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

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 N-2-rich with N-2 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/He-3 (similar to 9.95 x 10(11)) and CH4/He-3 (5.9 x 10(9)-1.65 x 10(10)), and low delta C-13 values of CO2 (similar to-15.4 parts per thousand) and CH4 (similar to-29.7 parts per thousand) 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 C-13 of CO2 (delta C-13: -8.4 parts per thousand and -5.5 parts per thousand) and low CO2/He-3 (2.3 x 10(9) and 2.4 x 10(9)) as well as CH4/He-3 (5.9 x 10(7) and 3.4 x 10(7)) 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 96 vol.%. The relatively high delta C-13 of CO2 (-4.2 to -7.7 parts per thousand), low CO2/He-3 ratios (7.9 x 10(8)-9.3 x 10(9)), 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/He-3 = 1.2 x 10(7)) 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. (C) 2014 Elsevier B.V. All rights reserved.