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

Abstract

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 75vol.%, while the contents of CO2 and CH4 are only 3.45–20.91vol.% and 0–3.58vol.%, respectively. Relatively high CO2/3He (~9.95×1011) and CH4/3He (5.9×109–1.65×1010), and low δ13C values of CO2 (~−15.4‰) and CH4 (~−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 13C of CO2 (δ13C: −8.4‰ and −5.5‰) and low CO2/3He (2.3×109 and 2.4×109) as well as CH4/3He (5.9×107 and 3.4×107) ratios. These patterns suggest that both CO2 and CH4 are abiotic, demonstrated by the S–M–L three-component mixing model displaying that more than 50% CO2 in these two sites are mantle-derived. CO2-rich gases are discovered in the southern Erdaogou Depot, the branch of Tuotuo River, and the Tanggula Mountain Depot with CO2 exceeding 96vol.%. The relatively high δ13C of CO2 (−4.2 to −7.7‰), low CO2/3He ratios (7.9×108–9.3×109), as well as the S–M–L three-component mixing model reveal that CO2 at least partially originates from the mantle source. Only one gas sample from the Tanggula Mountain Depot contains a trace amount of abiotic CH4 (CH4/3He=1.2×107) generated by abiotic reduction of mantle-derived CO2. However, the source of He is dominantly crust-derived with only 2.15–5.66% mantle-derived He. The emission of large quantity of mantle-derived CO2 is likely due to the presence of the Hoh Xil–Jinsha River suture and the Cenozoic volcanism in northern Tibet. Nevertheless, the small fraction of mantle-derived He may result from the limited degree of openness at the bottom of fault zones and possibly reflect a geological setting of strong extrusion and crustal thickening.