244Pu fission track thermometry and its application to stony meteorites

Abstract Revised cooling rates, obtained from mineral separates of several ordinary chondrites by means of the multiple fission track detector method, are presented. Especially for the H chondrites, these new data lead to major revisions of the previously published results. They are consistent with an ‘onion-shell’ structure of the parent asteroids of ordinary chondrites, in which petrologic type 6 materials resided near the central region, and types 5, 4 and 3 in the outer shells. The diameters of these asteroids are estimated to be between 100 and 140 km for the H body, greater than 200 km for the L body, and 220-300 km for the LL body.