U-SERIES DISEQUILIBRIUM ON MAGMA GENERATION AND TRANSPORTATION BENEATH MID-OCEAN RIDGE AXES
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摘要: 综述了20多年来洋中脊新生玄武岩中铀系不平衡研究成果,分析了铀系不平衡的形成机制和主要影响因素,并探讨了铀系不平衡研究中的一些问题。结果表明:(1)玄武质岩浆中。230Th/238U不平衡可能产生于石榴子石稳定的深部源区,也可能来自230Th的"内部增长"(in-growth),过剩230Th更可能形成于含铝单斜辉石;(2)226Ra/230Th不平衡可能来自岩浆的形成过程,也可能来自岩浆迁移过程;(3)铀系不平衡与岩浆滞留时间和上升速率紧密相关,226Ra的大量过剩显示,这个不平衡事件形成于8ka以内,如果不考虑玄武质岩浆迁移过程的事件,则意味着岩浆迁移时间不超过8ka。提出当前亟待解决的问题:①准确模拟地幔部分熔融条件并测定铀系子体在熔体/地幔残留相间的分配系数;②提出更合理的结晶分异模型;③探索流体对226Ra/230Th不平衡的影响。Abstract: The study of U-series disequilibrium in Mid Ocean Ridge basalts (MORB) of last 20 years has been comprehensively reviewed and the main results have revealed that:(1)230Th-238U disequilibrium of the basalt may be generated in deep sources where garnet is stable and also from the in-growth of the 230Th. The genesis of 230Th excess can be formed more likely in the presence of the clinopyroxene which contains aluminum. (2)The formation of 226Ra-230Th disequilibrium is attributed to the magma formation and also to its transportation. (3)Magma residence and transportation time can be reflected by U-series disequilibrium. Magma transportation time can not exceed 8 000 years regardless of other events during the magma transportation, because large 226Ra excess must be generated within 8 000 years. Further work needs to be done to improve our understanding of the formation of U-series disequilibrium:(1)Modeling of the partial melting of the mantle and determining the partition coefficients of the U-series daughters between the mantle solid and melt with accuracy. (2)Establishing suitable model of the fractional crystallization. (3)Unraveling the fluid influences on the 226Ra-230Th disequilibrium.
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Keywords:
- MORB /
- U-series disequilibrium /
- magma generation /
- magma transportation
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