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A cornucopia of presolar and early solar system
materials at the micrometer size range in primitive
chondrite matrix.
Bland P. A., Stadermann F. J., Floss C., Rost D., Vicenzi
E. P., Kearsley A. T. and Benedix G. K. (2007)
Meteorit. Planet. Sci. 42, 1417-1427.
ABSTRACT
We have used a variety of complementary microanalytical
techniques to constrain the mineralogy, trace-element
distributions, and oxygen-isotopic compositions in a 50 x 50
µm area of Acfer 094 matrix. The results reveal the
exceptional mineralogical and compositional heterogeneity of
this material at the sub-µm level. We observe
µm-scale and sub-µm grains with elemental
associations suggesting feldspar, metal with widely varying
Ni contents, and a Cr-Fe alloy (in addition to forsterite,
pyroxene, sulfide, ferrihydrite, and amorphous groundmass
previously described). A new class of µm-scale CAI
(µCAI) is also observed, which show sub-µm
compositional zoning, and a range of oxygen isotopic
compositions. Unlike the larger CAIs in Acfer 094, which are
uniformly 16O-enriched, two of the three mCAIs we
analyzed are isotopically normal. We also observed a Li-rich
hotspot that detailed analysis by ToF-SIMS suggests may be a
LiCr-oxide grain. Within the resolution of the NanoSIMS,
this grain has isotopically normal Li. Finally, in our 50 x
50 µm area, we positively identified a presolar grain
that is the most 18O-rich silicate found so far
in meteorites. The grain may originate from an asymptotic
giant branch (AGB) star, or more likely, a supernova. In
line with previous TEM studies, we find no evidence for
clastic material (e.g., fragmental chondrules) in the matrix
of Acfer 094: although the matrix is volatile-depleted, this
depletion does not appear to result from dilution of a
primordial starting material with (depleted) chondrule
fragments. Assuming that matrix experienced the depletion
event, our data on the detailed mineralogy of Acfer 094 are
currently equivocal in constraining the nature of that
event. We observe carrier phases for several elements
consistent with conditions approaching equilibrium
condensation; however, the presence of an amorphous
groundmass is suggestive of more rapid cooling.
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