09:30 〜 09:45
[MTT44-02] カナダ ラブラドル サグレック岩体の原太古代超苦鉄質岩の希ガス同位体組成
キーワード：希ガス、超苦鉄質岩、太古代
The low abundance of noble gases makes it possible to detect small changes in their isotopic composition caused by radiogenic contribution from other elements. Furthermore, they are chemically inert, so these two unique features make noble gases an invaluable tracer when considering mantle evolution. However, since not much research has been conducted on ancient noble gases, the evolution of the mantle noble gas isotopic composition remains unknown. Also, the previous analyses and model calculations on ancient noble gases (e.g., Richard et al. 1996, Seta et al. 2001, Matsumoto et al. 2002) focus only on He isotopes, so the data on other noble gas isotopes are lacking.
In order to solve these issues, we have conducted noble gas isotope analyses on Eoarchean ultramafic rocks from Saglek Block, Labrador, Canada. The previous work on the Saglek Block suggests this area had been cut in by 3.9Ga Saglek dyke, and experienced several large metamorphic events, represented by the granulite to amphibolite facies event at 2.7-2.8 Ga. The ultramafic rocks in this area are thought to be parts of ophiolite (Komiya et al. 2015). The analyses we have conducted are 1. Single-step bulk analysis, 2. Stepwise bulk analysis, and 3. Stepwise olivine analysis. In all analyses mentioned above, the gases were all extracted by crushing the samples by strokes of a magnetic-driven metal piston, or by a hydraulic press. All noble gas abundances (He, Ne, Ar, Kr, and Xe) and isotope ratios of He, Ne, and Ar were analyzed.
Helium isotope ratio (3He/4He) was about 1/500-1/1000 of the estimated value for Eoarchean ages of 80-90 Ra (Seta et al. 2001) in all analyses indicating strong radiogenic contribution from U and Th. This is consistent with the fact that the samples used in these analyses contains much more U and Th (Ishikawa et al. 2017) than the previous work on Archean ultramafic rocks (Matsumoto et al. 2002). Neon isotope compositions (20Ne/22Ne, 21Ne/22Ne) were all close to the present atmospheric values, with small excess in 21Ne and 22Ne, which are produced from nuclear reaction of 18O and 19F with alpha-particles (4He) derived from U and Th. 38Ar/36Ar showed atmospheric values, but 40Ar/36Ar varied from 715 in single-step bulk analysis to 31700 in stepwise olivine analysis, showing significant radiogenic 40Ar contribution from 40K. 84Kr/36Ar -132Xe/36Ar ratios are accounted for by mixing of three components, seawater, altered oceanic crust, and air (Nishiyama et al. 2020), which is consistent with the previous study that suggests that the ultramafic rocks in Saglek Block are thought to be parts of ophiolite. All of the results show that our attempt to constrain noble gas isotope characteristics of Eoarchean mantle has been hampered so far by the significant radiogenic or nucleogenic modifications, and also by contamination by Eoarchean and/or modern air for Ne and Ar.
In order to solve these issues, we have conducted noble gas isotope analyses on Eoarchean ultramafic rocks from Saglek Block, Labrador, Canada. The previous work on the Saglek Block suggests this area had been cut in by 3.9Ga Saglek dyke, and experienced several large metamorphic events, represented by the granulite to amphibolite facies event at 2.7-2.8 Ga. The ultramafic rocks in this area are thought to be parts of ophiolite (Komiya et al. 2015). The analyses we have conducted are 1. Single-step bulk analysis, 2. Stepwise bulk analysis, and 3. Stepwise olivine analysis. In all analyses mentioned above, the gases were all extracted by crushing the samples by strokes of a magnetic-driven metal piston, or by a hydraulic press. All noble gas abundances (He, Ne, Ar, Kr, and Xe) and isotope ratios of He, Ne, and Ar were analyzed.
Helium isotope ratio (3He/4He) was about 1/500-1/1000 of the estimated value for Eoarchean ages of 80-90 Ra (Seta et al. 2001) in all analyses indicating strong radiogenic contribution from U and Th. This is consistent with the fact that the samples used in these analyses contains much more U and Th (Ishikawa et al. 2017) than the previous work on Archean ultramafic rocks (Matsumoto et al. 2002). Neon isotope compositions (20Ne/22Ne, 21Ne/22Ne) were all close to the present atmospheric values, with small excess in 21Ne and 22Ne, which are produced from nuclear reaction of 18O and 19F with alpha-particles (4He) derived from U and Th. 38Ar/36Ar showed atmospheric values, but 40Ar/36Ar varied from 715 in single-step bulk analysis to 31700 in stepwise olivine analysis, showing significant radiogenic 40Ar contribution from 40K. 84Kr/36Ar -132Xe/36Ar ratios are accounted for by mixing of three components, seawater, altered oceanic crust, and air (Nishiyama et al. 2020), which is consistent with the previous study that suggests that the ultramafic rocks in Saglek Block are thought to be parts of ophiolite. All of the results show that our attempt to constrain noble gas isotope characteristics of Eoarchean mantle has been hampered so far by the significant radiogenic or nucleogenic modifications, and also by contamination by Eoarchean and/or modern air for Ne and Ar.