Olivine tholeiites (8-10 wt.% MgO) from Krafla show significant correlations between major elements (notably Fe) and incompatible trace elements. In particular, the samples with the highest Fe contents are the most enriched in elements such as K, Ti, and light rare earth elements (LREEs). The observed trends cannot be explained by fractional crystallization of olivine, plagioclase, or clinopyroxene from a single primary magma, nor are they likely to result from crustal contamination. The simplest explanation for the compositional variations is that they result from imperfect mixing of primary melts, produced at different levels in the upwelling asthenosphere, which later underwent olivine fractionation. Nd and Sr isotopic data hint at the possibility that some mixing between two (plume and non-plume) mantle sources may also be required. The average olivine tholeiite composition is compared with the average compositions of melts, predicted from parameterizations of melting experiments, produced from mantle with different potential temperatures. The predicted compositions were corrected for fractional crystallization before the comparison was made. The data compare well with the predicted average composition of melt from mantle with a potential temperature of approximately 1580-degrees-C. Differences between the observed and predicted compositions (notably higher Fe and lower Na in the Krafla basalts) are ascribed either to errors related to the modelling or to the effect of temperature- and velocity-structure of the mantle plume beneath Iceland. The average REE composition of the olivine tholeiites was then inverted to obtain the variation of melt fraction with depth. The predicted melt fraction rises from 0.0 at a depth of approximately 140 km (consistent with a potential temperature close to 1580-degrees-C) to a maximum value of approximately 0-3 at the surface. The predicted melt thickness (approximately 22 km when corrected for fractional crystallization) is consistent with geophysical estimates of crustal thickness.
