Referring to the observed results on the source composition of galactic cosmic rays, a possible mechanism for the formation of this composition is proposed by taking into account the fractionation of elements in dust grains formed in the interstellar molecular clouds. These dust grains become enriched with the elements whose condensation temperature is higher than about 1000K. Most of these nuclei enhanced in the source composition are identified as heavy and ultra-heavy ones, which must have been synthesized in the r-process initiated with the explosions of type II supernovae in the OB-association. The dust grains in the superbubble are later ionized and then accelerated through their interactions with shock waves of supershell formed from multiple explosions of nearby supernovae. This may mean that cosmic rays are significantly generated from the dust grains drifting in the local interstellar space.
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