ISOTOPIC CONSTRAINTS ON THE INFLUENCE OF THE ICELANDIC PLUME

Thermally buoyant mantle, in the form of a plume, rises beneath Icelan d creating a major topographic anomaly along the Mid-Atlantic Ridge an d in the surrounding ocean basin. However, the influence of the Icelan d plume on the composition of lavas erupted on adjacent ridges remains a contentious issue. Trace element systematics and radiogenic isotope ratios of Sr, Nd and Pb suggest that the plume influences a region 12 00 km in length. In contrast, the He-3 anomaly associated with Iceland closely corresponds to the 2400 km ridge section affected by thermal uplift. We present evidence that the Sr, Nd and Pb isotope signature o f the Iceland plume is in fact as widespread as its thermal and He-3 a nomalies. Results imply that much of the source of North Atlantic ridg e basalts has been contaminated by lateral outflow of asthenosphere fr om the Icelandic plume. Consequently, estimates of the average composi tion of mid-ocean ridge basalt (MORB) sources are likely to be biased by including data from plume-contaminated regions. True MORB values, a nd perhaps upper mantle geochemistry, can be constrained only by consi dering data untainted by plume asthenosphere.