3:1 Kirkwood Gap Asteroids III. Icarus (in Press)
Link to Article [doi:10.1016/j.icarus.2015.04.034]

Copyright Elsevier

^1,2,3Nan Liu et al. (>10)*
¹Department of the Geophysical Sciences, The University of Chicago, Chicago, IL, 60637, USA
²Chicago Center for Cosmochemistry, Chicago, IL 60637, USA
³Materials Science Division, Argonne National Laboratory, Argonne, IL 60439, USA
*Find the extensive, full author and affiliation list on the publishers website

We present strontium, barium, carbon, and silicon isotopic compositions of 61 acid-cleaned presolar SiC grains from Murchison. Comparison with previous data shows that acid washing is highly effective in removing both strontium and barium contamination. For the first time, by using correlated 88Sr/86Sr and 138Ba/136Ba ratios in mainstream SiC grains, we are able to resolve the effect of 13C concentration from that of 13C-pocket mass on s-process nucleosynthesis, which points toward the existence of large 13C pockets with low 13C concentrations in asymptotic giant branch stars. The presence of such large 13C pockets with a variety of relatively low 13C concentrations seems to require multiple mixing processes in parent asymptotic giant branch stars of mainstream SiC grains.

Reference
Liu N et al. (2015) Correlated Strontium and Barium Isotopic Compositions of Acid-cleaned Single Mainstream Silicon Carbides from Murchison. Astrophysical Journal 803,12
Link to Article [doi:10.1088/0004-637X/803/1/12]

¹Tommaso Di Rocco,¹Andreas Pack
¹Geowissenschaftliches Zentrum, Abteilung Isotopengeologie, Georg-August-Universität, Goldschmidtstraße 1, D-37077 Göttingen, Germany

We have conducted time and ƒO2-dependent oxygen isotope exchange experiments between chondrule analogue melts and H2O in the phase. The aim of our study is to address the question whether the oxygen isotope composition of chondrules is the result of exchange with the ambient nebular gas or has been inherited from the precursor material. The silicate melt-H2O vapor exchange experiments were carried out in a vertical gas-mixing furnace using the metal loop technique at 1500°C. The duration ranged from 5 to 1440 minutes and ƒO2 was set between IW-3.8 and IW-1.3 using the H2O/H2 buffer. Our experiments show that 50% exchange between H2O gas and silicate melt occurs in ∼4 hours at ƒO2 = IW-3.8 and in ∼1 hour at ƒO2 = IW-1.3. At solar nebula conditions, significant exchange occurs only if chondrule-melting times were several hours.

Reference
Di Rocco T, Pack A (2015) Triple oxygen isotope exchange between chondrule melt and water vapor: an experimental study. Geochimica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2015.04.038]

Copyright Elsevier

The most popular papers on Cosmochemistry Papers in April were:

1-Touboul M, Puchtel IS, Walker RJ (2015) Tungsten isotopic evidence for disproportional late accretion to the Earth and Moon. Nature (in Press) Link to Article [doi:10.1038/nature14355]

2-Martín-Torres FJ et al. (2015) Transient liquid water and water activity at Gale crater on Mars. Nature Geoscience (in Press) Link to Article [doi:10.1038/ngeo2412]

3-Kraus RG, Root S, Lemke RW, Stewart ST, Jacobsen SB, Mattsson TR (2015) Impact vaporization of planetesimal cores in the late stages of planet Formation. Nature Geoscience 8, 269–272 Link to Article [doi:10.1038/ngeo2369]

4-Kruijer TS, Kleine T, Fischer-Gödde M, Sprung S (2015) Lunar tungsten isotopic evidence for the late veneer. Nature (in Press) Link to Article [doi:10.1038/nature14360]

5-Mastrobuono-Battisti A, Perets HB, Raymond SN (2015) A primordial origin for the compositional similarity between the Earth and the Moon. Nature 520, 212–215 Link to Article [doi:10.1038/nature14333]