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, KAr 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 LaSm and 87Sr86Sr variations. A third EM-I type component, apparently entrained by the206Pb-rich plume, is suggested. We show that the discrepancy simply reflects that the LaSm, 87Sr86Sr (and143Nd144Nd) 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.