Partial melting models for the petrogenesis of Reykjanes Peninsula basalts,Iceland Implications for the use of trace elements and strontium and neodymium isotope ratios to record inhomogeneities in the upper mantle

The mixing of magmas derived from two major compositional layers in a vertically stratified mantle has been favoured by Zindler et al. 1] in their interpretation of the REE and Sr and Nd isotope data for basalts from the Reykjanes Peninsula. However, a model involving the dynamic partial melting of a regionally homogeneous, veined mantle can also explain the major and trace element data and be reconciled with an alternative interpretation of the time relationships of the lavas to that presented by Jakobsson et al. 2]. Moreover, it is possible to explain the constant⁸⁷Sr/⁸⁶Sr but variable¹⁴³Nd/¹⁴⁴Nd ratios of the lavas by this model if the vein and wall rock components of the mantle source have equilibrated for Sr but not for Nd isotopes — a state that has been interpreted for some veined mantle nodules 13]. The model presented also involves more realistic degrees of partial melting than the alternative magma mixing models and satisfactorily explains the erupted volumes of the different magma types found in the area. Interpreting the basalt geochemistry in these terms suggests that Sr isotope ratios of the lavas monitor different scales of heterogeneity in the precursor mantle sources than Nd isotope ratios.