Characterization of presolar silicate and oxide
grains in primitive carbonaceous chondrites.
Nguyen A. N., Stadermann F. J., Zinner E., Stroud R. M.,
Alexander C. M. O'D., and Nittler L. (2007)
Astrophys. J. 656, 1223-1240.
Raster ion imaging of the oxygen isotopes with the
NanoSIMS ion microprobe has been used to identify presolar
grains in two primitive meteorites. Eleven presolar
silicates and eight presolar oxides were identified in the
primitive carbonaceous chondrite Acfer 094 for abundances of
325 and 360 parts per million (ppm), respectively. In
addition, nine presolar silicates and five presolar oxide
grains were identified in the CO3 chondrite ALHA 77307, for
abundances of 320 and 200 ppm, respectively. These
abundances,which arematrix-normalized and corrected for
instrumental detection efficiencies, aremuch higher than
those of other presolar phases,with the exception of
nanodiamonds, although the latter may not all be presolar.
The chemical compositions of six presolar silicate grains
from ALHA 77307 were elucidated by Auger spectroscopy.
Transmission electron microscopy (TEM) analysis of one
presolar silicate grain revealed a nonstoichiometric
composition and an amorphous structure as indicated by the
diffuse electron diffraction pattern. The oxygen isotopic
compositions of the presolar silicates indicate origins in
red giant and asymptotic giant branch stars. Analysis of the
Si isotopic compositions of 10 presolar silicates provides
further constraints on the effects of Galactic chemical