Mantle-derived noble gases in natural gases from Songliao Basin, China

Abstract

Abundances and isotopic compositions of noble gases have been measured in six natural gas samples (CO 2 and CH 4-rich) from the Songliao Basin, Jilin Province, in northeastern China. The samples contain noble gases of mantle origin. The 3 He 4 He ratio reaches 5.0 times the atmospheric ratio. In a three-isotope plot of neon, the 20 Ne 22 Ne (up to 10.9) and 21 Ne 22 Ne (up to 0.051) ratios make a positive correlation array together with natural gases from other continental areas. Compared with a correlation band for MORB, the natural gases have a lower slope with more nucleogenic 21Ne. The natural gas samples contain radiogenic argon with 40 Ar 36 Ar ratios up to 7700. A positive correlation between 40 Ar 36 Ar and 20 Ne 22 Ne ratios indicates occurrence of mantle-derived Ar. Slight excess of 38Ar can be attributed to a nuclear reaction like 35C1 (α, p) 38 Ar. Apparent excesses of 1292'Xe, 132–136Xe are recognized in four samples. The excess of 129Xe (up to 3%) can be attributed to a decay of extinct 129I. Excess 132−136Xe is not large enough to determine if the origin of the excess is 238U or 244Pu. Anomaly in 129 Xe 130 Xe ratio is correlated with that of 136 Xe 130 Xe . The isotopic features of the natural gases with radiogenic 4He and nucleogenic 21Ne can be produced within the crust. Alternatively, they may reflect the geochemical features of the subcontinental mantle which has been enriched in U, Th. We can not distinguish the two possibilities. However, a natural gas from another basin in eastern China with a different reservoir age contains mantle derived neon which falls on the neon correlation line formed by the samples from the Songliao Basin. This consistency suggests that the isotopic features of the natural gases aren't necessarily ascribable to surface contamination of radiogenic and nucleogenic isotopes.