U-SERIES DISEQUILIBRIUM ON MAGMA GENERATION AND TRANSPORTATION BENEATH MID-OCEAN RIDGE AXES

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.