郯庐断裂带流体中氦的来源、空间分布及地质意义

Origin,spatial distribution,and geological implications of helium in fluids from the Tan-Lu fault zone

郯庐断裂带是中国东部规模最大、构造运动强烈以及地震频发的活动断裂带,其深部结构与现今活动状态引人注目。对郯庐断裂带郯城段和庐江段温泉与深井地下水逸出和溶解气体进行He同位素组成和气体组分分析结果显示,郯庐断裂带流体 ^(3)He/^(4)He测试值为大气参考值(记作1 Ra)的0.15~1.15倍。郯城段样品的 ^(3)He/^(4)He和4He/20Ne值分别为0.96~1.15 Ra和0.24~0.30,显示有大气和地下水氚衰变对He来源的贡献。庐江段样品的 ^(3)He/^(4)He空气校正值和4He/20Ne值分别为0.15~0.79 Ra和0.28~272,扣除大气贡献后He主要来源于地壳放射性元素衰变(90.4%~98.4%),局部受到地幔脱气作用的显著影响(幔源He占比最高为9.6%)。在空间上,庐江段幔源He占比呈现出随采样点远离断裂带而逐渐减小的趋势,反映了幔源流体运移过程中经历的稀释效应。幔源He空间分布与地震P波速度图像对比结果表明,庐江段深源流体可能受到地幔部分熔融作用的影响。全球典型走滑断裂带的流体 ^(3)He/^(4)He值和走滑速率之间呈正相关关系(R^(2)=0.8),表明断裂带活动性及其应力状态对幔源流体释放具有一定的控制作用。

The Tan–Lu fault zone(TLF),which is the largest active fault zone in eastern China,is characterized by strong tectonic activity and frequent seismic hazards.The status and deep structure of this fault zone have drawn considerable research attention.In this study,we examined the isotopic compositions of free and dissolved gases from hot springs and deep wells near the Tancheng and Lujiang segments of the TLF,with a focus on helium isotopes.Our results show that the fluid ^(3)He/^(4)He ratios are 0.15–1.15 times the atmospheric value(reported at 1 Ra).The ^(3)He/^(4)He and 4He/20Ne ratios of the Tancheng samples vary in ranges of 0.96–1.15 Ra and 0.24–0.30,respectively,indicating dominant contributions of atmospheric and tritiogenic 3He in groundwater.In contrast,the air-corrected ^(3)He/^(4)He and 4He/20Ne ratios of the Lujiang samples are 0.15–0.79 Ra and 0.28–272,respectively,indicating that He is mainly derived from the decay of radioactive elements in the crust(90.4%–98.4%),with localized mantle degassing contributions(up to 9.6%).For the Lujiang segment,the proportion of mantle He decreases with increasing distance between the sampling site and TLF,suggesting the dilution effects of mantle fluids during their transport in the crust.A comparison between the ^(3)He/^(4)He distribution and seismic P-wave velocity structure suggests that mantle degassing may be impacted by the upwelling and partial melting of the asthenospheric mantle.Moreover,a statistical analysis of ^(3)He/^(4)He ratios and slip rates of the representative continental strike-slip faults reveals that the mantle He proportions in fluids correlate positively with the slip rates(R^(2)=0.8).This indicates that the activity of the fault zone may be a controlling factor in the release of mantle-derived fluids.