华南东部地幔过渡带顶部低速层中的熔体含量估算  

Estimation of the melt fraction within the low velocity layer atop the mantle transition zone beneath the eastern South China

作　　者：周晓亚[1,2] 马麦宁[1,2] 徐志双[1,2] 韩林[1,2] 魏东平[1,2] 

机构地区：[1]中国科学院计算地球动力学重点实验室,北京100049 [2]中国科学院大学地球科学学院,北京100049

出　　处：《地球物理学报》2015年第9期3264-3271,共8页Chinese Journal of Geophysics

基　　金：国家自然科学基金(41274091;40774047);中国科学院知识创新方向性项目(KZCX2-EW-QN602);中国科学院与国家外国专家局创新团队国际合作伙伴计划项目(KZZD-EW-TZ-19)联合资助

摘　　要：地幔过渡带顶部低速层的成因及性质研究,对于认识地球内部物质运移及地幔对流过程等具有非常重要的动力学意义.最新的地震学研究显示,华南陆块东部地幔过渡带顶部的低速层存在着明显的区域性差异.一般认为,该低速层的形成与脱水引起的部分熔融有关.本文利用部分熔融体系的平衡几何模型,重点分析了熔体成分、位温、二面角和玄武质含量等因素对熔体含量的影响,并结合该低速层的分布特征,估算出研究区的南北两个子区域地幔过渡顶部熔体含量分别为～1.18vol.%和～2.02vol.%.这一熔体含量的显著差异可能与太平洋板片多期次俯冲作用的叠加有关。Genesis and property of the low velocity layer（LVL）atop the mantle transition zone（MTZ）are of important implications for interior geodynamic process,such as patterns of mantle convection and material migration.Recent seismology observed an LVL beneath the eastern South China,one of typical areas associated with intensive long-term subduction.There is an obvious lateral variation in seismic behaviors of the LVL across two subdomains（A and B areas）.The formation of the LVL is generally attributed to dehydration partial melting,therefore,a precise constraint of the melt fraction of the LVL is beneficial in understanding its intrinsic properties and corresponding geodynamic process.The equilibrium geometry model in a partial melting system is employed to estimate the effect of melt composition,potential temperature,dihedral angle and basalt fraction on the melt fraction.Five sets of typical melt compositions including MORB,dry peridotite（IT8720）,hydrous peridotite（IT8720＋2wt.%H2O;IT8720＋8wt.%H2O）and carbonated peridotite are introduced.The range of potential temperature is 1300～1800K（A area）and 1400～1800K（B area）.Dihedral angle varies from 5°to 30°and basalt fraction varies from 0to 40 vol.%.Under a given condition,the melt fraction will monotonically decrease with the potential temperature while its variation is relatively insensitive to melt composition,particularly the almost undistinguished discrepancy between the IT8720＋2wt.%H2O and carbonated peridotite.Additionally,our results show that the melt fraction will increase moderately with the basalt fraction and this tendency may represent relevant change in phase proportions.After evaluating the detailed mantle characteristics of the eastern South China,the creditable conditions of forementioned factors may be IT8720＋8wt.%H2O melt,a reference potential temperature of 1600 K,a dihedral angle of 10°and a basalt fraction of 20 vol.%.Therefore,the corresponding regional average values of melt fraction are about 2.02 vol.%（A

关 键 词：华南 地幔过渡带 低速层 部分熔融 熔体含量 

分 类 号：P315[天文地球—地震学]