Fig. 3: Noble gas signatures support the link of methane with the mantle influence during the Emeishan igneous period.
a Diagram of 4He/20Ne vs. 3He/4He (R/Ra) ratios. The 3He/4He ratio R is normalized to the atmospheric 3He/4He ratio (Ra = 1.384 × 10−6)67. Measured 4He/20Ne ratios in all samples ranged from 16.5 to 9.96 × 106. They are much higher than the ratio in air-saturated water (ASW = 0.288)68 or air (0.318)67. Therefore, the atmospheric or ASW-derived gas has negligible contribution to the He concentrations. To estimate mantle He contributions, a simple two-component mixing model44 was used between an upper crustal endmember (0.008Ra)48 and a mantle endmember relative to the sub-continental lithospheric mantle (SCLM) (6.1 ± 0.9Ra)69. b Diagram of 40Ar*/4He vs. 3He/4He (R/Ra) ratios. 40Ar* represents the resolved non-atmosphere derived excess 40Ar. The extrapolated mixing line between the crust and mantle endmembers was defined by Stuart et al. using unfractionated cases measured in Dae Hwa (South Korea) W-Mo deposit fluid inclusions44,49. The mantle 40Ar*/4He = 0.69 ± 0.06 is typical of unfractionated samples from the mantle50. In contrast, the crustal 40Ar*/4He = 0.007 is far lower than the crustal production ratio of ~0.2, typically representing a fluid derived from shallow cool regions of the crust48, in which crustal fluids often mix with a He-rich endmember due to preferential addition of 4He to the gas phase44.
