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

The currently active off-rift central volcano Or ae fajokull in south-east Iceland sits unconformably on much older (similar to 10-12 Ma) and eroded c rust. The composition of recent volcanics ranges from basalt to rhyolite, b ut the series is more sodic alkaline than the common rift zone tholeiitic s uites. In this study we present Sr, Nd, Pb and O isotopic data for a suite of Or ae fajokull samples. The complete suite shows typical mantle values f or oxygen isotopes. The Sr-87/Sr-86 ratios (average of 15 samples = 0.70370 2) of the modern Or ae fajokull rocks (basalts as well as rhyolites) are mu ch higher than observed so far for any other Icelandic rocks. The Nd-143/Nd -144 ratios (average= 0.512947; n = 15) are lower than for rift rocks, but similar to rocks of the off-rift Sn ae fellsnes: volcanic zone. Furthermore , the Or ae fajokull rocks are enriched in the Pb-207/Pb-204 and Pb-208/Pb- 204 isotope ratios compared to Icelandic rift basalts. The enriched nature of the suite indicates that Or ae fajokull samples a source component that has characteristics common with EN12 type mantle. Furthermore, it is conclu ded that the silicic rocks of Or ae fajokull formed by fractional crystalli zation from mafic melts rather than by partial melting of older crust. (C) 2001 Elsevier Science B.V. All rights reserved.