Anomalous strontium and lead isotope signatures in the off-rift ?r?faj?kull central volcano in south-east Iceland Evidence for enriched endmember(s) of the Iceland mantle plume?

The currently active off-rift central volcano ?r?faj?kull in south-east Iceland sits unconformably on much older (~10¯12 Ma) and eroded crust. The composition of recent volcanics ranges from basalt to rhyolite, but the series is more sodic alkaline than the common rift zone tholeiitic suites. In this study we present Sr, Nd, Pb and O isotopic data for a suite of ?r?faj?kull samples. The complete suite shows typical mantle values for oxygen isotopes. The 87Sr/86Sr ratios (average of 15 SAMPLES=0.703702) of the modern ?r?faj?kull rocks (basalts as well as rhyolites) are much higher than observed so far for any other Icelandic rocks. The 143Nd/144Nd ratios (average=0.512947; n=15) are lower than for rift rocks, but similar to rocks of the off-rift Sn?fellsnes volcanic zone. Furthermore, the ?r?faj?kull rocks are enriched in the 207Pb/204Pb and 208Pb/204Pb isotope ratios compared to Icelandic rift basalts. The enriched nature of the suite indicates that ?r?faj?kull samples a source component that has characteristics common with EM2 type mantle. Furthermore, it is concluded that the silicic rocks of ?r?faj?kull formed by fractional crystallization from mafic melts rather than by partial melting of older crust.