[B-CG09] 地球史解読：冥王代から現代まで

[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:
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