Osmium-oxygen isotopic evidence for a recycled and strongly depleted component in the Iceland mantle plume

Highly magnesian lavas characterised by strong light rare earth element depletion are a feature of Theistareykir and the Reykjanes Peninsula of Iceland, which are marginal to the proposed axis of the mantle plume. These lavas define positive covariations between whole rock osmium and olivine oxygen isotope ratios (Os-187/Os-188 = 0.1269-0.1369; delta(18)O(olivine) = 4.2-5.7parts per thousand) that extend the array defined by Hawaiian samples to more unradiogenic Os isotope ratios and lower delta(18)O. The Os-O variation is difficult to explain in terms of high level crustal assimilation of Icelandic crust, with the possible exception of a subset of large volume lava flows from Theistareykir. The strong coupling of Os and O isotopic compositions of the lavas in addition to large excesses in large ion lithophile elements (Rb, Ba, Sr), positive Eu anomalies, and deficiencies in Hf and Zr relative to the rare earth elements clearly distinguishes these recent picrites from mid-ocean ridge basalts. The Reykjanes and Theistareykir lavas appear to represent melting of a very ancient (Archaean) mantle source which has isotopic and elemental characteristics suggestive of recycled oceanic lithosphere. We suggest that tapping of the refractory and depleted part of such a mantle plume (i.e. low Os-187/Os-188 and delta(18)O) is only possible due to the fortuitous location of the Iceland plume beneath a spreading ridge, which permits more extensive melting than would occur in an intraplate setting (e.g. Hawaii). (C) 2001 Elsevier Science B.V. All rights reserved.