Doppler weather radar imaging enabled the rapid recovery of the Sutter's Mill meteorite after a rare 4-kiloton of TNT-equivalent asteroid impact over the foothills of the Sierra Nevada in northern California. The recovered meteorites survived a record high-speed entry of 28.6 kilometers per second from an orbit close to that of Jupiter-family comets (Tisserand's parameter = 2.8 ± 0.3). Sutter's Mill is a regolith breccia composed of CM (Mighei)-type carbonaceous chondrite and highly reduced xenolithic materials. It exhibits considerable diversity of mineralogy, petrography, and isotope and organic chemistry, resulting from a complex formation history of the parent body surface. That diversity is quickly masked by alteration once in the terrestrial environment but will need to be considered when samples returned by missions to C-class asteroids are interpreted.

We report the petrology, O isotopic composition, and Al-Mg isotope systematics of a chondrule fragment from the Jupiter-family comet Wild 2, returned to Earth by NASA's Stardust mission. This object shows characteristics of a type II chondrule that formed from an evolved oxygen isotopic reservoir. No evidence for extinct 26 Al was found, with ( 26 Al/ 27 Al) 0 < 3.0×10 −6 . Assuming homogenous distribution of 26 Al in the solar nebula, this particle crystallized at least 3 Myr after the earliest solar system objects-relatively late compared to most chondrules in meteorites. We interpret the presence of this object in a Kuiper Belt body as evidence of late, large-scale transport of small objects between the inner and outer solar nebula. Our observations constrain the formation of Jupiter (a barrier to outward transport if it formed further from the Sun than this cometary chondrule) to be more than 3 Myr after calcium-aluminum-rich inclusions.

Interstellar dust (ISD) from the local interstellar medium (LISM) streams into the solar system from approximately the direction of the constellation Ophiuchus. Prior to the return of the NASA Stardust spacecraft (1) no recognizable samples of this interstellar dust were available for laboratory study. Thus, our understanding of the properties of contemporary ISD has been derived primarily from astronomical observations of the ISM, including optical properties of the ISD and remote spectroscopy of the gas composition (2-4), and from in situ measurements by the dust analyzers on the Cassini, Ulysses and Galileo spacecraft (5-7). The canonical picture of ISD is that it is dominated by ~0.2 µm diameter (8) amorphous silicate grains, with or without carbonaceous mantles. However, the inferred properties of the particles, including size distribution, density and composition are heavily model dependent.

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...

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