Journal of Conference Abstracts

Geochemical Structure of the Mid-Atlantic Ridge Between 10° and 41°N

L. Dosso CNRS, IFREMER, Centre de Brest, B.P.70, 29280-Plouzané Cédex, France

laure@ifremer.fr

O. Bonnier UBO - CNRS, IFREMER, Centre de Brest, B.P.70, 29280 Plouzané Cédex, France

C. Bollinger UBO - CNRS, IFREMER, Centre de Brest, B.P.70, 29280 Plouzané Cédex, France

H. Bougault CNRS, IFREMER, Centre de Brest, B.P.70, 29280 Plouzané Cédex, France

J. Etoubleau CNRS, IFREMER, Centre de Brest, B.P.70, 29280 Plouzané Cédex, France

C. Langmuir Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY 10964, USA

New geochemical data (trace element concentrations and isotopic compositions Sr-Nd-Pb) obtained on more than 50 samples collected along the Mid-Atlantic Ridge (MAR) between 31° and 41°N, mostly during the FAZAR cruise (1992), allow a better description of the regional geochemical gradient observed south of the Azores (Schilling, 1986).

In particular, Sr isotopic ratios show that the gradient extends beyond Hayes Fracture Zone (FZ), all the way until Kane FZ whereas La/Sm (Nb/Zr) ratios or Pb-Nd isotopic ratios indicate a more step-like transition when crossing Hayes FZ.

In a Sr-Nd diagram, the data describe an array which is in alignment with the Walvis Ridge data and is distinct from the array described by samples located further south between 10° and 24°N. The most radiogenic samples in Sr (0.7040) come from immediately north of the Oceanographer FZ. They are also the least radiogenic in Nd (0.5127) but also in Pb (²⁰⁶Pb/²⁰⁴Pb=17.78) and they have anomalously high Ce/Pb ratios (>40). All samples coming from the ridge segment located just north of Oceanographer FZ, including "group A" samples located off axis of the same ridge segment and described previously by Shirey et al. (1987), have enriched trace element characteristics and display a positive correlation in a Nd vs. Pb isotopic plot whereas all other samples define a negative correlation. These new data bring strong support to earlier findings of a new type of enriched mantle in the area (Shirey et al., 1987; Rideout and Schilling, 1985).

The Northern MAR between 10° and 41°N samples at least 4 different mantle sources: Azores type (OIB), North Oceanographer type, 17°N type (extremely depleted - very low Sr-Pb, very high Nd isotopic ratios) and 14°N type (Sr-Nd isotopic signature close to so-called HIMU type sources). It is only along the restricted portion of ridge segment located between 19° and 24°N that so-called "N-MORB" (MORB whose chemistry is not influenced by plume material in their source) can be sampled.

References

Rideout, M. & Schilling, J.-G., Init. Rep. DSDP 82 , 483-496 (1985).

Schilling, J.-G., The Geology of North America, The Geological Society of America, 137-156 (1986).

Shirey, S.B., Bender, J.F. & Langmuir, C.H., Nature 325, 217-223 (1987).

White, W.M., Schilling, J.-G. & Hart, S.R., Nature 263, 659-663 (1976).