ヘリウム同位体によるエピソード的なマントルの融解と地殻の成長の証拠
1Department of Earth Sciences, University of Durham, South Road, Durham DH1 3LE, UK. stephen.parman@durham.ac.uk
Nature
|April 20, 2007
まとめ
海の島の玄武岩のヘリウムイソトープは,地球の大陸地殻の形成がエピソード的な爆発で起こったことを明らかにします. この発見は,以前に研究された同位体よりも,マントルの枯渇のより明確な記録を提供します.
科学分野:
- 地質化学 地質化学
- イソトープ地球化学 イソトープ地球化学
- 惑星科学 惑星科学
背景:
- 地球の大陸地殻の形成のタイミングは議論されているが,モデルは初期の形成,パルス成長,または安定した増加を示唆している.
- 大陸の地殻は,主にマントルの部分的な融解によって形成され,同位体シグネチャを残すると予想されるプロセスです.
- ほとんどのマントルの同位体系 (Sr,Nd,Pbなど) は,海洋地殻の沈下とマントルの混合によって遮られ,地殻形成の歴史の再構築を妨げています.
研究 の 目的:
- ヘリウムイソトープが,マントルの枯渇と大陸の地殻形成の歴史のより明確な記録を提供できるかどうかを調査する.
- 海洋島の玄武岩におけるヘリウムイソトープの比率を,提案された大陸の成長パルスと相関させるため.
- マントルの枯渇と地殻の成長に関する新しいモデルをテストする.
主な方法:
- 海洋島の玄武岩における4He/3He比のスペクトルの分析.
- ヘリウムイソトープのデータと,大陸の成長パルスの提案された年齢との相関.
- ヘリウムイソトープを,その非リサイクル性により,マントルの枯渇のトレーサーとして利用する.
主要な成果:
- 海洋島の玄武岩における4He/3He比のスペクトルは,提案された大陸の成長パルスのタイミングと密接に相関しています.
- この相関は,海中の玄武岩における支配的な4He/3Heピークをうまく予測している.
- このモデルは,地球の初期4He/3He比率の推定値も提供しています.
結論:
- 大陸の地殻形成はエピソード的であり,重要な,潜在的にグローバルな溶融イベントによって特徴づけられました.
- ヘリウムイソトープは,マントルの枯渇の歴史の信頼できるトレーサーとして機能し,他のイソトープ系よりも明確な記録を保持します.
- 提案されたヘリウム同位体進化モデルは,マントルの全体的なコンベクションをサポートし,原始的でガスのないマントルの貯蔵庫を必要としません.
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