Article

Discovery of abundant in situ silicate and spinel grains from red giant stars in a primtive meteorite

The Astrophysical Journal (Impact Factor: 6.28). 10/2004; DOI: 10.1086/424842
Source: OAI

ABSTRACT We report the discovery of 12 in situ presolar silicate and spinel grains, 140-590 nm in size, in the Acfer 094 meteorite. These grains represent a matrix-normalized abundance of presolar O-rich dust of 170 parts per million. Among the 10 silicate grains are three olivines, four pyroxenes, and three grains with glasslike composition. Eleven grains have large excesses in 17O with 17O/16O ratios of up to 2.9 times the solar ratio and slightly lower than or close-to-solar 18O/16O ratios. These grains most likely formed in 1.5-1.65 Msolar red giant branch (RGB) or asymptotic giant branch (AGB) stars with close-to-solar metallicity. One pyroxene grain has close-to-solar 17O/16O and 18O/16O of 3.8 times solar. Silicon- and Fe-isotopic ratios of this grain are suggestive of the formation in an RGB or AGB star, probably with higher-than-solar metallicity. 29Si/28Si and 30Si/28Si ratios of the silicate grains vary by ~16%, are positively correlated with one another, and fall to the 30Si-poor side of the Si mainstream line characteristic for presolar SiC from AGB stars. This gives independent confirmation for the view that the Si mainstream line reflects the Galactic chemical evolution of the Si isotopes, except for a small shift due to dredge-up of matter from the He shell in AGB stars.

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    ABSTRACT: Due to their small grain size (200-300 nm) and chemical similarity to the host meteorite matrix, the isotope analysis of presolar silicate particles must be performed using in situ techniques, such as secondary ion mass spectrometry (SIMS). The achievable spatial resolution of analyses depends on the element of interest, and for Mg isotope measurements it is comparable to or exceeds the size of individual presolar silicate grains. This compromises isotope ratio measurements by dilution from the neighbouring matrix. Here we present a method to prepare 200-300 nm presolar silicate grains of primitive meteorites for in situ isotope analysis with the NanoSIMS, a high spatial resolution secondary ion mass spectromreter, which minimises dilution effects imposed by the host meteorite matrix. This method follows the procedure developed by Nguyen et al. [A. N. Nguyen, S. Messenger, M. Ito, Z. Rahman, Mg isotopic measurement of FIB-isolated presolar silicate grains. 41 st Lunar Planet Sci Conf, abstr. 2413] for dense grain separates. Presolar silicate grains in a primitive carbonaceous chondrite (Acfer 094) were identified by their O isotope compositions, which can be measured at high spatial resolution (< 100 nm) with negligible dilution effect. After the grains had been identified in field-emission secondary electron microscope images, a focussed Ga ion beam was used to remove material around the grains in a 3 µm diameter circle to a depth of 1-1.5 µm, in order to minimise dilution effects. The major focus of our efforts has been the accurate analysis of Mg isotopes. Besides the potential of giving insight into nucleosynthetic processes affecting Mg isotope abundances, Mg isotope compositions of presolar silicate grains will also provide information on the Galactic chemical evolution of Mg.

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