No abstract

Deep Impact? On 15 February 2013, the Russian district of Chelyabinsk, with a population of more than 1 million, suffered the impact and atmospheric explosion of a 20-meter-wide asteroid—the largest impact on Earth by an asteroid since 1908. Popova et al. (p. 1069 , published online 7 November; see the Perspective by Chapman ) provide a comprehensive description of this event and of the body that caused it, including detailed information on the asteroid orbit and atmospheric trajectory, damage assessment, and meteorite recovery and characterization.

We report on the mineralogy, petrography, and in situ measured oxygen-and magnesiumisotope compositions of eight porphyritic chondrules (seven FeO-poor and one FeO-rich) from the Renazzo-like carbonaceous (CR) chondrites Graves Nunataks 95229, Grosvenor Mountains 03116, Pecora Escarpment 91082, and Queen Alexandra Range 99177, which experienced minor aqueous alteration and very mild thermal metamorphism. We find no evidence that these processes modified the oxygen-or Al-Mg isotope systematics of chondrules in these meteorites. Olivine, low-Ca pyroxene, and plagioclase within an individual chondrule have similar Oisotope compositions, suggesting crystallization from isotopically uniform melts. The only exceptions are relict grains in two of the chondrules; these grains are 16 O-enriched relative to phenocrysts of the host chondrules. Only the FeO-rich chondrule shows a resolvable excesses of 26 Mg, corresponding to an inferred initial 26 Al/ 27 Al ratio [( 26 Al/ 27 Al) 0 ] of (2.5±1.6)×10 6 (±2SE). Combining these results with the previously reported Al-Mg isotope systematics of CR chondrules (Nagashima et al., 2014, Geochem. J. 48, 561), 7 of 22 chondrules (32%) measured show resolvable excesses of 26 Mg; the presence of excess 26 Mg does not correlate with the FeO content of chondrule silicates. In contrast, virtually all chondrules in weakly metamorphosed (petrologic type 3.0-3.1) unequilibrated ordinary chondrites (UOCs), Ornans-like carbonaceous (CO) chondrites, and the ungrouped carbonaceous chondrite Acfer 094 show resolvable excesses of 26 Mg. The inferred ( 26 Al/ 27 Al) 0 in CR chondrules with resolvable excesses of 26 Mg range from (1.0±0.4)×10 6 to (6.3±0.9)×10 6 , which is typically lower than ( 26 Al/ 27 Al) 0 in the majority of chondrules from UOCs, COs, and Acfer 094. Based on the inferred ( 26 Al/ 27 Al) 0 , three populations of CR chondrules are recognized; the population characterized by low ( 26 Al/ 27 Al) 0 (<3×10 6 ) is dominant. There are no noticeable trends with major and minor element or Oisotope compositions between these populations. The weighted mean ( 26 Al/ 27 Al) 0 of 22 CR chondrules measured is (1.8±0.3)×10 6 . An apparent agreement between the 26 Al-26 Mg ages (using weighted mean value) and the revised (using 238 U/ 235 U ratio for bulk CR chondrites of 137.7789±0.0085) 207 Pb-206 Pb age of a set of chondrules from CR chondrites (Amelin et al., 2002, Science 297, 1678) is consistent with the initial 26 Al/ 27 Al ratio in the CR chondrite chondrule-forming region at the canonical level (~5.210 5 ), allowing the use of 26 Mg systematics as a chronometer for CR chondrules. To prove chronological significance of 26 Al for CR chondrules, measurements of Al-Mg and U-Pb isotope systematics on individual chondrules are required. The presence of several generations among CR chondrules indicates some chondrules that accreted into the CR chondrite parent asteroid avoided melting by later chondrule-forming events, suggesting chondrule-forming processes may have occurred on relativ...

The distribution of chemical elements in primitive meteorites (chondrites), as building blocks of terrestrial planets, provides insight into the formation and early differentiation of Earth. The processes that resulted in the depletion of some elements [such as chromium (Cr)] in the bulk silicate Earth relative to chondrites, however, remain debated between leading candidate causes: volatility versus core partitioning. We show through high-precision measurements of Cr stable isotopes in a range of meteorites, which deviate by up to ~0.4 per mil from those of the bulk silicate Earth, that Cr depletion resulted from its partitioning into Earth's core, with a preferential enrichment in light isotopes. Ab initio calculations suggest that the isotopic signature was established at mid-mantle magma ocean depth as Earth accreted planetary embryos and progressively became more oxidized.

Dynamic models of the protoplanetary disk indicate there should be large-scale material transport in and out of the inner Solar System, but direct evidence for such transport is scarce. Here we show that the ε50Ti-ε54Cr-Δ17O systematics of large individual chondrules, which typically formed 2 to 3 My after the formation of the first solids in the Solar System, indicate certain meteorites (CV and CK chondrites) that formed in the outer Solar System accreted an assortment of both inner and outer Solar System materials, as well as material previously unidentified through the analysis of bulk meteorites. Mixing with primordial refractory components reveals a “missing reservoir” that bridges the gap between inner and outer Solar System materials. We also observe chondrules with positive ε50Ti and ε54Cr plot with a constant offset below the primitive chondrule mineral line (PCM), indicating that they are on the slope ∼1.0 in the oxygen three-isotope diagram. In contrast, chondrules with negative ε50Ti and ε54Cr increasingly deviate above from PCM line with increasing δ18O, suggesting that they are on a mixing trend with an ordinary chondrite-like isotope reservoir. Furthermore, the Δ17O-Mg# systematics of these chondrules indicate they formed in environments characterized by distinct abundances of dust and H2O ice. We posit that large-scale outward transport of nominally inner Solar System materials most likely occurred along the midplane associated with a viscously evolving disk and that CV and CK chondrules formed in local regions of enhanced gas pressure and dust density created by the formation of Jupiter.