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NanoSIMS analysis and Auger electron spectroscopy of
silicate and oxide stardust from the carbonaceous chondrite Acfer 094.
Vollmer C., Hoppe P., Stadermann F. J., Floss C., and Brenker
F. E. (2009)
Geochim. Cosmochim. Acta, in press.
ABSTRACT
We have detected 138 presolar silicate, 20 presolar oxide and
three presolar complex grains within the carbonaceous chondrite Acfer
094 by NanoSIMS oxygen isotope mapping. These grains were further
investigated by scanning electron microscopy (SEM) and Auger electron
spectroscopy for morphological and chemical detailsand their
distribution within the meteorite matrix. The three complex grains
consist of Al-rich oxides (grossite and hibonite) attached to
non-stoichiometric Si- rich silicates. Refractory Al-rich oxides
therefore serve as seed nuclei for silicates tocondense onto, which is
proposed by condensation theory and astronomical observations. However,
in the majority of presolar silicates we did not find any indications
for large subgrains. Most of the grains (80%) belong to O isotope Group
I (17O-enriched) and come from 1 -2.5 Msolar
asymptotic giant branch(AGB) stars of close-to-solar or slightly
lower-than-solar metallicity. About 60% of these grains are irregular
in shape; ~40% display elliptical morphologies together with smooth,
platy surfaces. Three grains with large 17O enrichments (17O/16O
> 3x10-3) have highly irregular shapes and are very small
(< 250 nm); these grains may have formed in binary star systems or
around higher mass(~3 Msolar) AGB stars. About 10% of the
presolar silicates in this study can be assigned to the O isotope Group
IV, which most likely originate from type II supernovae (SNeII). These
grains are also generally smaller than 300 nm and are often irregular
in shape (88%), consistent with the SNII origin scenario. The presolar
grains are generally evenly distributed within the matrix on a mm
scale, although in one case a statistically significant clustering of
five grains in one 10x10 µm2 sized field is observed.
This could be an important hint that the distribution of presolar
material in the parental molecular cloud was heterogeneous on a very
fine scale. The matrix-normalized abundance of silicate stardust in
Acfer 094 is 163 ± 14 ppm, which is among the highest abundance
of O-rich stardust in primitive meteorites. Oxide stardust comprises 26
± 6 ppm of the matrix. Auger Nanoprobe measurements of 69
presolar silicates and oxides (30 on a quantitative, 39 on a
qualitative basis) indicate that most of the grains are Fe-rich
(Mg/(Mg+Fe) of 0.82 and lower), which is either due to non-equilibrium
condensation, secondary alteration, or both. (Mg+Fe)/Si ratios of the
silicates are mostly non-stoichiometric and scatter around
pyroxene-like rather than olivine-like compositions, which is
consistent with recent Auger and TEM observations and astrophysical
predictions. Mg-rich grains (Mg/(Mg+Fe)>0.5) more likely exhibit
elliptical, smooth surfaces (14 out of 18 grains), which is an
indication that these grains have not been strongly altered since their
circumstellar condensation. We identified only one GEMS-like grain with
a statistically significant sulfur content (> 2-3 at.%). It remains
unclear why the typical high-sulfur GEMS grains are only found in
interplanetary dust particles, but have not yet been unequivocally
identified in primitive meteorites.
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