Track record in meteorites

Abstract

Over geological time, meteoritical crystals have accumulated a record of tracks produced by heavily charged energetic particles from both internal and external sources, such as fissile nuclei and cosmic rays with Z >20. The paper highlights some of the areas where the track analysis method has made significant, and sometimes unique, contributions to our knowledge, not only of the history of meteorites but also of conditions prevailing in our Solar System from soon after the end of nucleosynthesis ( ca . 4.6 Ga ago) until the present day. The topics covered include the following, (i) Fission-track retention ages and cooling rates of meteoritic parent bodies. Ages range downwards from ca . 4.55 Ga, depending on the type of crystal and the time at which different crystals had cooled down sufficiently to retain tracks; typical early cooling rates are (a few times) 1 to 10 K Ma -1 . (ii) Charge and energy spectra of ancient cosmic rays in comparison with contemporary values. Methods of particle identification are described; no significant differences from Solar System abundances or contemporary cosmic rays are found for the v.h. (very heavy; Z « 20-28) and v.v.h. (very very heavy; Z > 30) nuclei, (iii) Determination of pre-atmospheric sizes of meteorites by fitting measured track-density values as a function of depth below a fusion crust to theoretical curves for track-production rate as a function of true pre-ablation depth. (iv) Search for superheavy elements (s.h.e.) in meteorites. By examining the silicate minerals at boundaries with metal in iron and stony-iron meteorites, where large amounts of excess tracks from the fission decay of siderophile superheavy elements should have been recorded in the silicate interface but are not found, upper limits ranging from ca. 1 fg g -1 to ca . 1 pg g -1 at fission-track retention time ( ca . 4.2 Ga ago) can be placed on the superheavy element content.