Mantle 3He in hydrothermal vents and plume of the Lucky Strike site (MAR 37°17′N) and associated geothermal heat flux

The helium isotopes composition of hydrothermal fluids at the Lucky Strike vent field has been determined by mass spectrometry. [4He] end-member concentration is in the 1.2 × 10−5–2.6 × 10−5 cm3 STP/g range and the 3He/4He ratio, R, is 8.13 ± 0.05 × Ra. The helium-3 budget over the active vent field was estimated from the 3He mapping of the plume advected along the rift valley, using a 1-D advection box-model constrained with current velocity measurements. This leads to a 3He flux of 2.3 × 10−8 mol/s. The scaling of the heat flux from the Lucky Strike site to that of 3He, using the 3He/heat ratio measured in the hydrothermal fluids (5.2 × 10−18–7.7 × 10−18 mol/J) allows an initial evaluation of the total geothermal heat output from the site, in the 3800 ± 1200 MW range. This flux, of the same order as that of the Endeavour segment (Juan de Fuca Ridge), indicates the present high level of the hydrothermal activity in this section of the Mid-Atlantic Ridge.