Historic magmatism on the Reykjanes Peninsula, Iceland: a snap-shot of melt generation at a ridge segment

We present new compositional data on a suite of historic lava flows from the Reykjanes Peninsula, Iceland. They were erupted over a short time period between c. 940 and c. 1340 ad and provide a snap-shot view of melt generation and evolution processes beneath this onshore, 65 km long, ridge segment. The lavas are tholeiitic basalts (MgO 6.5-9.2 wt%) and sparsely (a parts per thousand(a)5%) olivine and/or plagioclase phyric (+/- trace clinopyroxene). Individual eruptive events show remarkable compositional homogeneity. Despite a limited variation in Sr-Nd isotope compositions, high-precision double-spike Pb isotope data show tight coherent arrays that, together with correlations with incompatible trace element ratios, indicate control by binary mixing processes. Poor correlations with elemental abundances require that this mixing took place prior to extensive fractional crystallisation. Olivines in the historic lavas have light delta O-18 values (+4.2 to +4.3aEuro degrees), which is likely to be a feature of the enriched mantle source to Reykjanes Peninsula lavas. High precision Pb isotope analyses of other post-glacial Reykjanes Peninsula lavas show significant variability in Pb-207/Pb-204 and Pb-208/Pb-204 at lower Pb-206/Pb-204 values than in the historic lavas. This variation demonstrates that at least three compositionally distinct components within the mantle are required to explain the Pb isotope variations within the Reykjanes Peninsula as a whole.