Formation of the parent bodies of the carbonaceous chondrites

Abstract

We have carried out extensive particle track studies for several C2 chondrites. On the basis of these and the available data on spallogenic stable and radioactive nuclides in several C1 and C2 chondrites, we have constructed a scenario for the precompaction irradiation of these meteorites. We discuss the rather severe constraints which these data place on the events leading to the formation of the parent bodies of the carbonaceous chondrites. Our analyses suggest that the precompaction solar flare and solar wind irradiation of the individual components most probably occurred primarily while the matter had accreted to form swarms of centimeter- to meter-sized bodies. This irradiation occured very early, within a few hundred my of the birth of the solar system; the pressure in the solar system had then dropped below 10^-9 atm. Further, the model assumes that soon after the irradiation of carbonaceous matter as swarms, the small bodies coalesced to form kilometer-sized objects, in time scales of 10^5±1 years, a constraint defined by the low cosmogenic exposure ages of these meteorites. Collisions among these objects led to the formation of much-larger-sized parent bodies of the carbonaceous chondrites. Implicit in this model is the existence of "irradiated" components at all depths in the parent bodies, which formed out of the irradiated swarm material.