アークマグマのU-Thイソトープ: 吸収された地殻からの元素移転への影響
Hawkesworth1, Turner, McDermott
1C. J. Hawkesworth, S. P. Turner, D. W. Peate, and P. van Calsteren are with the Department of Earth Sciences, The Open University, Milton Keynes, MK7 6AA, UK. F. McDermott is with the Department of Geology, University College, Dublin, Eire.
まとめ
ウラン・ソリウム・イソトープは,流体と沈殿物の流出速度を,地質プレートからの流出速度を示している. 島弧の火山岩は,流体がウランを動員していることを示しており,堆積物は弧岩のトリウムに大きく貢献しています.
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
- テクトニクス (地質学) とは
背景:
- サブドクションゾーンは,地殻のリサイクルに不可欠です.
- 潜水板からマントルのへの物質の移転を理解することは,弧マグマティズムの解読の鍵です.
- ウラン (U) とトリウム (Th) の同位体は,地質学的過程のための強力なトレーサーを提供します.
研究 の 目的:
- 島弧の火山岩を用いて,下降板からマントルのへの流体と沈殿物の移動速度を調査する.
- 弧岩の組成に潜水した沈殿物の貢献を定量化するために.
- 流体と沈殿物の溶融輸送の時間スケールを決定するために.
主な方法:
- 島の火山岩におけるウラン-Th-Nd同位体の体系分析.
- 流体変化中のウランとトリウムの差分移動性を利用する.
- 源を区別するために,同位体共変数 (例えば,Th/Ce vs. 143Nd/144Nd) を解釈する.
主要な成果:
- ウランは液体の中で容易に移動し,トリウムは移動性が低い.
- Th/Ceと143Nd/144Ndの間の負の相関は,下降板からの重要な堆積物投入を示しています.
- 流体の推定移動時間は3万〜120万年,沈殿物の溶解時間は数百万年である.
- 沈殿した堆積物の約30%のトリウムのみが地殻に戻されます.
結論:
- 島弧火山学は,潜水中の流体と沈殿物の循環の複雑な相互作用を記録しています.
- 異なった同位体シグネチャーは,流体と沈殿物の溶融の異なる輸送機構を強調しています.
- 古代の地殻の組成は,異なる地殻生成プロセスを反映し,平均のU/Th比に影響を与えています.
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