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.
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
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.
deep thermal fluidheliumgenetic sourcemigration and accumulation mechanismYinggehai Basin
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