Genetic source,migration and accumulation of helium under deep hydrothermal fluid activities:A case study of Ledong diapir area in Yinggehai Basin,South China Sea
Genetic source,migration and accumulation of helium under deep hydrothermal fluid activities:A case study of Ledong diapir area in Yinggehai Basin,South China Sea
基于地球化学参数及分析数据,应用热守恒方程、质量平衡定律及瑞利分馏模型等方法,通过对莺歌海盆地乐东底辟区的壳源氦气原位产率和外部通量、幔源初始氦浓度及热驱动机制等定量分析,探讨深部热流体活动背景下氦气的成因来源及运聚机制.研究结果表明:①乐东底辟区氦气的来源以壳源氦为主,幔源为辅.其中,研究区幔源 3He/4He 值为(0.002～2.190)×10-6,R/R.值为 0.01～1.52,测算幔源 He 贡献占比为 0.09％～19.84％,占比较小;而壳源贡献占比则高达80％以上.②壳源氦的原位4He产量仅为(4.10～4.25)×10-4cm3/g,外部4He通量则显著高值,为(5.84～9.06)×10-2cm3/g,表明壳源氦气以外部输入为主,推测与地层流体受大气补给以及深部岩石-水相互作用有关.③底辟区深部热流体活动显著影响地温场,3He初始质量体积与对应焓的比值(W)为(0.004～0.018)× 10-iicm3/J,来自深部地幔的热贡献(XM)为7.63％～36.18％,揭示底辟热流体对幔源3He迁移具有一定的热驱动作用.④研究区氦气的初次运移方式以平流为主,二次运移受控于水热脱气和气液分离过程;氦气从深部至浅部的运移过程中,CO2/3He值由1.34×109升至486×109,指示受到壳幔混合和脱气效应影响,存在CO2规模性析出和3He的明显逸散.在深部热流体影响下,氦气运移聚集机制包括:深部热驱动扩散、平流释放、垂向水热脱气,浅部横向迁移、远离断裂的圈闭聚集,分压平衡、完整密闭的盖层封存等方面.
Abstract
Based on the geochemical parameters and analytical data,the heat conservation equation,mass balance law,Rayleigh fractionation model and other methods were used to quantify the in-situ yield and external flux of crust-derived helium,and the initial He concentration and thermal driving mechanism of mantle-derived helium,in the Ledong Diapir area,the Yinggehai Basin,in order to understand the genetic source and migration & accumulation mechanisms of helium under deep hydrothermal fluid activities.The helium in the Ledong diapir area is primarily derived from the crust and a small amount from the mantle.For mantle-derived helium,the 3He/4He values are(0.002-2.190)×10-6,the R/Ra values are 0.01-1.52,and the contribution is estimated to be 0.09％-19.84％,suggesting a relatively small percentage.In contrast,the contribution of crust-derived helium is more than 80％.For crust-sourced helium,the in-situ 4He yield is only(4.10-4.25)×10-4 cm3/g,while the external 4He flux is significantly high,being(5.84-9.06)× 1 0-2 cm3/g,indicating that crust-sourced helium is dominated by external input,which is speculated to be related to atmospheric recharge of formation fluids and deep rock-water interaction.Deep hydrothermal fluid in the diapir area significantly affects the geothermal field.The ratio of the initial mass volume of 3He to the corresponding enthalpy(W)is(0.006-0.018)×10-11 cm3/J,and the thermal contribution from the deep mantle(XM)is between 7.63％and 36.18％,suggesting that the diapir thermal fluid plays a certain role in driving the migration of mantle-sourced 3He.The primary migration of helium in the study area is dominated by advection,and the secondary migration is controlled by hydrothermal degassing and gas-liquid separation.During the migration of helium from deep to shallow,the CO2/3He value increases from 1.34× 109 to 486× 109,indicating the large-scale precipitation of CO2 and apparent escape of 3He due to the effect of crust-mantle mixing and degassing.Under the influence of deep hydrothermal fluid,the migration and accumulation mechanisms of helium include:deep heat driven diffusion,advection release,vertical hydrothermal degassing;shallow lateral migration,accumulation in traps far from faults;partial pressure balance,complete cover sealing;and so on.
关键词
深部热流体/氦气/成因来源/运移聚集机制/乐东底辟区/莺歌海盆地
Key words
deep thermal fluid/helium/genetic source/migration and accumulation mechanism/Yinggehai Basin
万方数据