利用地下流体氦同位素比值估算大陆壳幔热流比例

地下流体中的氦同位素 3He来自地幔的排气作用 ,4He则是铀、钍衰变的产物 .由于铀、钍元素在大陆地壳中富集 ,4He通量与地壳热流呈正相关关系 ;同时 3He通量与地幔热流之间呈正相关 .所以地下流体的氦同位素比值 （3He / 4 He）与大陆壳幔热流比值 （qc/qm）呈反相关关系 .根据欧亚大陆和加拿大地盾的地下流体氦同位素比值数据和相应的壳幔热流比值数据 ,统计出 qc/ qm 与 3He / 4 He之间的回归关系 :qc/ qm =0 81 5- 0 30 0ln（3He / 4 He） ;此处 3He/ 4 He的单位是RA（大气的 3He/ 4 He比值 ） .有了地表热流值和壳幔热流比值即可得到地壳热流和地幔热流 .利用该公式以及热流值估算了中国主要盆地的壳幔热流值 ;根据这些数值得出的热岩石圈厚度和地壳平均生热率结果与地震学研究成果一致 .氦同位素比值是区分大陆热流中地壳热流值和地幔热流值的有用参数 .

ESTIMATIONS OF THE RATIO OF CRUST/MANTLE HEAT FLOW USING HELIUM ISOTOPE RATIO OF UNDERGROUND FLUID

The helium isotope 3He in underground fluid comes from the mantle by degassing process, and is associated with mantle heat flux. While 4He is the decay product of uranium and thorium, mainly concentrated in the continental crust, and is approximately proportional to the heat flux of crust. Consequently, the ratio of crust/mantle heat flow （ q c?/ q m） in continents is inversely related to the 3He?/ 4He ratio in underground fluid. Based on data sets of 3He?/ 4He ratio and q c?/ q m in Eurasia and Canadian Shield, we obtain the regression relation: q c?/ q m=0.815-0.300 ln（ 3He?/ 4He）,in which the unit of 3He?/ 4He is R A （atmospheric 3He?/ 4He ratio）. Furthermore, the crust and mantle heat flow components can be estimated from surface heat flow and q c?/ q m ratio. On the basis of this formula and heat flow data in major sedimentary basins of China, the crustal and mantle heat flow values were estimated The inferred results on the thickness of the thermal lithosphere and the average crustal heat production rate are consistent with the results by seismology. Helium isotope ratio （ 3He?/ 4He） is a useful parameter for separating crust and mantle components from continental heat flow.