Presolar Stardust in Highly Pristine CM Chondrites Asuka 12169 and Asuka
  12236
  We report a NanoSIMS search for presolar grains in the CM chondrites Asuka
(A) 12169 and A12236. We found 90 presolar O-rich grains and 25 SiC grains in
A12169, giving matrix-normalized abundances of 275 (+55/-50, 1$\sigma$) ppm or,
excluding an unusually large grain, 236 (+37/-34) ppm for O-rich grains and 62
(+15/-12) ppm for SiC grains. For A12236, 18 presolar silicates and 6 SiCs
indicate abundances of 58 (+18/-12) and 20 (+12/-8) ppm, respectively. The SiC
abundances are in the typical range of primitive chondrites. The abundance of
presolar O-rich grains in A12169 is essentially identical to that in CO3.0
Dominion Range 08006, higher than in any other chondrites, while in A12236 it
is higher than found in other CMs. These abundances provide further strong
support that A12169 and A12236 are the least-altered CMs as indicated by
petrographic investigations. The similar abundances, isotopic distributions,
silicate/oxide ratio, and grain sizes of the presolar O-rich grains found here
to those of presolar grains in highly primitive CO, CR and ungrouped
carbonaceous chondrites (CCs) indicate that the CM parent body(ies) accreted a
similar population of presolar oxides and silicates in their matrices to those
accreted by the parent bodies of the other CC groups. The lower abundances and
larger grain sizes seen in some other CMs are thus most likely a result of
parent-body alteration and not heterogeneity in nebular precursors. Presolar
silicates are unlikely to be present in high abundances in returned samples
from asteroids Ryugu and Bennu since remote-sensing data indicate that they
have experienced substantial aqueous alteration.
