Origin of hydrogen-nitrogen gas seeps, Oman

Abstract

Six gas samples were collected from five thermal springs in the Semail Nappe ophiolite and the calcareous (calcite and dolomite) Hajar Formation, northern Oman. The 3He/ 4He, 4He/ 20Ne, 40Ar/ 36Ar and 38Ar/ 36Ar ratios, chemical compositions (H 2, N 2, CO 2, CH 4, O 2, Ar and He), and stable isotope compositions (δD H 2 , δD H 2O , δ 13C CO 2 , δ 13C CH 4 , and δ 15N N 2 ) are reported. Samples from the ophiolite region are significantly anoxic with major constituents of H 2, CH 4 and N 2, while those from calcite and dolomite regions are ordinary gas seeps, consisting of N 2, CO 2 and/or O 2. The former H 2-rich gas is characterized by relatively high 3He/ 4He ratio (0.4–0.8 R atm) with low He content (<5 ppm), atmospheric 40Ar/ 36Ar ratio, low N 2/Ar ratio (<55) and high δ 15N N 2 value (∼1 ‰). On the other hand, the latter N 2-rich gas shows relatively low 3He/ 4He ratio (0.1–0.4 R atm) with high He concentration (>300 ppm), slight radiogenic 40Ar/ 36Ar ratio, high N 2/Ar ratio (77–97) and low δ 15N N 2 value (<0‰). Observed δD H 2 value of −536‰ in H 2-rich gas is distinguished from the literature value of −699‰ in the ophiolite region, giving discrepant isotope formation temperatures.