Torfajökull: a radiogenic end-member of the Iceland Pb-isotopic array

Abstract

Torfajökull is the largest silicic center in Iceland and is located on the eastern branch of Iceland's neovolcanic zone. Torfajökull's location on the Iceland volcanic crust is unique because (1) it is situated behind the tip of a propagating ridge, and (2) several ridge jumps made it possible for lavas to extrude through relatively old basaltic crust (up to 10 Ma). Torfajökull is dominated by large amounts of silicic rocks, with less abundant basaltic and intermediate compositions. Several lavas show evidence of mixing of basaltic and rhyolitic melts. Varying degrees of assimilation of melts from hydrated and metamorphosed basaltic crust into primitive mantle-derived melts have been argued as the key mechanism in generating the variation of rock types and isotopic signatures found in the Icelandic lavas. Torfajökull, with its underlying old basaltic crust, offers the most favourable place to identify the contribution of crustal melts through radiogenic isotope analysis. Our data show that the Torfajökull lavas are among the most radiogenic Sr- and Pb-containing lavas in Iceland, but we find no correlation between major elements and the radiogenic isotope compositions. Basaltic, intermediate and rhyolitic lavas from the Torfajökull silicic center have 87Sr/ 86Sr- and 143Nd/ 144Nd-values of 0.70323–0.70342 and 0.512963–0.512999, respectively. Despite a large variation in SiO 2 concentrations (50%–74%), there are no significant differences in isotopic compositions between the different rock types. Torfajökull lavas plot towards the high 87Sr/ 86Sr end of the Icelandic Sr–Nd-isotope array. The Pb-isotope compositions are even more strikingly uniform ( 206Pb/ 204Pb 19.168–19.308, 207Pb/ 204Pb 15.513–15.587, 208Pb/ 204Pb 38.729–39.066) with no indication of radiogenic enrichment in the silicic lavas. The Pb-isotopic signature of the Torfajökull basaltic and silicic lavas clearly defines the most radiogenic end of the Iceland Pb-isotopic arrays. The radiogenic Sr- and Pb-isotope component of the Iceland isotopic array is well defined as a FoZo (focal zone) mantle component.