11:00 〜 13:00
[MTT44-P03] 隕石ジルコンを用いた宇宙線照射年代測定法
キーワード：隕石ジルコン、宇宙線照射年代
Meteoritic zircons have been found in Martian and Lunar meteorites (e.g., Liu et al. 2012; Bouvier et al. 2018) and in some asteroidal meteorites (e.g., Ireland and Wlotzka 1992; Misawa et al. 2005; Roszjar et al. 2014). Among the asteroidal meteorites, most zircons have been observed in the Vestan meteorites (e.g., eucrites and mesosiderites) (e.g., Haba et al., 2014; 2017). Because zircons have strong resistance against heating at high temperatures and impact shock, and are suitable minerals for U-Pb and Hf-W dating, they have been investigated to understand the early crustal evolution (e.g., Iizuka et al., 2015) and the timing of a large-scale collision on the parent body (Haba et al., 2019). Meanwhile, their robust nature inspires the idea that they would provide more information on the cooling history of the Vestan crust along with the delivery times and mechanism of Vestan meteorites to Earth. However, the rarity of meteoritic zircons and their small grain sizes, normally less than 20–30 μm have prevented such attempts. Recently, Haba et al. (2019) reported an effective separation method of zircons from mesosiderite samples using acids and revealed that the grain sizes of zircons in mesosiderites reach sizes of up to 200 μm, in proportion to the degree of recrystallization of the samples. These new findings enable us to test further possibilities of meteoritic zircons in cosmochemistry. In this study, we focused on the cosmogenic noble gas nuclides in meteoritic zircons to determine a cosmic ray exposure (CRE) age, corresponding to the periods spent as a meter-sized object in interplanetary space after launching from their parent bodies.
The noble gas compositions of zircons (40 μg) separated from the Estherville mesosiderite as well as those of the silicate part were obtained using a noble gas mass spectrometer. The zircons contain cosmogenic noble gas nuclides, and more importantly, cosmogenic 81Kr (t1/2 = 2.29 × 105 years) was successfully detected in the zircons. The CRE age dating using stable cosmogenic noble gas nuclides, such as 3He and 21Ne, requires their whole rock chemistry, which means we must prepare two different samples and sometimes leads a mismatch of their chemical compositions. Detection of cosmogenic nuclides from meteoritic zircons makes it possible to determine precise CRE ages without knowing the whole rock chemistry because of the limited target elements (Si and Zr) that produce cosmogenic nuclides in a zircon crystal. The 81Kr-Kr exposure age of the zircons was calculated to be 76 ± 5 million years (Ma). This age corresponds to the CRE ages obtained from cosmogenic 3He and 21Ne (82 ± 8 and 88 ± 9 Ma, respectively) of the silicate part and the previously reported 36Cl-36Ar age of the metal part (77 ± 9 Ma, Albrecht et al., 2000). The consistent CRE ages using different dating methods demonstrate that the 81Kr-Kr dating using meteoritic zircons is a new promising tool for determining the CRE age of meteorites.
The noble gas compositions of zircons (40 μg) separated from the Estherville mesosiderite as well as those of the silicate part were obtained using a noble gas mass spectrometer. The zircons contain cosmogenic noble gas nuclides, and more importantly, cosmogenic 81Kr (t1/2 = 2.29 × 105 years) was successfully detected in the zircons. The CRE age dating using stable cosmogenic noble gas nuclides, such as 3He and 21Ne, requires their whole rock chemistry, which means we must prepare two different samples and sometimes leads a mismatch of their chemical compositions. Detection of cosmogenic nuclides from meteoritic zircons makes it possible to determine precise CRE ages without knowing the whole rock chemistry because of the limited target elements (Si and Zr) that produce cosmogenic nuclides in a zircon crystal. The 81Kr-Kr exposure age of the zircons was calculated to be 76 ± 5 million years (Ma). This age corresponds to the CRE ages obtained from cosmogenic 3He and 21Ne (82 ± 8 and 88 ± 9 Ma, respectively) of the silicate part and the previously reported 36Cl-36Ar age of the metal part (77 ± 9 Ma, Albrecht et al., 2000). The consistent CRE ages using different dating methods demonstrate that the 81Kr-Kr dating using meteoritic zircons is a new promising tool for determining the CRE age of meteorites.