Devin L. Schrader^1,a, Kazuhide Nagashima^a, Alexander N. Krot^a, Ryan C. Ogliore^a and Eric Hellebrand^b

^aHawai‘i Institute of Geophysics and Planetology, School of Ocean and Earth Science and Technology, University of Hawai‘i at Mānoa, Honolulu, HI 96822, USA
^bDepartment of Geology and Geophysics, School of Ocean, Earth Science and Technology, University of Hawai‘i at Mānoa, Honolulu, HI 96822, USA
¹Present Address: Department of Mineral Sciences, National Museum of Natural History, Smithsonian Institution, PO Box 37012, MRC 119, Washington, D.C. 20013, USA

To better understand the environment of chondrule formation and constrain the O-isotope composition of the ambient gas in the Renazzo-like carbonaceous (CR) chondrite chondrule-forming region, we studied the mineralogy, petrology, and in situ O-isotope compositions of olivine in 11 barred olivine (BO) chondrules and pyroxene and silica in three type I porphyritic chondrules from the CR chondrites Gao-Guenie (b), Graves Nunataks (GRA) 95229, Pecora Escarpment (PCA) 91082, and Shişr 033. BO chondrules experienced a higher degree of melting than porphyritic chondrules, and therefore, it has been hypothesized that they more accurately recorded the O-isotope composition of the gas in chondrule-forming regions. We studied the O-isotope composition of silica as it has been hypothesized to have formed via direct condensation from the gas.
BO chondrules constitute ~4% of the total CR chondrule population by volume. On a three-isotope oxygen diagram (δ¹⁷O vs. δ¹⁸O), olivine phenocrysts in type I and type II BO chondrules plot along ~ slope-1 line; with the exception of a type II BO chondrule that plots along ~ slope-0.5 line. Olivine phenocrysts in type I and type II BO chondrules have similar but more restricted ranges of Δ¹⁷O values (~ -3.8 to ~ -1.3‰ and ~ -0.8 to ~ +1.4‰, respectively) than those in type I and type II porphyritic chondrules (~ -4.6 to ~ -0.3‰ and ~ -1.8 to ~ +0.9‰, respectively). The observation that olivine grains in type I BO chondrules have similar chemical and O-isotope compositions to those of olivine in their porphyritic counterparts argues against the hypothesis that olivine grains in type I porphyritic chondrules are xenocrysts and represent relict fragments of early formed planetesimals.
The compositional and O-isotope data suggest that BO chondrules experienced more extensive, but incomplete exchange with the ambient gas than porphyritic chondrules. We suggest that CR chondrules formed from relatively ¹⁶O-enriched solids in the presence of relatively ¹⁶O-depleted gaseous H₂O. The O-isotope compositions of chondrule olivine likely result from differences in the O-isotope composition of both the chondrule precursors and the ambient gas during chondrule formation. The inferred O-isotope composition of this gas (Δ¹⁷O ranges from ~ -3‰ to +3‰) is inconsistent with a high abundance of water from the outer Solar System, which has been predicted to be isotopically heavy.

Reference
Schrader DL, Nagashima K, Krot AN, Ogliore RC and Hellebrand E (in press) Variations in the O-isotope composition of gas during the formation of chondrules from the CR chondrites. Geochimica et Cosmochimica Acta
[doi:10.1016/j.gca.2014.01.034]
Copyright Elsevier

Link to Article

Stefan T.M. Peters^a,b, Carsten Münker^a,b,1, Harry Becker^c,2, Toni Schulz^d,3

^aInstitut für Geologie und Mineralogie, Universität zu Köln, Zülpicherstr. 49b, 50674 Cologne, Germany
^bSteinmann-Institut, Poppelsdorfer Schloss, 53115 Bonn, Germany
^cInstitut für Geologische Wissenschaften, Freie Universität Berlin, Malteserstr. 74-100, 12249 Berlin, Germany
^dDepartment of Lithospheric Research, Universität Wien, Althanstrasse 14, A-1090, Vienna, Austria

The decay of the rare nuclide ¹⁸⁴Os by alpha emission to ¹⁸⁰W has been theoretically predicted, but was previously never observed in experiments. Variable excesses of ¹⁸⁰W were recently observed for iron meteorites, but the contribution to these excesses by ¹⁸⁴Os-decay was regarded as insignificant. Here, we present combined ¹⁸⁰W and Os–W concentration data for meteorites and terrestrial rocks, now indicating that the ¹⁸⁰W heterogeneities can be explained by α-decay of ¹⁸⁴Os. A combined ¹⁸⁴Os–¹⁸⁰W isochron for iron meteorites and chondrites yields a decay constant value of λ¹⁸⁴Os(α) of 6.49±1.34×10⁻¹⁴ a⁻¹ (half life 1.12±0.23×10¹³ yr), in good agreement with theoretical estimates. The ¹⁸⁴Os–¹⁸⁰W decay system may constitute a viable tracer and chronometer for important geological processes like core formation, silicate differentiation or late accretion processes. This is illustrated by a measured ¹⁸⁰W-deficit in terrestrial basalts relative to chondrites by 1.16±0.69 parts in 10 000, consistent with core formation ~4.5 Ga ago.

Reference
Peters STM, Münker C, Becker H and Schulz T (2014) Alpha-decay of 184Os revealed by radiogenic 180W in meteorites: Half life determination and viability as geochronometer. Earth and Planetary Science Letters 391:69–76.
[doi:10.1016/j.epsl.2014.01.030]
Copyright Elsevier

Link to Article

Liping Jin1 and Min Li

College of Physics, Jilin University, Changchun, Jilin 130012, China

We show that the diversity of extrasolar planetary systems may be related to the diversity of molecular cloud cores. In previous studies of planet formation, artificial initial conditions of protoplanetary disks or steady state disks, such as the minimum mass nebula model, have often been used so that the influence of cloud core properties on planet formation is not realized. To specifically and quantitatively demonstrate our point, we calculate the dependence of disk properties on cloud core properties and show that the boundary of the giant planet formation region in a disk is a function of cloud core properties with the conventional core accretion model of giant planet formation. The gravitational stability of a disk depends on the properties of its progenitor cloud core. We also compare our calculations with observations of extrasolar planets. From the observational data of cloud cores, our model could infer the range and most frequent values of observed semimajor axes of extrasolar planets. Our calculations suggest that planet formation at the snowline alone could not completely explain the semimajor axis distribution. If the current observations are not biased, our calculations indicate that the planet formation at the snowline is inefficient. We suggest that there will be more observed planets with semimajor axis <9 AU than >9 AU, even with a longer duration of observations, if the planet formation at the snowline is inefficient.

Reference
Jin L and Li M (2014) Diversity of Extrasolar Planets and Diversity of Molecular Cloud Cores. I. Semimajor Axes. The Astrophysical Journal 783:37.
[doi:10.1088/0004-637X/783/1/37]

Link to Article