Mantle-derived helium in foreland basins in Xinjiang, Northwest China

- We have measured 3He/4He of natural gases in 5 sedimentary basins in Xinjiang, NW China.
- As much as approximately 7% of the helium in the Xinjiang basins is mantle-derived.
- Mantle-derived helium in foreland sub-basins reflects the injection of mantle fluids beneath the orogenic mountains through thrust faults.
- Purely radiogenic helium occurs extensively in the center of individual sedimentary basins.
- The possible mantle-derived CO2 in Tarim basin is proposed.

Hydrocarbon-rich natural gases from the Tarim, Junggar, Turpan-Hami and Santanghu basins in Xinjiang, Northwest China have measured 3He/4He ratios from 0.01 to 0.6 times higher than the atmospheric value, indicating 0-7% helium derived from the mantle. The mantle-derived helium is high in foreland basins associated with the Tianshan, Kunlun and Zhayier-Halalate orogenic mountains, but low towards the center of basins. This spatial distribution suggests that the mantle-derived helium originates either from fluids or small scale melts in the upper asthenospheric or lithospheric mantle which have found pathways into the root zones of the major faults defining these mountains, but do not significantly move into the basins themselves. During upward transport to near the surface, the mantle-derived helium is significantly diluted by radiogenic helium produced in the crust. Despite the lack of recent magmatic activity or extensional tectonics within the basins, this pattern shows strong evidence that the major faults play an important role in mantle-derived components degassing from the mantle to the surface.