Chemical stratigraphy, Iceland Research Drilling Project, Reydarfjordur, eastern Iceland

Major element data are reported for lavas and dikes from a 3.2-km section through the eastern Iceland lava pile. The section includes a 1.2-km subaerial exposure and a 1.9-km sequence drilled by the Iceland Research Drilling Project (IRDP). Alteration increases down-section, although primary magma chemistry may be interpreted from FeO* Al2O3, TiO2, P2O5 and (to a lesser extent) SiO2 in addition to immobile trace elements. Lava compositions range from MgO-rich tholeiite to SiO2-rich icelandite, while dikes range from tholeiite to tholeiitic andesite. Three stratigraphic divisions based on volcanologic and trace element criteria (e.g., Ce/Yb and Zr/Y) are confirmed by major elements. Upper and lower groups are predominantly basaltic while the middle group includes a high proportion of silicic lavas. Variation within groups conforms to simple models combining fractional crystallization and accumulation of phenocryst plagioclase. During build-up of the pile, eruptive phases commenced with silica-rich magma and frequently terminated with basaltic or intermediate plagioclase cumulates. Dike chemistry resembles that of upper group lavas and also the nearby Breiddalur volcano. Middle group lavas probably derive from a buried silicic complex, while the lower group may represent the flank of a more deeply buried complex. Chemical stratigrapy is consistent with iterative burial with spreading of lenticular basaltic-silicic complexes. Magma supply, fractionation, and mixing are considered in the framework of such kinematic models, although it is premature to draw direct analogies to the active Neogene zone of Iceland.