MANTLE HE-3 IN HYDROTHERMAL VENTS AND PLUME OF THE LUCKY-STRIKE SITE (MAR 37-DEGREES-17'N) AND ASSOCIATED GEOTHERMAL HEAT-FLUX

The helium isotopes composition of hydrothermal fluids at the Lucky St rike vent field has been determined by mass spectrometry. [He-4] end-m ember concentration is in the 1.2 x 10(-5)-2.6 x 10(-5) cm(3) STP/g ra nge and the He-3/He-4 ratio, R, is 8.13 +/- 0.05 x R-a. The helium-3 b udget over the active vent field was estimated from the He-3 mapping o f the plume advected along the rift valley, using a 1-D advection box- model constrained with current velocity measurements. This leads to a He-3 flux of 2.3 x 10(-8) mol/s. The scaling of the heat flux from the Lucky Strike site to that of 3He, using the He-3/heat ratio measured in the hydrothermal fluids (5.2 x 10(-18)-7.7 x 10(-18) mol/J) allows an initial evaluation of the total geothermal heat output from the sit e, in the 3800 +/- 1200 MW range. This flux, of the same order as that of the Endeavour segment (han de Fuca Ridge), indicates the present h igh level of the hydrothermal activity in this section of the Mid-Atla ntic Ridge. (C) 1998 Elsevier Science B.V. All rights reserved.