  Presolar grains are small particles found in meteorites through their
isotopic compositions which are considerably different from those of materials
in the Solar System. If some isotopes in presolar grains diffused out beyond
their grain sizes when they were embedded in parent bodies of meteorites, their
isotopic compositions could be washed out, and hence the grains cannot be
identified as presolar grains any more. We explore this possibility for the
first time by self-consistently simulating the thermal evolution of
planetesimals and the diffusion length of $^{18}$O in presolar silicate grains.
Our results show that presolar silicate grains smaller than $\sim$ 0.03 $\mu m$
cannot keep their original isotopic compositions even if the host planetesimals
experienced maximum temperature as low as 600 $^{\circ}$C. Since this
temperature corresponds to the one experienced by petrologic type 3 chondrites,
the isotopic diffusion can constrain the size of presolar silicate grains
discovered in such chondrites to be larger than $\sim$ 0.03 $\mu m$. We also
find that the diffusion lengths of $^{18}$O reach $\sim$ 0.3-2 $\mu m$ in
planetesimals that were heated up to 700-800 $^{\circ}$C. This indicates that,
if the original size of presolar grains spans a range from $\sim$ 0.001 $\mu m$
to $\sim$ 0.3 $\mu m$ like that in the interstellar medium, the isotopic
records of the presolar grains may be almost completely lost in such highly
thermalized parent bodies. We propose that isotopic diffusion could be a key
process to control the size distribution and abundance of presolar grains in
some types of chondrites.
