Stable isotopes of helium, nitrogen and carbon in a coastal submarine hydrothermal system

Geothermal gases from submarine and subaerial hot springs in Ensenada, Baja California Norte, Mexico, were sampled for determination of gas chemistry and helium, nitrogen and stable carbon isotope composition. The submarine hot spring gas is primarily nitrogen (56.1% by volume) and methane (43.5% by volume), whereas nearby subaerial hot spring gases are predominantly nitrogen (95–99% by volume). The N₂/Ar ratios and σ ¹⁵N values of the subaerial hot spring gas indicate that it is atmospheric air, depleted in oxygen and enriched in helium. The submarine hot spring gas is most probably derived from marine sediments of Cretaceous age rich in organic matter. CH₄ is a major component of the gas mixture (σ ¹³C = −44.05%₀), with only minor amounts of CO₂ (σ¹³C= −10.46%₀). The σ¹⁵N of N₂ is + 0.2%₀ with a very high N₂/Ar ratio of 160. The calculated isotopic equilibra tion temperature for CH₄---CO₂ carbon exchange at depth in the Punta Banda submarine geothermal field is approximately 200°C in agreement with other geothermometry estimates. The ³He/⁴He ratios of the hot spring gases range from 0.3 to 0.6 times the atmospheric ratio, indicating that helium is predominantly derived from the radioactive decay of U and Th within the continental crust. Thus, not all submarine hydrothermal systems are effective vehicles for mantle degassing of primordial helium.