  Presolar grains are trace components in chondrite matrices. Their abundances
and compositions have been systematically studied in carbonaceous chondrites
but rarely in situ in other major chondrite classes. We have conducted a
NanoSIMS isotopic search for presolar grains with O- and C-anomalous isotopic
compositions in the matrices of the unequilibrated ordinary chondrites
Semarkona, Meteorite Hills 00526, and Northwest Africa 8276. The matrices of
even the most primitive ordinary chondrites have been aqueously altered and/or
thermally metamorphosed, destroying their presolar grain populations to varying
extents. In addition to randomly placed isotope maps, we targeted recently
reported, relatively pristine Semarkona matrix areas to better explore the
original inventory of presolar grains. In all samples, we found a total of 122
O-anomalous grains (silicates + oxides), 79 SiC grains, and 22 C-anomalous
carbonaceous grains (organics, graphites). Average matrix-normalized abundances
are 151 ppm O-anomalous grains, 53 ppm SiC grains and 56 ppm carbonaceous
grains in Semarkona, 55 ppm (O-anom.), 22 ppm (SiC) and 3 ppm (carb.) in MET
00526 and 12 ppm (O-anom.), 15 ppm (SiC) and 1 ppm (carb.) in NWA 8276. In
relatively pristine ordinary chondrites and in primitive carbonaceous and
C-ungrouped chondrites, the O and C isotopic composition of presolar grains and
their matrix-normalized abundances are similar, despite the likely differences
in chondrite-formation time and nebular location. These results suggest a
relatively homogenous distribution of presolar dust across major
chondrite-forming reservoirs in the solar nebula. Secondary asteroidal
processes are mainly responsible for differences in presolar grain abundances
between and within chondrites, highlighting the need to identify and target the
most pristine chondrite matrices for such studies.
