抄録
Iceland is the largest landmass astride the Mid-Atlantic Ridge. Basaltic lava plateau in eastern Iceland was produced by crustal accretion in rift zones during the last 13 Myr. This area is an excellent field to study secular geochemical variation of Icelandic magmatism in order to understand the evolution of Iceland plume because it is straightforward to collect large numbers of samples from remarkably exposed successions of well-constrained stratigraphic relationships. Here we present major and trace element concentrations and Sr-Nd isotopic compositions of 120 basaltic samples broadly distributed throughout eastern Iceland.Tertiary volcanic rocks from eastern Iceland consist of tholeiitic basalts and volumetrically minor basaltic andesites. Tertiary basaltic rocks are enriched in incompatible trace element and radiogenic isotopes compared to postglacial tholeiitic basalts from active rift zones in Iceland and Reykjanes and Kolbeinsey Ridge MORBs. In the eastern Iceland lava section, trace element and Sr and Nd isotopic compositions systematically change with time. Fluctuations in La/Sm, Nb/Zr and 87Sr/86Sr are superimposed on a general trend of decreasing ratios from 13 to 1 Ma with positive peaks around 7-8 Ma. In contrast, Sm/Nd, Lu/Hf and 143Nd/144Nd show broad temporal increases with negative peaks around 7-8 Ma. Such secular geochemical variations might be due to a change in source mantle compositions and melt generation processes under Icelandic rift zones from 13 to 1 Ma. The isotope variation within lavas from eastern Iceland could be explained in terms of mixing of least two end-member components. Our data show that lavas erupted around 13 and 7-8 Ma had enriched isotopic compositions compared to lavas erupted during other periods. Contributions of the enriched source(s) might therefore be larger in 13 and 7-8 Ma magmatism than other periods.
