HELIUM AND METHANE MEASUREMENTS IN HYDROTHERMAL FLUIDS FROM THE MID-ATLANTIC RIDGE - THE SNAKE PIT SITE AT 23-DEGREES-N

Methane and helium isotopes concentrations have been measured in hydrothermal fluids of black smoker vents at the Snake Pit area (MAR, 23-degrees-N) as part of the Hydrosnake cruise with the submersible Nautile. The samples contain large quantities of helium and methane with endmember values of 4.6 x 10(-5) cm3 STP/g for helium and 1.38 x 10(-3) cm3 STP/g for methane. The helium isotopic ratio He-3/He-4 is 8.4 times the atmospheric ratio. These values are strikingly similar to those already obtained on the East Pacific Rise. The CH4/He-3 ratio is also quite similar (CH4/He-3 = 2.6 x 10(6)) suggesting a non-biogenic origin of this gas in the fluids. Assuming that the range of variation of the He-3 concentration data obtained so far from various tectonic settings is close to the genuine range of values encountered on mid-ocean ridges, the global mass flow involved in the hydrothermal circulation at the ridge's axis is estimated between 1000 and 3000 m3 of water/sec, leading to a residence time of the global ocean with respect to the hydrothermal circulation between 15 and 45 million years. This estimate implies that, in addition to the individual vents (smokers), there is an extensive diffuse circulation of hydrothermal fluids along the ridges, in agreement with recent observations concerning heat budgets at hydrothermal sites in the Eastern Pacific.