11:23 〜 11:38
[SGL21-03] 宇宙線照射起源39Arを用いた惑星表面のその場40Ar/39Ar年代測定
キーワード：宇宙線照射起源39Ar、惑星表面、その場40Ar/39Ar年代測定
In extraterrestrial space, high-energy cosmic-ray irradiation may induce 39K (n, p) 39Ar reactions in K-bearing materials. Detecting the cosmogenic 39Ar produced by this process will enable the dating of planetary surfaces during planetary landing missions. For example, the application could be used for lunar surface rocks by direct in situ 40Ar/39Ar measurements with equipment mounted on the lunar explorer based on the information on the K–Ar age and 40Ar/39Ar ratio of the lunar samples collected by the Apollo mission. As a under graduation thesis research under the guidance of Dr. Hironobu Hyodo, Takeshima (2001) carried out argon isotope analysis of bulk chondrules (1.7–3.5 × 10−7 g) of the Allende meteorite and succeeded in detecting a considerable amount of cosmogenic 39Ar (1.3–3.0 × 10−12 ccSTP/g within 7%–20% error). This reveals the possibility of in situ 40Ar/39Ar dating of planetary surface samples such as lunar samples.
The age calculation formula is as follows.
t = ln(1 + J40Ar/39Ar)/λ
The J value is obtained from the following formula.
J = (exp(λts) − 1)/(40Ar/39Ar)s
(40Ar/39Ar)s in this equation is the 40Ar/39Ar ratio of standard samples (such as basalts collected during the Apollo mission) with K-Ar ages (ts).
Since there is no atmospheric contamination on the lunar surface, this application would be easier than that on the Earth’s surface. On the other hand, the surface rocks contain the 36Ar implanted by the solar wind, inducing underestimation of 40Ar/36Ar. To minimize this underestimation, the experimental preheating technique would be necessary. The only drawback is that this approach cannot be applied to samples at different depths from the surface and/or samples with different irradiation histories.
Fabrication of a K-Ar isochron dating device for in-situ dating of planetary surface rocks was carried out in a doctoral thesis research at the University of Tokyo (Cho et al., 2015, 2016). Takeshima's research on the Allende meteorite chondrules was continued in her master's thesis. These dissertation data are composed of the EMP textural observation (BSE images and X-ray maps of the thirty-seven objects), the mineral chemistry, and the analytical data of the in-situ argon isotope analyses of minerals and the laser step heating 40Ar/39Ar analyses of chondrules. These data have not been published for a long time, but was published recently (Takeshima et al., 2023).
Those interested in in-situ dating of planetary surface rocks should refer to the following references. Here, I will talk about historically the research from a perspective that cannot be described in Journal papers.
References
*Cho et al. (2015) High-precision potassium measurements using laser-induced breakdown spectroscopy under high vacuum conditions for in situ K–Ar dating of planetary surfaces. Spectrochim. Acta Part B, 106, 28–35.
*Cho et al. (2016) An in-situ K–Ar isochron dating method for planetary landers using a spot-by-spot laser-ablation technique. Planet. Space Sci., 128, 14–29.
*Takeshima et al. (2023) In Situ Argon Isotope Analyses of Chondrule-Forming Materials in the Allende Meteorite: A Preliminary Study for 40Ar/39Ar Dating Based on Cosmogenic 39Ar. Minerals, 13, 31. https://doi.org/10.3390/min13010031
The age calculation formula is as follows.
t = ln(1 + J40Ar/39Ar)/λ
The J value is obtained from the following formula.
J = (exp(λts) − 1)/(40Ar/39Ar)s
(40Ar/39Ar)s in this equation is the 40Ar/39Ar ratio of standard samples (such as basalts collected during the Apollo mission) with K-Ar ages (ts).
Since there is no atmospheric contamination on the lunar surface, this application would be easier than that on the Earth’s surface. On the other hand, the surface rocks contain the 36Ar implanted by the solar wind, inducing underestimation of 40Ar/36Ar. To minimize this underestimation, the experimental preheating technique would be necessary. The only drawback is that this approach cannot be applied to samples at different depths from the surface and/or samples with different irradiation histories.
Fabrication of a K-Ar isochron dating device for in-situ dating of planetary surface rocks was carried out in a doctoral thesis research at the University of Tokyo (Cho et al., 2015, 2016). Takeshima's research on the Allende meteorite chondrules was continued in her master's thesis. These dissertation data are composed of the EMP textural observation (BSE images and X-ray maps of the thirty-seven objects), the mineral chemistry, and the analytical data of the in-situ argon isotope analyses of minerals and the laser step heating 40Ar/39Ar analyses of chondrules. These data have not been published for a long time, but was published recently (Takeshima et al., 2023).
Those interested in in-situ dating of planetary surface rocks should refer to the following references. Here, I will talk about historically the research from a perspective that cannot be described in Journal papers.
References
*Cho et al. (2015) High-precision potassium measurements using laser-induced breakdown spectroscopy under high vacuum conditions for in situ K–Ar dating of planetary surfaces. Spectrochim. Acta Part B, 106, 28–35.
*Cho et al. (2016) An in-situ K–Ar isochron dating method for planetary landers using a spot-by-spot laser-ablation technique. Planet. Space Sci., 128, 14–29.
*Takeshima et al. (2023) In Situ Argon Isotope Analyses of Chondrule-Forming Materials in the Allende Meteorite: A Preliminary Study for 40Ar/39Ar Dating Based on Cosmogenic 39Ar. Minerals, 13, 31. https://doi.org/10.3390/min13010031