Journal of Conference Abstracts

Hydrothermal Activity on the Mid-Atlantic Ridge from 15°N to 65°N: Helium Isotope Distribution and Constraints on Hydrothermal Fluxes

P. Jean-Baptiste CEA-CNRS, LMCE-CFR, CEN-Saclay, 91191 Gif-Sur-Yvette, France

pjb@obelix.cea.saclay.fr

J. C. Dutay CEA-CNRS, LMCE-CFR, CEN-Saclay, 91191 Gif-Sur-Yvette, France

A. Dapoigny CEA-CNRS, LMCE-CFR, CEN-Saclay, 91191 Gif-Sur-Yvette, France

P. Allard CEA-CNRS, LMCE-CFR, CEN-Saclay, 91191 Gif-Sur-Yvette, France

H. Bougault IFREMER, Centre de Brest, BP 70, 29280 Plouzané Cédex, France

J.-L. Charlou IFREMER, Centre de Brest, BP 70, 29280 Plouzané Cédex, France

Y. Fouquet IFREMER, Centre de Brest, BP 70, 29280 Plouzané Cédex, France

J. Radford Knoery IFREMER, Centre de Brest, BP 70, 29280 Plouzané Cédex, France

A. Vangriesheim IFREMER, Centre de Brest, BP 70, 29280 Plouzané Cédex, France

H. D. Needham IFREMER, Centre de Brest, BP 70, 29280 Plouzané Cédex, France

P. Rona Institute of Marine and Coastal Sciences, Rutgers University, New Brunswick, NJ 08903-0231 USA

Mid-ocean ridge hydrothermal fluids are highly enriched in helium, with ⁴He concentrations up to 1000 times that of the oceanic background. This helium, characterized by an isotopic ratio of the order of 7-9 R_a (with R_a= atmospheric ratio=1.38x10^-6), is degassed from the mantle as part of the processes generating new oceanic crust. A similar mantle signature is also detectable in mid-ocean ridge basalts (MORB) with values in the range 7-10 R_a, increasing up to 20-30 R_a in hot spot areas such as Iceland and Hawaii. The study of helium isotopes, in conjunction with the isotopic composition of Sr, Nd, Pb and ratios of trace elements, is a major tool in geochemistry to understand mantle structure and history.

In this study, we review helium isotope data in Mid-Atlantic Ridge (MAR) hydrothermal fluids, including unpublished results from the Lucky Strike (37°17'N), Menez Gwen (37°50'N) and Broken Spur (29°10'N) sites, as well as from the newly discovered site at 14°45'N. Also included are helium isotope measurements obtained in 1994 from geothermal fluids and gas emanations on the islands of Graciosa and Flores (Azores archipelago) situated on both sides of the MAR at 39°-40°N, close to the ridge axis. The hydrothermal and geothermal fluids results are consistent with ³He/⁴He ratios reported in the literature for MAR basalts and complement this data set in the 20°-30°N region where no measurements have yet been available.

Subsequently, the mantle ³He distribution in the deep North Atlantic is calculated, mainly from the Geosecs and TTO data, by correcting for the tritiugenic ³He component. This artificial ³He contribution, which is large north of 35°N due to the large bomb tritium injection in the North Atlantic Deep Waters, is computed from the simulation of the tritium invasion using a general circulation model (OPA model developed at LODyC, University of Paris). The reconstructed natural ³He field is further used, combined with the OPA model, to compute the distribution and size of the ³He hydrothermal input along the MAR from 15° to 65°N. To achieve this, ³He sources are evenly distributed along the ridge axis using a detailed bathymetry, with their strength proportional to the local spreading rate. Then, the sources are adjusted to match the corresponding ³He oceanic distribution, computed with the OPA model, with the real distribution.

This ³He flux, computed for the global North Atlantic, is compared to the ³He flux at the scale of a single segment, using dynamic hydrocasts ³He data and current velocity measurements obtained for the Lucky Strike segment (37°N) as part of the MARFLUX project.