  We conducted a TEM study of an unusual oxide-silicate composite presolar
grain (F2-8) from the unequilibrated ordinary chondrite Semarkona. The presolar
composite grain is relatively large, has an amoeboidal shape, and contains
Mg-rich olivine, Mg-Al spinel, and Ca-rich pyroxene. The shape and phase
assemblage are reminiscent of amoeboid-olivine-aggregates and add to the
growing number of TEM observations of presolar refractory inclusion-like (CAIs
and AOAs) grains. In addition to the dominant components, F2-8 also contains
multiple subgrains, including an alabandite-oldhamite composite grain within
the olivine and several magnetite subgrains within the Mg-Al spinel. We argue
that the olivine, Mg-Al spinel, and alabandite-oldhamite formed by equilibrium
condensation, whereas the Ca-rich pyroxene formed by non-equilibrium
condensation, all in an M-type AGB star envelope. On the other hand, the
magnetite subgrains are likely the result of aqueous alteration on the
Semarkona asteroidal parent body. Additional evidence of secondary processing
includes Fe-enrichment in the Mg-Al spinel and olivine, elevated Al contents in
the olivine, and beam sensitivity and a modulated structure for the olivine.
Compound presolar grains record condensation conditions over a wide range of
temperatures. Additionally, the presence of several different presolar phases
in a composite grain can impart information on the relative rates and effects
of post-condensation processing in a range of environments, including the
interstellar medium, solar nebula, and the host asteroid parent body. The TEM
observations of F2-8 provide insights across the lifetime of the grain from its
formation by condensation in an M-type AGB star envelope, its transit through
the interstellar medium, and aqueous alteration during its residence on
Semarkona's asteroidal parent body.
