全球幔源岩Pb-Sr-Nd同位素体系

根据各种同位素数据库得到的3万多个晚古生代以来的幔源岩(包括洋中脊玄武岩、洋岛玄武岩、岛弧火山岩、大陆与大洋溢流玄武岩以及大陆板内玄武岩)Pb-Sr-Nd同位素资料和图解分析,对各类火山岩的源区以及地幔的垂向与横向不均一性问题作了进一步讨论。笔者认为不存在具有公共性质的EM1、EM2和HIMU地幔端员,它们的源区可能来自上、下地幔过渡带,只在局部地区出现,独一无二。PREMA(FOZO)则是洋岛玄武岩和溢流玄武岩公共端员。DUAPAL异常现象不只是在洋中脊玄武岩中出现,在洋岛玄武岩、岛弧火山岩和大洋溢流玄武岩中也存在同步的地球化学分区现象。溢流玄武岩的同位素体系特征表明它们的源区涉及再循环地幔的壳幔混合、岩石圈减压熔融、上—下地幔过渡带和似原始-略亏损的下地幔。Pb同位素体系为鉴别俯冲带的存在提供了更严格的证据,这种鉴别表明,安第斯弧火山作用不是洋陆俯冲带产生的。

Pb-Sr-Nd isotopic systematics in mantle derived rocks

Based on more than 30000 Pb-Sr-Nd data of mantle derived rocks (including mid-ocean ridge, ocean island, intra-plate and island arc basalts, and flood basalts from continents and oceans) a discussion on magma sources of various volcanic rocks and vertical and lateral heterogeneities of mantle is given. It is thought that there are no common mantle end members of EM1,EM2 and HIMU, which only occur individually at very local areas, and their source region is probably related to a transition zone between the upper and lower mantle. PREMA(FOZO) is a popular end member of mantle for all ocean island and flood basalts, probably representing the lower mantle. DUPAL anomaly not only exists in mid-ocean basalts, but also in ocean island, island arc and oceanic flood basalts.Isotopic systematics of flood basalts indicate that their sources are involved in crust-mantle mixing in a recycled mantle, decompression melting of lithosphere. Lead isotopic systematics provide evidence for establishing the presence of subduction zones. For example, Pb isotopic evidence from volcanic rocks from the Andean magmatic arc indicates that there is no subducted plate from Pacific Ocean beneath the Andean margin.