非均衡脱ガス化であり,海洋島火山の火山活動におけるヘリウムの原始的な源である
Helge M Gonnermann1, Sujoy Mukhopadhyay
1Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts 02138, USA.
Nature
|October 26, 2007
まとめ
海洋島ベサルト (OIBs) の原始ヘリウム (3He) は放射性崩壊によるものではない. この研究は,マグマの脱ガス化モデルを使用して,OIBsと中海のリッジベザルト (MORBs) のヘリウム濃度パラドックスを説明します.
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
- マントルの地力学 マントルの地力学
背景:
- ウランとトーリウムの放射性崩壊はヘリウム-4 (4He) を生成し,地球マントルのヘリウム-3 (3He) は原始的で,蓄積過程で組み込まれている.
- オーシャン・アイランド・ベサルト (OIBs) は,伝統的にガスのないマントルの羽根に起因する,中洋のリッジ・ベサルト (MORBs) よりも高い3He/4He比を示している.
- MORBよりもOIBのヘリウム濃度とHe/Ne,He/Ar比が低いところには"ヘリウム濃度パラドックス"と呼ばれる不一致がある.
研究 の 目的:
- OIBとMORBの貴気ガス濃度に関する"ヘリウム濃度パラドックス"を解決する.
- MORBおよびOIBガラスにおける貴気ガス濃度の観測範囲について,自己一貫した説明を提供するため.
- マントルの羽根とその脱ガス過程の源と動態を再評価する.
主な方法:
- MORBおよびOIBガラスにおける貴気ガス濃度 (He, Ne, Ar) の分析.
- 噴出するマグマの不均衡のオープンシステムの脱ガスモデリング.
- OIBとMORBのマグマの異なるCO2含有量を持つ脱ガスモデルの比較.
主要な成果:
- "ヘリウム濃度パラドックス"とMORBとOIBのガラスにおける貴金属ガスの変動は,不均衡のオープンシステム脱ガスによって説明される.
- OIBマグマのCO2含有量の高さは,MORBマグマと比較して,より広範なヘリウム脱ガスにつながります.
- OIBの溶岩に含まれる高貴なガスは,大抵ガスを排出していない原始マントルの源泉から発生する可能性があります.
結論:
- 特にCO2含有量に影響されるマグマの脱ガス処理は,玄武岩の貴気体シグネチャーを制御する上で極めて重要です.
- OIBの従来のモデルは,ガスのないマントルの羽根のみを表すもので,精錬が必要である.
- この研究は,OIBとMORBのデータを脱ガスダイナミクスを通して調和させ,貴重ガス変数の統一された説明を提供します.
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