CHIME dating of monazite, xenotime and zircon in graphitic schist, Isua Supracrustal Belt, Greenland and problem for the measurement

Biogenic graphite in > 3.7 Ga metasediments, Isua Supracrustal Belt (ISB), West Greenland, has been reported as the oldest remnants of life. However, ecosystem spreaded in the >3.7Ga ocean is still poorly understood. Isua biogenic graphite is located in chlorite-rich layers of metasediments as...

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Bibliographic Details
Main Authors: 大友, 陽子, 79184, 加藤, 丈典, 79185, Ohtomo, Yoko, 79186, Kato, Takenori, 79187
Language:Japanese
Published: 名古屋大学宇宙地球環境研究所 2018
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Online Access:https://nagoya.repo.nii.ac.jp/record/26305/files/061-064_2017.pdf
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Summary:Biogenic graphite in > 3.7 Ga metasediments, Isua Supracrustal Belt (ISB), West Greenland, has been reported as the oldest remnants of life. However, ecosystem spreaded in the >3.7Ga ocean is still poorly understood. Isua biogenic graphite is located in chlorite-rich layers of metasediments as grains accompanied by monazite, xenotime and zircon. These rare earth element (REE)-bearing minerals could be supplied to sediments at primary sedimentary setting or precipitated from metamorphic fluids. Chemical Th-U-total Pb Isochron Method (CHIME) dating was conducted on these REE-bearing minerals to constraint the generation process. CHIME age of zircon in graphite-rich metasediments indicates 3.9−3.8 Ga, which is consistent with previous zircon age in banded iron formation collected close to our sampling area. CHIME age of monazite and xenotime indicate two age peaks of ~2.7 Ga and ~3.7 Ga. Previous studies suggest that Isua retrograde metamorphism has occurred at ~2.7 Ga (Frei et al., 2001), which is consistent with one of age peaks of monazite and xenotime. Nano-scale observation of monazite by transmission electron microscope showed that well-ordered, sheeted graphite was crystalized around monazite, whereas quartz and disordered graphite were observed as inclusion of monazite. Considering with CHIME results and nano-structures of monazite together, monazite was considered to be formed during early diagenesis with involving indigenous biogenic organic matter and quartz and suffered by subsequent metamorphic events. The occurrence of monazite resembles REE-bearing minerals observed in Australian Archaean sedimentary rocks reported by Rasmussen (1996), which could be derived from altered biogenic organic matter during diagenesis.最古の生命の痕跡は約38億年前の岩石が分布するグリーンランド・イスア表成岩帯で発見されているが,当時どのような微生物圏が地球表層に広がっていたのか詳細は不明である.本研究では,生物由来グラファイトと共存するモナザイトやゼノタイム,ジルコンのCHIME年代測定から,これらの鉱物の成因に制約を与えることを目的とした.分析の結果,ジルコンの形成年代は39~38億年前を示したのに対して,ゼノタイム及びモナザイトは約37億年前と約27億年前の2箇所にピークを持つことがわかった.また,電子顕微鏡観察結果から,モナザイトが欠陥の多いグラファイトや石英を包有していることがわかった.以上のことから,モナザイトは生物由来の有機物や周辺の石英を取り込みながら結晶化した可能性が挙げられる.しかしながらCHIME測定の成功率は低く未だ十分なデータ量が得られていないため,測定成功率を上げるべく試料準備に更なる工夫が必要と考えられる. departmental bulletin paper