大酸化イベントの頂点における上空酸化ウランの動員
Ann M Bauer1, Weiqiang Li1,2,3, Kyle S Rybacki4,5
1Department of Geoscience, University of Wisconsin-Madison, Madison, WI 53706.
まとめ
初期の地球
科学分野:
- 地化学
- イソトープ・ジオクロノロジー
- 地球 の 初期 史
背景:
- レドックス感受性元素は,地球の表面のレドックス状態の進化を理解するための鍵です.
- パレオプロテロゾイク期の大酸化現象は 大気中の酸素の有意な増加を示した.
- プレカンブリア時代の岩石における多段階の地熱の歴史は,地化学的な解釈を複雑にする.
研究 の 目的:
- ~2.06 Ga Kuetsjärvi火山群におけるウラン (U) 酸化の年齢と範囲を制限するために.
- U-Th-Pbイソトープを用いて,地表の酸化と地球深層の過程の関連性を調査する.
- 初期のU酸化と後のUとPb変異を区別する.
主な方法:
- U-Th-PbイソトープのジオクロノロジーはKuetsjärvi火山群に適用された.
- Pbイソトープの分析により,Uとソリウム (Th) の相対的移動性を比較する.
- 複数の地質年代測定システム (207Pb-206Pb,238U-206Pb,235U-207Pb,232Th-208Pb) を利用する.
主要な成果:
- 完全またはほぼ完全な酸化とUの除去は,約2.06億年前の噴火直後に発生した.
- このUの減少は 大気中の酸素濃度が高いことを示し 酸化気象と地球規模のUパルスに 海を駆り立てています
- マントルの貯蔵庫の観測された変化は,232Th/238Uの減少とHIMU源の発生を含む,このUパルスに関連しています.
結論:
- 紀元前2.06年頃のUの酸化損失は,後のUとPbの変異事件から地理年代学的に区別することができる.
- この研究は,多段階の地質史を持つ岩石における酸化還元素の複雑な振る舞いを強調しています.
- 元素の豊富さは,このような複雑な岩石の記録における酸化還元条件の不十分なプロキシです.
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