Nd and Sr isotope ratios and rare earth element abundances in Reykjanes Peninsula basalts evidence for mantle heterogeneity beneath Iceland

Abstract

Post-glacial tholeiitic basalts from the western Reykjanes Peninsula range from picrite basalts (oldest) to olivine tholeiites to tholeiites (youngest). In this sequence there are large systematic variations in rare earth element (REE) abundances (La/Sm normalized to chondrites ranges from 0.33 in the picrite basalts to 1.25 in the fissure tholeiites) and corresponding variations in 143Nd/ 144Nd (0.51317 in the picrite basalts to 0.51299 in the fissure tholeiites). The large viaration in 143Nd/ 144Nd, more than one-third the total range observed in most ocean islands and mid-ocean ridge basalts (MORB), is accompanied by only a small variation in 87Sr/ 86Sr (0.7031–0.7032). These 87Sr/ 86Sr ratios are within the range of other Icelandic tholeiites, and distinct from those of MORB. We conclude that the mantle beneath the Reykjanes Peninsula is heterogeneous with respect to relative REE abundances and 143Nd/ 144Nd ratios. On a time-averaged basis all parts of this mantle show evidence of relative depletion in light REE. Though parts of this mantle have REE abundances and Nd isotope ratios similar to the mantle source of “normal” MORB, 87Sr/ 86Sr is distinctly higher. Unlike previous studies we find no evidence for chondritic relative REE abundances in the mantle beneath the Reykjanes Peninsula; in fact, the data require significant chemical heterogeneity in the hypothesized mantle plume beneath Iceland, as well as lateral mantle heterogeneity from the Reykjanes Ridge to the Reykjanes Peninsula. The compositional range of the Reykjanes Peninsula basalts is consistent with mixing of magmas produced by different degrees of melting in different parts of the heterogeneous mantle source beneath the Reykjanes Peninsula.