ガーネットによるトリウム-ウラン分断:海洋ベースアルトの深い源と急速な上昇の証拠
まとめ
中洋ののベースルトと海洋島のベースルトは,トーリウム-230の過剰を示し,マントルの奥深く,特にガーネットの安定性フィールドで部分的な融解が始まることを示しています. この発見は,海洋の山脊の下のダイナミックな融解過程を必要とします.
科学分野:
- 地質化学 地質化学
- ペトロロジー・ペトロロジーとは
- イソトープ地球化学 イソトープ地球化学
背景:
- 中洋の山脊玄武岩 (MORBs) と海洋島の玄武岩 (OIBs) は,上層マントルの部分的な融解から生じる.
- これらの玄武岩は,その親体であるウラン-238 (238U) と比較して,トリウム-230 (230Th) の活動が特異的に過剰であることを示す.
研究 の 目的:
- 海洋ベースルトの観測された230Th-238U不均衡の原因となるマントルの融解プロセスを調査する.
- 地球上の上層マントルの初期部分融解の深さと条件を決定する.
主な方法:
- トリウム (Th) とウラン (U) の主要なマントルフェーズと融解の間の分割を実験的に測定した.
- マントルの融解過程の熱力学モデリング.
主要な成果:
- ガーネットは,ウランをソリウムに保持できる唯一の主要な上層マントル相として特定されており,これはクリノピロキセンの反対の分化である.
- このガーネットとウランの保持は,若い海洋ベースルトで観察された230Thの過剰を説明する.
- 計算によると,MORBにおける230Th-238Uの不均衡は,約80kmの深さから始まり,溶融の多孔度が≥0.2%であるダイナミックな部分溶融によるものである.
結論:
- MORBとOIBは,石榴石の安定性フィールド (~70km以下) 内で部分的に融解を開始します.
- 海洋の山脊の下の融解は広範囲に広がり,マグマは少なくとも0.9m/yrの速度で上昇しています.
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