Enriched and depleted primitive melts included in olivine from Icelandic tholeiites: origin by continuous melting of a single mantle column

Abstract

Iceland represents a type locality for mid-ocean ridge and plume-related magmatism. The petrogenesis of Icelandic lavas, however, is complicated by high-level crustal fractionation enhanced by the thick Icelandic crust, as well as assimilation of crustal material. Here we present results of major and trace elements studies of melt inclusions in high-magnesium olivines (Fo85.6–90.6) taken from the most primitive tholeiitic lavas found on Iceland. The compositions of the melt inclusions indicate that they represent very primitive trapped melts. Two populations of inclusions, enriched melt inclusions (EM: 0.07–0.23 wt% K2O, 0.07–0.52 wt% P2O5, and 0.54–1.78 wt% TiO2) and depleted melt inclusions (DM: 0.01–0.05 wt% K2O, 0.02–0.07 wt% P2O5, and 0.27–1.02 wt% (TiO2) can readily be distinguished on the basis of major and trace elements (e.g., (La/Sm)n ratios ranging between 0.14 and 1.89, (Sm/Yb)n ratios between 0.62 and 2.59, and Zr/Y ratios between 0.69 and 5.85). The compositions bracket the range of Icelandic primitive magmas, which we believe to be the result of mixing between these two endmembers.These two primary melt populations can be produced by critical (continuous) melting of a single mantle column, without the addition of material from the crust. In this model, the EM inclusions represent mixtures between enriched and depleted instantaneous melts in the ratios 0.6:0.4, respectively, where the first was formed in equilibrium with a garnet-bearing (up to 6 wt% of garnet) mantle source of primitive composition at a low degree of melting (F = 5.5%) and with 2.7 wt% of critical melt retained in residue. The most depleted inclusions represent unmixed instantaneous melts produced by 17–18% melting of a spinel lherzolite (either primitive or depleted composition) with slightly different amounts of critical melt (3.0 to 3.5 wt%) depending on the type of the source. In addition, Ba and Sr concentration anomalies noted in previous studies of Icelandic lavas are also present in the DM inclusions. They have been variously ascribed to assimilation-contamination processes, but can be explained by the presence of plagioclase in the source of the high level melt fractions. Thus, the complete range of Icelandic primary compositions can be produced by mineralogical variations in the mantle column in the framework of a dynamic melting model.