板块俯冲中水分运移过程及其地球物理含义

流体/水分在地球演化过程中具有重要作用,它的存在可以提高地质体的扩散及蠕变,降低岩石/地幔的固液相限,对地质体的地球物理性质、地质灾害诱发、地质构造演化等都有着重要的作用.在板块俯冲区,流体/水分的影响贯穿了整个动力学过程,但板块深部脱水还存在着争议,且目前系统地研究水分在整个俯冲中的迁移过程及其地球物理意义的工作还较少.本次研究以我国东北地区为例,借助于地球动力学数值模拟,利用二维岩石化学-热-力学耦合的数值计算程序,建立西太平洋板块向欧亚大陆岩石圈俯冲的数值模拟模型,模拟了板块俯冲整个连续动力学过程中流体/水分的迁移过程.模拟与分析结果表明,板块俯冲过程中水分的迁移过程可分为三个阶段:洋壳水化、板块浅(中)部脱水、板块深部脱水.进一步揭示了洋壳水化过程中水分渗流通道形成、水分渗流以及板块及地幔物质水化反应的机制;解释了板块浅(中)部脱水过程对火山岛弧、弧后盆地及低速异常带形成的作用,以及对~410 km不连续带的影响;模型演化过程中观察到的板块深部脱水现象说明了存在板块深部脱水的可能性,而且板块深部脱水可以较好地解释部分内陆火山以及部分地区地幔柱(岩浆羽)和深源地震的成因.研究展示了板块俯冲过程中水分运移的地球物理意义.

Process and geophysical implications of water migration in the plate subduction zone

Water(as hydrogen)plays an important role in geodynamics of our planet,not only because it enhances the diffusion and creep,but also because it lowers the melting temperature of the rock/mantle(i.e.,‘flux melting’).Furthermore,fluids significantly affect the geophysical properties of rock mass or the evolution of geological structures,even trigger geological disasters.Water-induced influence runs through the whole dynamic process of the plate subduction.However,the deep slab dehydration(at depth>300 km)is controversial,and the dynamic process of water migration in the subduction zone and its geophysical implications are still poorly understood.Two-dimensional chemical-thermal-mechanical coupling models,which used northeastern China as example,were implemented to simulate the slab subduction process,with particular attention paid to the whole continuous dynamics process of the fluid/water migration.The simulation and analysis results show that the process of water migration during plate subduction can be divided into three stages:ocean crust hydration,shallow(middle)slab dehydration,and deep slab dehydration.Furthermore,the mechanism of water seepage channel formation,water seepage and hydration reaction of plate and mantle material was revealed in the hydration process of ocean crust;the effects of shallow(middle)slab dehydration process on the formation of island arc,back-arc basin,low-velocity zone and the~410 km discontinuity were explained;the phenomenon of deep slab dehydration was observed in the numerical simulation,which can explain the causes of some inland volcanoes as well as mantle plumes(magma plumes)and deep-source earthquakes in some areas.This study demonstrates the geophysical implications of water migration during plate subduction.