Permalink : http://hdl.handle.net/2115/34825

Fossil charged-particle tracks and uranium contents were examined in major silicate phases in Yamato-74013, -74080, -74094, -74362 and -74459 chondrites. The tracks recorded in bronzite in diogenites and olivine in chondrites are expected to be mostly due to cosmic-ray heavy primaries, because of their extremely low uranium contents. Assuming that the cosmic-ray exposure age, 31 my, of Yamato-6902 is also applicable to other diogenites, the distance from the preatmospheric surface to the test crystals is estimated to be 10, 17 and 15 cm for Yamato-74013, -74037, and -6902, respectively. For Yamato-74014 and -74080 chondrites, the exposure age of about 3 ～ 5 and 14 my, each, can be evaluated, assuming the shallow burial depth of the samples, which have a fusion crust on one side. A small polished section with a fusion crust was etched to investigate the effect of the ablation heating. Cosmic-ray tracks were survived in olivine grains at the depth of more than 0.9 mm from the meteorite surface. According to the high-temperature annealing experiments, the temperature higher than 550℃ had not continued for 1 sec at the position. Pyroxene in chondrites usually contains much higher uranium contents and fossil tracks than in olivine. These tracks cannot be explained only by cosmic-ray and the spontaneous fission of 238U during 4.6 × 10^9 yrs. On the assumption that the excesses are due to the spontaneous fission of the presently extinct isotope, 244Pu (half life, 82 my), the time intervals between the end of nucleosynthesis and the onset of track retention, ΔT, were calculated. They are ranging from 3×10^7 to 3 × 10^8 yrs for five chondrites. No apparent relations were found between the results and the chemical and petrologic types of the chondrites.