The dynamic evolution of the Iceland mantle plume: the lead isotope perspective

We report Pb isotope compositions of 42 Tertiary basalts from two widely separated paleo-rift zones from eastern and western Iceland spanning the age range 15.3-2.7 Ma. These samples have previously been well characterized in terms of major and trace element compositions, igneous and zeolite metamorphic petrography, K/Ar ages, and paleo-magnetostratigraphy. The Pb isotope temporal variation is smooth and cyclic. A maximum in radiogenic Pb is observed around 7-8 Ma, which is coincident in time with maxima in the volcanic production rates on Iceland and along the Reykjanes Ridge. In Pb isotope space the Iceland Tertiary basalts do not fit a binary mixing model between the depleted MORB source and a single radiogenic plume source, as previously suggested by La/Sm and Sr-87/Sr-86 variations. A third EM-I type component, apparently entrained by the Pb-206-rich plume, is suggested. We show that the discrepancy simply reflects that the La/Sm, Sr-87/Sr-86 (and Nd-143/Nd-144) of the two enriched plume components proposed are too similar to be resolvable in ternary mixtures containing a significant fraction of the depleted MORB mantle source (i.e. a third component). We also show that the contribution of the radiogenic Pb-rich plume component positively correlates with the lava production rate. The cyclicity in Pb isotope composition is consistent with a pulsating or blob-like plume interacting with the asthenospheric flow related to the Mid-Atlantic ridge spreading plate boundary. (C) 1997 Elsevier Science B.V.