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Distinguishing Solar and Extrasolar Origins of Submicrometer Grains In A524 Goldschmidt Conference Abstracts 2005 Nearly Nano-Compositional Imaging Distinguishing solar and extrasolar NanoSIMS Mg isotope analyses of origins of submicrometer grains refractory inclusions in metal-rich in IDPs CB chondrites 1 1 1 SCOTT MESSENGER I.D. HUTCHEON , P.K. WEBER , S.J. FALLON 2 AND A.N. KROT Robert M Walker Laboratory for Space Science, NASA Johnson Space Center, Code KR, 2101 NASA Parkway, 1Lawrence Livermore National Laboratory, Livermore, CA Houston, TX 77058, USA ([email protected], 94551, USA ([email protected]) [email protected]) 2SOEST, Univ. of Hawaii, Honolulu, HI 96822, USA Interplanetary dust particles (IDPs) are primitive solar Hammadah al Hamra (HH) 237 is a metal-rich (CB) system objects from comets and asteroids that contain chondrite with unusual mineralogical and chemical signatures. abundant interstellar materials [1]. These materials include D- HH237 appears to be a remarkably pristine relic of large-scale and 15N-rich organic matter formed in a cold molecular cloud high temperature processes in the very early solar nebula [1]. environment and grains of stardust [2]. The presolar (stardust) We present Mg isotope analyses of refractory inclusions in grains are identified by their exotic isotopic compositions that HH237, collected with the LLNL NanoSIMS, demonstrating significantly differ from solar isotopic compositions [3]. large mass-dependent isotope fractionation and excesses of Silicates are the most abundant type of stardust, but because of radiogenic 26Mg. their small size (< 1 µm) were only recently discovered. The Mg isotope compositions were measured by static multi- Cameca NanoSIMS ion microprobe was key to this discovery. collection at a mass resolving power of ~4000, using a 20 pA, The NanoSIMS uses an ultra fine beam of (50 nm) Cs+ or (200 250 nm 16O- primary ion beam rastered over a 4x4 mm area. nm) O- and detects five mass lines simultaneously in imaging Isotope ratios were calculated using the sample-standard mode with high transmission at high mass resolving power. In bracketing approach [2]. Ca-Al-rich inclusion (CAI) HH237-s practice, presolar grains as small as 200 nm can be #1 is a compact CAI composed of grossite (CaAl4O7) distinguished in situ from solar system materials. surrounded by a melilite-pyroxene rim with enclosed metal Silicates are more abundant in IDPs (450 – 5,500 ppm) blebs; HH237-MW #4 is a fine-grained, hibonite-rich CAI than in meteorites (< 130 ppm), possibly because they are containing minor spinel, melilite and perovskite, surrounded from comets [3,4]. A key issue to be resolved is the true by layers of melilite and Al-diopside; HH237-104 CAI#1 is a proportion of presolar grains and solar system materials. Much spinel-hibonite spherule containing hibonite laths surrounded of the material in IDPs is too fine grained (< 200 nm) to by a spinel mantle and a melilite-pyroxene rim. NanoSIMS distinguish between solar and extrasolar origin. Some presolar analyses show large mass-dependent fractionation in all three silcates are amorphous (GEMS; 5), and may be mixtures of CAIs favoring the lighter Mg isotopes with FMg values ranging stardust and deposited material. The problem is further from -20 to -12 ‰/amu; 2σ is ~4‰/amu. Both hibonite-rich compounded by recent compositional mapping of GEMS CAIs exhibit large 26Mg excesses, with initial 26Al/27Al ratios grains by field emission TEM, showing that many GEMS of (7.5±3.3) x 10-5, while the grossite-rich inclusion contains grains are composite objects [6]. The isotopic measurement of no excess 26Mg with 26Al/27Al<2x10-6. GEMS grains may thus represent an average of still smaller The large enrichment of the lighter Mg isotopes is unusual subgrains. Resolving this issue may require higher precision for igneous CAIs and, together with group II REE patterns [1], isotopic measurements as the NanoSIMS is near the limiting indicate many CAIs in CB meteorites preserve primary size scale accessible to SIMS. condensation signatures inherited from the nebular gas. The isotope fractionation effects appear decoupled from References incorporation of 26Al. Additional refinement of initial 26Al [1] J. P. Bradley, S. A. Sandford, R. M. Walker, in Meteorites abundances will elucidate the time scales of condensation and and the Early Solar System, J. Kerridge and M.S. melting and test the hypopthesis grossite-rich CAIs formed Matthews, Eds. (Univ. Arizona Press, Tucson, AZ, 1988), before injection of 26Al into the solar nebula. pp. 861-895 [2] Messenger S. Nature 404, 968-971 References [3] S. Messenger, L. P. Keller, F. J. Stadermann, R. M. [1] Krot, A.N. et al. (2002) MAPS 37, 1451-1490. Walker, E. Zinner Science 300, 105 (2003). [2] Galy, A. et al. (2003) J. Anal. At. Spectr. 18, 1352-1356. [4] A. Nguyen, E. Zinner, Science 303, 1496 (2004) [5] Bradley, J.P. (1994) Science 265, 925 [6] L. P. Keller and S. Messenger (2005) Lunar Planetary Sci., submitted. .
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