ヘテロジェネスなハデアン・ハフニウム: 4.4~4.5 Gaの大陸地殻の証拠
T M Harrison1, J Blichert-Toft, W Müller
1Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia. mark.harrison@anu.edu.au
まとめ
初期の地球 初期の地球
科学分野:
- 地質化学 地質化学
- 初期の地球の条件.
- 大陸の地殻の進化について
背景:
- 現存する理論によると,大陸の地殻は40億年前から漸進的に発達した.
- 初期の地殻形成を理解することは,地球の地質の歴史の鍵です.
研究 の 目的:
- 初期の大陸の地殻形成のタイミングとメカニズムを調査する.
- 大陸地殻の漸進的な成長の仮説を検証する.
主な方法:
- 初期の176Hafnium/177Hafnium (Hf/Hf) の比率を古代デトリタルジルコンで分析した.
- 西オーストラリアのジャックヒルズのジルコンのHfイソトープ組成の測定 (4.014.37億年).
主要な成果:
- デトリタルジルコンは,Hf (地球の大体からの偏差) のエプシロン値が有意に正または負であることを示しています.
- 負のエプシロンHf値は,すでに45億年前に大陸の地殻貯水池 (Lu/Hf ~0.01) が形成されたことを示唆しています.
- 陽性なエプシロンHf値は,上層マントルの早期かつ広範な枯渇を示している.
結論:
- 大陸の地殻は44~45億年前に存在していた.
- この初期の地殻は,マントルに急速に再利用された可能性が高い.
- この発見は,大陸地殻の漸進的な発展というパラダイムに異議を唱えている.
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