最近落下した隕石ならびに南極やまと隕石の ^<14>C濃度測定(第15回名古屋大学タンデトロン加速器質量分析計シンポジウム(平成14(2002)年度)報告)

The storage time of a meteorite on the Earth's surface, its terrestrial age, can be determined by the decay of cosmic-ray-produced radionuclides. The radionuclide ^<14>C is very useful for determining the age, especially younger age than 〜30 kyr. We have attempted measuring reliable ^<14>C terrestrial ages of Antarctic meteorites by AMS at Nagoya University. There was a problem that a fixed blank exists when carbon was extracted from meteorite, and the blank effect was large due to little ^<14>C in meteorite. So we improved the previous carbon extraction method to combust the mixture of a meteorite sample and an iron standard including nearly ^<14>C-free carbon of 0.052% in the RF furnace (LECO HF-10) and extracted ^<14>CO_2 was diluted at the same time when the meteorite was combusted. The ^<14>C activities of recently fallen meteorites of Holbrook (L6) and Mt.Tazerzait (L5) were measured by using the method. The mean value was 56.5±3.2 dpm/kg. This value is suitable, compared with the reported value of 52.7±4.3 dpm/kg for recently fallen L6 chondrites. Terrestrial ages were measured for three Antarctic Yamato meteorite samples: 0.8±0.5 kyr of Y-74190 (L6), 1.7±0.5 kyr of Y-75097 (L6), and 1.8±0.5 kyr of Y-75108 (L6). These values are younger than the data by Minami and Nakamura (2002) but agree with the value reported by Jull et al. (1984). The five samples of Y-74190, Y-75097 and Y-75108, together with Y-75102 and Y-75271, are considered the same origin (Takaoka, 1987). From the results in this study and Minami and Nakamura (2002), Y-75097 and Y-75108 could be originated from the same meteoroid, while Y-74190 could have the different origin because its terrestrial age is younger. However, the large difference is not observed in major element compositions of these meteorites. Furthermore, there is a possibility that Y-74190 existed at deeper depth of a meteoroid than the other two samples from the data of ^3He/^<21>Ne, ^<22>Ne/^<21>Ne and ^<53>Mn (Takaoka, 1987). Y-74190 could have the near terrestrial age to Y-75097 and Y-75108, by shielding or depth corrections in a meteoroid. The shielding or depth corrections are needed for ^<14>C terrestrial age determination of a meteorite sample if the meteoroid was very large or very small. By normalizing the saturated activity of ^<14>C to that of ^<10>Be in a meteorite, more correct terrestrial age for the meteorite could be obtained.