日本地球惑星科学連合2018年大会

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[JJ] ポスター発表

セッション記号 B (地球生命科学) » B-CG 地球生命科学複合領域・一般

[B-CG09] 地球史解読:冥王代から現代まで

2018年5月22日(火) 10:45 〜 12:15 ポスター会場 (幕張メッセ国際展示場 7ホール)

コンビーナ:小宮 剛(東京大学大学院総合文化研究科広域科学専攻)、加藤 泰浩(東京大学大学院工学系研究科システム創成学専攻)、鈴木 勝彦(国立研究開発法人海洋研究開発機構・海底資源研究開発センター)

[BCG09-P04] The evaluation for the age bias with the grain size of detrital zircons from the Jack Hills

*田中 大貴1山本 伸次1小宮 剛2青木 翔吾3坂田 周平4大野 剛4 (1.横浜国立大学環境情報学府、2.東京大学大学院総合分化研究科広域科学専攻、3.岡山理科大学生物地球学部、4.学習院大学理学部化学科)

キーワード:冥王代、ジルコン、アパタイト、U-Pb年代

Terrestrial rocks recording the age during Hadean era are rarely preserved because of surface erosion, subduction, and intensive meteorite bombardment. The only clue to reveal the early earth environment is detrital zircon in metamorphosed sediments at the Mt Narryer and the Jack Hills in the Narryer Gneiss Terrane, the Yilgarn Craton, Western Australia [1], where grains as old as ~4,404Myr have been found so far [2]. It is known that Hadean zircons from Jack Hills area include a variety of minerals mainly such as quartz, K-feldspar, plagioclase, aluminosilicate, muscovite, biotite, hornblende, apatite, monazite, xenotime, Fe-Ti oxide, rutile, and biogenic carbon [3]. Our group especially focus on the chemical features of apatite. It is possible to indicate some information from the mineral such as the source magma reflected by the compositions of whole rocks [4] and the hydrogen isotope composition measured from the hydroxy group in apatite [5]. However, the frequency of the old zircons over 4.0 Ga is no more than 5% in all from the Jack Hills [6]. Moreover, the zircons including isolated apatite from clacks exist as rare as about 8% in the >4.0Ga zircons [3]. 5μm is necessary as the minimum to make an accurate analysis using secondary ion mass spectrometry (SIMS), so it is likely to find the inclusions in coarse grains. Hence, we evaluated the bias toward zircon ages of the samples concentrated from a metaconglomerate rock in Jack Hills and divided into three sets of grain size: >120μm, 120μm-75μm, <75μm. In the utilizing the technique of laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), the rates of zircons, which were found to be older than 3.8 Ga in 207Pb/206Pb age, were accordingly rated at 16.7%, 4.76%, 5.88%. We interpreted two presumable reasons from the relationship between α-decay damage calculated from the U-Th concentration and the concordance in zircon. First is the difference of the host rock in which each zircon group crystalized. Second is the artificial element when sample rocks are crushed on the method in a laboratory. As a result, it was suggested that large grain is suitable for collecting Hadean zircons including apatite with adequate size to analyze.

References:
[1] Spaggiari, C.V., Pidgeon, R.T., Wilde, S.A. The Jack Hills greenstone belt, Western Australia Part2: Lithological relationships and implications for the deposition of >4.0Ga detrital zircons. Precambrian Research 155 (2007) 261-286. [2] Wilde, S.A., Valley, J.W., Peck, W.H., Graham, C.M. Evidence from detrital zircons for the existence of continental crust and oceans on the Earth 4.4Gyr ago. Nature Vol.409 (2001) 175-178. [3] Bell, E.A., Boehnke, P., Hopkins-Wielicki, M.D., Harrison, T.M. Distinguishing primary and secondary inclusion assemblages in Jack Hills zircons. Lithos 234-235 (2015) 15-16. [4] Jennings, E.S., Marschall, H.R., Hawkesworth, C.J., Storey, C.D. Characterization of magma from inclusions in zircon: Apatite and biotite work well, feldspar less so. Geology (2011) V.39, No.9, 863-866. [5] Greenwood, J.P., Itoh, S., Sakamoto, N., Warren, P., Taylor, L., Yurimoto, H. Hydrogen isotope ratios in lunar rocks indicate delivery of cometary water to the Moon. Nature Geoscience (2011) Vol.4 79-82. [6] Holden, P., Lanc, P., Ireland, T.R., Harrison, T.M., Foster J.J., Bruce, Z. Mass-spectrometric mining of Hadean zircons by automated SHRIMP multi-collector and single-collector U/Pb zircon age dating: The first 100,000 grains. International Journal of Mass Spectrometry 286 (2009) 53-63.