ENRICHED AND DEPLETED PRIMITIVE MELTS INCLUDED IN OLIVINE FROM ICELANDIC THOLEIITES - ORIGIN BY CONTINUOUS MELTING OF A SINGLE MANTLE COLUMN

Iceland represents a type locality for mid-ocean ridge and plume-relat ed magmatism. The petrogenesis of Icelandic lavas, however, is complic ated by high-level crustal fractionation enhanced by the thick Iceland ic crust, as well as assimilation of crustal material. Here we present results of major and trace elements studies of melt inclusions in hig h-magnesium olivines (Fo(8.5-90.6)) taken from the most primitive thol eiitic lavas found on Iceland. The compositions of the melt inclusions indicate that they represent very primitive trapped melts. Two popula tions 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 inclusio ns (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)(R) ratios ranging between 0.14 and 1.89, (Sm/Yb)(n) ra tios between 0.62 and 2.59, and Zr/Y ratios between 0.69 and 5.85). Th e compositions bracket the range of Icelandic primitive magmas, which we believe to be the result of mixing between these two endmembers. Th ese two primary melt populations can be produced by critical (continuo us) melting of a single mantle column, without the addition of materia l from the crust. In this model, the EM inclusions represent mixtures between enriched and depleted instantaneous melts in the ratio 0.6:0.4 , respectively, where the first was formed in equilibrium with a garne t-bearing (up to 6 wt% of garnet) mantle source of primitive compositi on 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 unmix ed instantaneous melts produced by 17-18% melting of a spinel Iherzoli te (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 previ ous studies of Icelandic lavas are also present in the DM inclusions. They have been variously ascribed to assimilation-contamination proces ses, but can be explained by the presence of plagioclase in the source of the high level melt fractions. Thus, the complete range of Iceland ic primary compositions can be produced by mineralogical variations in the mantle column in the framework of a dynamic melting model.