Helium isotopes at the Rainbow hydrothermal site (Mid-Atlantic Ridge, 36°14′N)

Abstract

The 3He/ 4He ratio and helium concentration have been measured in the vent fluids and the dispersing plume of the Rainbow hydrothermal site, on the Mid-Atlantic Ridge (MAR). The 3He/ 4He ratio (7.51 R a) and 3He end-member concentration (25 pmol/kg) are in the range of observed values elsewhere on mid-ocean ridges, pointing to the relative homogeneity of the upper mantle with respect to helium isotope geochemistry. 3He is linearly correlated with methane and manganese throughout the plume, with CH 4/ 3He and Mn/ 3He ratios identical to those measured in the hot fluids. The bulk residence time of the plume in the rift valley estimated from the plume 3He budget is ∼20 days. The 3He flux transported by the plume, calculated from current-meter data, is 12.3±3 nmol/s, requiring a flux of 490 kg/s of high-temperature fluid. The scaling of the heat flux emitted by the Rainbow site to that of 3He, using the 3He/heat ratio measured in the hydrothermal fluids (9.3±2×10 −18 mol/J), indicates a heat output of 1320 MW. With a regional spreading rate of ∼25 mm/year, we calculate that the annual 3He flux for this section of the MAR is of the order of 0.5±0.2 mmol per kilometre of ridge per millimetre of newly formed crust. This figure compares well with the flux calculated for the neighbouring Lucky Strike segment. Although the uncertainties remain large, both fluxes are ∼40–50% above the world average (0.33 mmol/km/mm), thus supporting earlier suggestions that the intensity of the hydrothermal activity south of the Azores is higher than expected from the regional spreading rate.