1. Department of Geosciences, The University of Tulsa, 600 College Avenue, Tulsa, Oklahoma 74104, USA
2. Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts 02138, USA
3. Department of Marine Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, USA
4. Max-Planck-Institut für Chemie, Postfach 3060, 55020 Mainz, Germany
5. Lamont-Doherty Earth Observatory, Palisades, New York 10964, USA
6. College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, Oregon 97331, USA
7. School of Ocean and Earth Science and Technology, University of Hawaii, 1680 East-West Road, Honolulu, Hawaii 96822, USA
8. Alfred Wegener Institute for Polar and Marine Research, Columbusstrasse, D27568 Bremerhaven, Germany
9. Institute of Geosciences, University of Kiel, Olshausenstrasse 40, 24118 Kiel, Germany
10. Marine Science Institute, The University of Texas at Austin, 750 Channel View Drive, Port Aransas, Texas 78373-5015, USA

Correspondence to: P. J. Michael¹ Correspondence and requests for materials should be addressed to P.J.M. (Email: pjm@utulsa.edu).

Abstract

A high-resolution mapping and sampling study of the Gakkel ridge was accomplished during an international ice-breaker expedition to the high Arctic and North Pole in summer 2001. For this slowest-spreading endmember of the global mid-ocean-ridge system, predictions were that magmatism should progressively diminish as the spreading rate decreases along the ridge, and that hydrothermal activity should be rare. Instead, it was found that magmatic variations are irregular, and that hydrothermal activity is abundant. A 300-kilometre-long central amagmatic zone, where mantle peridotites are emplaced directly in the ridge axis, lies between abundant, continuous volcanism in the west, and large, widely spaced volcanic centres in the east. These observations demonstrate that the extent of mantle melting is not a simple function of spreading rate: mantle temperatures at depth or mantle chemistry (or both) must vary significantly along-axis. Highly punctuated volcanism in the absence of ridge offsets suggests that first-order ridge segmentation is controlled by mantle processes of melting and melt segregation. The strong focusing of magmatic activity coupled with faulting may account for the unexpectedly high levels of hydrothermal activity observed.

1. Department of Geosciences, The University of Tulsa, 600 College Avenue, Tulsa, Oklahoma 74104, USA
2. Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts 02138, USA
3. Department of Marine Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, USA
4. Max-Planck-Institut für Chemie, Postfach 3060, 55020 Mainz, Germany
5. Lamont-Doherty Earth Observatory, Palisades, New York 10964, USA
6. College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, Oregon 97331, USA
7. School of Ocean and Earth Science and Technology, University of Hawaii, 1680 East-West Road, Honolulu, Hawaii 96822, USA
8. Alfred Wegener Institute for Polar and Marine Research, Columbusstrasse, D27568 Bremerhaven, Germany
9. Institute of Geosciences, University of Kiel, Olshausenstrasse 40, 24118 Kiel, Germany
10. Marine Science Institute, The University of Texas at Austin, 750 Channel View Drive, Port Aransas, Texas 78373-5015, USA

Correspondence to: P. J. Michael¹ Correspondence and requests for materials should be addressed to P.J.M. (Email: pjm@utulsa.edu).