A cornucopia of presolar and early solar system
materials at the micrometer size range in primitive
chondrite matrix
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 µCAIs that we analysed 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 (Greshake, 1997), 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 by 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.
Bland et al. (2007) Meteorit. Planet. Sci. 42, 1417-1427.
Full publication: 2007_MAPS42_1417.pdf
Last revised: 11-2007