Using 43 documented drill core samples of the H_5-chondrite 'Jilin' the depth profile of GCR-and fission track densities was established. The track profile essentially reflects the GCR-track record of the seco...Using 43 documented drill core samples of the H_5-chondrite 'Jilin' the depth profile of GCR-and fission track densities was established. The track profile essentially reflects the GCR-track record of the second (4π-) irradiation stage of 'Jilin'. Relics of the track record of the first (2π-) exposure are indicated by a deviation from the theoretical depth profile of the 4π-exposure. No Pu-244 fission is observed ia the fission track background near the meteorite center. The atmospheric ablation thickness of the two drill cores A and B normal and parallel to the assumed surface of the 'Jilin' parent body was found to be 11±4 cm, respectively. A minimum shielding depth of the 'top' part of core A during the 2π-exposure is estimated to be～30 cm. Near surface deviations of the experimental track data from theory are ascribed to a short-term moderate heating event (equivalent to 24h,～370℃) no more than 0.4 Ma ago. The resulting partial track annealing in a surface layer of ～15—20 cm thickness of the preatmospheric body can explain the lack of GCR-tracks in Jilin samples from very shallow depths that are based on ^(60)Co measurements.展开更多
文摘Using 43 documented drill core samples of the H_5-chondrite 'Jilin' the depth profile of GCR-and fission track densities was established. The track profile essentially reflects the GCR-track record of the second (4π-) irradiation stage of 'Jilin'. Relics of the track record of the first (2π-) exposure are indicated by a deviation from the theoretical depth profile of the 4π-exposure. No Pu-244 fission is observed ia the fission track background near the meteorite center. The atmospheric ablation thickness of the two drill cores A and B normal and parallel to the assumed surface of the 'Jilin' parent body was found to be 11±4 cm, respectively. A minimum shielding depth of the 'top' part of core A during the 2π-exposure is estimated to be～30 cm. Near surface deviations of the experimental track data from theory are ascribed to a short-term moderate heating event (equivalent to 24h,～370℃) no more than 0.4 Ma ago. The resulting partial track annealing in a surface layer of ～15—20 cm thickness of the preatmospheric body can explain the lack of GCR-tracks in Jilin samples from very shallow depths that are based on ^(60)Co measurements.
