Origins of gas discharging from the Qiangtang Basin in the northern Qinghai-Tibet Plateau, China: Evidence from gas compositions, helium, and carbon isotopes

摘要

Gas compositions, helium isotopic ratios, and carbon isotopic compositions of CO2 and CH4 from six gas emission sites in the Qiangtang Basin and its adjacent areas were measured in order to unravel their origins. Gas samples from the Beiluhe, the Bucha Lake, and the eastern Tuotuo River are N2-rich with N2 over 75 voL%, while the contents of CO2 and CH4 are only 3.45-20.91 vol.% and 0-3.58 vol.%, respectively. Relatively high CO2/~3He (-9.95 × 10~(11)) and CR/He (5.9 × 10~9-1.65 × 10~(10)), and low δ~(13)C values of CO2 (—15.4‰) and CH, (— 29.7‰) suggest that both CO2 and CH4 from the Bucha Lake are biotic. The S-M-L (S, M, and L represent sedimentary organic carbon, mantle, and limestone, respectively.) three-component mixing model suggests that no mantle-derived CO2 exists in these samples, which is consistent with the result that He in these gas samples is all crust-derived. CO2 in gas samples from the Beiluhe and the eastern Tuotuo River are relatively enriched in ~(13)C of CO2 (δ~(13)C: - 8.4‰ and — 5.5‰) and low CO2/~3He (23 × 10~9 and 2.4 × 10~9) as well as CH4/~3He (5.9 × 10~7 and 3.4 × 107) ratios.