[S-GC51] Frontiers in noble gas isotope geosciences

[SGC51-P05] Possibility of K-Ar age mapping on the moon using cosmogenic ^{39}Ar

Yuko TAKESHIMA¹, *Hironobu HYODO², Tetsumaru ITAYA² (1.NEC Aerospace systems, 2.RINS, Okayama Univ. of Sci.)

Keywords:^{39}Ar, Cosmogenic, age distribution, ^{40}Ar/^{39}Ar age, moon surface

Rapid and wide range of age survey on the moon surface requires in situ measurement. ³⁹Ar of cosmogenic origin from ³⁹K in meteorite has been noticed since the early stage of noble gas research, and became a cue to the ⁴⁰Ar/³⁹Ar method. If the production rate of ³⁹Ar is considered uniform, the production and decay of ³⁹Ar reaches to equilibrium after a long time, since ³⁹Ar has a half life of 293 years. This half life is long enough to detect ³⁹Ar in meteorites and moon rocks, and short enough to reach equilibrium of the isotope. Thus, such rocks or minerals possess a certain amount of ³⁹Ar proportional to their potassium content. Using meteorites or rocks on moon surface under the same exposure condition, ⁴⁰Ar/³⁹Ar age can be determined. For a start, a precise date of one of those samples in a laboratory and ⁴⁰Ar/³⁹Ar ratio measurement of the same sample in the field are required to obtain J-value in the field. ⁴⁰Ar/³⁹Ar ratios measurement of the rest of unknowns in the field allow us to calculate ages. Interfering isotopes from calcium will not affect much as long as Ca/K ratio is low. Samples in a depth or a shadow from exposure are not suitable for the measurement since the neutron density attenuates with depth. However, no need for atmospheric contamination, mass measurement and ³⁶Ar measurement may provide us more mobility and rapid measurement.