A High-Re/Os Component in the North Atlantic Asthenospheric Mantle
 
 

H Becker 1(301-405-0084; e-mail: hbecker@geol.umd.edu)

K. Haase 2

R. J. Walker 1

1 Department of Geology, University of Maryland, College Park MD 20742, USA

2 Institut für Geowissenschaften, Universität Kiel, Olshausenstrasse 40-60, D-24098 Kiel, Germany
 
 

 Whether or not the convecting upper mantle contains a minor high-Re/Os component is crucial for its Re budget and the issue of whether its 187Os/188Os is more similar to the abyssal peridotite average of ~0.125 or estimates for a putative primitive upper mantle of ~0.130. Because of the low Os abundances, and associated alteration problems, the 187Os/188Os of MORB, the most straightforward proxy for the isotopic composition of the asthenosphere, are difficult to evaluate. Independent information may be provided by high-MgO alkaline basalts from solitary seamounts and off-ridge axis seamounts in the ocean basins. Alkaline basalts occur at young seamounts near the Jan Mayen fracture zone in the north Atlantic and at Vesteris Seamount, a solitary volcanic seamount located 400 km west of the Mohns ridge segment. These basalts have very similar Sr-Nd-Pb isotopic compositions and preferentially sample incompatible element-enriched components in the sublithospheric mantle. Samples that show relatively high Os abundances (70-135 ppt) have 187Os/188Os of 0.133-0.145. The young ages (0-1 Ma for Vesteris, <5 Ma for others) of the basalts and moderate 187Re/188Os (14-182) rule out the possibility that the radiogenic Os reflects radiogenic growth in the basalts. The elevated Os abundances of some of the samples minimizes alteration effects. Hence, like the radiogenic 87Sr/86Sr, 206Pb/204Pb and unradiogenic 143Nd/144Nd of the basalts, the radiogenic Os is likely inherited from the mantle source. The Sr-Nd-Pb-Os isotopic systematics of the seamount basalts can be explained by the presence of a recycled (mostly unaltered?) MORB component in the source that retained most of its budget of incompatible elements during subduction zone processing, and is preferentially sampled by the alkaline basalts. The basalts are distinguished from plume-related ocean island basalts of similar major element compositions by the factor of 2-3 higher Re abundances (482-755 ppt Re), probably a result of higher Re in their mantle source. These results suggest that a portion of the asthenospheric mantle underlying the North Atlantic is characterized by suprachondritic 187Os/188Os and Re/Os.