Gravity and bathymetry data along the Reykjanes Ridge indicate that hot asthenosphere derived from the Iceland mantle plume is dispersed southward beneath the spreading axis. Perturbations in the elevation of the axis relative to a predicted smooth decline in bathymetric depth away from Iceland indicate that some sections of the ridge lie above hotter mantle. These hotter, more elevated sections coincide with the intersection of chevron ridges and the spreading axis. Consequently, it can be inferred that the exhaust asthenosphere from the plume consists of hotter and cooler pulses.
We present evidence that the thermal pulsing of the plume affects the composition and mantle signature of ridge basalts over 1200km from the plume centre. Most of the trace element variation between hotter and cooler pulses can be explained by differences in the extent of melting, with melts from hotter pulses having lower Nb/Zr, La/Sm and Rb/Sr. However, superimposed on this melt variation is the chemical signature of the Iceland plume. This signature of low 143Nd/144Nd and high radiogenic Sr diminishes southward as far as 61.5°N. South of this latitude, each section of ridge produces a tight linear trend between Nd and Sr isotope ratios, with successive southward sections displaced to higher 87Sr/86Sr. These trends can be explained by mixing of rising MORB asthenosphere with variably depleted components from the Iceland plume. The most distal plume pulses appear to be the most depleted and have the highest Sr/Nd ratios, giving rise to the higher 87Sr/86Sr mixing trajectories. 3He/4He and Pb isotope ratios support the presence of plume mantle beneath the entire length of the ridge.