全球洋岛玄武岩氦同位素组成的地幔动力学意义  

The Mantle Dynamic Significance of the Helium Isotope Component of Oceanic Island Basalt

作　　者：汪洋[1] 

机构地区：[1]中国地质大学地球科学与资源学院,北京100083

出　　处：《矿物岩石地球化学通报》2001年第4期225-227,共3页Bulletin of Mineralogy, Petrology and Geochemistry

基　　金：国家自然科学基金项目 ( 4 9733110 ;49772 15 5 ) ;中国博士后科学基金 ;国土资源部国际合作与科技司科技发展计划 ( 2 0 0 0 10 10 2 0 6;2 0 0 0 10 10 2 0 2 );中国地质调查局 (K1 4 1 2 )项目联合资助

摘　　要：洋岛玄武岩 (OIB)氦同位素组成 (3 He 4 He)在地理分布上具有非均匀性特征。3 He 4 He值为 5～ 6Ra 的OIB主要分布在南半球 ,而分布于冰岛和夏威夷 (包括Loihi)等地的OIB3 He 4 He值为 10～ 35Ra。低3 He 4 He值OIB具有富集大离子亲石元素U、Th的源区 ,由于U、Th衰变释放的4He同位素的积累导致其3 He 4 He值降低。该源区的形成是俯冲作用导致深海沉积物与地幔混合 ,其地理分布受Pangea大陆周边的古俯冲带制约。高3 He 4 He值OIB的源区则是亏损U、Th的地幔胞 (mantleblob) ,该地幔胞是由极度亏损U、Th的再循环洋壳或大陆下地壳与未排气的地幔胞混合形成的。经过 1～ 2Ga的演化形成即相对亏损4He同位素而3 He 4 He值高的源区。同时 ,这种地幔胞富集难融组分 ,所以较洋中脊玄武岩 (MORB)的源区更为稳定 ,即高3 He 4 He值源区的部分熔融需要更高的地幔温度。超级地幔柱上升可以导致地幔升温和高3 He 4 He值源区的熔融。3 He 4 He值OIB地理分布的非均匀性反映出全球地幔对流系统复杂的半球非对称性格局。The helium isotope component ( 3He/ 4He) of oceanic island basalt (OIB) exhibits geographic diversity. Helium isotope ratio is 5-6 Ra (atmospheric 3He/ 4He ratio) in some OIB, which are mainly located in Southern Hemisphere (such as St Helena, Gough and Tristan, French Polynesia and others), and 10-35 Ra in others (i.e., Iceland, Loihi, Hawaii, etc.). The low 3He/ 4He OIB (5-6 R A) has a lithosphile element rich reservoir, in which enriched U, Th product more radiogenic 4He. The enriched reservoir of these OIB results from contamination by pelagic sedimentation; and its spatial distribution is controlled by ancient subduction slabs under Pangea. However, the reservoir of high 3He/ 4He OIB is related to the mantle blob that is mixed by U and Th depleted recycle oceanic crust or lower continental crust and the undegassed mantle blob. After 1-2 Ga evolution, the reservoir will have much less radiogenic 4He and higher 3He/ 4He ratio. It should be noted that the mixing blob is relative refractory, and this means that the reservoir of high 3He/ 4He OIB is more stable than MORB reservoir. The melting of high 3He/ 4He reservoir is related to the increase of mantle temperature, which can result from super-plume upwelling. The diverse geographic distribution of high and low 3He/ 4He OIB infers the complex hemispheric asymmetry patterns of global mantle convection.

关 键 词：氦同位素组成 洋岛玄武岩 地幔对流 半球非对称性 

分 类 号：P588.1[天文地球—岩石学]