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

Torfajokull is the largest silicic center in Iceland and is located on the eastern branch of Iceland's neovolcanic zone. Torfajokull'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 l avas to extrude through relatively old basaltic crust (up to 10 Ma). Torfaj okull is dominated by large amounts of silicic rocks, with less abundant ba saltic 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-derive d melts have been argued as the key mechanism in generating the variation o f rock types and isotopic signatures found in the Icelandic lavas. Torfajok ull, with its underlying old basaltic crust, offers the most favourable pla ce to identify the contribution of crustal melts through radiogenic isotope analysis. Our data show that the Torfajokull lavas are among the most radi ogenic Sr- and Pb-containing lavas in Iceland, but we find no correlation b etween major elements and the radiogenic isotope compositions. Basaltic, in termediate and rhyolitic lavas from the Torfajokull silicic center have Sr- 87/Sr-86- and Nd-143/Nd-144-values of 0.70323-0.70342 and 0.512963-0.512999 , respectively. Despite a large variation in SiO2 concentrations (50%-74%), there are no significant differences in isotopic compositions between the different rock types. Torfajokull lavas plot towards the high Sr-87/Sr-86 e nd of the Icelandic Sr-Nd-isotope array. The Pb-isotope compositions are ev en more strikingly uniform ((206)pb/(204)pb 19.168-19.308, (207)pb/(204)pb 15.513-15.587, (208)pb/(204)pb 38.729-39.066) with no indication of radioge nic enrichment in the silicic lavas. The Pb-isotopic signature of the Torfa jokull basaltic and silicic lavas clearly defines the most radiogenic end o f the Iceland Pb-isotopic arrays. The radiogenic Sr- and Pb-isotope compone nt of the Iceland isotopic array is well defined as a FoZo (focal zone) man tle component. (C) 1999 Elsevier Science B.V. All rights reserved.